GB2472569A - An asynchronous block CDMA system uses serial interference cancellation, based on the order of arrival of signals, to cancel multi-user interference - Google Patents

Abstract

A block spread code division multiple access transmission system comprises a plurality of user terminal transmitters which are configured to transmit signals to a base station receiver. The receiver uses a successive interference cancellation (SIC) scheme to mitigate against multiple access interference (MAI). Conventional SIC schemes process user signals in decreasing order of signal strength, but here asynchronously received signals are processed in the order in which they are received. Preferably, first and second blocks received from a first user are decoded and used to reconstruct interference, which is subtracted from a second user signal before decoding the first block of the second user signal. The transmitter applies precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount to mitigate against inter-block interference (IBI).

Description

A block spread code division multiple access transmission system and a method of and receiver for cancelling interference in such a transmission system This invention relates to a block spread code division multiple access transmission system and a method of and receiver for cancelling interference in such a transmission system.

CDMA is a popular multiple access technique that is used to support multiple users simultaneously in a network. In synchronous CDMA each user is allocated a code to modulate their signal such that the codes of each user are mutually orthogonal. This means that the dot product of vectors representing each code is zero. This is referred to as the cross-correlation of the orthogonal codes being equal to zero. Since the codes are orthogonal, signals constructed using different codes do not interfere with each other and can be separated from each other at the receiver.

Many variants of CDMA exist, including direct sequence (DS) CDMA, multi-carrier (MC) CDMA, cyclic prefixed (CP) CDMA, and chip interleaved block spread (CIBS) CDMA. In addition to these variants, many receiver architectures are often available for implementation in CDMA systems, such as the well-known RAKE receiver, interference cancellation receivers, and receivers that rely on channel equalisation.

Some CDMA schemes are interference limited; that is to say, as the number of users in the network increases, residual interference caused by each user eventually cripples the network, thus rendering simultaneous multiple access nearly impossible.

This residual interference generally results from the loss of orthogonality amongst users, which primarily occurs when the channel is temporally dispersive. The channel may be temporally dispersive, for example, due to multipath which occurs when a signal propagates from a transmitter to a receiver over a plurality of routes due to, for example, reflection or refraction, which means that the length of each path of the signal varies resulting in different propagation times for the different paths of the signal.

Several recent developments in block CDMA systems have led to multi-user interference (MUI) free transmission techniques.

Zhengdao Wang and G.B. Giannakis, "Wireless Multicarrier Conirnunications", IEEE Signal Processing Magazine, Vol. 17, May 2000, pp 29-48, the contents of which are incorporated herein by reference, describes so-called generalised MC-CDMA' (GMC-CDMA).

Shengli Zhou and G.B. Giannakis, "Chip-interleaved block-spread code division multiple access", IEEE Transaction on Communications, Vol. 50, Feb. 2002, pp. 235- 248, the contents of which are incorporated herein by reference, describes chip-interleaved block-spread CDMA "Performance comparison of distributed FDMA and localised FDMA with frequency hopping for EUTRA uplink," NEC Group and NTT DoCoMo, TSG RAN WG1 Meeting 42 RI -050791, Aug. 2005 and D. Galda and H. Rohiling, "A low complexity transmitter structure for OFDM-FDMA uplink systems," in Proc. of the IEEE Vehicular Technology Conference (VTC), vol. 4, May 2002, pp. 173 7-1741, the contents of which are incorporated herein by reference, describe a single-carrier frequency division multiple access (SC-FDMA) (DFT-spread OFDM).

S. Tomasin and F. Tosato, "Throughput Efficient Block-Spreading CDMA: Sequence Design and Performance Comparison," in Proc. of the IEEE Global Telecommunications Conference (Globecom), Nov.-Dec. 2005, the contents of which are incorporated herein by reference, describes a throughput-efficient block CDMA system.

J. P. Coon, "Precoded Block-Spread CDMA with Maximum User Support and Frequency-Domain Equalization", in Proc. of the IEEE International Conference on Communications (ICC), Glasgow, 2007, and GB-A-2433397, the contents of which are incorporated herein by reference, describe a configurable scheme which has led to MUI free transmission techniques. The scheme achieves higher bandwidth efficiency compared to conventional CDMA systems such as CIBS-CDMA.

In these systems, any number of users -up to a given maximum number -can theoretically transmit simultaneously without causing any degradation in system performance. Beyond this maximum number of allowable users, the system becomes interference limited in a similar manner to other CDMA systems.

Below, we refer to the system model based on J. P. Coon, "Precoded Block-Spread... Equalization" and GB-A-2433397 as the bandwidth efficient BS-CDMA system. Such systems include the orthogonal frequency-division multiple access (OFDMA) systems using discrete fourier transform (DFT) precoding, and single-carrier based distributed frequency division multiplexing access (FDMA) systems.

Although prior art methods can provide Mill free and inter-block interference (IBI) free transmission using the block CDMA techniques, this can generally only be achieved with ideal assumptions. One of these assumptions is that the receiver receives signals from user terminals with perfect synchronization. However, in practice MUI and IBI occur when the signal reception among different users cannot be perfectly synchronised due to delays in transmission or due to delays from the channel. In these cases the system is not synchronous but is asynchronous, which means that the MUI free and IBI free models described in the prior art are not practically implementable.

Jn other cases, MUI and IBI could occur due to the insufficient cyclic prefix as described in X. Peng, T. S. Dharma, F. Chin and A. S. Madhukumar, "Novel interference cancellation methods for BS-CDMA in uplink broadband mobile communication systems", in IEEE Communication Letters, vol. 12, No. 8, August 2008, the contents of which are incorporated herein by reference. This document considers interference cancellation for BS-CDMA and proposes a scheme where the mobility condition of the mobile subscribers is used for specifying the order of detection. High mobility subscribers are considered as more severe interferers and are detected first such that their effect on others may be cancelled at an early stage. A multistage successive multi-user interference cancellation (SMC) is also proposed that demonstrates high performance gains when the number of stages is large.

In the case of asynchronous reception, transmitted blocks are no longer mutually shift orthogonal, and M1JI and IBI occur, as described in the applicant's patent application no. GB 0817521.8, the contents of which are incorporated herein by reference. Depending on the order of signal reception, very high error floors can be observed. The MUI and IBI are suppressed in the referenced patent application by using a minimum mean squared error (MMSE) equaliser that takes the interference into account. Although the interference can be effectively suppressed by the proposed MMSE equaliser, to achieve optimal performance, the MMSE equaliser can be complex to implement.

According to a first aspect of the invention there is provided a method of cancelling interference in a block spread code division multiple access transmission system comprising a plurality of user terminal transmitters which are configured to (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix, the method comprising: (a) receiving asynchronously at a receiver signals from a plurality of user terminals; (b) processing the signal from a first user terminal; (c) reconstructing the interference caused by the signal transmitted by the first user terminal; (d) using the reconstructed interference to process signals received from the other user terminals; and (e) repeating steps (b) to (d) for the signals received from the other user terminals, wherein the signals are processed in the order of arrival at the receiver.

According to a second aspect of the invention there is provided a method of cancelling interference in a block spread code division multiple access transmission system comprising a plurality of user terminal transmitters which are configured to (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix, the method comprising: (a) receiving asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals Ma; (b) processing a first block n from a first user terminal m1; (c) processing a second block (n+1)1 received from the first user terminal rn immediately after receipt of the first block n1,; (d) reconstructing the interference caused by the first block n and the second block (n+1)1 received from the first user terminal rn; (e) using the reconstructed interference to process a third block n,+j, the third block fl1+j being transmitted by a second user terminal m1+j and being the next block received at the receiver immediately after receipt of the first block n-received from the first user terminal rn; and (f) repeating steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block n,+j no blocks subsequent to the second block (n+1)1 from the previous user terminal rn1 are processed before the third block ni is processed.

The current invention is concerned with the reconstruction of the MIEJI from the main interference contributors and subtracting it recursively from the received signal.

The detection order in this case is in ascending order of delay, i.e., processing blocks of data based on the order of arrival of the blocks and/or the order of arrival of signals from the user terminal. Interference reconstruction depends on knowledge of delay and channel state information (the channel taps), which can be calculated using known techniques at the receiver/base station (uplink). Note that asynchronous reception is not a significant problem in the downlink, in transmission from typically the base station to user terminals.

The order of processing of blocks may be given by: Initialization: a 1, n(O) 1 and in(O) l, O for tltOMa*T4 if n(t-1) T a + =1; end if if n(t-1) =1 or m(t-1) Ma, n(t) n(t-1)+m(t-1)-a+1; m(t)a; else m(t) (m(t-1) % M0) +1; n(t) m(t-l) + n(t-1) -end if end for where: Ma is the number of active user terminals; T is the number of blocks transmitted by each user terminal; t is a time index; a is an index; and n(t) and m(t) denote the nth block of the mth user terminal respectively decoded at time t.

The blocks may be transmitted using a low-complexity single-carrier frequency division multiple access scheme or a low complexity orthogonal frequency division multiple access scheme.

The step of processing a block may comprise: despreading the block; decoding the despread block; and equalising the decoded block.

The step of processing a block further may comprise: soft bit mapping the equalised block; de-interleaving the mapped block; and channel decoding the de-interleaved block.

The equalising step may comprise equalising the block with a conventional equaliser used for synchronous reception.

According to a further aspect of the invention there is provided a block spread code division multiple access transmission system comprising: a plurality of user terminal transmitters which are configured to: (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix; and a receiver for receiving asynchronously signals from the plurality of user tenninals, the receiver being configured to: (a) process the signal from a first user terminal; (b) reconstruct the interference caused by the signal transmitted by the first user terminal; (c) use the reconstructed interference to process signals received from the other user terminals; and (d) repeat steps (a) to (c) for the signals received from the other user terminals, wherein the receiver is configured to process received signals in the order of arrival at the receiver.

According to a further aspect of the invention there is provided a block spread code division multiple access transmission system comprising: a plurality of user terminal transmitters which are configured to: (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix; and a receiver for receiving asynchronously signals from the plurality of user terminals, the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals M0; (b) process a first block n from a first user terminal rn; (c) process a second block (n+])1 received from the first user terminal rn immediately after receipt of the first block n,; (d) reconstruct the interference caused by the first block n and the second block (n+1)1 received from the first user terminal rn; (e) use the reconstructed interference to process a third block n+1, the third block fl+j being transmitted by a second user terminal rn+j and being the next block received at the receiver immediately after receipt of the first block n received from the first user terminal rn; and (f) repeat steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block n,1 no blocks subsequent to the second block (n+1), from the previous user terminal rn are processed before the third block n+1 is processed.

According to a further aspect of the invention there is provided a receiver for cancelling interference in block spread code division multiple access transmission system signals transmitted by a plurality of user terminal transmitters, the signals having been transmitted by: (i) applying precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spreading the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) applying a guard interval in the form of a cyclic prefix; the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals Ma; (b) process the signal from a first user terminal; (c) reconstruct the interference caused by the signal transmitted by the first user terminal; (d) use the reconstructed interference to process signals received from the other user terminals; and (e) repeat steps (b) to (d) for the signals received from the other user terminals, wherein the receiver is configured to process received signals in the order of arrival at the receiver.

According to a further aspect of the invention there is provided a receiver for cancelling interference in block spread code division multiple access transmission system signals transmitted by a plurality of user terminal transmitters, the signals having been transmitted by: (i) applying precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spreading the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) applying a guard interval in the form of a cyclic prefix; the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals Ma; (b) process a first block n1 from a first user terminal rn; (c) process a second block (n+1)1 received from the first user terminal rn immediately after receipt of the first block n,,; (d) reconstruct the interference caused by the first block n and the second block (n+1)1 received from the first user terminal rn; (e) use the reconstructed interference to process a third block n1+j, the third block n+1 being transmitted by a second user terminal in+1 and being the next block received at the receiver immediately after receipt of the first block n received from the first user terminal rn; and (f) repeat steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block fl.4-j no blocks subsequent to the second block (n+1), from the previous user terminal rn are processed before the third block n,�1 is processed.

According to a further aspect of the invention there is provided a carrier medium carrying computer readable code for controlling a microprocessor to carry out the method described above.

The present invention can be implemented in any convenient form, for example using dedicated hardware, or a mixture of dedicated hardware and software. The present invention may be implemented as computer software implemented by one or more networked processing apparatuses. The network can comprise any conventional terrestrial or wireless communications network, such as the Internet, The processing apparatuses can comprise any suitably programmed apparatuses such as a general purpose computer, personal digital assistant, mobile telephone (such as a WAP or 3G-compliant phone) and so on. Since the present invention can be implemented as software, each and every aspect of the present invention thus encompasses computer software implementable on a programmable device. The computer software can be provided to the programmable device using any conventional carrier medium. The carrier medium can comprise a transient carrier medium such as an electrical, optical, microwave, acoustic or radio frequency signal carrying the computer code. An example of such a transient medium is a TCP/IP signal carrying computer code over an IP network, such as the Internet. The carrier medium can also comprise a storage medium for storing processor readable code such as a floppy disk, hard disk, CD ROM, magnetic tape device or solid state memory device.

The invention will be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic representation of the arrival timing delays from user terminals of a code division multiple access (CDMA) transmission system; Figure 2 is a schematic representation of CDMA transmission system according to an embodiment of the invention; Figure 3 is a block diagram of a transmitter of the system of Figure 2; Figures 4a to 4d are block diagrams of receivers that can be used as the receiver of the system of Figure 2; Figure 5 is a flow chart showing a method for determining the order in which blocks received at the receiver are to be processed; Figure 6 is a schematic representation of the order in which user signals are considered at the receiver of Figure 2; Figures 7a and 7b are tables showing the order in which blocks are processed for a 5 block per user 3 user system and a 4 block per user 5 user system respectively; Figures 8a and 8b are lookup tables corresponding to Figures 7a and 7b respectively; and Figures 9 and 10 are graphs showing numerical simulations of the performance of the method of the invention; This invention provides a successive multi-user interference (MTJI) cancellation (SMC) architecture for block spread code division multiple access (BS-CDMA) transmissions (including low-complexity single-carrier frequency division multiple access (SC-FDMA) and orthogonal frequency-division multiple access (OFDMA)) where DFT spreading codes are used.

Figure 1 illustrates the general system model where interfering signals from other user terminals arrive before and after a signal from a specified user terminal.

Compared to the signals from other user terminals, two distinct scenarios arise, namely 1) signals from the other user terminals arrive prior to the arrival of signals from the specified user terminal; and 2) signals from the other user terminals signals after the arrival of the signal from the specified user terminal. For example, when considering the symbols transmitted by user terminal a it can be seen that corresponding symbols from user terminal b arrive before the symbols transmitted by user terminal a and corresponding symbols transmitted by user terminal c arrive after the symbols transmitted by user terminal a. The delay between the ith user and the specified user is denoted as Ti, where i can be positive or negative, corresponding to the case where the ith user terminal's signal arrives after or before the specified user terminal's signal, respectively. In this embodiment, for mathematical simplicity we consider the case where the delay is a multiple of the symbol duration, i.e., -r. � 1, �2,... symbols and it will be evident to the person skilled in the art that this invention also applies to cases where the delay is not an integer multiple of the symbol duration.

Figure 2 is a schematic representation of a CDMA transmission system 100 according to an embodiment of the invention. The system 100 comprises a plurality of transmitters 200 and a receiver 300 such as a base station. The transmitters 200 communicate with the receiver 300 wirelessly through a transmission channel 400 with blocks from different transmitters 200 arriving at the receiver 300 asynchronously.

Each transmitter 200 has a structure as illustrated in Figure 3. Data coming from a source 210 is encoded in an encoder 220 before it is interleaved in an interleaver 230 and subsequently mapped in a mapping module 240. The mapped symbols are then precoded in a precoding module 250 by applying precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount. The precoded blocks are then spread in time or frequency using discrete Fourier transform spreading codes in a spreading module 260. Alternatively, the precoding module 250 and the spreading module 260 can be interchanged, i.e., blocks of symbols are spread in time or frequency using discrete Fourier transform spreading codes and precoding is applied to each sub-block of data in the spread blocks by progressively rotating the phase of each sub-block identically by a user-dependent amount (as described in page 8 and the first two paragraphs of page 9 and as shown in Figure 1 of GB-A-2433397). Before transmission, a guard interval is applied in the form of a cyclic prefix (CP) in a CP module 270 to separate different spread blocks.

The signal is then amplified and transmitted by means of an antenna 280. In an alternative embodiment, one or both of the encoding 220 and interleaving modules 230 may be omitted.

In greater detail, the transmitted message is constructed at the transmitters 200 in the form of a stream of bits that may be encoded and/or interleaved. These bits are mapped to complex baseband symbols that are members of a given constellation, such as rn-phase shift keying (PSK) or rn-quadrature amplitude modulation (QAM), where m is the number of elements in the constellation. The resulting complex constellation symbols are arranged into blocks of K symbols, the i th block being denoted by s:= (s(iK),...,sp((i+1)K_1))T. In the proposed framework, each symbol block is precoded 250 with a F x K user-specific (possibly time-dependent) precoding matrix where F �= K, and subsequently spread 260 on a block-by-block basis by a length-M spreading code c, : (c(iM),. . .,c((i +1)M -1) to give = c1 � A1s1 where Ca" with C' denoting the set of complex n-tuples and 0 is the Kronecker product operator. The skilled person will recognise that this block spreading' and precoding can be interchanged. Also, a skilled person will recognise that this block spreading' and precoding can be achieved by simple symbol spreading as in a conventional direct sequence (DS) CDMA system followed by an appropriate chip interleaver and precoding module. Note that if L is the memory order of the channel impulse response, it is important that K �= L + 1 holds true in order to perform low-complexity de-spreading and detection at the receiver. Following the spreading operation, a cyclic prefix is appended to the user's message in the CP module 270. It is assumed that the CP consists of Q �= L symbols so that inter-block interference is mitigated. It is further assumed that the channel remains static for the duration of each individual block.

The blocks transmitted by the transmitters 200 are received at the receiver 300.

The receiver 300 of Figure 2 has the structure of one of the receivers 300a, 300b, 300c, 300d respectively shown in Figures 4a, 4b, 4c and 4d.

In a case where the blocks of all user terminals are received synchronously, the blocks would be superimposed one on the other and the receiver would just change the spreading code it uses for the despreading operation to obtain the data from the desired user. In practice this would be implemented with a single received path and a bank of spreading codes, where one spreading code is chosen at a time. Alternatively, decoding of each of the synchronous user terminals' blocks could be performed simultaneously with the received signal, which comprises the blocks of all user terminals without delays between them, being replicated the desired number of times and each copy decoded with the desired spreading code.

In the present embodiment where reception is asynchronous, multiple paths are present which arrive asynchronously at the receiver 300. To decode a block from a later user terminal it is necessary to recover the data from the earlier user terminals since they contribute the most to the interference. This recovered data is then processed (using interference reconstruction) and subtracted from the received signal so that the receiver views its version of the signal received from the later user terminal as one in which interference from previous user terminals is minimal. For example, to decode data from user terminal a in Figure 1, it is required that data from the user terminal b (which arrives before data from user terminal a) is recovered. This recovered data is then subtracted from a replica of the received signal before the despreading and decoding operation of user terminal a, and a similar procedure is followed for subsequent user terminals.

It is assumed that the receiver 300 has knowledge of the delays between the different signals originating from the different user terminals. The receiver firstly synchronises to the start of the received signal, which corresponds to the start of the earliest (first) user terminal's first block. There are two possible approaches to the order in which received blocks from different user terminals are processed.

According to an embodiment of this invention, a first approach is to decode all the blocks of the first user terminal and reconstruct the interference when processing signals from the later user terminals. The receiver then moves its synchronisation point to the start of the second user terminal's signal, decodes all its blocks, reconstructs the interference on the third and later user terminals and so on.

According to another embodiment of this invention, a second method is to decode the blocks transmitted by later user terminals as soon as possible and this is the method used in the present invention as described below with reference to Figure 4, which shows the structure of the receiver 300 which processes the asynchronously received blocks of the Ma user terminals.

The wireless signals transmitted by users a to M0 of Figure 1 are received at an antenna 310. The first block to be processed by the receiver 300 is the first block received by the receiver 300 and the receiver 300 must synchronise to the start of that block before processing that block. As shown in Figure 1, the signals from the various user terminals experience different delays before arriving at the receiver 300 such that the blocks arrive asynchronously. To process the signals the users are renumbered according to the order in which they arrive at the receiver 300 -user terminal a as transmitted becomes user tenninal 2 as received and user b as transmitted becomes user terminal 1 as received, and this new ordering of the user terminals is shown in Figure 6.

Following this renumbering, each block of renumbered user terminal 1 begins to arrive before the corresponding block of renumbered user terminal 2 and each block of renumbered user terminal 2 begins to arrive before the corresponding block of user terminal 3 etc. Referring to Figure 4a, at the receiver 300a, a wireless signal transmitted by user terminal 1 from transmitter TXb is received at an antenna 310 and the output of the antenna 310 is passed to a CP module 320 which removes the guard interval. After this point the asynchronously received signals are shown in Figure 4 as being processed by different paths.

In relation to the signal received from renumbered user terminal I which has had its guard interval removed, the signal is de-spread in a de-spreading module 330. Each of the de-spreading modules 330 in the receiver 300 uses a different de-spreading code, chosen from a bank of de-spreading codes, which corresponds to the spreading code used for that signal in the respective transmitter 200. The de-spread signal is decoded using a linear decoder 340 that is suitable for use with the respective linear precoders 250. The decoded signal is processed with an equalising module 350 and the subsequent signal is de-mapped in a soft bit mapping module 360. Note that the equalizer 350 used in the present invention can be any conventional equaliser used for synchronous reception, such as a linear minimum mean-square error (MMSE) frequency domain equaliser (FDE) or decision feedback equaliser (DFE), as opposed to the optimal MTMSE equaliser used for asynchronous reception as described in the applicant's patent application no. GB 0817521.8.

The subsequent signal, which corresponds to the interleaved and encoded signal produced by user terminal b is deinterleaved in a deinterleaving module 380 and then decoded by a channel decoder 390.

In an alternative embodiment where there is one of or no encoder 220 and interleaver 230 in the transmitter 200, one or both of the de-interleaving 380 and channel decoding 390 modules may be omitted.

Note that the signal output from the equalising module 350 is also passed to an interference reconstruction module 370. Once the interference to which the block was subjected is reconstructed in the interference reconstruction module 380 the estimated interference from user terminal 1 is passed to a second chain of dc-spreading to soft bit mapping modules 330-360, as shown in Figure 4, and this reconstructed interference is used to cancel the estimated interference contributed to by the processed block of the user terminal 1 on blocks received from the second user terminal 2. This process is repeated for all Ma user terminals of the system 100.

In an alternative embodiment as shown in Figure 4b, the output from the equalising module 350 is passed to an interference reconstruction module 370, and the reconstructed interference is removed from subsequent receive chains prior to the decoding module 340.

In an alternative embodiment as shown in Figure 4c, the output from the equalising module 350 is passed to an interference reconstruction module 370, and the reconstructed interference is removed from subsequent receive chains prior to the equalising module 350.

In an alternative embodiment as shown in Figure 4d, the output from the equalising module 350 is passed to an interference reconstruction module 370, and the reconstructed interference is removed from subsequent receive chains prior to the soft bit mapping module 360. Note that in this case, the equaliser modules 350 must be linear, such as zero forcing (ZF) or linear MMSE equalisers, for all receive chains except for the last chain, which can use any equaliser.

In case of perfect synchronisation between the different users in the system, IBI and MIJI can be easily mitigated by the careful use of spreading codes, equalisers and decoders. In practical cases, residual interference prevents the use of simple techniques at the receiver. More complex equalisers are required at the receiver to suppress ff1 and MUI. Examples of such equalisers can be found in the applicant's patent application no. GB 0817521.8. The present invention allows for quasi-synchronous reception for each user terminal in an asynchronous system such that low complexity equalisation designed for the synchronous reception can be used to achieve performance that is close to that of synchronous reception.

For a block-spread CDMA system with signals from N user terminals arriving later than the synchronized user terminal's signal and signals from Nb user terminals arriving prior to the synchronized user terminal's signal, the ith received block after the removal of the CP can be written as: r H1x1. +1x0, +Ax01_1) � + 2b,1 + A7 Xb 1�1) + , (1) where: subscript a' refers to user terminals whose signal arrive after the signal from the synchronised user terminal while b' corresponds to those signals from user terminal arriving before.

Xai denotes the ith transmitted block of the ath user Hm denotes the MP x MP circulant channel matrix of the mth user, where P is the block length and Mis the length of the spreading codes.

is the sparse matrix, size iviP xMP, with the only non-zero entries residing in a sub-matrix in columns MP--(L +T0)+2 to MP-L and row ito row L - + r, -1. This sub-matrix is an upper triangular Toeplitz matrix with its first row being [_h0(L -i),. ,ha(Lp -r0 +1)] , where t0 is the delay of the ath user compared to the given user, h0 (1) denotes the ith channel tap of the ath user, and L is the length of the channel impulse response, and is the length of cyclic prefix.

A4 is an upper triangular Toeplitz matrix with first row being ha (L -1) h (L -Ta + 1)1.

is a lower triangular Toeplitz matrix with first colunm [01xMpT hb (0) ... -hb (Tb -1)]T.

A7 is an MP >< MP sparse matrix, with non-zero entries in a Tb x Tb sub-matrix situated from column MP - +1 arid row MP -Tb +1. The sub-matrix forms a hb(O) 0 *.* 0 lower triangular Toeplitz matrix given by.11 hb(Tb-1) hb(0) and v. is the equivalent noise term.

Assuming the receiver has knowledge of the channel state information and the relative delays of each user, it is possible to decode the received signal for each subscriber in ascending order of delay. In equation (1), the interference that comes from a user terminal that arrives later than the reference user terminal is: + + A4 Xa,i_i) This term can be ignored without incurring major performance penalties, as decribed in the applicant's patent application no. GB 0817521.8, because it can be shown that the interference from a later user to an earlier user is usually very small.

Especially, if the length of the cyclic prefix is greater than the memory of the channel added to the delay, v0, interference is actually zero. The procedure for recovering the data for the earliest of the users is identical to the receiver structure in GB-A-2433397.

For the signal from user terminal 2 that arrive second, the effects of all later user terminals can be ignored. The earlier user terminal contributes to the term (fIbxbI + L2 Xbj + A7 xbI�1), which accounts for interference to signals from this later user terminal.

The principle idea behind this invention is to reconstruct the interference from all earlier user terminals and then subtract it from the signal received from the currently synchronised user terminal. For example, to detect the data of the user terminal 2 that arrives second, the value of (2 Xbi + A7 x1,,+1) is calculated, since the data from the first user terminal 1 has already been recovered. (Note that the term including the circulant channel matrix is not considered since it does not contribute to any sort of interference, as described in GB-A-2433397, and the term HaXaj can be removed by the despreading operation later, due to the orthogonality of the spreading codes).

Recovering the data from the second user terminal 2 allows for the reconstruction of interference from the first and the second user terminals onto all later users. This sequential subtraction process is carried out for all user terminals.

As stated above, there are two possible implementation techniques for this interference cancellation system. According to a first embodiment of the invention, all the blocks from the earliest user terminal 1 are decoded and recovered and then signals arriving from the second terminal 2 are decoded and recovered, and so on. The downside of this method however is that since it is necessary to decode the entire signals from all earlier user terminals there might be a considerable delay in decoding signals from the later user terminals. In a second embodiment of the invention, the interference reconstruction and decoding is carried out on a block-by-block basis, or an array decoding process. Referring to the terms in (A2 XbI + A76 x1), it is observed that interference on the ith block is caused by blocks i and i +1 from earlier user terminals.

An illustration of the order of decoding the blocks is given in Figures 7 and 8. In each case, the first two blocks transmitted by the first user terminal whose signal arrives as the receiver 300 are first decoded, followed by block 1 arriving from user terminal 2 (which corresponds to user terminal a of Figure 1). To obtain interference on the second block of the second user terminal 2, block 3 of the first user terminal 1 must be known. Obtaining the second block of user terminal 2 allows for recovering the first block of user terminal 3 (which corresponds to user terminal c of Figure 1). Each time a block for a later user terminal is decoded, it is necessary for an extra block for the previous user terminal to be decoded. The process continues until an extra block of the first user terminal 1 is decoded. For a three user terminal system where each user terminal transmits 5 blocks, the block processing order is shown in Figures 7a and 8a and for a five user terminal system where each user terminal transmits 4 blocks, the block processing order is shown in Figures 7b and 8b.

Though the second technique involves more control on the decoding order, it allows fairness in decoding the data among all the user terminals. There are two possible implementation techniques for the array decoding. The first approach is to use a look-up table, for example as shown in Figures 7 and 8, to determine the order of decoding. Alternately, blocks of data from all user terminals can be decoded in an order determined by the algorithm as illustrated in Figure 5.

It is observed that for decoding the ith block of user terminal in, it is necessary that block i+f-m of user terminaif has been decoded, wheref< in. After decoding the itb block received from one user terminal, the receiver checks if the condition mentioned is met for the following user terminal. If not, the block of earlier user terminals needs to be decoded.

Assume that there are Ma active user terminals, each of which has transmitted a total ofT blocks. Let t denote the time index. At time t, the nth block of the mth user terminal is decoded, denoted as n(t) and m(t), respectively. For example, n(3) 3 and m(3) 1 means the third block of the first user terminal is decoded at t=3. The order of decoding is described below with reference to Figure 5.

In step S405 the algorithm is initialized such that: a I, n(0) =1 and m(0) 1, t=0.

In step S410, t is incremented and a "for" loop is begun, which runs from t =1 to Ma * T1 In step S4 15, if the block at the previous time t-1, is equal to the number of blocks transmitted Tby each user terminal, i.e. if n(t-1) =T, then the algorithm proceeds to step S420, where a is incremented, i.e., a + =1; else the algorithm proceeds to step S425.

Instep S425, if the block at the previous time t-1, is equal to unity, i.e. if n(t-1) =1 then the algorithm proceeds to step S430; else the algorithm proceeds to step S435.

In step S43 5, if the user terminal at the previous time t-1, is equal to the total number of user terminal, i.e. if m(t-l) Ma, then the algorithm proceeds to step S430; else the algorithm proceeds to step S440.

In step S430 the value of the block number n(t)at that time t is set as: n(t) n(t-1)+m(t-1)-a+ 1; and the value of the user number m(t) at that time is set as m(t) a; Alternatively, in step S440 the value of the user number m(t)at that time t is set as: m(t)(m(t-1)%Ma)+1; and the value of the block number n(t) at that time is set as n(t) = m(t-l) + n(t-l) -m(t); At step S445 the algorithm checks if all n blocks of all Ma user terminals have been decoded. If not all n blocks of all Ma user terminals have been decoded then the algorithm repeats steps S410 to S445. If all ii blocks of all Ma user terminals have been decoded then the algorithm ends at step S450.

The pseudocode for the algorithm shown in Figure 5 is given by: Initialization: a 1, n(O) =1 and m(O) =1, t=O for t1tOMa*T4 if n(t-1)T a + =1; end if if n(t-1)l orm(t-1)-Ma; n(t) = n(t-l)+,n(t-1)-a+l; m(t) = a; else m(t) = (m(t-l) % IkIa) +1 n(t) = m(t-1) + n(t-1) -m(t); end if end for Following the algorithm shown in Figure 5 arid as described above for a three user terminal system where each user terminal transmits five blocks results in the block processing order shown in Figure 8a and following the algorithm for a five user terminal system where each user terminal transmits four blocks results in the block processing order shown in Figure 8b.

Figure 9 and 10 give the plot when the proposed successive interference cancellation method is used, with 16 active users and QPSK modulation. In the figures, user terminal 1 is assumed to be the synchronous user with delay of it being 0. The delays of the rest of the users terminals are 1,1,1,2,2,2,3,3,3,4,4,4,5,5,5 relative to the first user terminal. Moreover, in all the simulations, we consider an exponentially decaying channel with 6 channel taps. The length of cyclic prefix is 8, and the size of each data block is 16. The performance shown in Figure 9 is that with perfect channel state information at the receiver 300, while that in Figure 10 shows a more realistic case where the channels are estimated by the least-squared (LS) estimator by transmitting two Chu sequences In the examples shown in Figures 1 and 6 to 10 and as described in the algorithm shown in Figure 5 we discuss a case where there are Ma user terminals where each user terminal transmits the same number, n, of blocks and the first block transmitted by each user terminal arrives at the receiver 300 asynchronously but in a time frame less than the length of each block, i.e., all the first blocks transmitted by each of the user terminals 200 begins to arrive at the receiver 300 during the time that the first block of the first received user arrive at the receiver 300.

In practice, there is not necessarily such synchronisation between the transmittal timing of the user terminals and when each user terminal sends a burst of n, blocks the number of blocks in each burst will vary and the overlap between the arrival time of the blocks will vary.

Hence a more general description of the order in which blocks arriving from multiple users can be given as follows.

When a plurality of blocks are received at a receiver from each of a plurality of user terminals Ma; a first block n, received from a first user terminal rn, is processed to determine the transmitted symbols. After that block has been processed, a second block (n+ 1), received from the same first user terminal rn immediately after receipt of the first block n, is processed next, to determine the transmitted symbols in that block. Once these two blocks have been processed, the interference produced by the first block n, and the second block (n+ l) received from the first user terminal m1 is reconstructed.

Next the symbols transmitted in a third block n+, are derived. The third block n1+i is a block which is transmitted by a second user terminal m*+j and is the next block received at the receiver 300 immediately after receipt of the first block n, received from the first user terminal m. The symbol derivation comprises using the reconstructed interference to cancel the interference caused by the first block n, and the second block (n+1) to the third block n+j and then processing the third block n,1 to determine the transmitted symbols.

Once the first and second blocks received from the first user terminal and the third block received from the second user terminal are processed, the above steps are repeated for all received blocks. According to the present invention, for each third block n,+j no blocks subsequent to the second block (n+1)1 from the previous user terminal rn1 are processed before the symbols are derived for the third block n1+j.

It is observed that the performances of all user terminals are very close to the case of synchronous reception, which corresponds to the plot of user terminal 1 with or without perfect channel state information at the receiver 300. Furthermore, error propagation is limited to a single block rather than the whole transmission data. The larger the size of a transmitted block is with respect to the order of the channel, the smaller is the degradation due to error propagation. Note that the performance degradation due to error propagation and the size of each transmitted block are inversely proportional, i.e., a long transmitted block facilitates system operation with very little error propagation.

The principle advantage of the receiver employing the successive interference cancellation method is that the system can achieve performance close to the synchronous reception. In the applicant's patent application no. GB 0817521.8, the optimal MMSE equaliser demonstrated error floor reduction in asynchronous reception at the price of high computational complexity where a matrix inversion operation may be involved. This present invention uses the low complexity conventional equalisers designed for synchronous reception while still effectively suppressing the MUI and IBI.

Furthermore, the terms and A7b are cyclic shift of each other. The process of reconstructing these matrices can therefore be simplified.

Various modifications will be apparent to those in the art and it is desired to include all such modifications as fall within the scope of the accompanying claims.

Claims (15)

1. CLAIMS: 1. A method of cancelling interference in a block spread code division multiple access transmission system comprising a plurality of user terminal transmitters which are configured to (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix, the method comprising: (a) receiving asynchronously at a receiver signals from a plurality of user terminals; (b) processing the signal from a first user terminal; (c) reconstructing the interference caused by the signal transmitted by the first user terminal; (d) using the reconstructed interference to process signals received from the other user terminals; and (e) repeating steps (b) to (d) for the signals received from the other user terminals, wherein the signals are processed in the order of arrival at the receiver.
2. 2. A method of cancelling interference in a block spread code division multiple access transmission system comprising a plurality of user terminal transmitters which are configured to (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix, the method comprising: (a) receiving asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals M; (b) processing a first block n from a first user terminal rn; (c) processing a second block (n+1)1 received from the first user terminal rn immediately after receipt of the first block ne,; (d) reconstructing the interference caused by the first block n1 and the second block (n+1)1 received from the first user terminal rn; (e) using the reconstructed interference to process a third block n1j, the third block n+j being transmitted by a second user terminal m1+1 and being the next block received at the receiver immediately after receipt of the first block n, received from the first user terminal m,; and (f) repeating steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block n,1 no blocks subsequent to the second block (n+1)1 from the previous user terminal rn1 are processed before the third block n+i is processed.
3. 3. A method as claimed in claim 2, wherein the order of processing of blocks is given by: Initialization: a 1, n(O) =1 and m(O) 1, t0 for tltOMa*T4 if n(t-1)T a + end if if n(t-1) 1 or rn(t-l) Ma; n(t) = n(t-1)+m(t-1)-a+1; m(t) = a; else m(t) (m(t-1) % M) +1; n(t) = m(t-1) + n(t-1) -end if end for where: Ma is the number of active user terminals; T is the number of blocks transmitted by each user terminal; t is a time index; a is an index; and n(t) and m(t) denote the nth block of the rnth user terminal respectively decoded at time t.
4. 4. A method as claimed in any one of the preceding claims, wherein the blocks are transmitted using a low-complexity single-carrier frequency division multiple access scheme.
5. 5. A method as claimed in any one of claims I to 3, wherein the blocks are transmitted using a low complexity orthogonal frequency division multiple access scheme.
6. 6. A method as claimed in any one of the preceding claims, wherein the step of processing a block comprises: despreading the block; decoding the despread block; and equalising the decoded block.
7. 7. A method as claimed in claim 6, wherein the step of processing a block further comprises: soft bit mapping the equalised block; de-interleaving the mapped block; and channel decoding the de-interleaved block.
8. 8, A method as claimed in claim 6 or 7, wherein the equalising step comprises equalising the block with a conventional equaliser used for synchronous reception.
9. 9. A block spread code division multiple access transmission system comprising: a plurality of user terminal transmitters which are configured to: (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in.the form of a cyclic prefix; and a receiver for receiving asynchronously signals from the plurality of user terminals, the receiver being configured to: (a) process the signal from a first user terminal; (b) reconstruct the interference caused by the signal transmitted by the first user terminal; (c) use the reconstructed interference to process signals received from the other user terminals; and (d) repeat steps (a) to (c) for the signals received from the other user terminals, wherein the receiver is configured to process received signals in the order of arrival at the receiver.
10. 10. A block spread code division multiple access transmission system comprising: a plurality of user terminal transmitters which are configured to: (i) apply precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spread the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) apply a guard interval in the form of a cyclic prefix; and a receiver for receiving asynchronously signals from the plurality of user terminals, the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals M; (b) process a first block n-from a first user terminal m,; (c) process a second block (n+1), received from the first user terminal rn immediately after receipt of the first block n,; (d) reconstruct the interference caused by the first block n, and the second block (n+l,)1 received from the first user terminal rn1; (e) use the reconstructed interference to process a third block n1�j, the third block n+ being transmitted by a second user terminal m1 and being the next block received at the receiver immediately after receipt of the first block n received from the first user terminal rn; and (f) repeat steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block fl+j no blocks subsequent to the second block (n+1) from the previous user terminal m, are processed before the third block fl+j is processed.
11. 11. A system as claimed in claim 10, wherein the order of processing of blocks is given by: Initialization: a I, n(0) =1 and m(0) =1, 1=0 for t=ltOMa*T1 if n(t-1)T a + =1; end if if n(t-1)1 orrn(t-l)Ma; n(t) n(t-1)+m(t-l)-a+1; m(t) -a; else m(t)(m(t-1) %Ma)+1; n(t) m(t-1) + n(t-l) -end if end for where: Ma is the number of active user terminals; T is the number of blocks transmitted by each user terminal; t is a time index; a is an index; and n(t) and m(t) denote the nth block of the mth user terminal respectively decoded at time t.
12. 12. A receiver for cancelling interference in block spread code division multiple access transmission system signals transmitted by a plurality of user terminal transmitters, the signals having been transmitted by: (i) applying precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spreading the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) applying a guard interval in the form of a cyclic prefix; the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals Ma; (b) process the signal from a first user terminal; (c) reconstruct the interference caused by the signal transmitted by the first user terminal; (d) use the reconstructed interference to process signals received from the other user terminals; and (e) repeat steps (b) to (d) for the signals received from the other user terminals, wherein the receiver is configured to process received signals in the order of arrival at the receiver.
13. 13. A receiver for cancelling interference in block spread code division multiple access transmission system signals transmitted by a plurality of user terminal transmitters, the signals having been transmitted by: (i) applying precoding to each block of symbols by progressively rotating the phase of each block of symbols identically by a user-dependent amount; (ii) spreading the precoded blocks in time or frequency using discrete Fourier transform spreading codes; and (iii) applying a guard interval in the form of a cyclic prefix; the receiver being configured to: (a) receive asynchronously at a receiver a plurality of blocks from each of a plurality of user terminals Ma; (b) process a first block n, from a first user terminal rn; (c) process a second block (n+])1 received from the first user terminal m immediately after receipt of the first block n1,; (d) reconstruct the interference caused by the first block n and the second block (n+1)1 received from the first user terminal rn; (e) use the reconstructed interference to process a third block n+j, the third block n+1 being transmitted by a second user terminal and being the next block received at the receiver immediately after receipt of the first block n, received from the first user terminal in,; and (f) repeat steps (c) to (e) for all received blocks, in each case using the reconstructed interference to process subsequently received blocks, wherein for each third block n+j no blocks subsequent to the second block (n+1)1 from the previous user terminal m1 are processed before the third block fl*+j is processed.
14. 14. A receiver as claimed in claim 13, wherein the order of processing of blocks is given by: Initialization: a = 1, n(0) 1 and m(0) 1, 1=0 for tltOMa*T1 if n(t-1) T a + end if if n(t-1) 1 or m(t-1) Ma, n(t) = n(t-1)+m(t-1)-a+1; m(t) = a; else m(t) (m(t-1) %Ma) +1; n(t) = m(t-1) + n(t.-1) -end if end for where: M is the number of active user terminals; T is the number of blocks transmitted by each user terminal; t is a time index; a is an index; and n(t) and tn(t) denote the nth block of the,nth user terminal respectively decoded at time t.
15. 15. A carrier medium carrying computer readable code for controlling a microprocessor to carry out the method of any one of claims 1 to 8.