WO2008069555A1

WO2008069555A1 - Apparatus and method for channel estimation and synchronization in ofdm/ofdma relay system

Info

Publication number: WO2008069555A1
Application number: PCT/KR2007/006258
Authority: WO
Inventors: Hyojin Lee; Hyun-Kyu Chung; Yong-Soo Cho; Kyung-Soo Woo; Hyun-Il Yoo; Heesoo Lee
Original assignee: Electronics And Telecommunications Research Institute; Chung-Ang University Industry-Academic Cooperation Foundation
Priority date: 2006-12-05
Filing date: 2007-12-04
Publication date: 2008-06-12
Also published as: KR100922739B1; KR20090074136A; KR20080052300A; KR100922729B1

Abstract

Provided are an apparatus and method for channel estimation and synchronization in an orthogonal frequency division multiplexing (OFDM)/ orthogonal frequency division multiplexing access (OFDMA) relay system. When data is received through a relay, channel estimation performance deterioration due to propagation delay can be improved in a transparent relay system or a cooperative relay system by extracting pilot signals based on a signal received from transmitters to a mobile station, estimating propagation delay between each of the transmitters, that transmitted the extracted pilot signals, and the mobile station based on the extracted pilot signals and preset reference pilot signals, and estimating a channel between the corresponding transmitter and the mobile station by estimating propagation delay weight of the corresponding transmitter based on the estimated propagation delay. Also, effects of a carrier frequency offset and timing offset of a link between the corresponding transmitter and the mobile station can be removed by storing the existing carrier frequency offset and timing offset of each transmitter, when the mobile station receives a signal, identifying a transmitter that transmitted the signal, and compensating the signal by using the stored carrier frequency offset and timing offset of the identified transmitter.

Description

[ 4 ]A carrier frequency offset due to a phase jitter, Doppler Shift, or the like occurs in a signal received through a channel, and a symbol tinning offset occurs according to a distance between a transmission terminal and a reception terminal. The carrier frequency offset generates interference between carrier waves, which changes power and phase of the signal, and the symbol timing offset generates ISI and inter-channel interference when the signal deviates from a CP section of an OFDM signal. Accordingly, when timing between the transmission terminal and the reception terminal remarkably deviates or a carrier frequency is different, interference occurs, and thus orthogonality between channels cannot be maintained. Consequently, an error rate while estimating a channel or interpreting a received signal increases. Also, the symbol timing offset generates linear phase distortion in a frequency domain, and a channel estimation performance deteriorates when a conventional method for channel estimation using a pilot is used.

[ 5 ]Accordingly, before performing a fast Fourier transform (FFT), the reception terminal has to synchronize a carrier frequency and timing between the reception terminal and the transmission terminal. In a conventional mobile communication system using an OFDM or orthogonal frequency division multiplexing access (OFDMA), a transmission terminal generally transmits a preamble signal for initial synchronization in front of a downlink frame. Also, in the conventional mobile communication system, a synchronization apparatus of a reception terminal acquires a carrier frequency and symbol timing synchronization with a base station (BS) using a preamble symbol transmitted from the BS.

[ 6 ] Since it is expected that a future mobile communication system will use a high frequency band higher than a band below or equal to 2 GHz used in the current mobile communication system, a cell radius may decrease and a shade region may increase accordingly. Also, improving a transmission speed in a limited transmission power causes a decreased signal to noise ratio (Eb/No), and thus performance of the mobile communication system deteriorates. [ 7 ]Accordingly, a multi hop relay system is being actively studied. The multi hop relay system is a type of mobile communication system for maximizing band efficiency by increasing transmission reliability or acquiring a multiplex gain, by transmitting a signal using a single or a plurality of relays scattered between the transmission terminal and the reception terminal. In a mobile communication system using a multi hop relay, a BS and a mobile station (MS) can directly communicate, but a MS on a cell boundary or in a propagation shade area may communicate with a BS through a fixed relay station (FRS) in a certain location. Since a multi hop is a general idea that can be shown in a structure wherein a 'relay' relays a signal, a multi hop relay system will hereinafter be referred to as a relay system.

[ 8 ] Through a relay function of a relay, a serviceable area can be expanded to a non-service area or a cell boundary, and also a throughput can be improved by reducing interference between neighboring cells. In a relay system, the BS and the relay are connected wirelessly, frequencies can be spatially reused by recognizing which MS is connected to the BS and the relay, and the BS centrally and intensively manages resources of the MSs connected to the relay.

[ 9 ] Types of a relay can be classified into a fixed relay, a nomadic relay, and a mobile relay according to portability. The fixed relay is fixed on a certain location like the BS. The fixed relay can be used to resolve a shade area and to expand a cell radius while the MS receives a signal. The nomadic relay can change its location when necessary, and can be used in a place for events or when the number of users increases in a short period of time. The mobile relay can be used in a train or bus.

[ 1 0 ] Alternatively, the relay can be classified into a transparent relay or a transparent mobile multi hop relay, which has a compatibility with a conventional mobile communication system since a relay system containing the transparent relay is designed in such a way that a MS do not need to know the existence of the relay, and a cooperative relay, which transmits a signal to a MS cooperatively with the BS in order to increase reliability of the signal received by the MS in a cooperative relay system. [ 1 1 ] FIG. 1 is a diagram illustrating each relay according to its use in an OFDM/ OFDMA relay system.

[ 1 2 ] Referring to FIG. 1 , throughput enhancement relay stations (TEs) 120 and 130 improve a throughput of MSs 121 and 131 in a low signal to interference plus noise ratio (SINR) area in a cell. The TEs 120 and 130 only transmit data, and a link between the TE 120 and the MS 121 and a link between the TE 130 and the MS 131 are controlled by a BS 110. Also, coverage extension relay stations (CEs) 140 and 150 expand a ceil area for MSs 141 and 151 outside the cell area that cannot be managed by the BS 110. The CEs 140 and 150 transmit a downlink and uplink control signal along with data, and a control signal between the CE and the MS 140 and 141 , or 150 and 151 is directly transmitted by the CE 140 or 150.

[ 1 3 ] In a transparent relay system, which includes a transparent relay, compatibility with a conventional mobile communication system is required, and thus a MS of the transparent relay system is not aware of the existence of the transparent relay. Accordingly, when the TEs 120 and 130 are the transparent relays, the MSs 121 and 131 in the areas of the TEs 120 and 130 perform synchronization with the BS 110, but do not perform synchronization with the TEs 120 and 130. Consequently, when data is received through the TEs 120 and 130, a channel estimation performance may deteriorate due to propagation delay.

[ 1 4 ] Also, in a cooperative relay system including a cooperative relay instead of a transparent relay, propagation delay of signals transmitted through cooperation occurs differently according to each transmission subject, and thus a channel estimation performance may deteriorate due to propagation delay.

[ 1 5 ] ln addition, when the MS moves in the transparent relay system, which includes the transparent relay, a carrier frequency offset of a link between the BS and the MS and a carrier frequency offset of a link between the relay and the MS largely differ. Accordingly, when synchronization with the BS is adjusted and data is received through the relay according to a wrongly estimated carrier frequency offset, performances of channel estimation and received signal interpretation deteriorate. In other words, when a carrier frequency offset is estimated according to a conventional method of a mobile communication system, the carrier frequency offset of the link between the relay and the MS cannot be estimated, and thus the performance of the mobile communication system deteriorates. Disclosure of Invention Technical Problem

[ 1 6 ] The present invention provides an apparatus for estimating a channel between a transparent relay and a mobile station, which can improve channel estimation performance deterioration due to propagation delay, in an orthogonal frequency division multiplexing (OFDM)/ orthogonal frequency division multiplexing access (OFDMA) transparent relay system.

[ 1 7 ] The present invention also provides an apparatus for estimating a channel between each transmitter and a mobile station, which can improve channel estimation performance deterioration due to propagation delay, in an OFDM/OFDMA cooperative relay system, which includes the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters.

[ 1 8 ]The present invention also provides an apparatus for synchronization, which synchronizes a transmitter, which transmitted a signal, and a mobile station in an OFDM/OFDMA transparent relay system.

[ 1 9 ]The present invention also provides a method for estimating a channel between a transparent relay and a mobile station, which can improve channel estimation performance deterioration due to propagation delay, in an OFDM/OFDMA transparent relay system.

[ 2 0 ]The present invention also provides a method for estimating a channel between each transmitter and a mobile station, which can improve channel estimation performance deterioration due to propagation delay, in an OFDM/OFDMA cooperative relay system, which includes the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters. [ 2 l ]Thθ present invention also provides a method for synchronization, which synchronizes a transmitter, which transmitted a signal, and a mobile station in an OFDM/OFDMA transparent relay system.

Technical Solution

[ 2 2 ] According to an aspect of the present invention, there is provided an apparatus for estimating a channel between a transparent relay and a mobile station in an orthogonal frequency division multiplexing (OFDM)/ orthogonal frequency division multiplexing access (OFDMA) transparent relay system, the apparatus comprising: a pilot extractor, which extracts pilot signals based on a frequency domain signal into which a signal that is received from the transparent relay by the mobile station and synchronized based on a signal transmitted from a base station to the mobile station is fast Fourier transformed; a propagation delay estimator, which estimates propagation delay between the transparent relay and the mobile station based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimator, which estimates a propagation delay weight for estimating the channel between the transparent relay and the mobile station based on the estimated propagation delay.

[ 2 3 ] According to another aspect of the present invention, there is provided an apparatus for estimating a channel between each of transmitters and a mobile station in an OFDM/OFDMA cooperative relay system, which comprises the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters, the apparatus comprising: a pilot extractor, which extracts pilot signals transmitted from the transmitters based on a frequency domain signal into which a signal that is received from the transmitters by the mobile station and synchronized based on a signal transmitted from the base station to the mobile station is fast Fourier transformed; a propagation delay estimator, which estimates propagation delay between each of the transmitters and the mobile station based on the extracted pilot signals, and reference pilot signals, which is preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimator, which estimates propagation delay weight for estimating the channel between each of the transmitters and the mobile station based on the estimated propagation delay between each of the transmitters and the mobile station.

[ 2 4 ]According to another aspect of the present invention, there is provided an apparatus for synchronization, which synchronizes a transmitter, which transmitted a signal to a mobile station, from among transmitters and the mobile station, in an OFDM/OFDMA transparent relay system comprising the transmitters including a base station and a transparent relay, and the mobile station, the apparatus comprising: an offset storage unit, which stores a pre-estimated carrier frequency offset and timing offset between each of the transmitters and the mobile station; a transmitter identifier, which identifies the transmitter, which transmitted a signal, when the mobile station receives the signal; and a compensator, which generates a compensated signal by compensating the signal received by the mobile station by using the stored carrier frequency offset and timing offset of the identified transmitter.

[ 2 5 ] According to another aspect of the present invention, there is provided a method of estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system, the method comprising: a pilot extracting operation which extracts pilot signals based on a frequency domain signal into which a signal that is received from the transparent relay by the mobile station and synchronized based on a signal transmitted from a base station to the mobile station is fast Fourier transformed; a propagation delay estimating operation which estimates propagation delay between the transparent relay and the mobile station based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimating operation which estimates a propagation delay weight for estimating the channel between the transparent relay and the mobile station based on the estimated propagation delay.

[ 2 6 ]According to another aspect of the present invention, there is provided a method of estimating a channel between each of transmitters and a mobile station in an OFDM/OFDMA cooperative relay system, which comprises the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters, the method comprising: a pilot extracting operation which extracts pilot signals transmitted from the transmitters based on a frequency domain signal into which a signal that is received from the transmitters by the mobile station and synchronized based on a signal transmitted from the base station to the mobile station is fast Fourier transformed; a propagation delay estimating operation which estimates propagation delay between each of the transmitters and the mobile station based on the extracted pilot signals, and reference pilot signals, which is preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimating operation which estimates propagation delay weight for estimating the channel between each of the transmitters and the mobile station based on the estimated propagation delay between each of the transmitters and the mobile station.

[ 2 7 ] According to another aspect of the present invention, there is provided a method for synchronization, which synchronizes a transmitter, which transmitted a signal to a mobile station, from among transmitters and the mobile station, in an OFDM/OFDMA transparent relay system comprising the transmitters including a base station and a transparent relay, and the mobile station, the method comprising: an offset storing operation which stores a pre-estimated carrier frequency offset and timing offset between each of the transmitters and the mobile station; a transmitter identifying operation which identifies the transmitter, which transmitted a signal, when the mobile station receives the signal; and a compensating operation which generates a compensated signal by compensating the signal received by the mobile station by using the stored carrier frequency offset and timing offset of the identified transmitter. Advantageous Effects

[ 2 8 ]According to the present invention, channel estimation, which can overcome performance deterioration due to propagation delay in a transparent multi hop relay system, can be performed. In other words, according to a conventional method of estimating a channel, when propagation delay exists, the performance of the entire system deteriorates since the effect of the propagation delay cannot be removed while estimating the channel. However, by using the method and apparatus of estimating a channel according to the present invention, the effect of the propagation delay can be removed, and thus the performance of estimating the channel can be improved. Also according to the method and apparatus for estimating a channel of the present invention, the channel can be estimated by estimating propagation delay weight by using a conventional pilot signal without using an additional training signal.

[ 2 9 ] Also, by using the present invention, the performance of estimating the cannel can be improved while estimating a channel that can overcome the performance deterioration due to the propagation delay in a cooperative multi hop relay system. In other words, when the propagation delay exists, the performance of the entire system deteriorates by using the conventional method of estimating a channel since the effect of the propagation delay cannot be removed. However, by using the method and apparatus for estimating a channel according to the present invention, the performance of estimating the channel can be improved by removing the effect of the propagation delay.

[ 3 O ] In addition, the application of the method and apparatus for estimating a channel of the present invention is not limited to a case when the total number of the base station and the relay are 2, but to a case when the total number is more than 2.

[ 3 1 ] Moreover, by using the present invention, performance of an OFDM/ OFDMA transparent relay system can be improved compared to using a conventional technology, in which the performance deteriorates due to the effects of carrier frequency offset and timing offset occurred as the mobile station does not synchronizes with the transparent relay. In other words, the performance of the entire system deteriorates since the effects of carrier frequency and timing offsets between the relay and the mobile station are not removed during the synchronization, but by using the method and apparatus for synchronization according to the present invention, the effects of the carrier frequency and timing offsets between the transparent relay and the mobile station can be removed.

Description of Drawings [ 3 2 ]The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: [ 3 3 ] FIG. 1 is a diagram illustrating each relay according to its use in an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiplexing access (OFDMA) relay system; [ 3 4 ] FIG. 2 is a diagram illustrating locations of a base station, relays, and mobile stations in an OFDM/OFDMA relay system; [ 3 5 ] FIG. 3A is a diagram illustrating a downlink frame block in an

OFDM/OFDMA transparent relay system; [ 3 6 ] FIG. 3B is a diagram illustrating an effect of propagation delay by a transparent relay on channel estimation in an OFDM/OFDMA transparent relay system; [ 3 7 ] FIG. 3C is a diagram illustrating an up/downlink frame block for analyzing symbol timing in an OFDM/OFDMA transparent relay system; [ 3 8 ] FIG. 4A is a diagram illustrating an apparatus for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to an embodiment of the present invention; [ 3 9 ] FIG. 4B is a diagram illustrating an apparatus for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to another embodiment of the present invention; [ 4 O ] FIG. 4C is a diagram illustrating a propagation delay estimator of an apparatus for estimating a channel between a transparent relay and a mobile station according to an embodiment of the present invention; [ 4 1 ] FIG. 4D is a diagram illustrating a weight estimator of an apparatus for estimating a channel between a transparent relay and a mobile station according to an embodiment of the present invention; [ 4 2 ] FIG. 4E is a diagram illustrating a channel estimator of an apparatus for estimating a channel between a transparent relay and a mobile station according to an embodiment of the present invention; [ 4 3 ] FIG. 5A is a flowchart illustrating a method for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to an embodiment of the present invention; [ 4 4 ] FIG. 5B is a flowchart illustrating a propagation delay estimation operation (S530) of the method of FIG. 5A according to an embodiment of the present invention; [ 4 5 ] FIG. 5C is a flowchart illustrating a weight estimation operation (S540) of the method of FIG. 5A according to an embodiment of the present invention; [ 4 6 ] FIG. 5D is a flowchart illustrating a channel estimation operation

(S550) of the method of FIG. 5A according to an embodiment of the present invention; [ 4 7 ] FIG. 6A is a diagram illustrating an up/downlink frame block for analyzing symbol timing according to a location of a mobile station in an

OFDM/OFDMA cooperative relay system; [ 4 8 ] FIG. 6B is a diagram for describing how a symbol timing offset occurs according to propagation delay in an OFDM/OFDMA cooperative relay system; [ 4 9 ] FlG. 7A is a diagram illustrating an apparatus for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to an embodiment of the present invention; [ 5 O ] FIG. 7B is a diagram illustrating a propagation delay estimator of the apparatus of FIG. 7A according to an embodiment of the present invention; [ 5 1 ] FIG. 7C is a diagram illustrating a weight estimator of the apparatus of

FIG. 7A according to an embodiment of the present invention; [ 5 2 ] FIG. 7D is a diagram illustrating a channel estimator of the apparatus of FIG. 7A according to an embodiment of the present invention; [ 5 3 ] FIG. 8A is a diagram illustrating an apparatus for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to another embodiment of the present invention; [ 5 4 ] FIG. 8B is a diagram illustrating propagation delay estimators of the apparatus of FIG. 8A according to an embodiment of the present invention; [ 5 5 ] FIG. 8C is a diagram illustrating weight estimators of the apparatus of

FIG. 8A according to an embodiment of the present invention; [ 5 6 ] FIG. 8D is a diagram illustrating channel estimators of the apparatus of

FIG. 8A according to an embodiment of the present invention; [ 5 7 ] FIG. 9A is a flowchart illustrating a method for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to an embodiment of the present invention; [ 5 8 ] FIG. 9B is a flowchart illustrating a propagation delay estimation operation (S930) of the method of FIG. 9A according to an embodiment of the present invention; [ 5 9 ] FIG. 9C is a flowchart illustrating a weight estimation operation (S940) of the method of FIG. 9A according to an embodiment of the present invention; [ 6 O ] FIG. 9D is a flowchart illustrating a channel estimation operation

(S950) of the method of FIG. 9A according to an embodiment of the present invention; [ 6 1 ] FIG. 1OA is a diagram illustrating an OFDM/OFDMA transparent relay system for analyzing an effect of a carrier frequency offset; [ 6 2 ] FIG. 10B is a diagram illustrating an occurrence of Doppler Shift according to movement of a mobile station in the OFDM/OFDMA transparent relay system of FIG. 10A; [ 6 3 ] FIG. 11 is a diagram illustrating an up/downlink frame block for analyzing a carrier frequency offset in an OFDM/OFDMA transparent relay system; [ 6 4 ] FIG. 12A is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to an embodiment of the present invention; [ 6 5 ] FIG. 12B is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to another embodiment of the present invention; [ 6 6 ] FIG. 12C is a diagram illustrating an estimator of an apparatus for synchronization according to an embodiment of the present invention; [ 6 7 ] FIG. 12D is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to another embodiment of the present invention; and [ 6 8 ] FIG. 13 is a diagram illustrating a method for synchronization in an

OFDM/OFDMA transparent relay system according to an embodiment of the present invention.

Best Mode [ 6 9 ] Hereinafter, the present invention will be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, like reference numerals denote like elements. [ 7 0 ] First, an apparatus and method of estimating a channel between a transparent relay and a mobile station in an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiplexing access

(OFDMA) relay system will be described. [ 7 I ] FIG. 2 is a diagram illustrating locations of a base station 110, relays

RSO and RS1 , and mobile stations in an OFDM/OFDMA relay system. [ 7 2 ] Referring to FIG. 2, a mobile station (MS) in a location 1 is synchronized with the base station 110, and receives data from the base station 110 or from the relay RSO for improving throughput. An MS in a location 2 is synchronized with the base station 110, and receives data from the relay RSO for improving throughput. The MS in a location 3 is synchronized with the relay RS1 for expanding a cell area, and receives data from the relay RS1 for expanding a cell area. [ 7 3 ] FIG. 3A is a diagram illustrating a downlink frame block in an

OFDM/OFDMA transparent relay system. [ 7 4 ]The OFDM/OFDMA transparent system of FIG. 3A is on the premise of the condition of FIG. 2. An MSO is in the location 1 and receives a signal from the base station 110, and an MS1 is in the location 1 and receives a signal from the relay RSO for improving throughput. An MS2 is in the location 2 and receives a signal from the relay RSO for improving data throughput, and an MS3 is in the location 3 and receives a signal from the relay RS 1 for expanding a cell area.

[ 7 5 ] Referring to FIG. 3A, P denotes a preamble for initial synchronization, FCH denotes a frame control header, which is a control signal, and MAP denotes MAP information. P, FCH, and MAP are all transmitted from the base station 110 or the relay RS1 for expanding a cell area. BS tx denotes a downlink frame transmitted from the base station 110, RSO tx denotes a downlink frame transmitted from the relay RSO for improving throughput, and RS1 tx denotes a downlink frame transmitted from the relay RS 1 for expanding a cell area. BS->MS/RS denotes a frame section transmitted by the base station 110, and RS->MS denotes a frame section transmitted by the relays RSO and RS1.

[ 7 6 ]Since the MSO receives the signal from the base station, the MSO performs synchronization with the base station 110 through a preamble transmitted from the base station 110. Also, the MS2 performs synchronization with the relay RS1 for expanding a cell area through a preamble transmitted by the relay RS1. The MS1 receives the signal from the relay RSO, but cannot be synchronized with the relay RSO. In other words, propagation delay of δ_BRo + δ_RaM] occurs in the signal received from the relay RSO, and propagation delay of δ_BM] occurs in a section synchronized with the base station 110, and thus propagation delay of δ_R(= δ_BRo + δ_RoMι -δ_mι ) occurs by receiving data from the RSO.

Propagation delay occurs similarly in the MS2. [ 7 7 ] FIG. 3B is a diagram illustrating an effect of propagation delay by a transparent relay on channel estimation in an OFDM/OFDMA transparent relay system. [ 7 8 ] Referring to FIG. 3B, an MS estimates symbol timing of the base station 110 for the signal received by the MS, removes a length of a cyclic prefix (CP) after the estimated symbol timing from the signal, and then performs fast Fourier transform (FFT) on the signal which the length of the CP is removed from, and thus an inter-symbol interference (ISI) does not occur. However, propagation delay of δ_R occurs in a frequency domain as phase rotation, and thus performance of the OFDM/OFDMA transparent relay system deteriorates while compensating a channel.

[ 7 9 ] Similar to the occurrence of the above-mentioned propagation delay, performance deterioration due to a carrier frequency offset occurs when the MS1 receives the signal from the relay RSO and is not synchronized with the relay RSO, but is synchronized with the base station 110. The carrier frequency offset does not affect the MSO, which receives a signal from the base station 110 that transmits the preamble for initial synchronization, and the MS3, which receives a signal from the relay RS1 that transmits the preamble for initial synchronization. As described above, the performance of an MS that receives a signal from a transparent relay for improving throughput in a transparent relay system may deteriorate due to an effect of a carrier frequency offset.

[ 8 O ] FIG. 3C is a diagram illustrating an up/downlink frame block for analyzing symbol timing in an OFDM/OFDMA transparent relay system.

[ 8 1 ] Referring to FIG. 3C, a reason for an occurrence of propagation delay in the OFDM/OFDMA transparent relay system including a transparent relay for improving throughput is shown. An MSO, which is located in a location 1 and receives data from a base station 110, receives a signal transmitted from the base station 110 continuously from a preamble symbol to the last downlink symbol. Since an MS1 , which is located in the location 1 and receives data from a relay RSO, and an MS2, which is located in a location 2 and receives data from the relay RSO, are both synchronized with the base station 110, and thus the MS1 and MS2 receive data from the relay RSO after being delayed from a symbol starting location acquired by the MS1 and MS2. In other words, a symbol timing offset due to propagation delay is not additionally occurred in the MSO, since the MSO continuously receives the signal from the base station 110 during the entire downlink section. However, a symbol timing offset due to propagation delay is additionally occurred in the MS1 and MS2, since the MS1 and MS2 receive the signal from the relay RSO despite that the MS1 and MS2 have acquired accurate synchronization using a preamble symbol of the base station 110. When the additional symbol timing offset exists, phase distortion is generated in a frequency domain, and thus a channel cannot be accurately estimated using a conventional method of estimating a channel. Accordingly, performance of the OFDM/OFDMA transparent relay system deteriorates.

[ 8 2 ] An apparatus and method of estimating a channel described in FIGS. 4 and 5 can be applied in a transparent relay system, which includes a base station, a transparent relay, and a mobile station, when the mobile station synchronizes with the base station and receives a data signal from the transparent relay.

[ 8 3 ] FIG. 4A is a diagram illustrating an apparatus for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to an embodiment of the present invention.

[ 8 4 ] FIG. 4B is a diagram illustrating an apparatus for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to another embodiment of the present invention.

[ 8 5 ] FIG. 5A is a flowchart illustrating a method for estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system according to an embodiment of the present invention.

[ 8 6 ] Operations of the method illustrated not only in FIG. 5A but also in FIGS. 5B₁ 5C, and 5D can be performed by elements of the apparatus illustrated not only in FIG. 4A but also in FIGS. 4B, 4C, 4D, and 4E. Accordingly, FIGS. 4 and 5 will be described together. Also, overlapping elements of FIGS. 4A and 4B will be described together.

[ 8 7 ] Referring to FIG. 4A, the apparatus includes a pilot extractor 410, a pilot storage unit 420, a propagation delay estimator 430, a weight estimator 440, a channel estimator 450, and a channel compensator 460.

[ 8 8 ] Referring to FIG. 4B, the apparatus further includes a synchronizer 404, a CP remover 405, a fast Fourier transformer 406, and a demodulator 407. In addition, the apparatus may further include an RF receiver 401 , an analog digital converter (A/D) 402, and a controller 403.

[ 8 9 ]The RF receiver 401 receives a time domain signal from the transparent relay through a reception antenna. The received time domain signal is digitalized in the A/D 402. The digitalized signal is synchronized in the synchronizer 404 based on a signal received from the base station to the RF receiver 401 , that is, a preamble transmitted from the base station. A CP of the synchronized signal is removed in the CP remover 405, and then the CP removed signal is converted to a frequency domain signal in the fast Fourier transformer 406. The frequency domain signal is inputted to the pilot extractor 410 and the channel compensator 460.

[ 9 O ] In order to describe the pilot storage unit 420 and the pilot extractor 410, a pilot signal will now be described. The pilot signal is a signal used to acquire time synchronization, acquire frequency synchronization, search for a cell, i.e. classify a base station, estimate a channel, and measure channel quality information.

[ 9 1 ] Specifically, in order to estimate a time-varying channel in the OFDM/OFDMA relay system, a transmission side transmits pilot signals, already known by the reception side, using pilot subcarriers allocated to some subcarriers in a symbol. Then, the reception side estimates a channel for subcarriers which actually include data and are transmitted to the reception side, by interpolation with the pilot signals.

[ 9 2 JAccordingly, the pilot signals, already known by the reception side, that is, the mobile station, are stored in the pilot storage unit 420 as reference pilot signals in operation S510.

[ 9 3 ] In operation S520, the pilot extractor 410 extracts the pilot signals received by the mobile station from the transparent relay included in the frequency domain signal, based on pre-known information about through which subcarrier in which symbol a pilot signal is transmitted, that is, based on a pre-known pilot location.

[ 9 4 ] The propagation delay estimator 430 estimates propagation delay between the transparent relay and the mobile station based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is corresponding to each of the extracted pilot signals in operation S530. Here, the reference pilot signals stored in the pilot storage unit 420 may be used.

[ 9 5 ] The weight estimator 440 estimates propagation delay weight for estimating the channel between the transparent relay and the mobile station based on the propagation delay estimated in the propagation delay estimator 430 in operation S540. Here, the propagation delay weight can be expressed as a channel transmission function, which compensates an effect of the estimated propagation delay on the frequency domain signal received by the mobile station.

[ 9 6 ] The channel estimator 450 estimates the channel between the transparent relay and the mobile station by using the pilot signals extracted by the pilot extractor 410, the reference pilot signals stored in the pilot storage unit 420, and the propagation delay weight estimated by the weight estimator 440 in operation S550.

[ 9 7 ] The channel compensator 460 performs channel compensation on the signal received by the mobile station based on the channel between the transparent relay and the mobile station estimated by the channel estimator 450 in operation S560.

[ 9 8 ] FIG. 6A is a diagram illustrating an up/downlink frame block for analyzing symbol timing according to a location of the mobile station in the OFDM/OFDMA cooperative relay system.

[ 9 9 ] FIG. 6B is a diagram for describing how the symbol timing offset occurs according to the propagation delay in the OFDM/OFDMA cooperative relay system.

[ 1 0 0 ] FIG. 6 assumes that the mobile station is placed between the base station and the relay, and receives a cooperative space-time block code (STBC) signal from the base station and the relay.

[ 1 0 1 ] Referring to FIG. 6A₁ the mobile station has already been synchronized with the base station, and thus propagation delay between a signal received from the base station and a signal received from the relay occurs.

[ 1 0 2 ] Referring to FIG. 6B, a symbol timing offset is additionally occurring due to the propagation delay. [ 1 0 3 ] Since the mobile station, receiving the cooperative STBC signal from two relays, also has been synchronized with the base station, symbol timing offset is additionally occurring in signals received from the two relays due to the propagation delay. In this case, when a channel is estimated using pilot signals, performance of the OFDM/OFDMA cooperative relay system deteriorates due to the channel estimation. And as degree of a modulation method increases, the performance of the OFDM/OFDMA cooperative relay system also deteriorates.

[ 1 0 4 ] The apparatus and method described with reference to FIGS. 7 through 9 can be applied in a cooperative relay system, which includes a base station, at least one cooperative relay, and a mobile station, when a signal transmitted from a transmitter, such as the base station or the cooperative relay, is transmitted to the mobile station.

[ 1 0 5 ] FIG. 7A is a diagram illustrating the apparatus for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to an embodiment of the present invention.

[ 1 0 6 ] FIG. 8A is a diagram illustrating the apparatus for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to another embodiment of the present invention.

[ 1 0 7 ] FIG. 9A is a flowchart illustrating the method for estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to an embodiment of the present invention.

[ 1 0 8 ] Operations of the method of not only FIG. 9A but also FIGS. 9B, 9C, and 9D can be performed by elements of the apparatus of not only FIG. 7A but also FIGS. 7B, 7C, and 7D, and thus FIGS. 7 and 9 will be described together. Also, overlapping elements of FIGS. 7 and 8 will be described together. In addition, referring to FIG. 8, a mobile station receives a signal from a base station and a relay 1 , or the relay 1 and a relay 2. [ 1 0 9 ] Referring to FIG. 7A, the apparatus includes a pilot extractor 710, a pilot storage unit 720, a propagation delay estimator 730, a weight estimator 740, a channel estimator 750, and a decoder 760.

[ 1 1 0 ] Referring to FIG. 8A, the apparatus includes first and second pilot extractors 811 and 812 corresponding to the pilot extractor 710 of FIG. 7A, first and second pilot storage units 821 and 822 corresponding to the pilot storage unit 710 of FIG. 7A, a propagation delay estimator 830 corresponding to the propagation delay estimator 730 of FIG. 7A, first and second weight estimators 841 and 842 corresponding to the weight estimator 740 of FIG. 7A, and first and second channel estimators 851 and 852 corresponding to the channel estimator 750 of FIG. 7A. The apparatus also includes a synchronizer 804, a CP remover 805, a fast Fourier transformer 806, and a modulator 807. In addition, the apparatus further includes an RF receiver 801, an analog digital converter (A/D) 802, and a controller 803.

[ I l l ] The RF receiver 801 receives a time domain signal from each transmitter, such as the base station or the relay, through a reception antenna. The time domain signal is digitalized in the A/D converter 802. The digitalized signal is synchronized in the synchronizer 804 based on a signal received by the RF receiver 801 , which is a preamble transmitted from the base station. A CP of the synchronized signal is removed in the CP remover 805, and then the CP removed signal is converted to a frequency domain signal in the fast Fourier transformer 806. The frequency domain signal is inputted to the pilot extractors 710, 811 , and 812 and decoders 760 and 860.

[ 1 1 2 ] The pilot storage unit 720 stores pilot signals of each of transmitters pre-known by the mobile station as reference pilot signals in operation S910.

[ 1 1 3 ] The pilot extractor 710 extracts the pilot signals transmitted from the transmitters and included in the frequency domain signal received by the mobile station in operation S920 by using pre-known information about which subcarriers of which symbol of each transmitter transmit pilot signals, i.e., by using pre-known pilot locations of each transmitter. [ 1 1 4 ] The propagation delay estimator 730 estimates propagation delay between a transmitter, which transmitted the extracted pilot signals, and the mobile station in operation S930 based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals. Here, the reference pilot signals stored in the pilot storage unit 720 may be used.

[ 1 1 5 ] The weight estimator 740 estimates propagation delay weight for estimating the channel between each transmitter and the mobile station in operation S940 based on the propagation delay between each transmitter and the mobile station estimated in the propagation delay estimator 730. Here, the propagation delay weight of each transmitter can be expressed as a channel transmission function which compensates an effect of the estimated propagation delay of the corresponding transmitter on the frequency domain signal received by the mobile station.

[ 1 1 6 ] The channel estimator 750 estimates a channel between each transmitter and the mobile station in operation S950 by using the pilot signals of each transmitter, that transmitted the signal to the mobile station, extracted by the pilot extractor, the reference pilot signals of the corresponding transmitter stored in the pilot storage unit 720, and the propagation delay weight of the corresponding transmitter estimated by the weight estimator 740.

[ 1 1 7 ] The decoder 760 decodes the signal received by the mobile station in operation S960 based on the channel between each transmitter and the mobile station estimated by the channel estimator 750.

[ 1 1 8 ] Hereinafter, an apparatus and method for synchronization in an OFDM/ OFDMA transparent system will be described.

[ 1 1 9 ] FIG. 1OA is a diagram illustrating an OFDM/OFDMA transparent relay system for analyzing an effect of a carrier frequency offset.

[ 1 2 0 ] FIG. 10B is a diagram illustrating an occurrence of Doppler Shift according to movement of a mobile station in the OFDM/OFDMA transparent relay system of FIG. 10A. [ 1 2 1 ] Specifically in FIG. 1OA, the effect of the carrier frequency offset is analyzed when a relay is a transparent relay for improving throughput. From among received signals, a path determined by path selection is indicated in a solid line. Referring to FIG. 1OA, BS denotes a base station, RSO and RS1 denote relays for improving throughput, and MSO and MS1 denote mobile stations. ε_BR , ε_m , and ε_m respectively denote a carrier frequency offset of a link between the base station and the relay, a carrier frequency offset of a link between the base station and the mobile station, and a carrier frequency offset of a link between the relay and the mobile station. Also, a suitable subscript is added according to a related relay and mobile station. A carrier frequency offset of the MS1 is estimated by using ε_Bm , which is a carrier frequency offset between the base station and the mobile station, and the MS1 receives data from the RSO. Since the received signal is received with ε_Rom _> which is a carrier frequency offset between the relay and the mobile station, and thus an actual carrier frequency offset of the MS1 is ε_BMi ~ ^ε _R0M\ ■ 'ⁿ the transparent relay system, which includes the transparent relay for improving throughput, the mobile station and the transparent relay both synchronize with the base station, and thus the carrier frequency offset between the base station and the mobile station and the carrier frequency offset between the relay and the mobile station can be assumed to be ε_Bm « ε_Rom .

[ 1 2 2 ] The above description assumes a difference of oscillators of a transmitter and receiver when the mobile station is stopped, without considering Doppler Shift. As described above, since the mobile station and the relay both estimate a carrier frequency based on the base station, S_BM\ ^ancl ^εκm_\ ^are equal to integral frequency offsets, and an error occurs only in a fractional frequency offset. In other words, when the mobile station is stopped, an error in estimating the carrier frequency of the oscillator causes deterioration of the performance of the OFDM/ OFDMA transparent relay system.

[ 1 2 3 ] Referring to FIG. 1OB, the mobile station moves to the base station at V km/h. 4 ] Since Doppler Shift is proportional to the speed of the mobile station and the carrier frequency, Doppler Shift should be considered in the next generation mobile communication system, which supports high speed mobile environment and has a high carrier frequency. When the mobile station moves towards the base station, the carrier frequency offset between the base station and the mobile station increases due to Doppler Shift, and when the mobile station moves away from the base station, the carrier frequency offset between the base station and the mobile station decreases. In other words, the carrier frequency offset between the base station and the mobile station is ε_mx + ε_Doppler , and the carrier frequency offset between the relay and the mobile station is ^εmu_\ - ε _Dop≠_er - Here, s _Doppler denotes a carrier frequency offset due to Doppler shift. The mobile station in the transparent relay system, which includes the transparent relay for improving throughput, synchronizes with the base station, and thus frequency offset between the base station and the mobile station is estimated as the carrier frequency offset. However, since the relay is fixed, the carrier frequency offset between the base station and the relay is not affected by Doppler shift. Accordingly when the mobile station moves, the carrier frequency offset between the base station and the mobile station and the carrier frequency offset between the relay and the mobile station are different. In other words, since the mobile station does not synchronizes to the carrier frequency between the relay and the mobile station while receiving data from the relay, interference between adjacent subcarriers occurs. When the mobile station receives data from the transparent relay for improving throughput in a mobile environment, the carrier frequency offset is ε_m = ε_M + K * ε_Dop≠er . Here, k denotes a constant determined according to a moving direction of the mobile station, and has a range between -2 to +2. The embodiment illustrated in FIG. 1OB is when the absolute value of k is maximum. The value of k is -2 when the mobile station moves towards the base station, and -2 when the mobile station moves towards the relay, on a straight line between the relay and the base station. [ 1 2 5 ] As described above, since the mobile station does not synchronize with the relay in the transparent relay system, the carrier frequency offset occurs, and thus the interference between adjacent subcarriers occurs after fast Fourier transform.

[ 1 2 6 ] FIG. 11 is a diagram illustrating an up/downlink frame block for analyzing a carrier frequency offset in an OFDM/OFDMA transparent relay system.

[ 1 2 7 ] Referring to FIG. 11 , a section 1100, in which performance deteriorates due to wrong carrier frequency offset, is illustrated when a signal received from a relay is compensated by estimating the carrier frequency using a front frame of a signal transmitted from a base station while a mobile station moves in the transparent relay system like FIG. 10. A carrier frequency offset between the relay and the mobile station should be separately acquired so that interference between adjacent subcariers due to the carrier frequency offset does not occur in the signal received from the relay.

[ 1 2 8 ] As described above, the carrier frequency offset between the base station and the carrier frequency offset between the relay and the mobile station are generally equal to an integral carrier frequency offset, but have an error of Kε_Doppler to a fractional carrier frequency offset.

Accordingly, the present application provides a method and apparatus for synchronization, in which the carrier frequency offsets are estimated and compensated by a carrier frequency estimator.

[ 1 2 9 ] A conventional carrier frequency and timing offsets tracking unit of a mobile communication system estimates carrier frequency and timing offsets by using a downlink symbol behind a preamble after initial synchronization. A mobile station in a system including a transparent relay for improving throughput receives a preamble and MAP from a base station, and data from the relay. In this case, when the conventional carrier frequency and timing offsets tracking unit is used, only timing and carrier frequencies of a signal received from the base station are tracked. Accordingly, when the mobile station receives a signal from the relay, performance of the mobile communication system deteriorates due to the carrier frequency offset. Consequently, a carrier frequency offset between the relay and the mobile station is tracked by using a signal received from the relay to the mobile station, and thus the mobile station should estimate a carrier frequency offset in a signal received from the relay from among downlink symbols, and then compensate the carrier frequency offset.

[ 1 3 0 ] However, interference between adjacent subcarriers due to wrongly estimated carrier frequency offset cannot be completely removed by only acquiring the carrier frequency offset between the relay and the mobile station, because a received signal should be pre-compensated with a value estimated in the previous operation while tracking the carrier frequency and timing offsets. In other words, when the mobile station receives a control signal, such as MAP, from the base station after estimating ε_m upon receiving the data from the relay, the mobile station compensates the received signal to ε_m , and then estimates ε_m by reading the received signal. The interference between the adjacent subcarriers occurs because the received signal is compensated with the wrong carrier frequency offset when the mobile station receives the received signal from the relay or the base station. Accordingly in the present invention, the method and apparatus for synchronization, which separately estimate and compensate the carrier frequency offset of the signal received from the base station and of the signal received from the relay, are provided.

[ 1 3 1 ] FIG. 12A is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to an embodiment of the present invention.

[ 1 3 2 ] FIG. 13 is a diagram illustrating a method for synchronization in an OFDM/OFDMA transparent relay system according to an embodiment of the present invention.

[ 1 3 3 ] Operations of the method of FIG. 13 can be performed by elements of the apparatus of FIG. 12A, and thus FIGS. 12A and 13 will be described together.

[ 1 3 4 ] Before describing FIGS. 12A and 13, processes of initializing an OFDM/ OFDMA transparent relay system, in which the current embodiments can be applied, will be described. [ 1 3 5 ] When a mobile station is turned on, initial synchronization is performed using a downlink signal received by the mobile station. In the initial synchronization, frame search, fractional carrier frequency offset estimation, symbol timing offset estimation, and cell search are performed. During the cell search, an integral carrier frequency offset is estimated by acquiring a segment and a cell ID, in which the mobile station belongs to.

[ 1 3 6 ] After performing a downlink initial synchronization, the mobile station performs an uplink initial ranging in an uplink section and simultaneously determines a path by using power of an uplink signal. The base station assigns a path (a base station or a relay) for transmitting data to the mobile station by using an uplink reception power.

[ 1 3 7 ] Through the initial synchronization, the mobile station estimates the carrier frequency and timing offsets with the base station. Also, the mobile station estimates the carrier frequency and timing offsets of a signal received from the relay, and estimates the carrier frequency offset between the relay and the mobile station by adding the estimated carrier frequency offset and an integral carrier frequency offset formed during the initial synchronization. An offset storage unit 1210, which will be described below, stores the carrier frequency and timing offsets of the base station and the carrier frequency and timing offsets of the relay, acquired through such processes of initializing an OFDM/ OFDMA transparent relay system, after estimating the carrier frequency offset between the relay and the mobile station is completed.

[ 1 3 8 ] Referring to FIG. 12A, the apparatus includes the offset storage unit 1210, a transmitter identifier 1220, a compensator 1230, an estimator 1240, and an updater 1250.

[ 1 3 9 ] The offset storage unit 1210 includes an offset storage unit

1211 for a base station, which stores pre-estimated carrier frequency offset and timing offset of the base station, and an offset storage unit

1212 for a relay, which stores pre-estimated carrier frequency offset and timing offset of the relay. (Operation S1310) [ 1 4 0 ] The transmitter identifier 1220 determines whether a signal received by the mobile station is transmitted from the base station or the relay based on a preamble included in a frame of the received signal. Here, when it is determined that the received signal is transmitted from the base station, i.e. when it is determined that the received signal is an MAP signal and data (BM signal) received from the base station, the received signal is inputted to a compensator 1231 for a base station. When it is determined that the received signal is data (BM signal) received form the relay, the received signal is inputted to a compensator 1232 for a relay. (Operation S1320)

[ 1 4 1 ] The compensator 1230 includes the compensator 1231 for a base station and the compensator 1232 for a relay, classified according to a subject that transmits the received signal.

[ 1 4 2 ] The compensator 1231 for a base station acquires έ_m and δ_m , which are respectively a carrier frequency offset and timing offset of the base station, from the offset storage unit 1211 of a base station, and compensates the received signal by using ε_m and δ_m .

[ 1 4 3 ] The compensator 1232 for a relay acquires έ_m and δ_m , which are respectively a carrier frequency offset and timing offset of the relay, from the offset storage unit 1212 of a relay, and compensates the received signal by using ε_m and δ_m .

[ 1 4 4 ] As described above, the compensator 1230 compensates the received signal from each transmitter by using the offsets of the corresponding transmitter. (Operation S1330)

[ 1 4 5 ] The estimator 1240 includes an estimator 1241 of a base station and an estimator 1232 of a relay, classified according to a subject that transmits the received signal.

[ 1 4 6 ] If the received signal is transmitted from the base station, and thus the received signal is compensated in the compensator 1231 of a base station, the compensated signal becomes an input b1 of the estimator 1241 of a base station.

[ 1 4 7 ] If the received signal is transmitted from the relay, and thus the received signal is compensated in the compensator 1232 of a relay, the compensated signal becomes an input b2 of the estimator 1242 of a relay.

[ 1 4 8 ] The estimator 1241 of a base station estimates the carrier frequency offset and timing offset between the base station and the mobile station in the compensated signal.

[ 1 4 9 ] The estimator 1242 of a relay estimates the carrier frequency offset and timing offset between the relay and the mobile station in the compensated signal.

[ 1 5 0 ] The estimator 1240 newly estimates carrier frequency offset and timing offset between the transmitter that transmitted a signal and the mobile station in order to update the pre-estimated carrier frequency offset and timing offset. (Operation S1340)

[ 1 5 1 ] The updater 1250 includes an updater 1251 of a base station and an updater 1252 of a relay classified according to a subject that transmitted the received signal.

[ 1 5 2 ] If the received signal is transmitted from the base station, and thus the carrier frequency offset and timing offset of the base station are estimated in the estimator 1241 of a base station, the estimated carrier frequency offset and timing offset become an input c1 of an updater 1251 of the base station.

[ 1 5 3 ] If the received signal is transmitted from the relay, and thus the carrier frequency offset and timing offset of the relay are estimated in the estimator 1242 of a relay, the estimated carrier frequency offset and timing offset become an input c2 of an updater 1252 of the relay.

[ 1 5 4 ] The updater 1251 of a base station transmits the estimated carrier frequency offset and timing offset of the base station to the offset storage unit 1211 of a base station so that the offset storage unit 1211 of a base station can update the stored carrier frequency offset and timing offset of the base station to the estimated carrier frequency offset and timing offset of the base station.

[ 1 5 5 ] The updater 1252 of a relay transmits the estimated carrier frequency offset and timing offset of the relay to the offset storage unit 1211 of a relay so that the offset storage unit 1211 of a relay can update the stored carrier frequency offset and timing offset of the relay to the estimated carrier frequency offset and timing offset of the relay.

[ 1 5 6 ] By updating the carrier frequency offset and timing offset between the transmitter that transmitted a signal and the mobile station, the updater 1250 can compensates the updated carrier frequency offset and timing offset for a following signal transmitted. (Operation S1350) Mode for Invention

[ 1 5 7 ] FIG. 4C is a diagram illustrating the propagation delay estimator 430 of the apparatus according to an embodiment of the present invention.

[ 1 5 8 ] FIG. 5B is a flowchart illustrating a propagation delay estimation operation (S530) of the method according to an embodiment of the present invention.

[ 1 5 9 ] Referring to FIG. 4C, the propagation delay estimator 430 includes a complex multiplier 431 , a conjugate complex number converter 432, an exponential function generator 433, an accumulation processor 434, an absolute value calculator 435, and a maximum value searcher 436.

[ 1 6 0 ] In operation S531 , the complex multiplier 431 generates a first complex value Y,(p)R^*(p)e^j2πpn/N proportional to a pilot signal Y,(p) , which is extracted from a frequency domain signal at a pilot location p of /th symbol by the pilot extractor 410, a conjugate complex number conversion value R^*(p) , in which a reference pilot signal corresponding to the pilot signal Y₁ (p) is converted to a conjugate complex number by the conjugate complex number converter 432, and an exponential function _e ^j2πpn/N , which is generated by the exponential function generator 433 in the pilot location p . Here, n denotes a time domain sample, and N denotes the size of fast Fourier transform. [ 1 6 1 ] In operation S532, the accumulation processor 434 generates a second complex value ^_jY,(p)Rj(p)e^j2"^p"'^N by accumulating the first

complex values Y,{ρ)R_ι ^*(p)e^j2'^φ"^IN of the extracted pilot signals. Here, S_p denotes a set of the pilot locations p . [ 1 6 2 ] The absolute value calculator 435 generates an absolute value

of the second complex value ∑y_t{p)R]{p)e^J2!φnlN in operation S533. peS,

[ 1 6 3 ] The maximum value searcher 436 estimates the propagation delay between the transparent relay and the mobile station by searching for a time domain sample δ_R , in which the absolute value of the second complex value reaches the maximum value in operation S534.

[ 1 6 4 ] FIG. 4D is a diagram illustrating the weight estimator 440 of the apparatus according to an embodiment of the present invention.

[ 1 6 5 ] FIG. 5C is a flowchart illustrating a weight estimation operation (S540) of the method according to an embodiment of the present invention.

[ 1 6 6 ] Referring to FIG. 4D, the weight estimator 440 includes a delta function generator 442 and a delayer 441.

[ 1 6 7 ] The weight estimator 440 generates a delta function δ(ή) in the time domain by using the delta function generator 442, and generates

δ(n-δ_{p Dday}) by delaying the time of the delta function δ(ri) by the

propagation delay estimated in the propagation delay estimator 430 by using the delayer 441 in operation S541. Also, the weight estimator 440 requests the fast Fourier transformer 406 to fast Fourier transform the delta function in the time domain with delayed time, and determines the propagation delay weight as the result of fast Fourier transforming the

delayed delta function δ(n -δ_{p Delαy}) in operation S542.

[ 1 6 8 ] Here, the fast Fourier transform is the basic technology of an OFDM technology, and thus the weight estimator 440 may perform the fast Fourier transform by itself or request an element other than the fast Fourier transformer 406 to perform the fast Fourier transform.

[ 1 6 9 ] FIG. 4E is a diagram illustrating the channel estimator 450 of the apparatus according to an embodiment of the present invention. [ 1 7 0 ] FlG. 5D is a flowchart illustrating a channel estimation operation (S550) of the method according to an embodiment of the present invention.

[ 1 7 1 ] Referring to FIG. 4E, the channel estimator 450 includes a phase distortion remover 451 , an interpolator 452, and a channel estimation determiner 453.

[ 1 7 2 ] Also, the phase distortion remover 451 includes a conjugate complex number converter 451a , a complex multiplier 451b, and a complex divider 451c.

[ 1 7 3 ] The phase distortion remover 451 acquires the extracted pilot signal Y,(p) from the pilot extractor 410. The extracted pilot signal Y₁ (p) is inputted to the complex multiplier 451 b. Also, the phase distortion remover 451 acquires propagation delay weight W(p) in the pilot location p of the extracted pilot signal Y,(p) by using the estimated propagation delay weight in the weight estimator 440. Conjugate complex number conversion on the propagation delay weight W{p) is performed after W(p) is inputted to the conjugate complex number converter 451a. Then, the result W^*(p) and the extracted pilot signal Y₁(P) are inputted to the complex multiplier 451b, and then multiplied. In operation S551 , the complex divider 451c acquires a phase distortion removed channel H(p) , in which phase distortion occurring due to propagation delay between the transparent relay and the mobile station in each pilot location p is removed in each pilot location p by dividing Y,(p)W^*(p) by a reference pilot signal R₁ (p) of the pilot location p stored in the pilot storage unit 420.

[ 1 7 4 ] In operation S552, the interpolator 452 acquires an interpolated channel H(k) by performing interpolation on a frequency domain of the frequency domain signal based on the phase distortion removed channel H(P) .

[ 1 7 5 ] In operation 553, the channel estimation determiner 453 determines the channel between the transparent relay and the mobile station based on the interpolated channel and the propagation delay weight. The channel can be determined by multiplying the interpolated channel H(k) and the propagation delay weight W(Jc) estimated by the weight estimator 440. The result of multiplication is delivered to the channel compensator 460. [ 1 7 6 ] The apparatus for estimating a channel between the transparent relay and the mobile station in the OFDM/OFDMA transparent relay system will now be described with reference to FIGS. 4 and 5 based on

Equations. [ 1 7 7 ] In the transparent relay system, a signal of /th OFDM symbol transmitted from a transmission side, i.e. a subject that transmits a signal, such as the base station or the relay, can be expressed as follows.

[ 1 7 8 ] x_ι {ή) = —γ_dX_ι {k)e^JlmklN ... Equation 1

[ 1 7 9 ] Here, x,(n) and X₁(Jc) respectively denote a time domain signal of an nth time domain sample of the /th OFDM symbol and a k th frequency domain signal of the / th OFDM symbol. Also, N denotes the size of the fast Fourier transform.

[ 1 8 0 ] A signal which is received from the relay by the mobile station, when the OFDM symbol transmitted by the transmission antenna passes through the channel, and a carrier frequency offset, symbol timing offset, and propagation delay exist, can be expressed as follows.

[ 1 8 1 ] y,(n) = —∑ H(k)X_l (k)e^{J2πik+ε)0>+δ+δ«)/N} ... Equation 2

[ 1 8 2 ] Here, y,(ή) denotes a received signal of the rath time domain sample of the /th OFDM symbol, H(k) denotes a frequency response of the channel between the transparent relay and the mobile station, i.e. a channel transmission function, ε denotes a normalized carrier frequency offset (CFO) between the base station and the mobile station, δ denotes a normalized symbol timing offset (STO) between the base station and the mobile station, and δ_R denotes normalized propagation delay between the relay and the mobile station. [ 1 8 3 ] A signal which is carrier frequency synchronized and symbol timing synchronized with the base station by using a conventional method via the synchronizer 404 of FIG. 4B can be expressed as follows.

[ 1 8 4 ] y_l (ή) = —y_jH{k)X,(k)e^j2'^A{"^+δ"^)IN ... Equation 3

[ 1 8 5 ] When Equation 3 is fast Fourier transformed and then shown in a frequency domain signal, it is as follows.

[ 1 8 6 ] Y_l(k) = H(,k)X,(k)e'²*^δ*'^N ... Equation 4

[ 1 8 7 ] As shown in Equation 4, the frequency domain signal with the propagation delay is still affected by the propagation delay due to the relay even if the frequency domain signal has acquired synchronization with the base station by using the estimated carrier frequency offset and symbol timing offset. Accordingly, an offset due to the propagation delay should be accurately estimated, and a process of estimating the propagation delay is as follows.

... Equation 5

[ 1 8 9 ] The propagation delay between the transparent relay and the mobile station is estimated using Equation 5. Here, p denotes a pilot location, which is a preset frequency location in the /th OFDM symbol which the frequency domain signal belongs to, S_p denotes a set of pilot locations p , δ_R denotes an estimated propagation delay, n denotes a time domain sample, Y,(p) denotes a pilot signal extracted by the pilot extractor 410 in the pilot location p , Rj (p) denotes a conjugate complex number conversion signal of a reference pilot signal in the pilot location p stored in the pilot storage unit 420, and N denotes the size of the fast Fourier transform. [ 1 9 0 ] When Y,(p) and R,(p) are the same signal,

∑ F, 00*; QOe jlπpnl N , which is an output of the absolute value calculator peS p

435 of FIG. 4C, has a form of a channel impulse response starting from a location corresponding to the propagation delay. Assuming that the channel impulse response exponential functionally decreases, the propagation delay can be estimated by acquiring a location of the time domain sample in which the output of the absolute value calculator 435

j2φn/N

∑7_;Q0Λ;Q0e reaches the maximum value. At this time, an peS, p integral carrier frequency offset should not occur in order to estimate accurate propagation delay, and thus the propagation delay is estimated using a signal that passed through the synchronizer 404 of FIG. 4B. 1 9 1 ] The propagation delay estimated in Equation 5 is inputted to the weight estimator 440, and a process of estimating propagation delay weight by using the estimated propagation delay can be expressed as follows.

N_l -jlnknlN

[ 1 9 2 ] W(k) = ∑δ(n- δ_R)e ... Equation 6 n=0

[ 1 9 3 ] Here, δ(ή) denotes a delta function and W(Jk) denotes the propagation delay weight estimated by the weight estimator 440 in a frequency location k of a subcarrier, which transmitted the received signal, on the frequency domain. The propagation delay weight is a value for estimating the channel between the transparent relay and the mobile station. Equation 6 means that fast Fourier transform is performed on a delayed delta function which expresses that the delta function is delayed by the propagation delay.

[ 1 9 4 ] The channel estimator 450 removes phase distortion occurring due to the propagation delay in the received frequency domain signal, performs interpolation on the phase distortion removed channel, and then estimates the channel for the entire subcarriers. Then, the channel estimator 450 distorts the phase of the estimated channel so that the phase distorted channel is equal to a channel which the actually received signal passed through. A process of removing the phase distortion occurring due to the propagation delay in the received frequency domain signal can be expressed as follows.

H(p) =

R₁(P)

[ 1 9 5 ] Equation 7

= H(p)

[ 1 9 6 ] Here, H(p) , Y₁(P) , and W^* (p) respectively denote a phase distortion removed channel, in which phase distortion due to the propagation delay in the pilot location p is removed, a frequency domain signal received by the mobile station in the pilot location p , and a conjugate complex number conversion value of the propagation delay weight estimated by the weight estimator 440 in the pilot location p .

[ 1 9 7 ] A process of estimating the channel value for the entire subcarriers from Equation 7 can be expressed as follows.

[ 1 9 8 ] H(_j,) ^Li"^earl"^terp°^°ⁿ ^(fc) Equation 8

[ 1 9 9 ] Here, H(k) denotes an estimated channel for the entire subcarriers.

[ 2 0 0 ] A process of compensating H(A;) for the phase distortion due to the propagation delay, in order to estimate a channel actually experienced by the received signal can be expressed as follows.

[ 2 0 1 ] H(Jc) = H(K)W (k) = H(k)e^j2πkδ"^IN ... Equation 9

[ 2 0 2 ] Here, H(A:) denotes an estimation value of a channel actually experienced by the received frequency domain signal. [ 2 0 3 ] When Equation 4 is substituted by Equation 9, it can be expressed as follows. [ 2 0 4 ] Y,(k) = H(Jc)X₁ (Jc) ... Equation 10

[ 2 0 5 ] By dividing both members by H(k) , X₁(Ic) can be calculated. Accordingly, the channel estimator 460 can estimate the transmitted signal X₁(Ic) by performing channel compensation on the frequency domain signal Y₁(Ic) , that the mobile station received, through the channel H(k) estimated by the channel estimator 450.

[ 2 0 6 ] An apparatus and method of estimating a channel between each transmitter and a mobile station in an OFDM/OFDMA cooperative relay system according to embodiments of the present invention will now be described.

[ 2 0 7 ] In order to increase reliability of a received signal, a diversity gain can be acquired in a cooperative multi hop relay system that cooperatively transmits the received signal using a base station and relays. Since a mobile station recognizes the relay in the cooperative multi hop relay system, a path selection may be considered in the mobile station by using a preamble transmitted from the base station and the relay. However, propagation delay and a frequency offset are different each in a signal received from the base station and a signal received from the relay, and thus acquiring accurate carrier synchronization and timing synchronization is difficult.

[ 2 0 8 ] FIG. 7B is a diagram illustrating the propagation delay estimator 730 of the apparatus according to an embodiment of the present invention.

[ 2 0 9 ] FIG. 8B is a diagram illustrating the propagation delay estimators 831 and 832 of the apparatus according to an embodiment of the present invention.

[ 2 1 0 ] FIG. 9B is a flowchart illustrating a propagation delay estimation operation (S930) of the method according to an embodiment of the present invention.

[ 2 1 1 ] Referring to FIG. 7B₁ the propagation delay estimator 730 includes a complex multiplier 731 , a first accumulation processor 732, an absolute value processor 733, a second accumulation processor 734, a maximum value searcher 735, an exponential function generator 736, and a conjugate complex number converter 737.

[ 2 1 2 ] Referring to FIG. 8B, the propagation delay estimator 830 includes a complex multiplier 831, a first accumulation processor 832, an absolute value calculator 833, a second accumulation processor 834, a maximum value searcher 835, an exponential function generator 836, and a conjugate complex number converter 837.

[ 2 1 3 ] In operation S931 , the complex multiplier 731 or 831 generates first complex values proportional to i) the pilot signals extracted by the pilot extractor 710 or the first and second pilot extractors 811 and 812, ii) conjugate complex number converted values of reference pilot signals corresponding to the extracted pilot signals in the conjugate complex number converter 737 or 837, and iii) exponential functions generated by the exponential function generator 736 or 836 based on the corresponding pilot locations, in each of symbols in a predetermined section from among symbols that are transmitted by the each corresponding transmitter that transmitted a signal to the mobile station. Herein, first and second symbols in FIG. 8 are described as the symbols in the predetermined section.

[ 2 1 4 ] The first accumulation processor 732 or 832 generates a second complex value of each of the symbols in the predetermined section for each transmitter in operation S932 by accumulating the first complex values on pilot locations of the reference pilot signals of each transmitter in each symbol in the predetermined section.

[ 2 1 5 ] The absolute value calculator 733 or 833 generates an absolute value of the second complex value in operation S933.

[ 2 1 6 ] The second accumulation processor 734 or 834 generates an accumulation value of each transmitter in operation S934 by accumulating the absolute values of the second complex values for each transmitter.

[ 2 1 7 ] The maximum value searcher 735 or 835 estimates propagation delay between each transmitter and the mobile station in operation S935 by searching for a time domain sample, in which the accumulation value of each transmitter reaches the maximum value. [ 2 1 8 ] FIG. 7C is a diagram illustrating the weight estimator 740 of the apparatus of according to an embodiment of the present invention.

[ 2 1 9 ] FIG. 8C is a diagram illustrating the weight estimators 841 and 842 of the apparatus according to an embodiment of the present invention.

[ 2 2 0 ] FIG. 9C is a flowchart illustrating a weight estimation operation (S940) of the method according to an embodiment of the present invention.

[ 2 2 1 ] Referring to FIG. 7C, the weight estimator 749 includes a delta function generator 741 and a delayer 742.

[ 2 2 2 ] Referring to FIG. 8C, the first weight estimator 841 includes a first delta function generator 841a and a first delayer 841 b, and the second weight estimator 842 includes a second delta function generator 842a and a second delayer 842b.

[ 2 2 3 ] The weight estimator 740 or the first and second weight estimators 841 and 842 generate a time delayed delta function of each transmitter in operation S941 by generating a delta function δ{ή) of each transmitter in a time domain by using the delta function generator 741 or the first and second delta function generators 841a and 842a, and delaying the time of the delta function δ(ή) of each transmitter by propagation delay between the corresponding transmitter and the mobile station estimated in the propagation delay estimator 730 or 830 by using the delayer 742 or the first and second delayers 841 b and 842b. The first and second weight estimators 841 and 842 requests the fast Fourier transformer 806 to fast Fourier transform the time delayed delta function of each transmitter, and determines the propagation delay weight of the corresponding transmitter as the result of fast Fourier transform in operation S942.

[ 2 2 4 ] Here, the fast Fourier transform is the basic technology of an OFDM technology, and thus the weight estimator 740 and the first and second weight estimators 841 and 842 may perform the fast Fourier transform by themselves or request an element other than the fast Fourier transformer 806 to perform the fast Fourier transform. [ 2 2 5 ] FIG. 7D is a diagram illustrating the channel estimator 750 of the apparatus according to an embodiment of the present invention.

[ 2 2 6 ] FIG. 8D is a diagram illustrating the channel estimators 851 and 852 of the apparatus according to an embodiment of the present invention.

[ 2 2 7 ] FIG. 9D is a flowchart illustrating a channel estimation operation (S950) of the method according to an embodiment of the present invention.

[ 2 2 8 ] Referring to FIG. 7D₁ the channel estimator 750 includes a phase distortion remover 751 , an interpolator 752, an average calculator 753, and a channel estimation determiner 754.

[ 2 2 9 ] Referring to FIG. 8D, the first channel estimator 851 includes a first phase distortion remover 881 , a first interpolator 882, a first average calculator 883, and a first channel estimation determiner 884, and the second channel estimator 852 includes a second phase distortion remover 885, a second interpolator 886, a second average calculator 887, and a second channel estimation determiner 888.

[ 2 3 0 ] Also, each of the first and second phase distortion removers 881 and 885 includes minor conjugate complex number converters, minor complex multipliers, and minor complex dividers.

[ 2 3 1 ] Also, the first and second interpolators 882 and 886 include minor interpolators.

[ 2 3 2 ] The phase distortion remover 751 of FIG. 7D acquires pilot signals, which are extracted from pre-determined pilot locations in each of symbols in a predetermined section for each transmitter, from the pilot extractor 710. Also, the phase distortion remover 751 acquires propagation delay weights in pilot locations of the extracted pilot signals from the weight estimator 740. Then, each of signal values, in which conjugate complex number conversions are performed on the propagation delay weights, and the corresponding pilot signal extracted by the pilot extractor 710 are multiplied. Next, a reference pilot signal stored in the pilot storage unit 720 in the corresponding pilot location is acquired, and the result of multiplication is divided by the value of the corresponding reference pilot signal. Accordingly, in operation S951, the phase distortion remover 751 acquires phase distortion removed channels, in each of which, phase distortion of the channel between each transmitter and the mobile station occurring due to propagation delay in each pilot location of the extracted pilot signals - the pre-determined pilot location - in each of symbols in a predetermined section from among symbols, which are included in the signal received by the mobile station and transmitted by the transmitters, is removed.

[ 2 3 3 ] The interpolator 752 acquires interpolated channels by performing interpolation on the phase distortion removed channels in a frequency domain of a frequency domain signal in operation S952. The interpolation may be linear interpolation.

[ 2 3 4 ] Since the number of acquired interpolated channel is equal to the number of symbols in the predetermined section for each transmitter, the average calculator 753 calculates an average of the number of interpolated channels equal to the number of the symbols of each transmitter in operation S953.

[ 2 3 5 ] The channel estimation determiner 754 estimates a channel between a transmitter and the mobile station based on the average of the interpolated channels and the propagation delay weight between the corresponding transmitter and the mobile station in operation S954. The estimated channel is transmitted to the decoder 760.

[ 2 3 6 ] A case when a signal is received through two transmitters, such as a case when the mobile station receives a cooperative STBC signal from the base station and the relay 1 , or the relay 1 and the relay 2 as illustrated in FIG. 8, will now be described in detail.

[ 2 3 7 ] Here, an STBC method has been developed in various forms, such as an STBC method suggested by S. M. Alamouti in ¹A simple transmitter diversity scheme for wireless communications', IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, Oct. 1998, and a STBC method suggested by Vahid Tarokh in 'Space time block coding from orthogonal designs', IEEE Trans, on Info., Theory, Vol. 45, pp. 1456-1467, July 1999. Since the mobile station receives a signal from two transmitters, a transmission antenna diversity (Tx.ANT diversity) method suggested by S. M. Alamouti will be described first, and FIG. 8 will be described based on the STBC method of S. M. Alamouti. [ 2 3 8 ] First, the transmission antenna diversity method is applied when two transmission antennas are used in a transmitter of a multiple-input multiple-output (MIMO) mobile communication system. Here, let's assume that xixj denotes serial modulation symbols inputted to the transmitter. Accordingly, space-time block encoding is performed on the serial modulation symbols xixj as expressed in Equation 11 by the transmission antenna diversity method.

X₁ X ,

[ 2 3 9 ] X₁₁ = ... Equation 11

- x,

[ 2 4 0 ] A matrix of X_l} in Equation 11 is an encoding matrix according to the transmission antenna diversity method, and * denotes a complex conjugate calculation.

[ 2 4 1 ] The matrix of Equation 11 is an encoding matrix of modulation symbols transmitted through the two transmission antennas. In the matrix, elements of each row correspond to the two transmission antennas, and elements of each column correspond to the two transmission antennas in the corresponding space-time.

[ 2 4 2 ] !n other words, in a first space-time tι, x, is transmitted through a first transmission antenna (Tx.ANT 1) and X_j is transmitted through a second transmission antenna (Tx.ANT 2), and in a second space-time t|+1 , - x] is transmitted through the first transmission antenna and x^* is transmitted through the second transmission antenna.

[ 2 4 3 ] A case when one base station and one relay exist in an OFDM/OFDMA cooperative relay system, which includes a base station and at least one cooperative relay, wherein the cooperative relay system uses a space-time coding (STC), here, STBC, according to the transmission antenna diversity method will now be described.

[ 2 4 4 ] When the numbers of transmission antennas in the base station and the relay are each one in the cooperative relay system, signals of first and second OFDM symbols, in which STBC is performed, transmitted from the transmitters can be expressed as follows.

[ 2 4 5 ] ... Equation 12

[ 2 4 6 ] Here, ^χ _BsΛⁿ) and

respectively denote n th transmission signals in a time domain sample of first and second OFDM symbols transmitted from the base station, x_ns^(n) and x_!iSι,(n) respectively denote rath transmission signals in a time domain sample of first and second OFDM symbols transmitted from the relay, and X_λ(k) and X_s(k) respectively denote signals transmitted by Hh subcarrier in a frequency domain. Assuming that the number of reception antenna of the mobile station is one, reception signals of the mobile station when OFDM symbols transmitted from the transmission antennas pass through a channel between the corresponding transmitter and the mobile station, and a carrier frequency offset, symbol timing offset, and propagation delay exist can be expressed as follows.

[ 2 4 7 ]

^■<V fc=0 N k=Q

[ 2 4 8 ] ... Equation 13

[ 2 4 9 ] Here, y_λ(ή) and y₂in) respectively denote reception signals of n th sample of first and second OFDM symbols, H_{m λ} (k) and H_ma(k) respectively denote frequency responses of a channel between the base station and the mobile station in a Hh subcarrier of the first and second OFDM symbols, H_{m ι}(k) and H_{m 2}(k) respectively denotes frequency responses of a channel between the relay and the mobile station in a &th subcarrier of the first and second OFDM symbols, ε denotes a normalized carrier frequency offset, δ denotes a normalized symbol timing offset, and δ_BM and δ_m respectively denote normalized propagation delay between the base station and the mobile station, and the relay and the mobile station. A reception signal, in which carrier frequency synchronization and symbol timing synchronization are performed with the base station by using the synchronizer 804 of FIG. 8A, can be expressed as follows.

(k)X₂(k)e^"^^IN

[ 2 5 1 ] ... Equation 14

[ 2 5 2 ] When Equation 14 is fast Fourier transformed, and then shown in a frequency domain signal, it can be expressed as follows.

Y_x (k) = H_m, {k)X_x (k)e^j2M™^IN + H_m, (K)X₂ (k)e^j2^'^IN

Equation 15

[ 2 5 4 ] When an average of channels during 2 continuous OFDM symbols is acquired for STBC decoding from Equation 15, it can be expressed as follows. H _m (k) = ^•-

[ 2 5 5 ] Equation 16

2 5 6 ] Equation 15 can be expressed in a following matrix via a conventional STBC decoding method by using Equation 16.

[ 2 5 7 ] ... Equation 17

[ 2 5 8 ] After conjugate complex number conversion is performed on Y₂(Jc) of Equation 17, STBC decoding processes can be expressed as follows.

[ 2 6 0 ] ... Equation 18

[ 2 6 1 ] As expressed in Equation 18, the propagation delay still affects the received frequency domain signal even when synchronization of the estimated carrier frequency offset and estimated symbol timing offset are acquired. Accordingly, propagation delay can be estimated using Equation 19 below. [ 2 6 2 ]

... Equation 19

[ 2 6 3 ] Propagation delay between each transmitter and the mobile station is estimated using Equation 19, and here, p_mλ , p_{BM<2 >} Pm _{\ >} and p_{m 2} respectively denote preset pilot locations in the first and second OFDM symbols, transmitted from the base station, between the base station and the mobile station and preset pilot locations in the first and second OFDM symbols between the relay and the mobile station, ^m. iiP BM. x) _^ ^RBMAPBM,₂) , ^RRMAPRM,I) > ^and RRMΛPM,I) respectively denote pilot signals of first and second OFDM symbols, transmitted from the base station to the mobile station, in the corresponding pilot location, and pilot signals of first and second OFDM symbols, transmitted from the relay to the base station, in the corresponding pilot location, i.e. reference pilot signals, and δ_m and δ_m respectively denote estimated propagation delay between the base station and the mobile station, and the relay and the mobile station.

[ 2 6 4 ] When an operation as Equation 19 is performed and Y_λ(p_{BM λ}) and R_BMΛPBM,I) . Y₂(P BMa) ^{and R}BMAPBM,₂) ■ ⁷i 0W ^and ^RmAPm_,ι) > and Y₂(PmJ and R_ma(p_ma) are the same signals,

from among four output values I" / ^

J²"PBλf,2"'^N

Σ ^Y2 iPBM,2 )^RB*M.2 iPBM,2)e ftSU ,2^eS/)Λ/,2

j1πp_m,₂ι>IN of the absolute value calculator 833

of FIG. 8B, the former two output values have channel impulse response forms starting from a location corresponding to the propagation delay between the base station and the mobile station, and the latter two output values have channel impulse response forms starting from a location corresponding to the propagation delay between the relay and the mobile station. When the channel impulse response exponential functionally decreases, the propagation delay between each transmitter and the mobile station can be estimated by acquiring a location of a time domain sample which has the sum of output values of the corresponding transmitter as the maximum value as shown in Equation 19. Here, in order to estimate accurate propagation delay, an integral carrier frequency offset should not occur, and thus the propagation delay is estimated using a signal that passed through the synchronizer 804. [ 2 6 5 ] The propagation delays between the base station and the mobile station, and the relay and the mobile station estimated in Equation 19 are respectively inputted to the first and second weight estimators 841 and 842, and a process of estimating propagation delay weight between each transmitter and the mobile station by using the estimated propagation delay between each transmitter and the mobile station can be expressed as follows.

N-X

[ 2 6 6 ] ^₁ ... Equation 20

n=0

[ 2 6 7 ] In equation 19, δ(ή) denotes a delta function, W_BM (k) and W_m (k) respectively denote propagation delay weight, in a frequency location, i.e. a location k on a frequency domain of a subcarier wave that transmitted the reception signal, estimated by the first and second weight estimators 841 and 842. The propagation delay weights are used to estimate a channel between each transmitter and the mobile station. Equation 20 is equal to a case when fast Fourier transform is performed on the delta function of each transmitter after the time of the delta function is delayed by the propagation delay between the corresponding transmitter and the mobile station.

[ 2 6 8 ] The channel estimators 851 and 852 of FIG. 8A estimate channel values of the entire subcarriers of each transmitter by removing phase distortion due to propagation delay in each transmitter from the received frequency domain signals, estimating the channels, and then performing interpolation. Processes of compensating the phase distortion due to propagation delay in each transmitter from the received frequency domain signals can be expressed as follows.

= H RM,, (P RM,, ) C/ = 1,2)

... Equation 21

[ 2 7 0 ] Here, H_{m λ} {p_{m λ}) and H_{BM 2} (p_BMa) respectively denote

phase distortion removed channels, which are estimated channels after phase distortion due to propagation delay in the corresponding pilot location of the first and second OFDM symbols received from the base station to the mobile station is removed, Y₁ (P _BMιi) and Y₂(p_BMa) respectively denote frequency domain signals received by the mobile station in the corresponding pilot location of the first and second OFDM symbols received from the base station to the mobile station, and ^ OW a^{nd W} _BM (P_BM,2) respectively denote conjugate complex number conversion components of the propagation delay weight estimated by the first weight estimator 841 in the corresponding pilot location of the first and second OFDM symbols received from the base

station to the mobile station. Also, H_m>](p_BM^) and H_BMa{p_BMa)

respectively denote phase distortion removed channels, which are estimated channels after phase distortion due to propagation delay in the corresponding pilot location of the first and second OFDM symbols received from the relay and the mobile station is removed, Y_\ (p_mλ) and Y₂(p_{m 2}) respectively denote frequency domain signals received by the mobile station in the corresponding pilot location of the first and second OFDM symbols received from the relay to the mobile station, and wm (P_RM,\) ^{and w}m iPm_,i) respectively denote conjugate complex number conversion components of the propagation delay weights estimated by the second weight estimator 842 in the corresponding pilot location of the first and second OFDM symbols received from the relay to the mobile station.

[ 2 7 1 ] Processes of estimating channel values of the entire subcarriers from Equation 21 are as follows. u\

W

TT ( \ Linear Interpolation _v T T / τ \

[ 2 7 2 ] ^ UW > H^BM, ₂ W _{Equation 2}2 aViGW ^{Linearlnterpolation} > /w*)

[ 2 7 3 ] Here, H_m,(k) , H_ma(k) , H_m, (k) and H_ma(k)

respectively denote estimated channels of the entire subcarriers of the first and second OFDM symbols received from the base station to the mobile station, and estimated channels of the entire subcarriers of the first and second OFDM symbols received from the relay to the mobile station. [ 2 7 4 ] Then, in order to estimate a channel actually experienced by the received frequency domain signal, an average of the channel of each

transmitter is acquired by using H_{BM X}{k) , H_BMa(k) , H_mΛ(k) , and

H_{m 2}(k) , and phase distortion due to propagation delay of the

corresponding transmitter in the average of the channel of each transmitter is compensated. The result of compensation is used for STBC demodulation, which can be expressed as Equation 23.

Equation 23

[ 2 7 6 ] Here, E_m (k) and H_m (k) respectively denote estimated channels, which are actually experienced by the received signal, between the base station and the mobile station, and the relay and the mobile station.

[ 2 7 7 ] When the channels estimated in Equation 23 are substituted in Equation 18, it can be expressed as follows.

Equation 24

[ 2 7 9 ] Based on Equation 24, the decoder 860 can estimate X^k) and X₂(k) , which are source signals of the frequency domain signals Y_x(k) and Y₂(k) received from the transmitters to the mobile station, by using H_m (k) and H_m(k) , which are the channels estimated by the first and second channel estimators 851 and 852.

[ 2 8 0 ] The embodiment described with reference to FIG. 8 is about the method and apparatus for estimating a channel in the cooperative relay system, when the numbers of the base station and the relay are each one, and the mobile station receives a signal from the base station and the relay. However, one of ordinary skill in the art can easily understand that the embodiment of FIG. 8 can be applied in the cooperative relay system, when the total numbers of the base station and the relay increases from two to four. Also, the embodiment of FIG. 8 can be applied not only in STBC but also in SFBC in the cooperative relay system.

[ 2 8 1 ] The embodiment of FIG. 12A can be modified to an embodiment of FIG. 12B in order to minimize hardware required while realizing the embodiment of FIG. 12A.

[ 2 8 2 ] FIG. 12B is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to another embodiment of the present invention.

[ 2 8 3 ] Referring to FIG. 12B, identifying a transmitter that transmitted the received signal by using a control signal of a controller 1261 corresponds to the function of the transmitter identifier 1220 of FIG. 12A. Also, each element of FIG. 12B corresponds to each element of the apparatus of FIG. 12A₁ and thus details thereof will be referred to FIG. 12A.

[ 2 8 4 ] FIG. 12C is a diagram illustrating the estimator 1240 of the apparatus for synchronization according to an embodiment of the present invention. Here, ε_m and δ_m respectively denote the estimated carrier frequency and timing offsets of a signal received from the base station, and ε_m and δ_m respectively denote the estimated carrier frequency and timing offsets of a signal received from the relay.

[ 2 8 5 ] Referring to FIG. 12C, the estimator 1240 includes an estimator 1241 for a base station and and an estimator 1242 for a relay.

[ 2 8 6 ] The estimator 1241 for a base station includes a first conjugate complex number converter 1241a, a first complex multiplier 1241b, a first accumulator 1241c, a first maximum value searcher 1241d, and a first phase tracker 1241e.

[ 2 8 7 ] The estimator 1242 for a relay includes a second conjugate complex number converter 1242a, a second complex multiplier 1242b, a second accumulator 1242c, a second maximum value searcher 1242d, and a second phase tracker 1242e.

[ 2 8 8 ] Functions of the elements of the estimator 1240 can be grasped via Equation 25. After compensating the carrier frequency and timing offsets of the transmitter that transmitted the received signal by using the compensator 1230 of FIG. 12A, carrier frequency and timing offsets are estimated by using data signal received from the relay or the base station.

Ml c,(n) = ∑ ∑/(n + aN_D +l + N)y(n + aN_D +l) [ 2 8 9 ] ^«=MV ^/=^I δ, = maxfjc, O)|), ε_t = — arg(c, (S₁ ))

... Equation 25

[ 2 9 0 ] Here, i denotes a classification symbol of data communication path determined based on from which transmitter the signal is received, N_{1 1} denotes a starting point of a signal received through the data communication path / in a frame, M₁ denotes the number of symbols used in relation to estimating the data communication path / , L denotes the length of repeating signal, generally corresponding to the size of a CP, n denotes a time domain sample, N_D denotes the length of a symbol including the CP, and N denotes the size of fast Fourier transform.

[ 2 9 1 ] FIG. 12D is a diagram illustrating an apparatus for synchronization in an OFDMA/OFDMA transparent relay system according to another embodiment of the present invention.

[ 2 9 2 ] Referring to FIG. 12D₁ a synchronizer 1203, including a controller 1203a and a frequency offset and timing offset synchronizer 1203b, can be realized by the embodiments of the apparatus illustrated in FIGS. 12A and 12B. In other words, the controller 1203a corresponds to the transmitter identifier 1220 of FIG. 12A and the frequency offset and timing offset synchronizer 1203b corresponds to the remaining elements excluding the transmitter identifier 1220 of FIG. 12A. 3 ] While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, various alternative embodiments can exist, such as i) an apparatus for estimating and compensating a channel when two relays perform STBC or SFBC in a cooperative relay system, and ii) an apparatus for estimating and compensating a channel when relays perform STBC or SFBC, wherein the total number of a base station and the relays is 3 to 4. The preferred embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.

Claims

[ 1 ]An apparatus for estimating a channel between a transparent relay and a mobile station in an orthogonal frequency division multiplexing (OFDM)/ orthogonal frequency division multiplexing access (OFDMA) transparent relay system, the apparatus comprising: a pilot extractor, which extracts pilot signals based on a frequency domain signal into which a signal that is received from the transparent relay by the mobile station and synchronized based on a signal transmitted from a base station to the mobile station is fast Fourier transformed; a propagation delay estimator, which estimates propagation delay between the transparent relay and the mobile station based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimator, which estimates a propagation delay weight for estimating the channel between the transparent relay and the mobile station based on the estimated propagation delay.

[ 2 ]The apparatus of claim 1 , further comprising a pilot storage unit, which stores the reference pilot signals, wherein the propagation delay estimator estimates the propagation delay between the transparent relay and the mobile station by using the reference pilot signals stored in the pilot storage unit.

[ 3 ]The apparatus of claim 1 , wherein the propagation delay estimator comprises: a complex multiplier, which generates first complex values proportional to each of the extracted pilot signals, a conjugate complex number conversion value of the reference pilot signal corresponding to each of the extracted pilot signals, and an exponential function based on a pilot location of each of the extracted pilot signals; an accumulation processor, which generates a second complex value by accumulating the first complex values; an absolute value calculator, which generates an absolute value of the second complex value; and a maximum value searcher, which estimates the propagation delay between the transparent relay and the mobile station by searching for a time domain sample, in which the absolute value of the second complex value reaches the maximum value.

[ 4 ]The apparatus of claim 1 , wherein the weight estimator estimates the propagation delay weight by delaying a delta function of the time domain by the estimated propagation delay and acquiring a result of fast Fourier transforming the delayed delta function of the time domain.

[ 5 ]The apparatus of claim 1 , further comprising a channel estimator, which estimates the channel between the transparent relay and the mobile station based on the extracted pilot signals, the reference pilot signals, and the propagation delay weight.

[ 6 ]The apparatus of claim 5, wherein the channel estimator comprises: a phase distortion remover, which acquires a phase distortion removed channel, in which phase distortion occurring due to propagation delay between the transparent relay and the mobile station in each pilot location of the extracted pilot signals is removed based on each of the extracted pilot signals, each of the reference pilot signals which is corresponding to each of the extracted pilot signals, and the propagation delay weight in the pilot location of each of the extracted pilot signals; an interpolator, which acquires an interpolated channel by performing interpolation on a frequency domain of the frequency domain signal based on the phase distortion removed channel; and a channel estimation determiner, which determines the channel between the transparent relay and the mobile station based on the interpolated channel and the propagation delay weight.

[ 7 ]The apparatus of claim 6, wherein the interpolation is linear interpolation.

[ 8 ]The apparatus of claim 5, further comprising a channel compensator, which performs channel compensation on a signal received by the mobile station based on the channel between the transparent relay and the mobile station estimated by the channel estimator. [ 9 ]The apparatus of claim 1 , wherein the transparent relay is a transparent relay for improving throughput.

[ 1 0 ] An apparatus for estimating a channel between each of transmitters and a mobile station in an OFDM/OFDMA cooperative relay system, which comprises the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters, the apparatus comprising: a pilot extractor, which extracts pilot signals transmitted from the transmitters based on a frequency domain signal into which a signal that is received from the transmitters by the mobile station and synchronized based on a signal transmitted from the base station to the mobile station is fast Fourier transformed; a propagation delay estimator, which estimates propagation delay between each of the transmitters and the mobile station based on the extracted pilot signals, and reference pilot signals, which is preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimator, which estimates propagation delay weight for estimating the channel between each of the transmitters and the mobile station based on the estimated propagation delay between each of the transmitters and the mobile station.

[ 1 l ]The apparatus of claim 10, further comprising a pilot storage unit, which stores the reference pilot signals for each of the transmitters, wherein the propagation delay estimator estimates the propagation delay between each of the transmitters and the mobile station by using the reference pilot signals stored in the pilot storage unit.

[ 1 2 ]The apparatus of claim 10, wherein the propagation delay estimator comprises: a complex multiplier, which generates first complex values proportional to each of the pilot signals extracted from each of symbols in a predetermined section from among symbols, which are included in the signal received by the mobile station and transmitted by the transmitters, a conjugate complex number conversion value of the reference pilot signal corresponding to each of the extracted pilot signals, and an exponential function based on a pilot location of each of the extracted pilot signals; a first accumulation processor, which generates a second complex value of each of the symbols in the predetermined section for each of the transmitters by accumulating the first complex values on pilot locations of the reference pilot signals of each of the transmitters in each of the symbols in the predetermined section; an absolute value calculator, which generates an absolute value of the second complex value of each of the symbols in the predetermined section for each of the transmitters; a second accumulation processor, which generates an accumulation value of each of the transmitters by accumulating the absolute values of the second complex values for each of the transmitters ; and a maximum value searcher, which estimates the propagation delay between each of the transmitters and the mobile station by searching for a time domain sample, in which the accumulation value of each of the transmitters reaches the maximum value.

[ 1 3 ]The apparatus of claim 10, wherein the weight estimator estimates the propagation delay weight of each of the transmitters by delaying a delta function of the time domain of each of the transmitters by the estimated propagation delay between the corresponding transmitter and the mobile station, and acquiring a result of fast Fourier transforming the delayed delta function of the time domain.

[ 1 4 ]The apparatus of claim 10, further comprising a channel estimator, which estimates the channel between each of the transmitters and the mobile station based on the extracted pilot signals, the reference pilot signals of the corresponding transmitter and the propagation delay weight of the corresponding transmitter.

[ 1 5 ]The apparatus of claim 14, wherein the channel estimator comprises: a phase distortion remover, which acquires phase distortion removed channels, in each of which, phase distortion of the channel between each of the transmitters and the mobile station occurring due to propagation delay in each pilot location of the extracted pilot signals in each of symbols in a predetermined section from among symbols, which are included in the signal received by the mobile station and transmitted by the transmitters, is removed; an interpolator, which acquires interpolated channels by performing interpolations on the phase distortion removed channels in a frequency domain of the frequency domain signal; an average calculator, which calculates an average of the interpolated channel in each transmitter; and a channel estimation determiner, which determines the channel between each of the transmitters and the mobile station based on the average of the interpolated channel and the propagation delay weight between the corresponding transmitter and the mobile station.

[ 1 6 ] The apparatus of claim 15, wherein the interpolation is linear interpolation.

[ 1 7 ]The apparatus of claim 14, wherein each transmitter transmits a signal forming the signal received by the mobile station by using a predetermined coding method for multiple-input multiple-output (MIMO), and the apparatus further comprising a decoder, which decodes the signal received by the mobile station based on the channel between each of the transmitters and the mobile station estimated by the channel estimator.

[ 1 8 ] The apparatus of claim 17, wherein the predetermined coding method for MIMO is a space-time coding (STC) method.

[ 1 9 ]An apparatus for synchronization, which synchronizes a transmitter, which transmitted a signal to a mobile station, from among transmitters and the mobile station, in an OFDM/OFDMA transparent relay system comprising the transmitters including a base station and a transparent relay, and the mobile station, the apparatus comprising: an offset storage unit, which stores a pre-estimated carrier frequency offset and timing offset between each of the transmitters and the mobile station; a transmitter identifier, which identifies the transmitter, which transmitted a signal, when the mobile station receives the signal; and a compensator, which generates a compensated signal by compensating the signal received by the mobile station by using the stored carrier frequency offset and timing offset of the identified transmitter.

[ 2 0 ]The apparatus of claim 19, further comprising: an estimator, which estimates an carrier frequency offset and timing offset between the identified transmitter and the mobile station based on the compensated signal; and an updater, which updates the carrier frequency offset and timing offset of the identified transmitter stored in the offset storage unit to the estimated carrier frequency offset and timing offset.

[ 2 l ]The apparatus of claim 19, wherein the transparent relay is a transparent relay for improving throughput.

[ 2 2 ]A method of estimating a channel between a transparent relay and a mobile station in an OFDM/OFDMA transparent relay system, the method comprising: a pilot extracting operation which extracts pilot signals based on a frequency domain signal into which a signal that is received from the transparent relay by the mobile station and synchronized based on a signal transmitted from a base station to the mobile station is fast Fourier transformed; a propagation delay estimating operation which estimates propagation delay between the transparent relay and the mobile station based on the extracted pilot signals and reference pilot signals, which are preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimating operation which estimates a propagation delay weight for estimating the channel between the transparent relay and the mobile station based on the estimated propagation delay.

[ 2 3 ] The method of claim 22, further comprising a pilot storing operation which stores the reference pilot signals, wherein the propagation delay estimating operation estimates the propagation delay between the transparent relay and the mobile station by using the stored reference pilot signals

[ 2 4 ] The method of claim 22, wherein the propagation delay estimating operation comprises: a complex multiplying operation which generates first complex values proportional to each of the extracted pilot signals, a conjugate complex number conversion value of the reference pilot signal corresponding to each of the extracted pilot signals, and an exponential function based on a pilot location of each of the extracted pilot signals; an accumulation processing operation which generates a second complex value by accumulating the first complex values; an absolute value calculating operation which generates an absolute value of the second complex value; and a maximum value searching operation which estimates the propagation delay between the transparent relay and the mobile station by searching for a time domain sample, in which the absolute value of the second complex value reaches the maximum value.

[ 2 5 ] The method of claim 22, wherein the weight estimating operation comprises: delaying a delta function of the time domain by the estimated propagation delay; and estimating the propagation delay weight by acquiring a result of fast Fourier transforming the delayed delta function of the time domain.

[ 2 6 ] The method of claim 22, further comprising a channel estimating operation which estimates a channel between the transparent relay and the mobile station based on the extracted pilot signals, the reference pilot signals, and the propagation delay weight.

[ 2 7 ] The method of claim 26, wherein the channel estimating operation comprises: a phase distortion removing operation which acquires a phase distortion removed channel, in which phase distortion occurring due to propagation delay between the transparent relay and the mobile station in each pilot location of the extracted pilot signals is removed based on each of the extracted pilot signals, each of the reference pilot signals which is corresponding to each of the extracted pilot signals, and the propagation delay weight in the pilot location of each of the extracted pilot signals; an interpolating operation which acquires an interpolated channel by performing interpolation on a frequency domain of the frequency domain signal based on the phase distortion removed channel; and a channel estimation determining operation which determines the channel between the transparent relay and the mobile station based on the interpolated channel and the propagation delay weight.

[ 2 8 ] The method of claim 27, wherein the interpolation is linear interpolation.

[ 2 9 ]The method of claim 26, further comprising a channel compensating operation which performs channel compensation on a signal received by the mobile station based on the channel between the transparent relay and the mobile station estimated by the channel estimating operation.

[ 3 0 ]The method of claim 22, wherein the transparent relay is a transparent relay for improving throughput.

[ 3 I ]A method of estimating a channel between each of transmitters and a mobile station in an OFDM/OFDMA cooperative relay system, which comprises the transmitters consisting of a base station and at least one cooperative relay, and the mobile station receiving signals transmitted from the transmitters, the method comprising: a pilot extracting operation which extracts pilot signals transmitted from the transmitters based on a frequency domain signal into which a signal that is received from the transmitters by the mobile station and synchronized based on a signal transmitted from the base station to the mobile station is fast Fourier transformed; a propagation delay estimating operation which estimates propagation delay between each of the transmitters and the mobile station based on the extracted pilot signals, and reference pilot signals, which is preset signal values, and each of which is a corresponding signal to each of the extracted pilot signals; and a weight estimating operation which estimates propagation delay weight for estimating the channel between each of the transmitters and the mobile station based on the estimated propagation delay between each of the transmitters and the mobile station.

[ 3 2 ]The method of claim 31 , further comprising a pilot storing operation which stores the reference pilot signals for each of the transmitters, wherein the propagation delay estimating operation estimates the propagation delay between each of the transmitters and the mobile station by using the stored reference pilot signals.

[ 3 3 ]The method of claim 31 , wherein the propagation delay estimating operation comprises: a complex multiplying operation which generates first complex values proportional to each of the pilot signals extracted from each of symbols in a predetermined section from among symbols, which are included in the signal received by the mobile station and transmitted by the transmitters, a conjugate complex number conversion value of the reference pilot signal corresponding to each of the extracted pilot signals, and an exponential function based on a pilot location of each of the extracted pilot signals; a first accumulation processing operation which generates a second complex value of each of the symbols in the predetermined section for each of the transmitters by accumulating the first complex values on pilot locations of the reference pilot signals of each of the transmitters in each of the symbols in the predetermined section; an absolute value calculating operation which generates an absolute value of the second complex value of each of the symbols in the predetermined section for each of the transmitters; a second accumulation processing operation which generates an accumulation value of each of the transmitters by accumulating the absolute values of the second complex values for each of the transmitters; and a maximum value searching operation which estimates the propagation delay between each of the transmitters and the mobile station by searching for a time domain sample, in which the accumulation value of each of the transmitters reaches the maximum value.

[ 3 4 ]The method of claim 31 , wherein the weight estimating operation comprises: delaying a delta function of the time domain of each of the transmitters by the estimated propagation delay between the corresponding transmitter and the mobile station; and estimating the propagation delay weight of each of the transmitters by acquiring a result of fast Fourier transforming the delayed delta function of the time domain.

[ 3 5 ]The method of claim 31 , further comprising a channel estimating operation which estimates the channel between each of the transmitters and the mobile station based on the extracted pilot signals, the reference pilot signals of the corresponding transmitter and the propagation delay weight of the corresponding transmitter.

[ 3 6 ] The method of claim 35, wherein the channel estimating operation comprises: a phase distortion removing operation which acquires a phase distortion removed channel, in which phase distortion of the channel between each of the transmitters and the mobile station occurring due to propagation delay in each pilot location of the extracted pilot signals in each of symbols in a predetermined section from among symbols, which are included in the signal received by the mobile station and transmitted by the transmitters, is removed; an interpolating operation which acquires an interpolated channel by performing interpolation on a frequency domain of the frequency domain signal based on the phase distortion removed channel; an average calculating operation which calculates an average of the interpolated channel in each transmitter; and a channel estimation determining operation which determines the channel between each of the transmitters and the mobile station based on the average of the interpolated channel and the propagation delay weight between the corresponding transmitter and the mobile station. [ 3 7 ] The method of claim 36, wherein the interpolation is linear interpolation.

[ 3 8 ] The method of claim 35, wherein each transmitter transmits a signal forming the signal received by the mobile station by using a predetermined coding method for multiple-input multiple-output (MIMO), and the method further comprising a decoding operation which decodes the signal received by the mobile station based on the channel between each of the transmitters and the mobile station estimated in the channel estimating operation.

[ 3 9 ]The method of claim 38, wherein the predetermined coding method for MIMO is a space-time coding (STC) method.

[ 4 O ]A method for synchronization, which synchronizes a transmitter, which transmitted a signal to a mobile station, from among transmitters and the mobile station, in an OFDM/OFDMA transparent relay system comprising the transmitters including a base station and a transparent relay, and the mobile station, the method comprising: an offset storing operation which stores a pre-estimated carrier frequency offset and timing offset between each of the transmitters and the mobile station; a transmitter identifying operation which identifies the transmitter, which transmitted a signal, when the mobile station receives the signal; and a compensating operation which generates a compensated signal by compensating the signal received by the mobile station by using the stored carrier frequency offset and timing offset of the identified transmitter.

[ 4 l ]The method of claim 40, further comprising: an estimating operation which estimates an carrier frequency offset and timing offset between the identified transmitter and the mobile station based on the compensated signal; and an updating operation which updates the stored carrier frequency offset and timing offset of the identified transmitter to the estimated carrier frequency offset and timing offset. ]The method of claim 40, wherein the transparent relay is a transparent relay for improving throughput.