KR20060136277A - Method for determinning handoff in a mobile communication system - Google Patents

Abstract

According to the present invention, when a handoff process occurs due to a change in a communication environment of a user in a specific cell or sector in a cellular communication system in a cellular environment, the accuracy of the handoff determination in the system is increased and the data transmission delay occurring in the handoff process It provides a handoff process that reduces the computation required for the subscriber station while reducing.

To this end, whether or not a specific user enters the handoff process is determined by the user's uplink reception signal level received at the corresponding serving base station or serving sector and the uplink of the user received at neighboring target base stations or neighboring sectors. The user reports the link reception signal level and optionally the downlink reception signal level of the serving base station or the serving sector to the serving base station or the serving sector and the downlink reception signal level of the neighboring base station or the neighboring sector. The information reported to the serving base station or the serving sector is collectively used to determine whether the user's handoff occurs.

intercell handoff, sector handoff, network initiated handoff, network controlled handoff, uplink signal based handoff

Description

How to determine handoff in mobile communication system {METHOD FOR DETERMINNING HANDOFF IN A MOBILE COMMUNICATION SYSTEM}

1 schematically illustrates the structure of a mobile communication system in a typical cellular environment.

2 shows an approximate operation of a terminal initialization and terminal assist handoff scheme as one of the handoff schemes described above.

3 is a diagram illustrating signaling performed for a handoff scheme considering only an uplink channel environment of a use frequency band proposed in the first embodiment of the present invention.

4 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 3.

FIG. 5 is a diagram illustrating signaling performed for a handoff scheme considering an uplink channel environment of a full frequency band proposed in the first embodiment of the present invention. FIG.

6 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 5.

FIG. 7 is a diagram illustrating signaling performed for a handoff scheme in consideration of an uplink / downlink channel environment proposed in the first embodiment of the present invention.

8 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 7.

9 is a diagram illustrating signaling performed for the handoff scheme proposed in the second embodiment of the present invention.

FIG. 10 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 9.

The present invention relates to a handoff method in a mobile communication system in a cellular environment, and more particularly, to a handoff determination method considering the level of an uplink received signal.

Typically, a mobile station (hereinafter referred to as "MS") moves to another service area instead of staying in only one service area (cell or sector) as it is easy to move. In this case, the mobile communication system supports handoff for continuous provision of voice or data services. Of course, handoff does not only consider the positional movement of the MS. For example, changes in the propagation environment, the proliferation of MSs in specific service areas, and the like may be further considered.

The handoff may be a procedure of efficiently recognizing a change in a communication environment and newly establishing a communication path with an optimal base station (hereinafter referred to as "BS"). That is, the handoff corresponds to a procedure of forming a communication path to the neighbor base station when there is a neighbor BS having a better communication environment than the serving BS. The serving base station is a base station where a mobile terminal is currently being served, and the neighbor base stations are base stations adjacent to the serving base station. A neighbor BS to perform handoff among the neighbor BSs is a target BS.

1 is a diagram schematically illustrating a structure of a mobile communication system in a typical cellular environment. In FIG. 1, a cell is assumed to be a service area.

Referring to FIG. 1, first to third MSs 112 to 116 are located in a first service area by the first base station 110. The third MS 116 of the MSs is located in an area where a handoff from the first BS 110 to the second BS 120 is required. In this case, for the first MS 116, the first BS 110 is a serving base station, and the second BS 120 is a target base station.

Typically the handoff method can be classified as follows.

First, it may be classified into a break-before-make (BBM) method and a make-before-break (MBB) method according to a disconnection time with a serving base station. The BBM handoff method is a method of establishing a connection with a target base station after terminating a connection with a serving base station when handoff is required. The MBB handoff scheme establishes a new connection with the target base station while maintaining the connection with the serving base station. This is to prevent the ping-pong phenomenon that may occur in the BBM handoff scheme and to obtain a spatial diversity gain. Only after the MS is completely out of the handoff region, the MS disconnects from the serving base station.

Secondly, according to the subject performing the handoff, it may be classified into a terminal decision handoff method, a network decision handoff method, and a terminal assist handoff method. That is, it may be classified according to the location where the apparatus for determining whether a handoff occurs and determining a target base station among neighbor base stations exists.

Lastly, according to a method of initializing radio wave environment measurement for determining whether a handoff occurs, it may be classified into a terminal initialization handoff method and a base station initialization handoff method.

2 is a diagram illustrating an approximate operation of a terminal initialization and a terminal assist handoff scheme as one of the above-described handoff schemes.

2, the MS connected to the serving base station periodically monitors the radio channel environment for the downlink with the neighboring base station (step 210, step 212, step 214). The monitoring is possible through the measurement of pilot signal strengths from the neighbor base stations. The MS requests a handoff from the serving base station when a pilot signal strength of more than a threshold is measured. At this time, the MS reports the identifier of the neighboring base station from which the pilot signal was measured and the strength of the measured pilot signal to the serving base station.

The serving base station determines whether to handoff by the strength of the pilot signal reported from the MS. The serving base station reports the decision to a higher layer and transmits a handoff command to the MS.

As described above, the handoff scheme based on the radio channel environment for the downlink is used for handoff between base stations using the same frequency band. That is, the present invention is mainly applied to a mobile communication system which acquires code synchronization using a searcher and a finger. This is because the MS can measure the strength of the received signal received from the neighboring base station by using the searcher in the idle state even during data communication with the serving base station. Therefore, there is no need to set an additional device or a separate search section. The search section is a section for periodically measuring the received signal strength from an adjacent base station.

However, in case of handoff using different frequencies between neighboring base stations or in a wideband wireless access system using multiple carriers, a downlink reception signal from a neighboring base station must be set separately from a serving base station and a search period in which data is not transmitted / received. Intensity can be measured. The MS performs an operation for searching for a downlink radio channel environment and periodically reports the result through uplink resource allocation for the serving base station.

As described above, in order to apply the existing handoff scheme to a broadband wireless access system using multiple carriers, a separate search period should be set. In addition, an uplink resource for reporting a search result should be allocated.

Accordingly, the present invention provides a method for performing handoff based on an uplink channel environment.

In addition, the present invention provides a handoff method for reducing data transmission delay due to search interval setting in a broadband wireless access system using multiple carriers.

The present invention also provides a handoff method for reducing the operation required in the mobile terminal associated with the handoff in a broadband wireless access system using multiple carriers.

The present invention also provides a method for performing handoff based on an uplink channel environment in a serving base station (or serving sector) and an uplink channel environment in a neighboring base station (or neighboring sector).

The present invention also provides a method for selectively considering a downlink channel environment in performing a handoff based on an uplink channel environment.

In a first aspect for achieving the above, the present invention provides a process for measuring the level of the first uplink reception signal by the serving base station in the mobile communication system through the monitoring of the uplink with the mobile terminal, the serving base station Reporting the level of the second uplink received signal measured by the at least one neighboring base station; and the serving base station considering the level of the first uplink received signal and the level of the second uplink received signal; A method of determining whether a mobile terminal is handed off is determined by a process of determining whether the mobile terminal is handed off.

Preferably, in consideration of the level of the downlink signal reported from the mobile terminal to determine whether to handoff to the mobile terminal.

More preferably, the mobile terminal requests to transmit a signal through a frequency band not currently used in the uplink, and further considering the level of the uplink received signal received in response to the request. Determine whether it is off.

In a second aspect to achieve the above, the present invention is a process for measuring the level of the first uplink received signal by the neighboring base station in the mobile communication system through the uplink monitoring with the mobile terminal, the neighboring base station Reporting the level of the second uplink received signal measured by the serving base station; and the neighboring base station considering the level of the first uplink received signal and the level of the second uplink received signal to the mobile terminal; A method for determining whether a mobile terminal is handed off is determined by a process of determining whether a handoff is performed.

Preferably, in consideration of the level of the downlink signal reported from the mobile terminal to determine whether to handoff to the mobile terminal.

More preferably, the mobile terminal requests to transmit a signal through a frequency band not currently used in the uplink, and further handoffs the mobile terminal by further considering the level of the uplink received signal received in response to the request. Determine whether or not.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

In the present invention to be described later, the uplink signal level (uplink channel environment) of the MS and the serving base station (or serving sector), the MS and neighbor base stations (or neighbor sectors) in order to guarantee the mobility of the MS in a cellular communication system of cellular structure. We propose a handoff method for determining whether a handoff occurs and an appropriate target base station (or target sector) based on the "

In addition, in the present invention, in order to prevent handoff malfunction when the uplink reception signal level of the serving base station is less than or equal to a threshold value, the uplink signal level of the current MS or the uplink signal level of the entire frequency band or the downlink signal of the serving base station The level should be further considered to determine whether to handoff. To this end, the serving base station requests the MS to transmit a signal pattern capable of receiving a received signal level, or the MS reports a downlink signal level.

On the other hand, the handoff method is divided into various scenarios according to the handoff initialization subject, parameters used to determine whether the handoff has occurred, and a subject that determines whether the handoff has occurred. The present invention distinguishes between a case in which a serving base station determines whether a handoff is determined by a target base station and a case in which the target base station determines another.

Therefore, in the detailed description to be described later, a first embodiment will be described how the serving base station determines whether to handoff according to the present invention. In the second embodiment, a description will be given of a method of determining whether the target base station is hand rope as proposed by the present invention. Meanwhile, the detailed description to be described later considers only handoff between cells, but it may be obvious to those skilled in the art that the same applies to handoff between sectors. That is, in the detailed description to be described later, the serving base station may be applied by changing the serving sector, the neighboring base station to the neighboring sector, and the target base station to the target sector.

A. First Embodiment

Hereinafter, a first embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the first embodiment of the present invention, a handoff scheme will be proposed in which the serving base station initiates a handoff process and determines whether to handoff with the aid of a neighboring base station. To this end, the serving base station determines the uplink channel environment and uses this to determine whether to handoff the corresponding MS. The serving base station may further consider the uplink channel environment of neighboring base stations in determining whether to handoff the corresponding MS. When the uplink channel environment is less than expected, the serving base station refers to the uplink channel environment of the frequency band not used by the MS, as well as the downlink channel environment reported from the MS.

In the first embodiment of the present invention to be described below, the operation of performing the handoff will be described in consideration of the level of the uplink reception signal at the serving base station and the neighboring base station. In addition, a case in which an uplink channel environment of a frequency band not used by the MS is considered and a case in which a downlink channel environment reported from the MS is considered will be described.

A-1. Handoff only considering uplink channel environment of frequency band

Hereinafter, an operation of performing a handoff by considering a level of an uplink reception signal in a frequency band used by a specific MS will be described in detail with reference to the accompanying drawings.

In brief, the description will be described later, the serving base station measures the strength of the uplink received signal with all MSs belonging to it, and calculates the level of the uplink received signal by the measured uplink received signal strength. The handoff initialization process is performed by determining whether to issue a handoff for each MS based on the calculated level of the uplink received signal. The handoff initialization process includes determining a target base station corresponding to each MS and transmitting a handoff command to the target base station and the MS.

3 is a diagram illustrating signaling performed for a handoff scheme considering only an uplink channel environment of a use frequency band proposed in the first embodiment of the present invention.

Referring to FIG. 3, in step 310, data (hereinafter referred to as "uplink data") is transmitted from the MS to the serving base station through uplink, and data (hereinafter referred to as "downward") is transmitted from the serving base station to the MS through downlink. Link data) is transmitted.

In step 312, the serving base station monitors an uplink with all MSs belonging to the serving base station, thereby measuring a level (P _used ) of an uplink reception signal for each MS. At this time, the level of the uplink received signal may be calculated by measuring the signal strength of the section in which the uplink traffic is transmitted, and accumulating for a specific time period after compensating for the difference caused by the uplink power control or the change in the transmit power. Alternatively, the strength of the pilot signal transmitted from the MS may be measured, and may be calculated by accumulating for a specific time interval after compensating for a difference due to uplink power control or transmission power change.

The serving base station determines whether the level (P _used ) of the uplink received signal calculated in step 314 is smaller than the threshold value TH1. If the uplink reception signal level P _used is greater than or equal to the threshold value TH1, the channel situation may be considered good. Accordingly, the serving base station continues with steps 310 and 312. However, if the uplink reception signal level P _used is smaller than the threshold value TH1, the handoff process is initialized by determining that the channel situation is degraded. That is, in step 328, the target base station corresponding to the MS is determined, and in steps 330 and 332, a message (HO_REQ) requesting handoff is transmitted to the target base station and the corresponding MS.

In the above description, it is assumed that the serving base station determines the target base station for the corresponding MS in advance. However, the serving base station may further grant signaling for determining a target base station for the corresponding MS.

To this end, the serving base station transmits a message (NBR_MON_REQ) requesting the monitoring of the uplink to the neighboring base station in step 316 to step 318 if the uplink received signal level (P _used ) is less than the threshold value TH1. The neighbor base stations monitor uplink from the corresponding MS (step 320 and step 322).

The following three methods may be proposed for the method of measuring the uplink signal of the MS that the neighboring base station is handing off through monitoring.

The first is that the serving base station additionally provides context information related to the MS (uplink frequency in use, time alignment information, CID, etc.) to the neighbor base stations so that the neighbor base stations can provide the uplink signal of the MS. Receive method (Passive Mode).

The second is a method in which the serving base station requests channel sounding to the terminal to be handed off in uplink and the serving base station and the neighboring base station measure the uplink signal based on the sounding signal (Active Mode).

Lastly, for more accurate uplink channel estimation, there is a method in which the target base station does not use resources currently used by the MS in its cell by using context information of the MS received from the serving base station. .

The neighbor base stations measure the level of the uplink received signal for the MS handing off by taking one of the methods described above. The measured level of the uplink reception signal is transmitted to the serving base station through a report message NBR_MON_RSP (steps 324 and 326).

The serving base station determines at least one target base station for each MS to be handed off in consideration of the level of the uplink received signal reported from the neighboring base station in step 328. The target base station is determined as an adjacent base station having a level of the uplink received signal that is larger by a predetermined threshold TH4 than the level of the uplink received signal at the serving base station. Conditions for designating the target base station may be defined as in Equation 1 below.

P _Neighbor -P _used > TH4

Here, P _Neighbor means the level of the uplink reception signal reported from neighboring base stations.

On the other hand, if there is only one neighboring base station satisfying the above-described condition corresponding to a specific MS, the neighboring base station is designated as the target base station. However, when there are a plurality of neighboring base stations, one or a plurality of neighboring base stations may be designated as target base stations according to the handoff method. That is, in the case of the BBM handoff method, only the neighbor base station having the best level of the uplink reception signal among the plurality of neighbor base stations is designated as the target base station. However, in the MBB handoff scheme, all of the plurality of neighbor base stations are designated as target base stations.

The serving base station transmits a message (HO_REQ) for requesting handoff to the MS and the target base station when handoff is determined and the target base station is determined.

4 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 3. The control flow shown in FIG. 4 may be equally applied to all MSs belonging to the serving base station. However, in the following, it should be noted that for the convenience of description, a single MS.

Referring to FIG. 4, the serving base station measures uplink received signal strength from a periodically received uplink data signal and initializes the handoff by the level of the uplink received signal calculated by the uplink received signal strength. Do this. At this time, if necessary, consider the level of the uplink reception signal of the neighbor base stations.

To this end, the serving base station receives the uplink data in step 410, and transmits the downlink data. In step 412, the serving base station monitors uplink by the uplink data. The uplink received signal strength is measured through the monitoring, and the level (P _used ) of the uplink received signal is calculated using the uplink received signal strength.

The level of the uplink received signal (P _used ) is measured by measuring the strength of the uplink received signal in a section in which uplink traffic is transmitted, and accumulating for a specific time period after compensating for the difference caused by uplink power control or transmission power change. Can be calculated Otherwise, it may be calculated by measuring the strength of the pilot signal transmitted from the MS, accumulating for a specific time interval after compensating for the difference due to uplink power control or transmission power change.

The serving BS determines whether less than a level of the received uplink signal measured at the step 414 (P _used) a threshold value TH1. The threshold TH1 may be defined as a signal level such that handoff is necessary, which is predetermined in advance.

The serving base station determines that the current channel state is good enough that no handoff is necessary if the measured level P _used of the uplink received signal is greater than or equal to the threshold value TH1. Therefore, the process proceeds to step 410 to receive uplink data or transmit downlink data.

However, if the measured level (P _used ) of the uplink received signal is less than the threshold value TH1, the serving base station performs an initialization process for handoff. That is, the serving base station transmits a handoff command to the MS and the target base station in step 424. As described above, in order for the serving base station to determine the handoff by considering only the level of the uplink received signal, the serving base station may be aware of the channel condition between neighboring base stations and the MS.

Otherwise, if the serving base station is not aware of the channel condition between the neighbor base stations and the MS, an additional procedure is needed to confirm the channel condition between the neighbor base stations and the MS. To this end, FIG. 4 further discloses a process for checking a channel condition between neighboring base stations and the MS.

When the serving base station determines that handoff is necessary by the level (P _used ) of the uplink received signal, the serving base station initiates a handoff procedure by requesting neighbor base stations to monitor uplink with the MS in step 416. In response, the neighbor base stations monitor the uplink with the MS and report the monitoring result to the serving base station.

The serving base station receives the monitoring results from the neighbor base stations in step 418. The serving base station uses the level of the uplink received signal of the serving base station and neighboring base stations to determine whether a handoff occurs and an appropriate target base station.

The serving base station has a neighboring base station having a level of an uplink reception signal having a threshold level TH4 or more greater than its uplink reception signal level (P _used ) among the reported levels of uplink reception signals of neighboring base stations. Check the operation (step 420). If there is no neighbor base station satisfying the condition, the serving base station returns to step 410 because there is no target base station for the MS to handoff.

Otherwise, if there is a neighbor base station satisfying the condition, the serving base station designates the neighbor base station as a target base station for the MS. In the target base station, one neighbor base station may be designated in the BBM handoff scheme, and a plurality of neighbor base stations may be designated in the MBB handoff scheme.

When the target base station is designated for the MS, the serving base station transmits a handoff request message (HO_REQ) to the MS and at least one target base station in step 424. Upon receiving the handoff request message, the MS performs a handoff to the target base station. The target base station receiving the handoff request message starts the service for the MS through a handoff procedure.

A-2. Handoff Considering Uplink Channel Environment in All Frequency Bands

Hereinafter, an operation in which the serving base station performs handoff in consideration of the level of the uplink reception signal in the entire frequency band will be described in detail with reference to the accompanying drawings.

To summarize briefly, the following description will be made. If a specific MS belonging to a serving base station is a user who uses only a part of a frequency region rather than the entire frequency band in a multi-carrier system, the serving base station deteriorates the propagation environment of some bands in determining whether to handoff. In order not to be mistaken for the occurrence of a handoff situation, the level of an uplink reception signal of a frequency band not currently used may be considered. That is, when the serving base station's current level (P _used ) is lower than the threshold value TH1, the serving base station determines whether to handoff by considering the level of the uplink received signal of another frequency band not currently used.

FIG. 5 is a diagram illustrating signaling performed for a handoff scheme considering an uplink channel environment of an entire frequency band proposed in the first embodiment of the present invention. In FIG. 5, steps (516 and 518) of monitoring the unused frequency bands in the signaling of FIG. 3 described above and determining whether to handoff are added thereto. Therefore, it should be noted that a detailed description of the same steps as in FIG. 3 is omitted.

Referring to FIG. 5, uplink / downlink data is transmitted in step 510. The serving base station monitors the uplink for each MS in step 512 to measure the level (P _used ) of the uplink reception signal. In step 514, the serving base station identifies the MS whose level (P _used ) of the uplink received signal is smaller than a threshold value TH1.

In step 516, the serving base station monitors a frequency band not used by the identified MSs. To this end, the serving base station requests to transmit a signal through a frequency band not currently used by the identified MSs. MSs receiving the request transmit uplink signals in a frequency band not used by the MSs. The serving base station measures the level P _unused of the uplink received signals received from the MSs in response to the request. The level P _unused of the uplink reception signal in the frequency band which is not currently used can be measured in the same manner as the method for calculating P _used .

When the serving base station completes the monitoring of the unused frequency band, the serving base station determines whether to initiate handoff for each MS in step 518. For example, if the difference between the current level of the uplink reception signal P _used and the _average value of the uplink reception signal level P _unused of the _unused frequency band P _average is smaller than the threshold value TH3, the handoff process is initiated. do. This prevents the MS that uses some frequency bands from being used as uplink because the received signal is degraded due to the frequency selective fading, etc., but the handoff process is misunderstood as a need for a different frequency band. Is an optional process.

The serving base station transmits a message (NBR_MON_REQ) requesting monitoring for uplink to neighboring base stations for handoff initialization (steps 520 and 522). The neighbor base stations monitor uplink from the corresponding MS (steps 524 and 526). The neighboring base stations report the level of the uplink received signal measured through the monitoring to the serving base station through a report message NBR_MON_RSP (steps 528 and 530).

The serving base station determines at least one target base station for each MS to be handed off in consideration of the level of the uplink reception signal reported from the neighboring base station in step 530. The serving base station transmits a message (HO_REQ) for requesting handoff to the MS and the target base station when the target base station is determined to be handoff or not (steps 534 and 536).

FIG. 6 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 5. The control flow shown in FIG. 6 may be equally applied to all MSs belonging to the serving base station. However, in the following, it should be noted that for the convenience of description, a single MS.

Referring to FIG. 6, the serving base station receives uplink data and transmits downlink data in step 610. In step 612, the serving base station monitors uplink by the uplink data. The uplink received signal strength is measured through the monitoring, and the level (P _used ) of the uplink received signal is calculated using the uplink received signal strength.

The serving BS determines whether less than the measured level in step 614 of the uplink receiving signal (P _used) a threshold value TH1. The serving base station proceeds to step 610 when the measured level P _used of the uplink received signal is greater than or equal to the threshold value TH1. However, if the measured level (P _used ) of the uplink received signal is smaller than the threshold value TH1, the serving base station monitors the unused frequency band in step 616.

In order to monitor the unused frequency band, the serving base station requests the MS to transmit an uplink signal through a frequency band not currently in use. In response to the request, a level P _unused of an uplink reception signal received from the MS is calculated. Thereafter, the serving base station proceeds to step 618 to determine whether to perform handoff initialization. Whether to perform the handoff initialization may be determined by the conditions disclosed in Equation 2 below.

Here, P _average means an _average value (P _average ) of the level (P _unused ) of the uplink reception signal of the frequency band not currently used.

If the condition is met, the serving base station initiates a handoff procedure by requesting neighbor base stations to monitor uplink with the MS in step 620. In step 622, the serving base station receives a monitoring result from the neighbor base stations in response to the request.

The serving base station is a neighbor having a level of an uplink reception signal having a threshold value of TH4 or more greater than its own uplink reception signal level (P _used ) among the levels of uplink reception signals of the neighboring base stations received as a result of the monitoring. In step 624, the base station is checked for existence.

The serving base station designates a neighbor base station satisfying the condition as a target base station for the MS. In step 626, the serving base station transmits a handoff request message (HO_REQ) to the MS and at least one target base station.

A-3. Handoff Considering Uplink Channel Environment and Downlink Channel Environment in All Frequency Bands

Hereinafter, an operation in which the serving base station performs handoff in consideration of the level of the uplink reception signal in the entire frequency band will be described in detail with reference to the accompanying drawings.

To summarize briefly, the serving base station may consider both the level of the uplink received signal of the serving base station and the neighboring base station and the level of the downlink signal of the serving base station in order to determine whether a handoff occurs and an appropriate target base station. . That is, the serving base station determines whether to hand off using a larger value among the average of the level of the uplink reception signal and the level of the downlink reception signal.

The simultaneous consideration of the uplink and the downlink prevents a mistake in the uplink channel environment of a specific frequency band as a handoff caused by the symmetry of the uplink and downlink radio channels of the TDD system. To do this.

FIG. 7 is a diagram illustrating signaling performed for a handoff scheme considering the uplink / downlink channel environment proposed in the first embodiment of the present invention. In FIG. 7, a request for a downlink channel environment is performed in the signaling of FIG. 3 described above, and thus, information (steps 724 and 726) of the downlink channel environment is obtained. Therefore, it should be noted that a detailed description of the same steps as in FIG. 3 is omitted.

Referring to FIG. 7, in step 710, uplink / downlink data is transmitted. In step 712, the serving base station monitors the uplink for each MS to measure the level of the uplink received signal (P _{UL_used} ). In step 714, the serving base station identifies the MS whose level (P _{UL_used} ) of the uplink received signal is smaller than the threshold value TH1.

The serving base station transmits a message (NBR_MON_REQ) requesting monitoring for uplink to neighboring base stations for handoff initialization (steps 716 and 718). The neighbor base stations monitor uplink from the corresponding MS (steps 720 and 722). The neighboring base stations report the level of the uplink received signal measured through the monitoring to the serving base station through a report message NBR_MON_RSP (steps 728 and 730).

Meanwhile, the serving base station requests the neighbor base station to measure the level of the uplink reception signal of the corresponding MS and requests the corresponding MS to search for a downlink channel state with the serving base station. The MS needs to periodically receive a preamble or pilot signal from the serving base station to maintain synchronization with the base station and reports the received values to the serving base station according to the downlink channel state search request of the serving base station. This can be distinguished from the conventional MS has been monitoring the downlink of the neighbor base station in that there is no need to set an additional search interval.

If there is a result of measuring the downlink signal level with the neighboring base station in the idle mode (idle mode) prior to the request of the serving base station, and if the result is valid, it can be additionally used for specifying the correct target base station.

The MS transmits the downlink monitoring result to the serving base station through a report message DL_MEASUREMENT_REPORT.

In step 732, the serving base station determines whether a handoff has occurred and an appropriate target base station. Wherein for this purpose the serving base station uses the quality of the uplink reception signal of the serving base station (P _{UL_used)} to the level of uplink received signals of the adjacent base station (P _{UL_neighbor)} and the level of the downlink signal of the serving base station (P _DL). The condition for determining whether the handoff occurs may be defined by Equation 3 below, and the condition for determining an appropriate target base station may be defined by Equation 4 below.

The serving base station will determine that a handoff is required for the corresponding MS when the condition of Equation 3 is satisfied, and designates an adjacent base station satisfying the condition of Equation 4 as a target base station for the MS. do.

The serving base station transmits a message (HO_REQ) requesting the handoff to the MS and at least one target base station when the target base station is determined to be handoff (steps 734 and 736).

FIG. 8 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 7. The control flow shown in FIG. 8 may be equally applied to all MSs belonging to the serving base station. However, in the following, it should be noted that for the convenience of description, a single MS.

Referring to FIG. 8, the serving base station receives uplink data and transmits downlink data in step 810. The serving base station monitors uplink by the uplink data in step 812. The uplink received signal strength is measured through the monitoring, and the level P _{UL_used} of the uplink received signal is calculated using the uplink received signal strength.

In step 814, the serving base station determines whether the measured level UL signal P _{UL_used} is smaller than a threshold value TH1. The serving base station proceeds to step 810 if the measured level _{PUL_used} of the uplink received signal is greater than or equal to the threshold value TH1. However, if the measured level UL received signal P _{UL_used} is less than the threshold value TH1, the serving base station requests neighbor base stations to monitor the uplink for the corresponding MS in step 816. The request is made by sending an NBR_MON_REQ message. In step 820, the serving base station requests the MS to measure a downlink channel state.

The neighbor base stations monitor the uplink with the MS and report the monitoring result to the serving base station. The MS monitors the downlink with the serving base station and reports the monitoring result to the serving base station.

The serving base station receives the monitoring result from the neighboring base stations in step 822, and receives the monitoring result from the MS in step 824. The monitoring result reported from the neighbor base stations may be information about the level of the uplink reception signal corresponding to the MS. The monitoring result reported from the MS may be information about a level of a downlink signal between the MS and the serving base station.

In step 826, the serving base station determines whether a handoff occurs and at least one target base station in consideration of the monitoring results from the neighboring base stations and the MS and the level of the uplink received signal measured by the serving base station. That is, it is determined whether to perform handoff initialization. The occurrence of the handoff may be determined by Equation 3, and the target base station may be determined by Equation 4.

If it is determined that handoff initialization is necessary, the serving base station proceeds to step 828 and transmits a handoff request message (HO_REQ) to the MS and at least one target base station previously determined.

B. Second Embodiment

Hereinafter, a second embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the second embodiment of the present invention to be described later, the neighboring base station is distinguished from the first embodiment described above in that the neighboring base station is a subject for initializing handoff. That is, in the second embodiment, the neighbor base station determines the handoff by finally collecting the level of the uplink / downlink received signal between the MS and the serving base station or the neighbor base station.

To this end, the neighboring base station periodically identifies the uplink channel environment transmitted by the MSs belonging to its neighbor cells and initializes the handoff procedure of the corresponding MS. In addition, the neighbor base station may additionally consider the downlink channel state between the serving base station and the MS, and the level of the uplink received signal of the unused frequency band to prevent handoff malfunction.

9 is a diagram illustrating signaling performed for the handoff scheme proposed in the second embodiment of the present invention.

Referring to FIG. 9, uplink data and downlink data are periodically transmitted between the serving base station and the MS (steps 910 and 912). Meanwhile, neighboring base stations periodically monitor uplink with the MS in step 914. The monitoring targets a traffic channel or an uplink pilot signal with the MS. The neighbor base stations measure the level (P _{UL_neighbor} ) of the uplink received signal through the monitoring. The serving base station provides context information related to the MS as necessary so that the neighboring base station can measure the level of an uplink reception signal with the MS. In this case, the serving base station may inform the quality of service (QoS) of the MS together.

The neighboring base stations determine the level (P _{UL_neighbor)} of the measurement by the uplink received signal in step 916 is greater than the threshold value TH2. This means that an uplink signal of sufficient level to perform a handoff is received from the MS.

When the neighbor base stations receive an uplink received signal sufficient to perform handoff, the neighbor base station transmits a message UL_MEASUREMENT_REQ to the serving base station in step 918.

When the serving base station receives the UL_MEASUREMENT_REQ message, the serving base station monitors uplink with the MS. In addition, the level (P _{UL_serving} ) of the uplink reception signal measured through the monitoring is transmitted to the neighbor base stations through a measurement report message (UL_MEASUREMENT_REPORT).

The neighboring base stations finally collect the level P _{UL_neighbor} of the uplink reception signal measured by the neighboring base station and the level P _{UL_serving} of the uplink reception signal provided from the serving base station to determine whether to perform handoff. ). When it is determined that the handoff is performed, the neighboring base station transmits a handoff request message (HO_REQ) to the MS and the serving base station (step 938 and step 940).

On the other hand, when the neighbor base station needs to consider the level of the downlink signal, it can request the monitoring of the downlink to the serving base station in step 922. This corresponds to the neighbor base station transmitting a downlink measurement request message (DL_MEASUREMENT_REQ) to the serving base station.

When the serving base station receives a downlink measurement request message DL_MEASUREMENT_REQ from the neighboring base station, the serving base station transmits it to the MS (step 924). When the MS receives a downlink measurement request message (DL_MEASUREMENT_REQ) from the serving base station, the MS monitors the downlink formed between the serving base station and itself, and measures the level of the downlink signal obtained through the monitoring report (DL_MEASUREMENT_REPORT). In step 926, the serving base station transmits to the serving base station. When the serving base station receives the measurement report message DL_MEASUREMENT_REPORT from the MS, the serving base station transmits it to the neighbor base station (step 928). This allows the neighboring base station to consider the level of the downlink signal in determining the initialization of the handoff.

Meanwhile, when the neighboring base station wants to monitor the uplink channel state of a specific frequency band, it may request the MS to transmit the signal of the specific frequency band (step 930). In this case, a UL_SOUNDING_REQ message is transmitted from the neighbor base station to the serving base station. When the serving base station receives the UL_SOUNDING_REQ message, it transmits it to the MS (step 932). When receiving the UL_SOUNDING_REQ message, the MS transmits an uplink signal to the neighbor base station in a specific frequency band (934).

As a result, the adjacent base station can measure the level of the uplink reception signal received in the specific frequency band. In determining the initialization of the handoff, the neighboring base station further considers the level of the uplink received signal measured in the specific frequency band.

FIG. 10 is a diagram illustrating a control flow of a serving base station for performing the signaling of FIG. 9. The control flow shown in FIG. 10 may be equally applied to all MSs belonging to the serving base station. However, in the following, it should be noted that for the convenience of description, a single MS. In FIG. 10, the options shown in FIG. 9 are not considered, but it will be apparent to those skilled in the art to reflect the options.

Referring to FIG. 10, the neighbor base station receives uplink data and transmits downlink data in step 1010. In step 1012, the neighbor BS performs monitoring on the uplink by the uplink data. The uplink received signal strength is measured through the monitoring, and the level P _{UL_neighbor} is calculated using the uplink received signal strength.

The neighbor base station determines the level (P _{UL_neighbor)} of the uplink signal received from the measuring step 1014 is greater than the threshold value TH2. The threshold TH2 may be defined as a signal level to the extent that handoff is possible, which is predetermined in advance.

The neighboring base station determines that the current channel state is not good enough to perform handoff when the measured level of the uplink received signal P _{UL_neighbor} is less than or equal to the threshold value TH2. Therefore, the process proceeds to step 1010 to receive uplink data or transmit downlink data.

However, if the measured level of the uplink received signal P _{UL_neighbor} is greater than the threshold value TH2, the neighboring base station requests the serving base station to monitor the uplink with the MS in step 1016. In response, the serving base station monitors an uplink with the MS and reports the monitoring result to the neighboring base station.

The neighbor base station receives the monitoring result from the serving base stations in step 1018. The monitoring result is the level (P _{UL_serving} ) of the uplink reception signal at the serving base station. The neighboring base station uses the quality of the uplink reception signal by their measurement (P _{UL_neighbor)} to the level of the uplink received signal measured at the serving base station (P _{UL_serving)} to determine the hand-off occurs if the target itself base station do.

The neighboring base station determines whether the level P _{UL_neighbor} of the uplink received signal measured by the neighboring base station is greater than or equal to the threshold value TH4 greater than the level P _{UL_serving} measured by the serving base station in step 1020. This may be expressed as in Equation 5 below.

If the neighboring base station satisfies the condition of Equation 5, it designates a destination as a target base station for the MS. The neighbor base station transmits a handoff request message (HO_REQ) to the MS and the serving base station in step 1022.

When the target base station transmits a handoff request message to the MS, the target base station also transmits the level (P _{UL_neighbor} ) of the uplink reception signal measured by the target base station. This is to prepare for a case in which a plurality of neighboring base stations may exist in the handoff area in the case of the overlapped cell structure, in which a plurality of target base stations simultaneously transmit a handoff request message to one MS. If the MBB handoff scheme is supported, handoff may be performed for all target base stations that have requested handoff. However, in case of supporting BBM handoff, the MS can select one type of base station by comparing the level of uplink received signals reported from the target base stations.

As described above, the embodiment of the present invention does not use the downlink pilot signal strength received from neighboring cells requiring periodic search interval setting, operation in the terminal, reporting through uplink, etc. in the handoff initialization process. In addition, we proposed a handoff initialization method that can initialize the handoff process based on the level of the uplink received signal and prevent data transmission delay, complexity, and uplink resource waste occurring during the handoff process.

In addition, in an embodiment of the present invention, the radio wave environment between the serving base station and the MS is poor as well as a relatively good environment by using the level of uplink reception signals between neighboring base stations located near the MS and the serving base station in the handoff decision process. In the case of better propagation environment between target base stations and MS, we proposed a handoff method that can increase the average data transmission in the cell rather than the handoff method based only on the uplink signal level of the serving base station by handoff. .

In addition, in the embodiment of the present invention, if the level of the uplink received signal between the MS and the serving base station is poor, the MS measures the level of the downlink received signal that should be basically measured during data transmission and reports it to the serving base station. The serving base station additionally uses the reported measurement in determining whether to handoff, and thus accurately determines whether a handoff occurs while preventing transmission delay due to discovery interval setting compared to the downlink signal level retrieval method for periodic neighboring base stations. An additional method has been proposed.

In addition, the handoff method based on the level of the uplink reception signal proposed in the embodiment of the present invention is also applied to the inter-sector handoff, so that the level of the uplink reception signal between the MS and the serving sector and the uplink reception between the MS and the neighboring sectors. In order to support the intersectoral handoff method based on the signal level and optionally the downlink received signal level between the MS and the serving base station.

According to the above-described embodiments of the present invention, the data transmission time can be prevented from being delayed and additional operations and devices of the MS can be prevented. In addition, it is possible to prevent a handoff malfunction due to a waste of resources and an error caused by periodically transmitting a downlink reception signal level between neighboring base stations and the MS to the base station through uplink.

In addition, the handoff scheme proposed in the present invention is handoff because the serving base station incorrectly recognizes the radio channel environment between the serving base station and the MS in the OFDMA system using the multi-user diversity due to the influence of the frequency selective channel environment. The performance degradation caused by mistake can be selectively prevented, and thus it can be applied as a method to guarantee mobility in a cellular system using a broadband wireless access method using multi-user diversity.

Claims (20)

A method for determining whether to handoff a mobile terminal in a mobile communication system, Measuring, by a serving base station, a level of a first uplink received signal through monitoring of uplink with the mobile terminal; Receiving, by the serving base station, reporting a level of a second uplink received signal measured from at least one neighboring base station; And determining, by the serving base station, whether to handoff the mobile station by considering the level of the first uplink received signal and the level of the second uplink received signal. The method as claimed in claim 1, wherein the level of the first uplink reception signal is calculated by monitoring data transmitted from the mobile terminal through the uplink. The method as claimed in claim 1, wherein the level of the first uplink reception signal is calculated by monitoring a pilot signal transmitted from the mobile terminal through the uplink. The method of claim 1, wherein the level of the first uplink reception signal is less than a first threshold value, and the level of the second uplink reception signal is greater than a level of the first uplink reception signal by a second threshold value. The method for determining a handoff for the mobile terminal. The target base station of claim 4, wherein the serving base station targets the mobile station to hand off an adjacent base station that reports a level of a second uplink received signal that is greater than a level of the first uplink received signal by a second threshold. The method as characterized in that the base station. The method of claim 1, wherein the serving base station determines whether to handoff the mobile terminal by further considering the level of the downlink signal reported from the mobile terminal. The method of claim 6, wherein the level of the downlink signal is obtained through monitoring of downlink between the mobile station and the serving base station. The method of claim 1, wherein the serving base station requests the mobile station to transmit a signal through a frequency band not currently being used in uplink, and further provides a level of an uplink received signal received in response to the request. And determining whether to handoff the mobile terminal. The method of claim 1, wherein the level of the uplink reception signal is obtained by accumulating for a predetermined time after compensating for a difference due to uplink power control or transmission power change to the strength of the uplink reception signal. Way. The method of claim 5, wherein the serving base station further comprises a step of instructing the mobile terminal and the target base station to perform a handoff when a handoff is determined for the mobile terminal. The method of claim 1, wherein the serving base station further comprises the step of transmitting the context information of the mobile terminal to the neighboring base station so that the neighboring base station can measure the level of the second uplink received signal. Way. A method for determining whether to handoff a mobile terminal in a mobile communication system, Measuring, by a neighboring base station, the level of the first uplink reception signal through monitoring of the uplink with the mobile terminal; Receiving, by the neighboring base station, the level of the second uplink received signal measured from the serving base station; And determining, by the neighboring base station, whether to handoff the mobile station by considering the level of the first uplink received signal and the level of the second uplink received signal. The method of claim 12, wherein the level of the first uplink reception signal is calculated through monitoring of data transmitted from the mobile terminal through the uplink. The method as claimed in claim 12, wherein the level of the first uplink reception signal is calculated by monitoring a pilot signal transmitted from the mobile terminal through the uplink. The method of claim 12, wherein the neighboring base station, the level of the first uplink received signal is greater than the first threshold value and the level of the second uplink received signal is a second compared to the level of the first uplink received signal And determining a handoff as the target base station for the mobile terminal when it is as large as a threshold. The method as claimed in claim 12, wherein the neighboring base station further determines whether to handoff the mobile station by further considering the level of the downlink signal reported from the mobile station. The method of claim 16, wherein the level of the downlink signal is obtained through monitoring of downlink between the mobile station and the serving base station. The method of claim 13, wherein the neighboring base station requests the mobile station to transmit a signal through a frequency band not currently being used in the uplink, and further increases the level of the uplink received signal received in response to the request. And determining whether to handoff the mobile terminal. The method of claim 13, wherein the level of the uplink reception signal is obtained by accumulating for a predetermined time after compensating for a difference due to uplink power control or transmission power change to the strength of the uplink reception signal. Way. The method of claim 15, wherein the serving base station further comprises notifying the mobile terminal and the serving base station that the handoff is determined when the handoff is determined for the mobile terminal.

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