GB2423675A

GB2423675A - Diversity transmitter

Info

Publication number: GB2423675A
Application number: GB0504061A
Authority: GB
Inventors: Abdol Hamid Aghvami; Mischa Dohler; Fatin Said; Afzal Mahmood Lodhi; Francesco Saverio Ostuni
Original assignee: Kings College London
Current assignee: Kings College London
Priority date: 2005-02-28
Filing date: 2005-02-28
Publication date: 2006-08-30
Anticipated expiration: 2025-02-28
Also published as: GB0504061D0; GB2423675B; US20060193245A1; US7609613B2

Abstract

A diversity transmitter for use in an OFDM transmission protocol which diversity transmitter comprises: ```a diversity generator (2) for example a space-time diversity block coder for receiving and diversifying OFDM transmit symbols, and outputting diversified OFDM symbol matrices (DOSM), DOSM symbols within each DOSM being divided into at least two primary streams each comprising different DOSM symbols, ```a transmit processor for receiving said at least two primary streams of DOSM symbols, and for transforming said each DOSM symbol from the frequency domain into the time domain for example by IFFT, and outputting time domain OFDM symbols (TDOSs), ```a cyclic delay circuit (41 ... 4P) for dividing at least one of said primary streams of TDOSs into at least two branches of identical TDOSs, each branch for supplying a respective spatial channel for transmission to a receiver, ```the arrangement being such that, in use, said cyclic delay circuit (41 ... 4P) applies a cyclic time shift to a TDOS symbol in at least one of said branches before transmission.

Description

DIVERSITY TRANSMITTER AND METHOD

FIELD OF THE INVENTION

The present invention relates to a diversity transmitter, to a base station, a mobile radio communication device and a broadcast transmitter each comprising such a diversity transmitter; and to a method of transmitting data in an OFDM system.

BACKGROUND TO THE INVENTION

Orthogonal frequency division multiplexing (OFDM) based systems are the most promising candidates for 4th generation broadband mobile communication networks. Such systems deployed with multiple-input multiple-output (MIMO) techniques promise satisfaction of the ever growing demands of multi-media services and applications. OFDM has been successfully used in standards for digital audio broadcasting (DAB), terrestrial video broadcasting (DVB-T), and wireless local area networks (WLANs), for example. It reduces receiver complexity in the equalization and symbol decoding stages by transmitting each symbol over a single flat sub- channel. However, OFDM's inability to extract multipath diversity (inherently present in the broadband wireless channel) and to guarantee symbol detection, when channel nulls occur on parallel sub-channels, are two adverse effects associated with its simplicity.

Diversity techniques are proven to be very effective for combating time varying multipath fading in broadband wireless channel environments. In these techniques, some less attenuated replicas of the transmitted signal, either in the time, frequency or spatial domain, or a combination of all three, are provided to the receiver. These replicas minimize the degrading channel imperfections and enhance the system performance. Temporal diversity is widely achieved by using forward error correction (FEC) coding in combination with (random) time interleaving, whereas frequency diversity can be exploited by using non-linear equalizers or Rake receivers in a single carrier system or with a FEC in an OFDM based system. Spatial diversity in the form of spatially separated, cell sectoring, or polarized antennas have also been of major interest in the research community. All of the above mentioned diversity methods are severely dependent on channel scenarios, transmission data rate, Doppler spread and channel delay spread. Therefore, it is very difficult to realize all forms of diversity in one particular system; for example, in case of slow fading channels with large delay spreads, random time interleaving with FEC or channel coding becomes ineffective. Similarly, frequency interleaving becomes useless for channel environments showing a typical frequency-flat profile. In contrast, spatial diversity is the best approach towards mitigating the channel impairments and enhancing performance, as long as signals at the transmit and receive antenna elements are sufficiently de-correlated.

In connection with diversity transmitters, different concepts are being discussed for multi-carrier, in particular OFDM systems. OFDM is ideally suited for broadband frequency selective channels as it gives the opportunity to use the existing transmit diversity techniques (designed for flat fading channels) in such environments. The design and performance criteria for broadband MIMO OFDM systems promises an excellent diversity level, which is multiplicative of transmit and receive antennae, and the number of multipath components of the broadband channel (with an ideal assumption like equal power in all multipath components and fixed delay between them) [1]. However, the orthogonal space-time block code (STBC) processing scheme proposed in [2] and generalized in [3] failed to extract any or almost no multipath diversity in OFDM. Space- time trellis codes (STTC) of [4], promise diversity as well as coding gain but become unattractive due to their complexity in practical realizations. Other transmit diversity techniques like non- orthogonal block codes also faced the same dilemma of lack of frequency or multipath diversity. This gave a research challenge to design new set of codes for OFDM based systems that would extract at least some of the promised advantages of MIMO OFDM.

Nevertheless, some transmitter diversity schemes, in particular Delay Diversity (DD), when modified to be used in OFDM systems gives excellent simplicity and performance. This technique can be found in many forms, where it differs slightly in terms of its placement in the system. Cyclic Delay Diversity (CDD) (a time domain equivalent of Phase Diversity (PD)) is an improved version of DD. In particular, CDD addresses the adverse effects of DD by introducing cyclic time delays instead of simply time delays [5]. For OFDM based systems, CDD is the simplest approach for extracting frequency diversity that itself has no built-in diversity. It converts the spatial diversity into frequency diversity by artificially increasing the channel delay spread. However, it requires an outer channel or a FEC facility to benefit from the induced selectivity.

Over the years, the search for optimal transmit diversity schemes for MIMO OFDM systems led to many transmitter diversity processing structures and configurations. All these proposed schemes tackled the problem of achieving the maximum (spatial plus multipath) diversity and coding gain in frequency selective environments. By trading complexity, additional processing and incorporating pre- coding arrangements, it was shown that theoretical diversity limits could be achieved.

The most noticeable diversity transmitters in this regard can be found in [6], [7], [8] and [9].

In [6], a MIMO-OFDM scheme with variable multiplexing gains was presented. This scheme traded data rate for full diversity (spatial and multipath) by employing an arbitrary space-time code (STC), and to achieve maximum spatial diversity OFDM sub-carriers were encoded. On top of this, an outer codec was used for achieving multipath diversity. The amount of frequency diversity is related to the redundancy introduced by this outer codec, making this scheme severely dependent on the outer codec and the number of resolvable multi-paths. Only a fraction of the available frequency diversity could be exploited when considering an affordable rate loss and practical scenarios.

In [7] and [8], linear constellation pre-coding (LCP) based OFDM diversity transmitters were presented. The design of LCP with STC techniques was discussed in [7]. This approach used existing STCs of [3] and [4], and relied on combining these codes with redundant or non- redundant pre-coders. This scheme achieved maximum diversity and coding gain at the expense of spectral efficiency. Another LCP based diversity transmitter was presented in [8]. This scheme did not rely on STC techniques and used digital phase sweeping (DPS) or circular block delay diversity (CBDD), which are the same as CDD or PD. To achieve the full diversity, this scheme was again dependent on the LCP. The design of this LCP has severe implications on realistic channel conditions and restricted the number of diversity braches.

The scheme in [9] used a mapping approach to design full diversity codes from the existing STC techniques for arbitrary power delay profiles, again suffering from severe rate loss for attaining maximum diversity.

Drawbacks of aforementioned techniques include loss of data rate and/or additional transmitter and receiver complexities. Incorporating LCP or some other codecs to extract multipath diversity may not be the best solution. In all wireless and mobile communication systems channel or FEC coding techniques have become an integral part. These techniques can provide a much simpler and cost effective solution in extracting the frequency diversity in broadband scenarios. We have realised that a hybrid of STC schemes and CDD may offer an improved diversity transmitter and method measured in terms of performance, cost and complexity for multi-carrier systems.

WO 03/015334 (Hottinen) discloses a diversity transmitter for use in CDMA systems. The transmitter applies fixed complex weights in the phase domain to symbols to be transmitted. Hottinen's scheme is not suitable for use in systems that employ OFDM for broadband, as it requires additional processing at the transmitter.

Hottinen's scheme would require significant modification to be useful in OFDM systems.

SUMMARY OF THE INVENTION

Preferred embodiments of the present invention are based on the insight that in a broadband multi-carrier system it is possible to improve performance by utilising diversity coding techniques to extract spatial diversity, and at the same time to use cyclic time delays to improve frequency diversity in the broadband channel.

It is an aim of at least preferred embodiments of the present invention to provide an improved and less complex diversity transmitter and a transmission method for OFDM based systems, in which the drawbacks of complexity and spectral efficiency are mitigated.

According to the present invention there is provided a diversity transmitter for use in an OFDM transmission protocol which diversity transmitter comprises: a diversity generator for receiving and diversifying OFDM transmit symbols, and outputting diversified OFDM symbol matrices (DOSM), DOSM symbols within each DOSM being divided into at least two primary streams each comprising different DOSM symbols; a transmit processor for receiving said at least two primary streams of DOSM symbols, and for transforming said each DOSM symbol from the frequency domain into the time domain, and outputting time domain OFDM symbols (TDOSs); and a cyclic delay circuit for dividing at least one of said primary streams of TDOSs into at least two branches of identical TDOSs, each branch for supplying a respective spatial channel for transmission to a receiver; the arrangement being such that, in use, said cyclic delay circuit applies a cyclic time shift to a TDOS in at least one of said branches before transmission. The diversity transmitter may be implemented entirely in software or hardware, or a combination of both. One particular advantage of the present invention is that channel selectivity is improved; more frequency selective channels help the receiver to extract multipath diversity gain in conjunction with channel coding (although with Trellis codes for example channel coding is not mandatory to achieve this advantage). Furthermore the use of additional antenna branches to transmit cyclically delayed replicas of symbols output from the diversity generator does not incur a rate loss. Still further the additional branches of the transmitter do not require any changes at the receiver other than channel estimation to cater for longer impulse responses that are artificially created by the cyclic time shift. Thus the diversity transmitter is very simple to implement. The diversity transmitter can form part of a MISO (multiple input single output) and/or MIMO link.

It will be appreciated that the number of branches in a cyclic delay circuit can be varied (and be different between cyclic delay circuits) according to the channel characteristics where the transmitter is to be used. For example, increasing the number of branches (i.e. the number of cyclic time shifted symbol replicas) can help to make the wideband channel more frequency selective at the carrier frequencies of the OFDM system. For wideband channels that are already highly frequency selective, the use of one or two extra branches enables similar performance with fewer total spatial channels than diversity transmitters that use only space-time

coding for example.

Thus the diversity transmitter is very simple whilst achieving good spectral efficiency. Further features are set out in claims 2 to 17 to which attention is hereby directed.

According to another aspect of the present invention there is provided a method of transmitting data in an OFDM system, which method comprises the steps of: (1) using a diversity generator to receive and diversify OFDM transmit symbols, and output diversified OFDM symbol matrices (DOSM); (2) dividing DOSM symbols within each DOSM into at least two primary streams each comprising different DOSM symbols; (3) transforming each DOSM symbol from the frequency domain into the time domain, and outputting time domain OFDM symbols (TDOSs); (4) dividing at least one of said primary streams of TDOSs into at least is two branches of identical TDOSs, each branch for supplying a respective spatial channel for transmission to a receiver; and (5) applying a cyclic time shift to a TDOS symbol in at least one of said branches before transmission.

Further steps of the method are set out in claims 22 to 36 to which attention is hereby directed.

A diversity transmitter and method according to the invention may bring about some of the following advantages: improvement of performance without any additional complexity at the receiver; increased frequency selectivity for low selective and low delay spread channels; conversion of spatial diversity into frequency diversity assisted by channel coding; spectral efficiency is dependent only on the diversity generator; simple implementation of cyclic time shifts (performance of the cyclic time shift in the time domain eases computational overhead for example); cyclic time shift can be set according to the system guard period; any diversity generator can be used, although full rate STBC are preferred for greater spectral efficiency.

There is also provided a diversity transmitter and a transmission method in which there is first channel coding, e.g. by a Turbo/Convolutional codes, the output is modulated, e.g. phase shift keying, the modulated output is subjected to diversification for example STBC, serial to parallel converted, inverse OFDM, each or at least one of the diversified output is subjected to a plurality of at least two branches, in each or at least one branch the symbol sequence is subjected to a cyclic time shift with in each symbol and then transmitted from the parallel spatial channels after insertion of guard period in each branch.

Thus by virtue of the present invention the drawbacks in terms of rate and complexity inherent to known prior art arrangements may be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of how the invention may be put into practice, preferred embodiments of the invention will be described, by way of example only, to the accompanying drawings, in which: Fig. I is a block diagram of a first embodiment of a diversity transmitter in accordance with the present invention; Fig. 2 is a block diagram of a second embodiment of a diversity transmitter in accordance with the present invention; Fig. 3 is a block diagram of a third embodiment of diversity transmitter in accordance with the present invention; Fig. 4 is a block diagram of a fourth embodiment of diversity transmitter in accordance with the present invention; Fig. 5 is a graph of BER versus SNR per bit for a computer simulation of another diversity transmitter in accordance with the present invention; Fig. 6 is a graph of BER versus SNR per bit for a computer simulation of another diversity transmitter in accordance with the present invention; Fig. 7 is a graph of BER versus SNR per bit for a computer simulation of another diversity transmitter in accordance with the present invention; Fig. 8 is a graph of BER versus SNR per bit for a computer simulation of various diversity transmitters in accordance with the present invention at two different channel orders; Fig. 9 is a graph of BER versus SNR per bit for a computer simulation of a diversity transmitter in accordance with the present invention and transmitter using STBC; and Fig. 10 is a graph of BER versus SNR per bit for a computer simulation of a diversity transmitter in accordance with the present invention and a transmitter using a space-time-multipath coding scheme.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the diversity transmitter and method according to the present invention there are multiple parallel transmissions out of at least three spatial channels (which may be antennas or beams) and preferably four or more. Out of the at least three parallel channels at least one of the said spatial channel is shifted by a cyclic shift, the cyclic shift being a fraction/multiple of the system guard period. There may be at least three (logical) parallel channels at a given receiver, although the invention does not place any structural requirements on the receiver.

Fig. 1. shows a generic block diagram of the diversity transmitter.

Transmissions out of antennas (representing an example of spatial channels) XjI,...,XIM,...,Xpl,...,XPM are experiencing the influence of respective transmission channels hII,...,hlM,...,hpI,...,hpM before reception at a receiver. In general, an OFDM symbol sequence/vector/matrix s to be transmitted is processed at the transmitter, transmitted via the transmission channels and received at the receiver, where it is subjected to reception processing in order to reconstruct the initially transmitted signal. Reception processing involves channel estimation in order to compensate for the influence of the transmission channels. (Note that the symbol s as well as the transmission channels, i.e. channel impulse response a thereof, are in matrix notation).

Referring to Fig. I, a diversity transmitter generally identified by reference numeral 10 comprises a transmit symbol input 1 for inputting OFDM symbols (symbol matrix or a sequence of symbols) into a diversity generator 2. The transmit symbol input I is the output from a channel encoder. Each OFDM symbol comprises a number of binary values that will be used to modulate the sub-carriers in the OFDM system. The diversity generator 2 facilitates diversity using orthogonal transmit diversity (OTD) or any other diversity mechanism by applying a generator (e.g. a generator matrix) to each OFDM symbol sequence/matrix and outputs a stream of diversified OFDM symbols s each of size N where N is the number of subcarrier frequencies in the OFDM system. The stream of diversified OFDM symbols is supplied to a processor 3 where the stream is divided into P streams. As shown in the Fig. 1, first diversified OFDM symbol s1 is supplied to a transmit processor 31, while the pth diversified signal s, is supplied to a transmit processor 3P. The processor 3a converts each OFDM symbol from the frequency domain into the time domain (inverse OFDM processing') e.g. using an inverse FFT, and outputs P time domain OFDM symbols. Each of the P streams is input to a respective cyclic delay circuit 41...4P. Each cyclic delay circuit 41...4P comprises M branches, each branch having a copy of the respective time domain OFDM symbol output by the processor 3a. Furthermore each branch m comprises a respective transmit antenna Xll, ***,XIM,...,Xpj,...,XPM. In use each cyclic delay circuit 41...4P applies a cyclic time shift to some of the time domain OFDM symbols using a cyclic delay matrix Cdm* The cyclic delay matrix Cdm effects a shift of SCYm samples of the time domain OFDM symbol modulo the total number of samples in a time domain OFDM symbol. The effect of this cyclic time shift is to increase the overall delay spread of the channel, at the receiver, as explained in greater detail below.

Each cyclic delay circuit 41...4P applies a cyclic time shift to one or more copies of the time domain OFDM symbol s on its M branches. For example the cyclic delay circuit 41 of Fig. I receives symbol x on both branches. The upper branch applies Scy = 0 (i.e. no time delay), whereas the lower branches applies Scy. This generates two output OFDM symbols: x11 to be transmitted from antenna X11 and x12 to be transmitted from antenna X12. The same procedure takes place at the other cyclic delay circuits 42.. .4P on the other time domain OFDM symbols. After this processing and addition of the usual guard period (or cyclic prefix) all OFDM symbols are transmitted substantially simultaneously from the antennas It will appear to the receiver that the signal comes from only P antennae, each with longer channel impulse responses, despite the fact that PM antennae have been used for transmission. To extract this multipath diversity reliance is placed on channel coding techniques (e.g. convolutional codes), already part of many system standards. If a STTC encoder is used as the diversity generator 2, reliance on channel coding to extract the multipath gain is not necessary.

- 10 - The cyclic delay matrix Cdm takes the form: Cd1 =[ I(m_lxL+1)]mE[IMI N-(m- l)(L+1) where A is the identity matrix of order AxA, N is the number of sub-carrier frequencies in the OFDM system, M is the total number of branches in the cyclic delay circuit 41. . .4P where Cdm is to be applied, and L is number of non-zero taps assumed in the channel transfer function. The transmitted OFDM symbols from different antennas are cyclically delayed replicas of each time domain OFDM symbol. Furthermore the construction of the cyclic delay matrix Cdm ensures that the delay applied to each OFDM symbol on the first antenna of each stream p is zero i.e. the original diversity encoded OFDM symbol is transmitted. Cyclically delayed versions are transmitted from the other antennas at the same time.

In Fig. I each cyclic delay circuit 41. . .4P is identical to the others and cyclic delay matrix Cdm effects only the lower branches (M = 2) in each circuit. Each cyclic delay circuit 41. . .4P can have differing numbers of branches. Alternatively, cyclic delay matrix Cdm may be used in the upper branch instead of the lower branch or in any number of branches. The symbols sequence/matrix s needs to be defined differently depending upon the diversity generator used. This will be explained in greater detail with reference to Fig. 2.

Referring to Fig. 2. a diversity transmitter generally identified by reference numeral 20 comprises four spatial channels X1,X2,X21,X22 i.e. bothP,M2 (transmit antennae) with the transmit symbol input 1 and the diversity generator 2 generally similar to those described in connection with Fig. 1. The diversity generator 2 is a full rate space-time block encoder such as that described in [2]. A first diversified OFDM symbol s1 is supplied to a transmit processor 31, while a second diversified OFDM symbol s2 is supplied to a transmit processor 32. As the symbols s and s2 belong to different branches of the space-time block codes, they become 11 - s = (c1 c2) and 2 = (-c c;) (or any combination of two OFDM symbols with orthogonal structure), where c1 and c2 are complex OFDM symbols and c* represent the complex conjugate of symbol c. The complex OFDM symbols are separable (in fact orthogonal) due to the properties of space-time block codes. Note that in this case the symbol rate remains at I since it takes two time intervals to transmit two symbols due to the properties of STBC. However, the diversity order is doubled.

When considering diversification with some other technique for example, STTC one need to define the symbols s differently depending upon the memory order of the trellis encoder.

Next the transmit processor 3a converts the OFDM symbols streams s1 and 2 from the frequency domain to the time domain (inverse OFDM') e.g. using an IFFT, and outputs two time domain OFDM symbols: c1 followed by -c on stream p=l, and c2 followed by c on stream p=2. Following this the time domain OFDM symbols in each stream are forwarded to respective cyclic delay circuits 41, 42, where they are processed by the cyclic delay matrices Cdm as follows. At time t0, symbol c1 is given a cyclic shift of Scy1 = 0 on branch m=1, and a cyclic shift of öcy2 = 16 samples on branch m2 of the cyclic delay circuit 41. At the same time symbol c2 is given a cyclic shift of 5cy1 = 0 on branch m=1, and a cyclic shift of öcy2 = 16 samples (assuming the guard period of the considered OFDM system is 16 samples) on branch m=2of the cyclic delay circuit 42. At time t1, symbol -c is given a cyclic shift of Scy1 =0 on branch m1, and a cyclic shift of Scy2 =16 samples on branch m=2 of the cyclic delay circuit 41. At the same time symbol c is given a cyclic shift of Scy1 = 0 on branch m1, and a cyclic shift of Scy2 = 16 samples on branch m=2 of the cyclic delay circuit 42. This results in the following symbols being transmitted from the diversity transmitter 20 at times t0 and t1: - 12 - Xli X12 X21 X22 t=o c1 c1.Scy2 c2 c2.Scy2 -c; -c; .s2 The choice of cyclic time shift is purely dependent on the channel memory (number of resolvable multipath components or taps). As the channel memory is assumed not known at the diversity transmitter, it can be compensated with the guard period of the OFDM system. The guard period (or cyclic prefix) is used in OFDM systems to compensate for the delay spread of the channel and helps to reduce intersymbol interference (lSI). Thus the magnitude of the cyclic time shift (or delay) may be chosen as a function of the guard period. For example in the Hiperlan II standards the number of OFDM sub-carriers is 64 and a compulsory guard period of to 16 samples is prescribed. When considering the diversity transmitter of Fig. 2 for 1-liperlan II standards, the cyclic delay matrices Cd2 should shift each OFDM symbol in the lower branch m=2 by 16 samples; the cyclic shift neither increases the length of the subject OFDM symbol nor increases compulsory guard period of the system.

The cyclic shift effects a shift of the OFDM symbol in the time domain that appears to the receiver as a multipath component (and therefore useable to obtain a diversity gain).

When considering more than two branches in each cyclic delay circuit 41...4P, the cyclic delays are chosen as multiple of guard period on successive parallel branches. This will be explained with reference to Fig. 3.

Referring to Fig. 3 a diversity transmitter generally identified by reference numeral 30 generally similar to the diversity transmitter 10, comprises six spatial channels X11,X12,X1,X,X2X3 with a full rate STBC as the diversity generator 2.

The diversity generator 2 generates P=2 diversified OFDM symbol streams, each of which is input to a cyclic delay circuit 41 and 42 respectively. Each cyclic delay circuit 41, 42 comprises M=3 branches and thus there are six spatial channels in total.

In each cyclic delay circuit 41, 42 the time domain OFDM symbols are - 13 - operated on by a respective cyclic delay matrix Cdm* The cyclic delay matrices Cdm in each cyclic delay circuit 41, 42 cyclically shift each time domain OFDM symbol by a different number of samples. The difference between cyclic time shift(s) in each branch of cyclic delay circuits 41, 42 is a maximum by which is meant that the shift in samples should be such that the symbols transmitted from the different spatial channels should not overlap, whereby the receiver sees longer impulse responses coming from the respective primary stream p, no matter how many branches there are in the corresponding cyclic delay circuit. This can be achieved by choosing the cyclic time shift (in samples) as different multiples of the system guard period in successive branches in each cyclic delay circuit 41, 42.

Taking the Hiperlan II example, the cyclic shift in each of upper branch of each cyclic delay circuit 41, 42 is zero i.e. no cyclic shift; and the original diversified and inverse OFDM processed symbol is transmitted; in second branch of both cyclic delay circuits 41, 42 the cyclic delay matrices Cd2 apply a cyclic shift Scy2 = 16 samples; lastly, in third branches, in both cyclic delay circuits 41, 42 the cyclic delay matrices Cd3 apply a cyclic shift Scy3 = 2.öcy2 32 samples. This results in the following symbols being transmitted from the diversity transmitter 30 at times t=Oand t=1.

Xli X12 X13 X21 X22 X23 t=0 c1 c.Scy2 c1.öcy3 c2 c2.öcy2 c2.Scy3 t=1 -c c.Scy2 -c.Scy3 c c.öcy2 c.öcy3 Any combination of P and M can be used. This will be explained with an example with reference to Fig. 4.

Referring to Fig. 4. a fourth embodiment of a diversity transmitter generally identified by reference numeral 40 generally similar to diversity transmitter 30 (like reference numerals indicate like parts), comprises six spatial channels X11,X12,X21,X22,X31X32 and a diversitygenerator 2 that uses sporadic or half rate STBC. in the diversity transmitter 40 the six spatial channels are provided with a combination of P=3 and M = 2, rather than P=2 and M=3 of transmitter 30.

- 14 - To assess the performance of the diversity transmitters 10, 20, 30 and 40, a computer simulation based on the HIPERLAN 2 Standards (see [10]) was carried out to examine Bit Error Rate (BER). The following results show BER versus SNR per bit (defined as the transmitted bit energy over the noise power spectral density (Eb/No)). A total bandwidth of 20MHz with 64 sub-carriers and a guard period of 16 samples was used to counteract ISI. A 64 point IFFT was employed to generate each time domain OFDM symbol as described above. A half rate convolutional encoder (R=l/2, (133, 171)8) was used for channel encoding, and a soft Viterbi decoder was used for channel decoding. Perfect channel estimation and a Maximum Ratio Combining (MRC) detection scheme was used at the receiver. All of the simulations were performed with one receive antenna. Unless otherwise state full rate space-time block codes (based on [2]) was used as the diversity generator 2.

To emphasize the importance of multipath diversity, the simulation used various transmit antenna configurations in the presence of varying channel taps. In all antenna combinations described below, only the number of branches M has been varied. Ignoring the cyclic prefix, the rate achievable with the diversity transmitters is I bitls/Hz using QPSK modulation. The channel taps are i.i.d. complex Gaussian distributed with variance cr, I /(L + 1); three channel orders L = 0, 2, 5 were used.

Referring to Figs 5, 6 and 7 (the simulation results for different diversity transmitters with varying channel taps. Referring to Fig. 5 a graph 50 shows the simulation results for a diversity transmitter with three antennae, two of which transmit time domain OFDM symbols with Scy1 = 0 and the other with Scy2 = 16. For a given Eb/No the BER can be seen to improve with increasing channel taps; this is as expected and results from the increased multipath diversity with more channel taps. Referring to Fig. 6 a graph 60 shows the simulation results for a diversity transmitter with four antennae, P=2 and M=2; one branch in each of the two cyclic delay circuits applies a cyclic delay of Scy2 = 16 samples. Comparing graph 60 with graph 50 it will be seen that approximately the same BER rates can be obtained with three antennae when L=5 as with four antennae when L=2. The graph 60 supports the conclusion that addition of one more cyclic delay chain gives better performance even with less number of multipaths available in the channel.

- 15 - Referring to Fig. 7 a graph 70 shows the simulation results for the diversity transmitter 40 i.e. with six antennae and P=2 and M=3. Again the further improvement in BER for a given EfN0 is apparent. In particular comparing graph 50 with graph 70 it will be seen that approximately the same BER rates can be obtained with three antennae and L = 5, as with six antennae (P=2 and M=3) when L0 i.e. only one signal path. Accordingly, for different channel scenarios it is possible to increase the number of branches M where naturally occurring multipath is limited, without increasing complexity at the receiver.

Referring to Fig. 8 a graph 80 shows the results obtained for a diversity transmitter with combinations of P and M as follows: P=2and M1 (three antennae in total); P=2 and M=2; and P=2 and M=4; all at L0,6 respectively. It is readily seen how increasing the number cyclic delay branches M improves BER.

This effect is yet further enhanced when the number of channel taps L (i. e. is multipath) is increased.

Following the first simulation, an 18-tap Rayleigh channel model corresponding to a typical large open space environment in Non Line of Sight (NLOS) conditions was further used to compare the diversity transmission method to the following earlier proposals: (1) the space-time block encoding scheme of [3], and (2) the space-time-multipath scheme proposed in [8]. The I 8-tap model has an overall and average rms delay spread of 730ns and 1 OOns respectively. The channel taps were generated using the model in Jakes [11].

Referring to Fig. 9 a graph 90 shows the results of comparison (1). In particular the BER curves are plotted against Eb/No for ST-block codes of [3] with two, four and six antennae respectively. The diversity transmitter of the present invention had the following combinations: P=2and M1; P=2and M=2; and P=2 and M=4. 1 6-QAM was used to compensate for the rate loss as reported in [3] when four and six antennae were simulated for STBCs; for all other simulations QPSK was used. It is immediately apparent that all of the simulated diversity transmitters according to the invention outperform the best performance available with ST-block codes Referring to Fig. 10 a graph 100 shows the results of comparison (2). The - 16 - space-time-multipath (STM) scheme was simulated with V2 rate convolutional codes with QPSK modulation; this makes the rates of the diversity transmission method of the invention and the STM scheme the same. According to the STM scheme employing four transmit antennae requires 64 sub-carriers as the model has a delay spread of 16 samples. Therefore STM becomes sub-optimal for more than four transmit antennae. The diversity transmitter of the present invention was simulated in the following combinations: P=2and M=1 i.e. three antennae in total), and P=2and M=2. The present invention outperforms STM scheme in both cases.

The spectral efficiency of the transmit scheme according to the present invention depends on the rate of the employed diversity generator 2. The ST-block codes for more than two diversification branches P>2 and complex signals suffer from a half or sporadic rate loss [3]. Loss in spectral efficiency makes the diversity transmitter of Fig. 4 less desirable and it is preferred that full rate STBC with any number of cyclically delayed branches are used. However, there is no limitation in using any diversity generator for better performance by trading spectral efficiency.

The diversity transmitter may be part of a mobile terminal or a base station for example, and the receiver may be a base station or another mobile terminal.

Alternatively the diversity transmitter may be part of a broadcast transmitter such as a digital broadcast transmitter (e.g. DVB). Furthermore, the spatial channel may include polarization diversity channels. Some of the branches can have different cyclic delays and may provide a different quality of service and performance. There may be different channel coding, different diversity generators and even parallel transmission with necessary changes to the receiver.

Documents cited in the specification

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23-28, 2000.

[2] S. M. Alamouti, "A simple transmit diversity technique for Wireless Communications," IEEE Journal on Selected Areasin Communications, Vol. 16, pp. 1451-1458, October 1998.

[3] V. Tarokh, H. Jafarkhani and A. R. Calderbank, "Space-time block codes - 17 - from orthogonal design," IEEE Transactions on Information Theory, Vol. 45, pp. 1456-1467, July 1999.

[4] V. Tarokh, N. Seshadri and A. R. Calderbank, "Space-time codes for high data rate wireless communication: Performance criterion and code construction," IEEE Transactions on Information Theory, Vol. 44, pp. 744764, March 1998.

[5] A. Dammann and S. Kaiser, "Standard conformable antenna diversity techniques for OFDM and its application to DVB-T system," IEEE Global Telecommunications Conference (GLOBECOM 2001), pp. 3100-3105, November2001.

[6] H. Bölcskei, M. Borgmann and A. Paulraj, "Space-frequency coded MIMO OFDM with variable multiplexing-diversity tradeoff," International Conference on Commun. (ICC), pp. 2837-284 1, May 2003.

[7] Y. Xin, Z. Wang and G. Giannakis, "Space-time diversity systems based on linear constellation precoding," IEEE Trans. on Wireless Commun., vol. 2,pp.

294-309, Mar. 2003.

[8] X. Ma and G. Giannakis, "Space-time-multipath coding using digital phase sweeping or circular delay diversity," to appear in IEEE Trans. Signal Processing, 2004.

[9] W. Su, Z. Safar, M. Olfat and K. J. Ray Liu, "Obtaining fulldiversity space frequency codes from space-time codes via mapping," IEEE Trans. on Signal Processing, vol.5 1, NO. 11, Nov. 2003, pp. 2905-2915.

[10] European Telecommunications Standard Institute ETSI, "Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Physical Layer", V1.1.I (2000-04) [11] W. C. Jakes, "Microwave Communications", Wiley, 1974.

Claims

- 18 - CLAIMS

1. A diversity transmitter for use in an OFDM transmission protocol which diversity transmitter comprises: a diversity generator for receiving and diversifying OFDM transmit symbols, and outputting diversified OFDM symbol matrices (DOSM), DOSM symbols within each DOSM being divided into at least two primary streams each comprising different DOSM symbols, a transmit processor for receiving said at least two primary streams of DOSM symbols, and for transforming said each DOSM symbol from the frequency domain into the time domain, and outputting time domain OFDM symbols (TDOSs), a cyclic delay circuit for dividing at least one of said primary streams of TDOSs into at least two branches of identical TDOSs, each branch for supplying a respective spatial channel for transmission to a receiver, the arrangement being such that, in use, said cyclic delay circuit applies a cyclic time shift to a TDOS symbol in at least one of said branches before transmission.

2. A diversity transmitter as claimed in claim 1, wherein said cyclic delay circuit applies no cyclic time shift in at least one of said branches, whereby said diversity transmitter transmits substantially simultaneously from different spatial channels at least the original TDOS symbol and a cyclically delayed replica thereof.

3. A diversity transmitter as claimed in claim I or 2, wherein there is a plurality of said branches, said cyclic delay circuit applying a different cyclic time shift in each branch.

4. A diversity transmitter as claimed in claim 3, wherein said cyclic time shift differs between branches by an amount sufficient to inhibit overlap between TDOSs transmitted from difference spatial channels.

5. A diversity transmitter as claimed in claim 3 or 4, wherein said cyclic time shift is such that the length of cyclic prefix or guard period is not increased.

6. A diversity transmitter as claimed in any preceding claim, wherein said cyclic - 19 - time shift is performed modulo the length of said TDOS symbol.

7. A diversity transmitter as claimed in claim 6, wherein said length is measured in a number of samples of said TDOS symbol, and said cyclic shift is performed by shifting the order of said samples.

8. A diversity transmitter as claimed in any preceding claim, wherein said cyclic time shift is a multiple of a guard period of said OFDM protocol.

9. A diversity transmitter as claimed in any preceding claim, wherein said cyclic time shift is determined according to system requirements and the number of primary streams.

10. A diversity transmitter as claimed in any preceding claim, wherein said cyclic time shift is dependent on the number of taps in the channel.

11. A diversity transmitter as claimed in any preceding claim, wherein said cyclic delay circuit comprises a cyclic delay matrix (Cdm)

12. A diversity transmitter as claimed in claim 11, wherein said cyclic delay matrix (Cdm) is obtainable from: Cdm =[ 0 (m_lxL+l)]mE[lMI N-(m-1) (L+1) 0 where A is the identity matrix of order AxA, N is the number of sub-carrier frequencies in the OFDM system, M is the number of branches in that cyclic delay circuit where Cdm is to be applied, and L is number of non-zero taps assumed in the channel transfer function.

13. A diversity transmitter as claimed in any preceding claim, further comprising a cyclic delay circuit per primary stream of TDOS symbols.

14. A diversity transmitter as claimed in any preceding claim, wherein at least one of said primary streams bypasses said cyclic delay circuit for supplying a spatial channel substantially directly, while the or each remaining primary stream delivers - 20 - TDOS symbols to one or more cyclic delay circuit.

15. A diversity transmitter as claimed in any preceding claim, wherein said diversity generator is adapted to subject said OFDM symbol matrices to at least one of orthogonal transmit diversity (OTD), orthogonal spacetime block code (STBC) processing, non-orthogonal STBC processing, spacetime Trellis code (STTC) processing, or space-time turbo code processing.

16. A diversity transmitter as claimed in any preceding claim, wherein said OFDM symbol matrices are generated from a channel coded sequence.

17. A diversity transmitter as claimed in claim 16, wherein said channel coded sequence has been generated by Turbo coding, convolutional coding, block coding, or Trellis coding.

18. A base station comprising a diversity transmitter as claimed in any of claims 1 to 17.

19. A mobile radio communication device comprising diversity transmitter as claimed in any of claims 1 to 17.

20. A broadcast transmitter comprising a diversity transmitter as claimed in any of claims 1 to 17.

21. A method of transmitting data in an OFDM system, which method comprises the steps of: (1) using a diversity generator to receive and diversify OFDM transmit symbols, and output diversified OFDM symbol matrices (DOSM); (2) dividing DOSM symbols within each DOSM into at least two primary streams each comprising different DOSM symbols; (3) transforming each DOSM symbol from the frequency domain into the time domain, and outputting time domain OFDM symbols (TDOSs); (4) dividing at least one of said primary streams of TDOSs into at least two branches of identical TDOSs, each branch for supplying a respective spatial channel for transmission to a receiver; and - 21 - (5) applying a cyclic time shift to a TDOS symbol in at least one of said branches before transmission.

22. A method according to claim 21, further comprising the step of applying no cyclic time shift in at least one of said branches, and transmitting substantially simultaneously from different spatial channels at least the original TDOS symbol and a cyclically delayed replica thereof.

23. A method according to claim 21 or 22, wherein there is a plurality of said branches, the method further comprising the step of applying a different cyclic time shift in each branch.

24. A method according to claim 23, wherein said cyclic time shift differs between branches by an amount sufficient to inhibit overlap between TDOSs transmitted from difference spatial channels.

25. A method according to claim 21, 22, 23 or 24, wherein said cyclic time shift is such that the length of cyclic prefix or guard period is not increased.

26. A method according to any of claims 21 to 25, further comprising the step of performing said cyclic time shift modulo the length of said TDOS symbol.

27. A method according to claim 26, wherein said length is measured in a number of samples of said TDOS symbol, and said cyclic shift is performed by shifting the order of said samples.

28. A method according to any of claims 21 to 27, wherein said cyclic time shift is a multiple of a guard period of said OFDM protocol.

29. A method according to any of claims 21 to 28, wherein said cyclic time shift is dependent on the number of channel taps.

30. A method according to any of claims 21 to 29, wherein step (5) comprises the step of applying a cyclic delay matrix (Cdm) to shift said TDOS symbol in time.

- 22 -

31. A method according to claim 30, wherein said cyclic delay matrix (Cdm) is obtainable from: r o i 1L11 Cdm = (m-)( +) ,mE[1,M] L N-(m-l)(L+I) J where A is the identity matrix of order AxA, N is the number of sub- carrier frequencies in the OFDM system, M is the number of branches in that cyclic delay circuit where Cdm is to be applied, and L is number of non-zero taps assumed in the channel transfer function.

32. A method according to any of claims 21 to 31, further comprising the step of applying at least one cyclic delay per primary stream of TDOSs.

33. A method according to any of claims 21 to 32, further comprising the step of bypassing symbols in at least one of said primary streams around said cyclic delay circuit to supplying a spatial channel substantially directly, while the or each remaining primary stream delivers TDOS symbols to one or more cyclic delay circuit.

34. A method according to any of claims 21 to 33, wherein step (1) is carried out by subjecting said OFDM symbol matrices to at least one of orthogonal transmit diversity (OTD), orthogonal space-time block code (STBC) processing, non- orthogonal STBC processing, space-time Trellis code (STTC) processing, or space- time turbo code processing.

35. A method according to any of claims 21 to 34, wherein said OFDM symbol matrices are generated from a channel coded sequence.

36. A method according to claim 35, further comprising the step of generating said channel coded sequence by Turbo coding, convolutional coding, block coding, or Trellis coding.

37. A computer program comprising computer executable instructions for causing a transmitter controller to perform the method steps of any of claims 21 to 36.

- 23 -

38. A computer program product storing computer executable instructions in accordance with claim 37.

39. A computer program product as claimed in claim 38, embodied on a record medium, in a computer memory, in a read-only memory or on an electrical carrier signal.