EP3991374A1

EP3991374A1 - A time interleaved block windowed burst orthogonal frequency division multiplexing communication system

Info

Publication number: EP3991374A1
Application number: EP20745281.4A
Authority: EP
Inventors: Marco Alexandre CRAVO GOMES; Vítor Manuel MENDES DA SILVA; Rui Miguel HENRIQUES DIAS MORGADO DINIS; Filipe João ABRANTES SOARES DA CONCEIÇÃO
Original assignee: Universidade Nova de Lisboa; Universidade de Coimbra; Instituto de Telecomunicacoes
Current assignee: Universidade Nova de Lisboa; Universidade de Coimbra; Instituto de Telecomunicacoes
Priority date: 2019-06-26
Filing date: 2020-06-25
Publication date: 2022-05-04
Also published as: WO2020261176A1; PT119526A

Abstract

To improve the spectral efficiency of TIBWB-OFDM, it is proposed a communication system that implements an alternative packing of the windowed OFDM component blocks that form a TIBWB-OFDM symbol. Adjacent windowed OFDM blocks are partially overlapped, instead of being juxtapose as in original proposed TIBWB-OFDM technique. Although the proposed packing method enables a highly efficient spectral transmission, this is achieved at the expense of intentionally introduced interference, by the transmitter, between consecutive sub-blocks that shape the TIBWB-OFDM symbol, which deteriorates the bit error rate performance. To cope with this additional interference, the receiver must entail both channel equalization and inter-block interference cancellation between OFDM component blocks of the TIBWB-OFDM symbol.

Description

DESCRIPTION

A TIME INTERLEAVED BLOCK WINDOWED BURST ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING COMMUNICATION SYSTEM FIELD OF THE INVENTION

The present invention is enclosed in the area of telecommunication systems.

PRIOR ART

With the current growth of active mobile users and the continuous increase in data traffic, the 5G (fifth generation) communication systems are expected to improve major key performance indicators, such as data rate, spectral and power efficiencies, latency, reliability and mobility, thus promising to deliver an enhanced experience to these mobile users, while increasing the capacity with respect to current long-term evolution systems.

In order to support data transmission in several scenarios, with different service qualities, delay requirements and different carrier frequencies, a new radio interface is being developed by 3rd Generation Partnership Project (3GPP) for 5G systems. The choice of the waveform for 5G New Radio culminated in the adoption of Orthogonal Frequency Division Multiplexing (OFDM), with the addition of cyclic prefix (CP) for the downlink and uplink transmissions, as in 4G, due to its robustness over multipath propagation and easy signal generation and equalization through the inverse fast Fourier transform (IFFT) and FFT algorithms, respectively. However, OFDM has several drawbacks such as limited spectral efficiency, while exhibiting a high out of band (OOB) radiation due to the rectangular pulse shaping in time domain. Additionally, OFDM signals may present large peak-to-average power ratio (PAPR). Thus, there was left open the possibility of using new waveform modulation types - not defined by 3GPP - that should be able to achieve the desirable spectral and power efficiencies and surpass major OFDM constraints. Therefore, alternative waveforms are being researched and proposed for 5G in order to compensate OFDM disadvantages - limited power and spectral efficiencies - such as Generalized Frequency Division Multiplexing, Filtered-OFDM and Filter Bank Multicarrier. An another promising OFDM-based waveform Contender is denoted as Time-Interleaved Block Windowed burst OFDM (TIBWB-OFDM).

The TIBWB-OFDM technique is based on grouping several windowed OFDM symbols into a single multi-symbol, named as the Block Windowed Burst OFDM (BWB-OFDM) symbol, followed by time-interleaving the samples of this one. The windowing operation allows a reduction in OOB radiation and, thus, a greater signal's spectral confinement when compared to CP-OFDM schemes, while the hybrid nature of the technique allows the detection of the received signal as of a block based single carrier transmission without the need of using a CP - a zero-padding (ZP) is used instead.

The transmitter scheme of a TIBWB-OFDM can be built based on a Block Windowed burst OFDM transmitter, in which the only difference relates to the block representing the time-interleave operation applied to OFDM symbols after the cyclic extension and windowing operations. After determining the bit-stream to be transmitted, the bit sequence is mapped into symbols of an M-ary signal constellation, in the frequency domain. The symbols modulated in the frequency domain in a specific sub-carrier result from direct mapping of a bit-stream with channel coding and bit interleaving applied. Afterwards, the symbol stream is spilt into N lower-rate sub streams through a S/P operation, generating N_s OFDM symbols compounded by N carriers. After applying an N- sized IDFT, performed by an IFFT algorithm, the OFDM symbols are obtained in the time domain. The new windowed symbol is obtained by applying a square root raised cosine (SRRC) window to which are applied a time- interleaved operation in order to interleave the samples of this symbols between each other, resulting in a set of interleaved symbols. Finally, each of these symbols is concatenated to generate a single mega-block consisting of interleaved symbols, forming a TIBWB-OFDM symbol. On the other hand, the main role of a TIBWB-OFDM receiver is to equalize the received signal and perform the time-deinterleave and matched filtering operations. The received signal is converted to the frequency domain by means of a long N_x- sized DFT and it is equalized using a technique that results from the application of the Minimum Mean Square Error (MMSE) method. The estimated signal is them converted to the time domain after applying an IDFT, following the time- deinterleaved operation. This operation is applied to the resulting signal and is complementary to the operation applied in the transmitter. The same window matched filtering - SRRC - is applied here, to determine the sub-symbols which are then converted back to the frequency domain in order to estimate the original OFDM symbols. Finally, it is a case for such, the bit-deinterleaving and channel decoding operations, complementary to those used in the transmission process, are applied to each of the estimated symbols to obtain an estimate of the original binary sequence.

TIBWB-OFDM, as presented in, claims several advantages to conventional CP-OFDM. The use of a zero-padding (ZP) improves power efficiency since no power is wasted on its transmission. A spectral efficiency gain is also claimed by the use of time domain square root raised cosine (SRRC) windowing, that improves the spectral confinement and reduces out of band (OOB) emissions of the OFDM-based blocks. Furthermore, in order to achieve higher robustness against channel's deep fades, a time interleave operation is performed between the samples of the OFDM-based blocks. Although spectral confinement increases for higher window's roll-off, the length of the TIBWB-OFDM block increases proportionally, due to the juxtaposition of the component symbols. This implies a reduction of symbol rate, which limits spectral efficiency gains. Also, the overall average power of the TIBWB-OFDM block is reduced thus implying an increase of the PAPR transmitted signal, which limits achievable power efficiency gains.

PROBLEM TO BE SOLVED

Both spectral and power efficiencies of the TIBWB-OFDM transmission scheme are limited due to the windowing operation performed in the transmitter. Although this operation is required to reduce OOB emissions, it results in temporal extension of the OFDM-based blocks and increases the signal peak-to-average power ratio (PAPR). It is therefore an object of the present invention a TIBWB-OFDM communication system, where adjacent OFDM sub-blocks are overlapped in time, leading to a smaller overall block, increasing the spectral efficiency and decreasing the PAPR of the transmitted signals when compared with standard TIBWB-OFDM.

SUMMARY OF THE INVENTION

To improve the spectral efficiency of TIBWB-OFDM, it is presented an alternative packing of the windowed OFDM component blocks that form a TIBWB-OFDM symbol. Adjacent windowed OFDM blocks are partially overlapped, instead of being juxtapose, as in original proposed TIBWB-OFDM technique.

Although the proposed packing strategy enables a highly efficient spectral transmission, this is achieved at the expense of intentionally introduced interference between consecutive sub-blocks that shape a TIBWB-OFDM symbol, which deteriorates the bit error rate (BER) performance. To cope with this additional interference, avoiding BER degradation, the receiver must entail both channel equalization and inter-block interference cancellation (IBIC) between OFDM component blocks of the TIBWB-OFDM symbol. Linear or iterative frequency domain equalization (FDE) can be applied to the received signal aiming to cancel out channel impairments, as for conventional TIBWB- OFDM without windowing time overlapping. In order to mitigate inter-block interference, a two-way (i.e. simultaneously forward and backward) interference successive cancellation (ISC) is proposed. While ISC can also be made iterative enabling better interference cancellation when combined with linear FDE, the IBIC procedure can be simplified after the initial iteration when this is combined with iterative FDE. Therefore, it is proposed a set of different receiver embodiments (iterative and non iterative) for detection of TIBWB-OFDM with windowing overlapping.

DESCRIPTION OF FIGURES

Figure 1 - representation of an embodiment of a transmitter architecture for the Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing - TIBWB- OFDM - communication system, wherein the reference signs represent: 1 - Serial-to-parallel conversion circuit;

2 - Inverse Fast Fourier Transform circuit of the OFDM modulator;

3 - Cyclic extension guard interval setting and windowing circuit;

4 - Time-overlapping circuit;

5 - Time-interleaving circuit;

6 - Parallel-to-serial conversion circuit;

7 - Frame assembly;

8 - Zero-padding circuit.

Figure 2 - representation of the TIBWB-OFDM original packing.

Figure 3 - representation of the TIBWB-OFDM packing with windowing time overlapping, wherein N_start represents a first overlapped sample from the block windowed burst-OFDM symbol and 0 < b < 1 is the roll-off factor of the OFDM windowed sub-symbols.

Figure 4 - represents the concept of overlapping operation between adjacent symbols. Figure 5 - representation of the operation of the successive cancellation method, that can be carried in order to recover s_{w i} from the estimated BWB with windowing time overlapping. The reference signs represent:

A - Forward successive cancellation - OFDM block 1, ..., N_s;

B - Backward successive cancellation - OFDM block N_s, ..., 1;

wl - windowed OFDM block 1;

w2 - windowed OFDM block 2;

w3 - windowed OFDM block 3;

Figure 6 - representation of an embodiment of a receiver architecture for the Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing -TIBWB- OFDM - communication system. More particularly, a linear equalizer at frequency domain is employed to cancel MIMO channel impairments, while a time domain linear equalizer of the type forward and backward is employed to cancel out windowing time overlapping distortion. The reference signs represent:

9 - frequency domain coefficient;

10 - Inverse Fast Fourier Transform circuit of the OFDM demodulator; 11 - TIBWB-OFDM unformatting circuit;

12 - Time-deinterleaving block;

13 - Windowing time overlapping compensation block;

14 - Block windowed burst- OFDM block;

15 - Demapper & Bit Deinterleaver & Channel decoding;

16 - Soft decision device;

17 - Noise estimation.

Figure 7 - representation of an embodiment of a receiver architecture for the Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing - TIBWB- OFDM - communication system. More particularly a linear equalizer at frequency domain is employed to cancel MIMO channel impairments, while an iterative time domain equalizer is employed to cancel out windowing time overlapping distortion. The time domain equalizer performs, in the first stage, a forward and backward successive cancelation of the windowing time overlapping distortion, while in the following stages uses the prior estimated TIBWB-OFDM signal. The reference signs represent:

9 - frequency domain coefficient;

10 - Inverse Fast Fourier Transform circuit of the OFDM demodulator;

11 - TIBWB-OFDM unformatting circuit;

12 - Time-deinterleaving block;

13 - Windowing time overlapping compensation block;

14 - Block windowed burst- OFDM block;

15 - Demapper & Bit Deinterleaver & Channel decoding;

16 - Soft decision device;

17 - Noise estimation;

18 - Mapper & Bit interleaver & Channel coding;

19 - TIBWB-OFDM formatting circuit.

Figure 8 - representation of an embodiment of a receiver architecture for the Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing - TIBWB- OFDM - communication system. More particularly, an iterative equalizer at frequency domain, of the type (IB-DFE (iterative block decision feedback equalizer) or MRC (maximum ratio combining) or EGC (equal gain combining), is employed to cancel MIMO channel impairments, while a time domain linear equalizer of the type forward and backward successive cancelation is employed to cancel out windowing time overlapping distortion. The reference signs represent:

9 - frequency domain coefficient;

10 - Inverse Fast Fourier Transform circuit of the OFDM demodulator;

11 - TIBWB-OFDM unformatting circuit;

12 - Time-deinterleaving block;

13 - Windowing time overlapping compensation block;

14 - Block windowed burst- OFDM block;

15 - Demapper & Bit Deinterleaver & Channel decoding;

16 - Soft decision device;

17 - Noise estimation;

18 - Mapper & Bit interleaver & Channel coding;

19 - TIBWB-OFDM formatting circuit, formed by a time-interleaving block, a windowing time overlapping compensation block and a BWB-OFDM formatting block;

20 - Fast Fourier Transform circuits.

Figure 9 - representation of an embodiment of a receiver architecture for the Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing - TIBWB- OFDM - communication system. More particularly, an iterative equalizer at frequency domain, of the type (IB-DFE (iterative block decision feedback equalizer) or MRC (maximum ratio combining) or EGC (equal gain combining), is employed to cancel MIMO channel impairments, while an iterative time domain equalizer is employed to cancel out windowing time overlapping distortion. The time domain equalizer performs in the first stage a forward and backward successive cancelation of the windowing time overlapping distortion, while in the following stages uses the prior estimated TIBWBOFDM signal. The reference signs represent:

9 - frequency domain coefficient;

10 - Inverse Fast Fourier Transform circuit of the OFDM demodulator;

11 - TIBWB-OFDM unformatting circuit; 12 - Time-deinterleaving block;

13 - Windowing time overlapping compensation block;

14 - Block windowed burst- OFDM block;

15 - Demapper & Bit Deinterleaver & Channel decoding;

16 - Soft decision device;

17 - Noise estimation;

18 - Mapper & Bit interleaver & Channel coding;

19 - TIBWB-OFDM formatting circuit;

20 - Fast Fourier Transform circuits.

DETAILED DESCRIPTION

In this document the following notation will be employed: capital bold lettering (e.g., S) is used to refer a block/vector of samples at the frequency domain, and lower-case bold lettering (e.g., s) to denote a block/vector of samples at the time domain, while non-bold capital (e.g., S) or lower-case lettering (e.g., s) are used to denoted the symbols/samples of each of those block/vectors, respectively.

TIBWB-OFDM waveform

A typical TIBWB-OFDM transmitter is built on the Block Windowed Burst Orthogonal Frequency Division Multiplexing (BWB-OFDM) transmitter, taking advantage of the greater confinement achieved in the signal's spectrum by decreasing the OOB radiation. The BWB-OFDM transmitter applies a cyclic extension to the conventional OFDM symbol and replaces the rectangular window with a roll-off dependent window, known as square root raised cosine (SRRC), extending each symbol to N_sym = JV(1 + b ) samples after discarding the tailing zeros from the windowing operation in the time domain, where b represents the window roll-off. This symmetric window multiplies in the time domain with the samples of the signal, allowing to achieve a reduction in the frequency side lobes, improving spectral efficiency.

Furthermore, in this waveform, N_s windowed OFDM symbols or blocks are packed together, and are added a single zero-pad (ZP) of length N_ZP, thereby improving power efficiency, since a single suffix of zeros is added, eliminating the need of a CP per OFDM symbol. The ZP length, N_ZP, added to the BWB-OFDM symbol, must be longer than the multipath channel's propagation delay in order to avoid ISI.

Considering that s_{w i} [ri\, i = 1 , .. . , N_S, are the N_s OFDM symbols resulting from the operations previously mention, a BWB-OFDM symbol/mega-block before the ZP insertion, s_B [n], n = 0 , . .. , N_symN_s — 1, can be described as sum of juxtaposed windowed OFDM with a delay proportional to N_sym and can be expressed through:

Hence, a BWB-OFDM mega-block has a total length of N_x = N_s N(1 + b ) + Nz. The BWB-OFDM technique can either achieve higher transmission rates than CP-OFDM schemes while maintaining the same spectrum or achieve better spectrum confinement maintaining the same data rate.

A wireless channel is regularly a frequency selective channel and has a coherence bandwidth. This bandwidth represents the frequency range where the frequency response of the channel is approximately flat. This means that there are certain spectral regions that are characterized by having deep inband fades, which can cause signal degradation in an OFDM system because they affect the sub-carriers located in that region. Given that the signal spectrum of the transmitted BWB-OFDM mega-block consists on a superimposed and phase-shifted spectrum of all N_s windowed OFDM symbol, S_{w i}, i = 1 , .. . , N_S, it can be deduced by the applying the discrete Fourier transform (DFT) to s_B [n], hence:

The detection of the transmitted signal is heavily conditioned by the frequency response of the channel since the spectral data that modulate a specific set of sub-carriers that lay inside the deep fade sections are completely corrupted.

In order to solve the problem of high sensitivity to deep fading, a multi carrier technique, called Time-Interleaved BWB-OFDM (TIBWB-OFDM), is known from the art allowing the signal to be resilient against deep inband fades. The TIBWB-OFDM symbols are generated by performing a time-interleave operation between the samples of the various OFDM sub-symbols (from a total of N_s) that make up the BWB-OFDM mega-block. This causes the replication of the spectral data over the available bandwidth, occupied by the BWB-OFDM signal, creating a diversity effect in the frequency domain, granting much better robustness against deep fading effect of the communication channel. This procedure allows partial recovery of the spectral data that had been affected by the channel through the unaffected compressed replicas created by the time-interleave operation. Therefore, this operation is similar to an expanding operation, by a factor of N_s, which results in the expanded windowed OFDM symbols, s_e,i, i = 1, ... , N_S given by

where c E TL, n = 0,1,—, N_symN_s — 1.

This operation is, then, followed by a unitary time delay between consecutive symbols, filling the zero gaps of each other, s_{n i}, i = 1, ... , N_s. A mega-block TIBWB-OFDM, s_Bint[ri\ can be expressed as follows:

Therefore, the spectrum of a TIBWB-OFDM mega-block remains a superimposition of the spectrum of all N_s expanded windowed OFDM symbol, s_{e i}, i = 1, ..., N_S. However, due to the expanding operation, their spectrum is now compressed and replicated in frequency N_s times,

This technique is considered an hybrid technique, since it combines features associated to single carrier and multicarrier transmission systems, keeping in mind that, on the one hand, it can be considered a single carrier technique with frequency domain equalization (SC-FDM) when N = 1 or, on the other hand, it can be considered a simple multicarrier OFDM system with a ZP, when N_s= 1.

Overlapped-TIBWB-OFDM

A. Transmitter architecture

In order to address both the issues presented by the TIBWB-OFDM scheme and achieve an improved spectral efficiency by avoiding the temporal expansion of the signal, a new waveform based on this one emerges. The Overlapped-TIBWB- OFDM waveform is created based on the operations performed on the TIBWB-OFDM waveform, followed by a partial overlap operation, between adjacent windowed OFDM symbols, in the time domain.

First, a set of N_s N- sized OFDM symbols are generated by mapping the coded and bit-interleaved bitstream onto symbols from an M-ary constellation, according to where / = 1 , . . . , N_s, n = 0 , . , . , N— 1, S_k. represents the constellation symbol loaded on sub-carrier k = 0, ... , N— 1, from the OFDM symbol / , and w[n] is a rectangular window.

Afterwards, the N_s OFDM symbols = [s_{n i}; n = 1, ... , N_S =

[s_w . s_N- ], i = 1, ... , JV_S, are cyclic extended and windowed in time domain resulting, forming the cyclic extended and windowed OFDM symbols, s_{w i}, i = 1, ... , N_s, which can be expressed by

i = ⁵;] Q ^hsRRc [n], i = 1, ... , N_S (eq.7)

Where h_SRRC is a SRRC window, written as: Then, after discarding the tailing zeros, the BWB-OFDM mega-block is obtained through (eq.l). The next step involves the overlapping operation between the cyclic extended and windowed OFDM sub-blocks, that is, the last samples of symbol i = 1, ..., N_S are added, in the time domain, with the first samples of the symbol i + 1. The new Overlapped-BWB-OFDM symbol can be expressed by

where N_start represents the first overlapped sample from the block. The number of overlapped samples, N_os, can be expressed through N_os = N_sym— N_start.

The spectrum of eq.9 can be expressed as

By analysing eq.2 and eq.lO and in order to maintain the same transmission rate for both signals, the first one must be sampled at a frequency higher than the second one. Consequently, the Overlapped-BWB-OFDM signal's spectrum provides a bandwidth saving which is proportion to the temporal growth of the signal. On the other hand, for the same spectrum usage, the Overlapped-BWB-OFDM mega block spectrum contains the superimposition of each windowed OFDM symbol's spectrum, allowing higher rate data streams, improving the spectral efficiency.

This way, the new waveform reduces the number of transmitted samples, i.e., the signal length from N_0B = N_start(N_s— 1) + N_sym, discarding the ZP, at the expense of intentionally introduced interference between the N_s blocks that shape the BWB-OFDM mega-block. The Overlapped-TIBWB-OFDM mega-block is generated performing the time-interleave operation, similarly to the TIBWB-OFDM case. However, in this case, in order to perform the time-interleave operation with interleaving factor N_s, the number of transmitted samples, N_0B must be a multiple of the number of blocks, N_s. Therefore, this waveform keeps its robustness when transmitting through time dispersive channels when the number of blocks and N_start are chosen accordingly. Finally, in case of a time dispersive channel, a single ZP of length N_ZP is added to the mega-block. Thus, the transmitted signal has a total length of N_x = N_0B + N_ZP.

B. Receiver architecture

Channel equalization - FDE

TIBWB-OFDM is seen as a hybrid modulation technique, where the received signal can be regarded as a block-based single-carrier transmission and it is equalized as a whole in the frequency domain. Therefore, both linear or iterative FDE can be employed. If the ZP added to the transmitted signal is longer than the channel's impulse response, the received signal, Y_k, k = 0, ... , N_X— 1, can be expressed, in the frequency domain, as a function of the DFT of the transmitted signal, S_k, k = 0, ... , N_x— 1, as Y_k = S_kH_k + h_1i, where H_k denotes the channel frequency response at kth sub carrier and ?7_fc represents the complex additive white Gaussian noise (AWGN) sample at that sub-carrier.

In order to get an estimate of the transmitted signal, S_k, k = 0, ... , N_x— 1, the received signal is equalized in the frequency domain, which can be performed by the equalization method minimum mean squared error (MMSE) as follows

where H_k denotes the channel frequency response at kth sub-carrier, H^* _k represents a complex conjugate of the channel's frequency response and Y is the Signal-to-noise ratio.

Afterwards, the estimated signal is converted to time domain, ^, ?! = 0, ... , N_x— 1, through a N_x- sized IFFT so its ZP can be removed. Then, the time- deinterleave operation is applied on the time domain signal. This operation reorders the signal so that it can be reverted to its original sequence, based on the number of blocks, N_s. The performance can be improved by employing a more powerful non-linear iterative FDE known as iterative block decision feedback equalizer (IB-DFE). At this point, after applying the equalization and time-deinterleave operations to the received signal, an Overlapped-BWB-OFDM signal estimate is obtained. In order to get the original BWB-OFDM signal, prior to the overlapping operation, it is necessary to develop inter-block interference cancellation (IBIC) to cancel its effect.

IBIC:

When windowing time overlapping is employed, the windowed OFDM blocks that composed the estimated BWB symbol are still overlapped. BWB unformatting thus requires the development of an IBIC algorithm to mitigate this effect, aiming to obtain an estimate of the N_s windowed OFDM symbols, s_{w i}. For that, a match filtering operation must be performed prior to the interference cancellation. This operation is employed in an overlapping approach, similar to the overlapping operation described in the transmitter while an equal number of zeros is added to each symbol so that their length reaches 2 N, resulting in the overlapped symbols s_{wo i} with length 2 N (only the N_sym = iV( 1 + b ) samples in middle have non null values). Leveraging on (eq.7), i.e. on the cyclic extension of each OFDM component symbol and the symmetric profile of the window, i.e., root raised cosine (RRC), r(n) = h_RRC [n] = h² _SRRC [n ], it is easily perceived that each windowed OFDM symbol contains redundant information, since each sample of the original OFDM symbol appears both on the left and the right halves of the windowed OFDM block, just multiplied by different coefficient of the window. Thus, an interference successive cancellation can be carried upon reception in order to recover s_{w i} from the estimated BWB with windowing time overlapping. Particularly, the left part of the first windowed OFDM has no interference from any other symbol, thus, leveraging on the window knowledge, it is possible to estimate and cancel the interference caused by the first symbol's right part in the left part of the second windowed OFDM symbol. The procedure can thus be successively repeated between the last windowed OFDM symbol whose interference on the left part was cancelled and the OFDM symbol that follows. A similar procedure can be carried simultaneously in the backward direction, since the right part of the last windowed OFDM has no interference from any other symbol, enabling to estimate and cancel the interference caused by this in the right part of the penultimate windowed OFDM symbol. Two different approaches can be carried on performing this forward and backward ISC, which are zero forcing (ZF) or MMSE methods:

ISC-ZF:

In the following analysis, the non-null samples of the distorted symbols

N N

s_{wo i} are given by indexes s_{wo ij}·, where j =—— (1 + b ), — (1 + b ). In the forward direction, the original samples j' from the distorted symbol s_{wo i} (i = 1, ... , N_S), can be estimated through symbol i— 1 according to: where j’ =— j, — j (l— /?). Additionally, in the backward direction, the original samples j" from the distorted symbol s_{wo i} can be estimated through symbol i + 1, following similar analysis:

ISC-MMSE:

In this case, the interference values and partial estimations of the symbols take into account the SNR. This means that a second Hadamard product is applied between the estimated samples and the window samples followed by a Hadamard division that includes the SNR, Y . The forward and backward cancellation method are defined by:

Forward:

Backward: Receiver designs:

Four different receiver embodiments for a TIBWB-OFDM with windowing time overlapping transmission, where iterative and noniterative strategies can be used for both channel FDE and IBIC. Receivers are presented for the single input single output (SISO) case only, although an extension to MIMO is straight forward. Besides, the interference cancellation equations are presented for the forward direction. In the backward direction, similar equations can be derived.

Linear FDE + ISC: The first receiver embodiment is represented in figure 6. It consists on a non-iterative MMSE FDE to cope with channel impairments, while an ISC operation of type MMSE is employed to cancel WTO distortion. In this way, the estimation of the transmitted TIBWB-OFDM symbol is performed as in (eq.ll). Then, the overlapping interference is compensated and the windowed OFDM blocks are estimated through (eq.14).

Iterative FDE + ISC: A second embodiment of the TIBWB-OFDM with windowing time overlapping receiver (figure 8) consists on an iterative FDE, of the IB-DFE type and an ISC operation of type MMSE. For this case, in each iteration l (except the first one), the equalized signal depends on the TIBWB-OFDM multi-symbol, estimated from the previous (Z— 1) FDE iteration. The iterative nature of this receiver allows an improvement in BER performance, which is expected to be much more pronounced for the TIBWB-OFDM with windowing time overlapping transmission since each iteration allows a better estimate of the transmitted signal, by applying iteratively the time domain ISC algorithm to the received signal. Thus, assuming the same notation, the MMSE ISC operation, in the forward direction, can be defined for each iteration l, by

Linear FDE + iterative IBIC: A third embodiment of the TIBWB-OFDM with windowing time overlapping receiver (figure 7) relies on a linear FDE, such as MMSE and on an iterative IBIC algorithm. In this case, the received signal is equalized in frequency domain as in (eq.ll). However, instead of applying iteratively the ISC operation to the received signal, each iteration (except the first one) performs the cancellation operation assuming that the prior estimated signal is the one that allows a perfect reconstruction of the distorted signal. Thus, for the first iteration, the IBIC operation corresponds to the MMSE ISC algorithm and the windowed OFDM symbols, in the forward direction, can be estimated as

Furthermore, for the remaining iterations l the samples j' belonging to the windowed OFDM blocks can be estimated through:

Iterative FDE + iterative IBIC: the fourth embodiment of the TIBWB-OFDM with windowing time overlapping receiver (figure 9) consists on a combination of both previously presented receivers, wherein an iterative FDE of the type IB-DFE and an iterative IBIC operation are employed. In the first iteration of this receiver, / = 1, the overlapped transmitted signal is estimated through (eq.ll) and the interference resulting from the overlapping operation, in the forward direction, is cancelled according to (eq.16). For the remaining iterations the equalized TIBWB-OFDM with windowing time overlapping symbol can be computed and the sub-symbol's overlapping distortion is partially eliminated according to

^§wlr = ^§wl,i,r - V ¾7- ^:> _+N ^{ec*-¹⁹⁾

One of the key features about this receiver is that it exhibits the same complexity as the IB-DFE receiver, since the signal reconstruction performed in the FB loop also enables the cancellation of the time domain interference resulting from the overlapping operation. Both the transmitter and the receiver architectures described may include a radio front-end comprised by at least one transmitting antenna and at least one receiving antenna, respectively. The referred radio front-end is provided with processing means configured to allow the operation of the transmitter and the receiver according to a SISO or a MIMO communication channel.

DESCRIPTION OF THE EMBODIMENTS:

The present application describes a TIBWB-OFDM communication system comprising at least one TIBWB-OFDM transmitter and at least one TIBWB-OFDM receiver. In a preferred embodiment, the transmitter is characterised by comprising a time-overlapping circuit configured to generate an overlapped block windowed burst- OFDM symbol, s_0B, by overlapping OFDM windowed sub-symbols s_{w i}, with adjacent sub-symbols, that is, the last samples of a current sub-symbol s_{w i} are added in the time domain with the first samples of the next sub-symbol s_{w i+1}; wherein the overlapped block windowed burst-OFDM symbol is expressed by:

where,

N_s is the number of OFDM blocks of length N;

i is the number of the OFDM block, i = 1, ... , N_s; and

N £ _star_t £ N_Sym’N_Sym = N(1 + b ) represents a first overlapped sample from the block windowed burst-OFDM symbol; and 0 < b < 1 is the roll-off factor of the OFDM windowed sub-symbols; the number of samples of the signal S_0B, N_0B, being a multiple of the number of blocks N_s and is given by N_0B = N_start. ( N_s— 1) + N_sym the generated s_0B being the input of a time-interleaving circuit, adapted to generate TIBWB-OFDM symbols, s_n; the transmitter further comprising a zero padding circuit configured to add a single zero padding - ZP- of length N_ZP to the signal to be transmitted, x, such that x = [ZP s_re]; the size of the transmitted signal N_x, is given by N_x = N_0B + N_ZP;

and wherein, On the other hand, the receiver comprises a TIBWB-OFDM unformatting circuit including:

a time-deinterleaving block programmed to reorder an estimation of the transmitted signal in the time domain, s^, n = 0, ... , N_X— 1, so that it can be reverted to its original sequence based on N_s;

a windowing time overlapping compensation block programmed to apply a matched filtered operation with the same window applied in the transmitter to the overlapped symbols outputted from the time-deinterleaving; and

a block windowed burst -OFDM unformatting block configured to execute an interference cancelation algorithm to be applied to the overlapped windowed signals, s_{wo i}, to estimate the received N_s windowed OFDM symbols, s_{w i}, with length N_Sym· In one embodiment of the system proposed, the number of overlapped samples of the of the signal s_0B, N_os, is given by N_os = N_sym - N_start.

In one embodiment of the system proposed, the transmitter further comprises:

— A radio front-end block, comprised by at least one transmitting antenna;

— A serial-to-parallel conversion circuit configured to split a modulated data symbol S_k, k = 0 , . . , N— 1, in the frequency domain, into N lower rates sub streams;

— An OFDM modulator, connected to the serial-to-parallel conversion circuit, comprised by N_s Inverse Fast Fourier Transform - IFFT - blocks of iV-size each;

— A cyclic extension setting and windowing circuit connected to the output of the OFDM modulator; the output of the cyclic extension setting and windowing circuit being the input of the time-overlapping circuit;

— A time interleaving circuit connected to the output of the time-overlapping circuit, configured to generate overlapped TIBWB-OFDM symbols, s_n, based on the overlapped block windowed burst-OFDM symbol s_0B. Yet in another embodiment of the system, the a ith IFFT block of the OFDM modulator comprises processing means programmed to generate an OFDM symbol s_{k i}, in the time domain, having N sub-carriers, that is s_{k i} = [s_{o i}, ... , s_{N-l i} , k = 0. N - l; i = 1. JV_S.

Yet in another embodiment of the system, the OFDM modulator is configured to:

— generate N_s OFDM symbols in the frequency domain, having N carriers, that is S_{k i}, k = 0, ... , N— 1, with i = 1, ... , N_S; and

— convert the frequency domain N_s OFDM symbols in the time domain, such that, denotes the ith OFDM symbol, and k is the sub-carrier, where k = 0, ... , N— 1 and i = 1, ... , N_S.

Yet in another embodiment of the system, N carrier of the generated N_s OFDM symbols is modulated into symbols of an M-ary signal constellation according to:

where,

i = 1, ... , N_S; n = 0, ... , N— 1; S_k. is a constellation symbol loaded on sub-carrier k = 0, ... , N— 1 from the OFDM symbol i; and w[n\ is a rectangular window of length N. Said M-ary constellation being mQAM or QPSK.

Yet in another embodiment of the system, the cyclic extension setting and windowing circuit is configured to apply a cyclic extension to the time domain N_s OFDM symbols, s^, and to apply a roll-off square root raised cosine window, b, to each symbol, generating OFDM windowed symbols s_{w i}; such symbols being expressed by:

i = ⁵;] Q ^hsRRc [n], i = 1, ... , N_S where, In another embodiment, the time interleaving circuit of the transmitter comprises processing means adapted to generate TIBWB-OFDM symbols, s_n, according to the following expression: s_n = s_0B ^^Ns

Yet in another embodiment the transmitter further comprises a parallel- to-serial circuit connected to the output of the time interleaving circuit.

In one embodiment of the system, the receiver is further comprised by:

— A radio front-end block, comprised by at least one receiving antenna;

— A frequency domain conversion circuit comprised by a plurality of N_x- sized DFT blocks, configured to convert the received time domain signal y_n = y[n] = 0, ... , N_x— 1, in the frequency domain, such that Y_k = DFT(y_n), k = 0, — ,N_X— 1;

— A serial-to-parallel conversion circuit configured to split a modulated data symbol Y_k, k = 0, ... , N_x— 1, in the frequency domain, into N_x lower rates sub streams;

— An equalization block configured to execute an equalization algorithm to estimate the transmitted signal, S_k, k = 0, ... , N_x— 1, in the frequency domain;

— An OFDM demodulator configured to convert the estimation of the transmitted signal S_k in the time-domain, s^, n = 0, ... , N_X— 1; said OFMD demodulator being comprised by N_x- sized IFFT blocks; the output of the OFDM demodulator being the input of the time-deinterleaving block; the output of the demodulator being the input of the TIBWB-OFDM unformatting circuit. In another embodiment of the system wherein the equalization block of the receiver is configured to execute a linear or an iterative equalization algorithm to estimate the transmitted signal, S_k, k = 0, ... , N_X— 1, in the frequency domain. Particularly, the equalization algorithm is a linear minimum mean square error algorithm and the estimation of the transmitted signal S_k is given by:

Where H_k denotes the channel frequency response at /cth sub-carrier, H^* _k represents a complex conjugate of the channel's frequency response and Y_k, k = 0, ..., N_X— 1, is the received signal; Y is the Signal-to-noise ratio.

Yet in another embodiment of the system, the equalization block of the receiver implements an Iterative Block Decision Feedback Equalizer or a Maximum Ration Combining equalizer or an Equal Gain Combining equalizer.

Yet in another embodiment of the system, the interference cancellation algorithm executed in the windowing time overlapping compensation block of the receiver is an interference forward-backward successive cancellation or an iterative inter-block interference cancellation.

Yet in another embodiment of the system, the equalization algorithm is a linear minimum mean square error algorithm and the interference cancellation algorithm executed in the windowing time overlapping compensation block is an iterative inter-block interference cancellation algorithm.

Yet in another embodiment of the system, the equalization algorithm is an iterative Block Decision Feedback Equalizer and the interference cancellation algorithm executed in the windowing time overlapping compensation block is an iterative inter-block interference cancellation algorithm.

Yet in another embodiment of the system, the interference cancellation algorithm executed in the windowing time overlapping compensation block of the receiver is an interference forward-backward successive cancellation implemented as a zero-forcing cancellation method, wherein, in a forward direction, original samples j' from s_{wo i}, i = 1, ... , N_S is estimated through symbol i— 1 according to:

and

in a backward direction, original samples j" from s_{wo i}, i = 1, ... , N_S is estimated through symbol i + 1 according to: and where,

the non-null samples of the symbols s_{wo i} are given by indexes s_{W ij}, where _/ =

Yet in another embodiment of the system, the interference cancelation algorithm executed in the windowing time overlapping compensation block of the receiver is an interference forward-backward successive cancellation implemented as a minimum mean squared error cancellation method, wherein,

In a forward direction, original samples from s_{wo i}, i = 1, ... , N_S is estimated through symbol i— 1 according to:

in a backward direction, original samples j" from s_{wo i}, i = 1, ... , N_S is estimated through symbol i + 1 according to:

where Y is the Signal-to-noise ratio,

and where,

the non-null samples of the symbols s_{wo i} are given by indexes s_{W ij}, where j = (1 + b), (1 + b) and r = h_RRC [n] = h² _SRRC [n] . Yet in another embodiment of the system, the windowing time overlapping compensation block of the receiver is configured to add a specific number of zeros to each OFDM symbol to enable the matched filtering operation equal to the one applied in the transmitter. Particularly, the number of zeros introduced by the windowing time overlapping compensation block of the receiver is the same that is introduced in the zero-padding circuit of the transmitter.

Yet in another embodiment of the system, the matched filtering operation performed in the windowing time overlapping compensation block of the receiver is equal to the one performed by the cyclic extension guard interval setting and windowing circuit of the transmitter.

Yet in another embodiment of the system, the receiver further comprising a demapper-bit deinterleaver and channel decoding circuit connected to the output of the TIBWB-OFDM unformatting circuit; the demapper-bit deinterleaver and channel decoding circuit being configured to decode bit streams of information.

Claims

1. A Time Interleaved Block Windowed Burst Orthogonal Frequency Division Multiplexing - TIBWB-OFDM - communication system comprising at least one TIBWB-OFDM transmitter and at least one TIBWB-OFDM receiver;

wherein, the transmitter is characterised by comprising a time overlapping circuit configured to generate an overlapped block windowed burst-OFDM symbol, s_0B, by overlapping OFDM windowed sub-symbols s_{w i}, with adjacent sub symbols, that is, the last samples of a current sub-symbol s_{w i} are added in the time domain with the first samples of the next sub-symbol s_{w i+1}; wherein the overlapped block windowed burst-OFDM symbol is expressed by:

and

where,

N_s is the number of OFDM blocks of length iV; i is the number of the OFDM block, i = 1, ... , N_S; and N £ N_start £ N_sym, N_sym = JV( 1 + b) ; N_start representing a first overlapped sample from the block windowed burst-OFDM symbol; where 0 < b < 1 is the roll-off factor of the OFDM windowed sub-symbols;

the number of samples of the signal S_0B, N_0B, being a multiple of the number of blocks N_s and is given by N_0B = N_start. (N_s - 1) + N_sym;

the generated s_0B being the input of a time-interleaving circuit, adapted to generate TIBWB-OFDM symbols, s_n; the transmitter further comprising a zero padding circuit configured to add a single zero padding - ZP- of length N_ZP to the signal to be transmitted, x, such that x = [ZP s_re]; the size of the transmitted signal N_x, is given by N_x = N_0B + N_ZP;

and wherein,

the receiver is characterised by comprising a TIBWB-OFDM unformatting circuit comprising: a time-deinterleaving block programmed to reorder an estimation of the transmitted signal in the time domain, s^, n = 0, ... , N_X— 1, so that it can be reverted to its original sequence based on N_s;

a windowing time overlapping compensation block programmed to apply a matched filtered operation with the same window applied in the transmitter to overlapped symbols outputted from the time-deinterleaving; and

a block windowed burst -OFDM unformatting block configured to execute an interference cancelation algorithm to be applied to the overlapped windowed signals, s_{wo i}, to estimate the received N_s windowed OFDM symbols, s_{w i}, with length Nsym-

2. System according to claim 1, wherein the number of overlapped samples of the of the signal s_0B, N_os, is given by: N_os = N_sym— N_start.

3. System according to any of the previous claims, wherein the transmitter further comprises:

— A radio front-end block, comprised by at least one transmitting antenna;

— A serial-to-parallel conversion circuit configured to split a modulated data symbol S_k, k = 0 , . . , N— 1, in the frequency domain, into N lower rates sub- streams;

— A time interleaving circuit connected to the output of the time-overlapping circuit, configured to generate time interleaved overlapped TIBWB-OFDM symbols, s_n, based on the overlapped block windowed burst-OFDM symbol s_0B.

4. System according to claim S, wherein the a ith IFFT block of the OFDM modulator comprises processing means programmed to generate an OFDM symbol s^, in the time domain, having N sub-carriers, that is

[so i, ... , 5_¾_i _j], i— 1, ... , N_S.

5. System according to claims S and 4, wherein the OFDM modulator is configured to:

— generate N_s OFDM symbols in the frequency domain, having N carriers, that is S_{k i}, k = 0, ... , N— 1, with i = 1, ... , iV₅; and

— convert the frequency domain N_s OFDM symbols in the time domain, such that, denotes the ith OFDM symbol, and k is the sub-carrier, where k = 0, ..., N— 1 and i = 1, ... , N_S.

6. System according to claim 5, wherein each N carrier of the generated N_s OFDM symbols is modulated into symbols of an M-ary signal constellation according to:

where,

i = 1, ..., N_S; n = 0, ..., N— 1; S_k. is a constellation symbol loaded on sub-carrier k = 0, ... , N— 1 from the OFDM symbol i; and w[n\ is a rectangular window of length N.

7. System according to claim 6, wherein the M-ary constellation is mQAM or QPSK.

8. System according to any of the previous claims S to 7, wherein the cyclic extension setting and windowing circuit is configured to apply a cyclic extension to the time domain N_s OFDM symbols, s^, and to apply a roll-off square root raised cosine window, b, to each symbol, generating OFDM windowed symbols s_{w i}; such symbols being expressed by:

i = ⁵;] Q ^hsRRc[n], i = 1, ... , N_S

where,

9. System according to any of the previous claims, wherein the time interleaving circuit comprises processing means adapted to generate TIBWB-OFDM symbols, s_n, according to the following expression s_n = s_0B ^^Ns

10. System according to any of the previous claims 3 to 9, wherein the transmitter further comprises a parallel-to-serial circuit connected to the output of the time interleaving circuit.

11. System according to any of the previous claims, wherein the length of N_ZP is longer that a channels delay spread.

12. System according to any of the previous claims, wherein the receiver further comprises:

— A radio front-end block, comprised by at least one receiving antenna;

— A frequency domain conversion circuit comprised by a plurality of N_x- sized DFT blocks, configured to convert the received time domain signal y_n = y[n] = 0, ... , N_x— 1, in the frequency domain, such that Y_k = DFT(y_n), k = 0,— , N_X— 1;

— A serial-to-parallel conversion circuit configured to split a modulated data symbol Y_k, k = 0, ... , N_x— 1, in the frequency domain, into N_x lower rates sub streams; — An equalization block configured to execute an equalization algorithm to estimate the transmitted signal, S_k, k = 0, ... , N_x— 1, in the frequency domain;

— An OFDM demodulator configured to convert the estimation of the transmitted signal S_k in the time-domain, s^, n = 0, ... , N_X— 1; the output of the OFDM demodulator being the input of the time-deinterleaving block; the output of the demodulator being the input of the TIBWB-OFDM unformatting circuit.

13. System according claim 12, wherein the equalization block is configured to execute a linear or an iterative equalization algorithm to estimate the transmitted signal, S_k, k = 0, ... , N_x— 1, in the frequency domain.

14. System according to claim 13, wherein the equalization algorithm is a linear minimum mean square error algorithm and the estimation of the transmitted signal S_k is given by:

Where H_k denotes the channel frequency response at kt sub-carrier; H^* _k represents a complex conjugate of the channel's frequency response; Y_k, k = 0, ..., N_X— 1, is the received signal; Y is the Signal-to-noise ratio.

15. System according to claim 13, wherein the equalization block implements an Iterative Block Decision Feedback Equalizer or a Maximum Ration Combining equalizer or an Equal Gain Combining equalizer.

16. System according to any of the previous claims 12 to 15, wherein the interference cancellation algorithm executed in the windowing time overlapping compensation block is an interference forward -backward successive cancellation or an iterative inter-block interference cancellation.

17. System according to claims 14 and 16, wherein equalization algorithm is a linear minimum mean square error algorithm and the interference cancellation algorithm executed in the windowing time overlapping compensation block is an iterative inter-block interference cancellation algorithm.

18. System according to claims 15 and 16, wherein the equalization algorithm is an iterative Block Decision Feedback Equalizer and the interference cancellation algorithm executed in the windowing time overlapping compensation block is an iterative inter-block interference cancellation algorithm.

19. System according to any of the previous claims 16 to 18, wherein the interference cancellation algorithm executed in the windowing time overlapping compensation block is an interference forward-backward successive cancellation implemented as a zero-forcing cancellation method, wherein,

in a forward direction, original samples j' from s_{wo i}, i = 1, ... , N_S is estimated through symbol i— 1 according to:

and

the non-null samples of the symbols s_{wo i} are given by indexes s_{W ij}, where j = - j Cl + jff), - ,j (l + 0); and r = /i_BBC[n] = ¾² _SRR_C W -

20. System according to claim 16, wherein the interference cancelation algorithm executed in the windowing time overlapping compensation block is an interference forward-backward successive cancellation implemented as a minimum mean squared error cancellation method, wherein,

where Y is the Signal-to-noise ratio,

and where,

the non-null samples of the symbols s_{wo i} are given by indexes s_{W ij}, where j = - j (1 + b), - , j (1 + b) and r = h_RRC [n] = h² _SRRC [n] .

21. System according to claims 19 or 20, wherein the windowing time overlapping compensation block of the receiver is configured to add a specific number of zeros to each OFDM symbol to enable the matched filtering operation equal to the one applied in the transmitter.

22. System according to any of the previous claims 21, wherein the number of zeros introduced by the windowing time overlapping compensation block of the receiver is N b.

23. System according to claims 8 and 22, wherein the matched filtering operation performed in the windowing time overlapping compensation block of the receiver is equal to the one performed by the cyclic extension guard interval setting and windowing circuit of the transmitter.

24. System according to any of the previous claims 12 to 23, wherein the receiver further comprising a demapper-bit deinterleaver and channel decoding circuit connected to the output of the TIBWB-OFDM unformatting circuit; the demapper-bit deinterleaver and channel decoding circuit being configured to decode bit streams of information.