GB2516874A - An orthogonal diversity encoding method - Google Patents

Abstract

Symbols are encoded for transmission over a plurality of transmit channels by applying a Walsh spreading code to each bit in a block of bits to produce a set of spread sequences, summing the spread sequences element by element to create a block of uncoded channel symbols and transforming blocks of uncoded channel symbols using a block diversity coder into a block of encoded channel symbols. The block diversity coder may be an Alamouti diversity encoder. Channel diversity may be provided by space and time or frequency diversity.

Description

AN ORTHOGONAL DIVERSITY ENCODING METHOD

TECHNICAL FIELD

The present invention relates generally to and in particular to transmitter diversity apparatus, methods and systems using multiple antennas and orthogonal block coding in order to maximize coding gain.

Introduction

Antenna diversity coding has been widely adopted in wireless systems due its ability to provide higher reliability and faster data rates. Where channel state information is known or can be estimated, it is possible to determine antenna weights to improve performance. However in some cases the channel response is unknown or not practical to estimate, for example in the case of one-way transmission of broadcast information.

Obtaining the benefits of multiple transmit antennas requires the use of special diversity coding schemes such as the well known Alamouti scheme [ALAMOIJTI] "S. NI.

Alamouti, "A simple transmit diversity technique for wireless communications," IEEE J. Sd. Areas Commun., vol. 16, pp. 145 1-1458, Oct. 1998". Other schemes are also known (see [SIBILL Sibille etal. "MIMO From theory to Implementation", Elsevier, 2011).

Diversity encoding schemes such as the Alamouti scheme arc generally referred to as "open-loop" because they operate without requiring channel state information at the transmitter. As evidence of their practical importance, they are widely used in third and fourth generation cellular networks.

The Alamouti scheme for block diversity encoding is atnactive since it allows low decoding complexity and provides significant performance improvement using only two antcnnas at the transmitter and requires only one at the receiver. However link performance does not match that achievable by closed loop schemes, and the data sent using open-loop schemes must be sent at a lower rate or suffer a higher error rate than can be achieved using closed loop schemes.

Therefore an improved open-loop scheme whose performance is better than existing open loop schemes, while maintaining low decoding complexity and not compromising coding rate, is highly desirable.

Summary

According to an aspect of thc present invention, thcrc is provided a transmission method in which symbols are transmitted over a plurality of transmit channels, the method comprising: applying a Walsh spreading code to each bit in a block of bits (16) to produce a set of spread sequences (17); summing the spread sequences (17) element by element to create a block of uncoded channel symbols (19); transforming blocks of uncoded channels symbols using a block diversity coder (15) into a block of encoded channel symbols (41- 48); and transmitting the encoded channel symbols. With embodiments of the invention, Performance limitations of prior art systems can be overcome, and an advance in the art is realized, by a block code arrangement where blocks of bits are transmitted over a plurality of transmit channels' The block diversity encoder (such as the Alamouti encoder) employs channel diversity via space diversity and either time diversity (so called space time block coding, STBC) or frequency diversity (so called space frequency block coding, SFBC). Space diversity is effected by redundantly transmitting over a plurality of antennas; time diversity is effected by redundantly transmitting at different times; and frequency diversity is effected by redundantly transmitting at different frequencies. Illustratively, using two transmit antennas and a single receive antenna, the disclosed embodiments provides the superior diversity gain to prior art schemes. The novel method does not require any bandwidth expansion or feedback from the receiver to the transmitter, and has linear decoding complexity. The principles of this invention not limited to arrangements with exactly two transmit antennas, and illustrative examples are presented with other numbers of antennas.

It is well known that the Alamouti diversity encoder introduces diversity without any loss of orthogonality or transmission rate. It thus provides a frill rate diversity orthogonal encoding scheme, since encoded channel symbols output by the diversity cncodcr are orthogonal, have divcrsity and havc the bit rate as before. In the particular case of QPSK, an average of two bits per symbol would be sent and bit transmission is orthogonal.

The method of the invention can also maintain orthogonality, diversity and full encoding rate. It also permits lower coding rates while employing the advantages of the invention, and also permits higher coding rates, albeit at the cost of orthogonality.

Embodiments of the invention will 110W be described, byway of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: FIGURE 1 is a block diagram of a system in accordance with the principles of this invention.

FIGURE 2 is a block diagram of a bit encoder in accordance with the principles of this invention.

FIGURE 3 illustrates the transmission of the two successive pairs of STBC symbols corresponding to a block of bits.

FIGURE 4 illustrates the transmission of the two successive pairs of STBC symbols according to embodiment 3.

FIGURE 5 illustrates the transmission of the two successive pairs ofSTBC corresponding to in block of input bits, according to embodiment 4.

FIGURE 6 illustrates an example of transmission using SFBC.

FIGURE 7 illustrates the transmission according to embodimentS.

FIGURE 8 illustrates the transmission according to embodiment 6.

FIGURE 9 illustrates the transmission according to embodiment 7.

In the following unless otherwise stated bipolar representation is used for binary values, thus a bit b takes the value +1, with b=1 corresponding to data bit 0 and b= -Ito data bit 1. Throughout this document the well known Walsh-Hadamard codes are referred to simply as Walsh codes. In accordance with common practice the notation x' indicates complex conjugation of x, whose real and imaginary parts are denoted Re(I) and Iarn(x).

The notation [.11' indicates matrix or vector transposition. The function Sign(x) is defined as -1 if x<0 otherwise +1. The notation [xi denotes the floor thnction, i.e. integer truncation towards -cc and [xl denotes the ceiling ftinction, i.e. integer truncation towards +cc. The term transmission opportunity" relates to a time or frequency resource which is sufficient to transmit one channel symbol; in the case of STBC this corresponds to a symbol period (where implicitly transmission is within a given frequency band), while for SFBC this corresponds to a carrier (during a particular symbol period).

FIGURE 1 presents a block diagram of an arrangement in accordance with an aspect of the invention. The arrangement includes multiple transmitter antennas (providing space diversity). Transmitter (10) illustratively comprises bit encoder 13, block diversity encoder 15 and antennas (11) and (12), and it transmits blocks of bits (16) each containing 2' bits, where N is 1 or greater. Each antenna transmits a sequence oft encoded channel symbols, each antenna transmitting one encoded channel symbol per transmission opportunity, thus providing an average transmission rate of 2 bits per transmission opportunity. Also illustratively, the arrangement includes a receiver (20) that comprises a single antenna (21).

The block diversity encoder (15) transforms each block of uncoded channel symbols into two sequences of encoded channel symbols, one per antenna, in the illustrative embodiments the block of uncodcd channel symbols contains two elements and the encoded channel symbols are transmitted in two transmission opportunities, thus the input rate and output rate are equal.

In certain illustrative embodiments the block diversity coder (15) uses space time block coding (STBC), where the output channel sequences on a given antenna are transmitted during separate symbol periods and are thus separated in time, but each transmitted simultaneously with a corresponding channel symbol the other antenna.

An instancc of STBC is illustrated in FIGURE 3. In the case of STBC, channel frequency is fixed and the channel symbol transmission opportunity corresponds to a symbol period (40). Encoded channel symbols (41,42,43,44) are transmitted on a first antenna (11) simultaneously, i.e. in the same symbol periods (40), with corresponding encoded channel symbols (45,46,47,48) being transmitted on a second antenna (12). Where STBC is used, each block takes 2N symbol periods to transmit which corresponds in the illustrative embodiment to 4 symbol periods.

Alternatively as illustrated in FIGURE 6 the block diversity encoder can use space frequency block coding (SFBC). In the case of SFBC transmission takes place simultaneously during one symbol period, and each transmission opportunity corresponds to transmission on given carrier (71). In this case the encoded channel symbols (41, 42, 43, 44) transmitted on a first antenna are each transmitted on separate frequency carriers and do not mutually interfere, but a corresponding set of encoded channel symbols (45, 56,47,48) is transmitted on the same frequency carriers at the same time on a second antenna (12). Where SFBC is used, each block is transmitted on 2' frequency carriers simultaneously.

Transmitter Embodiment I-Walsh Hadamard block coding FIGURE 2 illustrates the encoder 13 in accordance with a first transmitter embodiment. A block of bits (16) containing 2N+J bits to be transmitted is denoted {b1,b2 b2Nf). In this illustrative embodiment the bit encoder (13) encodes blocks of bits (16) that contains 8 bits [b1, b9}.

A spreader (14) uses a set of spreading codes to spread each of the 2 bits within the block of bits (16) to produce a set (17) of spread sequences that are submitted to the symbol encoder (18). The set of spreading codes is denoted {w1 w2N+1}, in this illustrative embodiment eight spreading codes are used {w1 w2 w8}. The first 2N spreading codes are constructed by taking the rows of a x 2NHadamard matrix. The remaining 2N spreading codes are constructed using same spreading codes multiplied by i.

Thus, in the illustrative embodiment, the spreading code for a given bit corresponds to a row of a 4 x 4 Hadamard matrix multiplied by 1 or i, and a total of 2' , i.e. eight, orthogonal codes is available comprising four real codes and four imaginary codes:-Wi 1 1 1 1 W2 1 -1 1 -1 1 1 -1 -1 -1 -1 -1 1 w5 -I I I I -i -i 1 1 -i -i w8 -i -i 1 Bit bk, 1 «=k«=2' , is spread by the spreading code Wk, resulting in the following set (17) of 8 spread sequences, 88 1T in the illustrative embodiment:-b b1 b1 b1 B2 b2 -b2 b2 -2 83 b3 b3 -b3 -b3 B4 b4 -b4 -b4 b4 B5 -lb5 lb5 lb5 lb5 B6 lb6 -lb6 lb6 -lb6 B7 lb7 lb7 -lb7 -lb7 Hg lb8 -lb8 -lb8 lb3 In the symbol encoder (18) the spread sequences are summed element by element, to form a block ofuncoded channel symbols (19) comprising 2 uncoded channel symbols, (in the illustrative embodiment 4) uncoded channel symbols:-[s 2 53 54 Where: (b1 + b2 + b3 + b4 + lb5 + lb6 + lb7 + 1b8) -(b1-b2+b3-b4+1b5-1b6+1b7-1b3) -(b1+b2-b3-b4+1b5+1b6-1b7-1b8) 54 (b1 -b2 -b3 + b4 + lb5 -lb6 -lb7 + 1b8) The 2A uncoded channel symbols (19) are presented in pairs to the block diversity encoder (15), in this illustrative embodiment an Alamouti space time block encoder. In the illustrative embodiment two pairs of symbols are transmitted on each of two antennas in successive symbol periods (40), as illustrated in FIGURE 3. Denoting the sequence of encoded channel symbols (41-44) transmitted on the first antenna (11) as t1 and the encoded channel symbols (45-48) transmitted on the second antenna (12) as t2, this results in the following 2N (in the illustrative embodiment 4) encoded channel symbols (41-48) transmitted on each of the respective antennas over four symbol periods (40): I-ti1 -s1 -S 53 -s; It21 -2 54 53 Transmitter Embodiment 2 This illustrative embodiment reduces the PAPR (peak to average power ratio) compared with transmitter embodiment 1, while achieving a similar performance advantage to the first embodiment. Operation is as described in embodiment 1 apart from the following modifications.

In accordance with FIGURE 2, the spreader (14) spreads each input bit in the block of input bits (16) using one of a set oft (in this embodiment 4) spreading codes as follows. The set of spreading codes is denoted [wk}, where 1«=k«=2N, in this illustrative example 1«=k«=4. The first tj are constructed by taking the rows of a 2'-x 2N-1 Hadamard matrix. The remaining 2N-' codes are constructed using same spreading codes multiplied by /. Thus, in this illustrative example, the spreading codes correspond to:-:t 1 Wz -1 -1 W3l I 1 -j Two bits are spread using each spreading code, thus 2' bits are spread with spreading codes. In this illustrative embodiment each available spreading code is used to sprcad two bits, in this case successive even and odd bits. The mapping of bits to spreading code consists of spreading pairs of bits with the same code, i.e. Bit bk, 1«=k«=2N_t,is spread by the spreading code W1k/1,resulting in the following set (17) of 8 spread sequences F12 88 IT:_ b1 b1 82 b2 b2 83 b3 -b3 84 -b4 -b4 -lb5 lb5 86 lb6 lb6 87 lb7 -lb7 B lb8 -lb8 The spread sequences (17) are summed by the symbol encoder (18) by even and odd rows, to form the alternate even and odd symbols in a sequence of uncoded channel symbols, the sequence ofuncoded channel symbols (19) comprising 2', i.e. in the illustrative embodiment 4, channel symbols:-2 3 4 jT Where: (b1+b3--ib5+ib7) -(b2+b4--ib6+ib8) S3 -(b1-b3--ib5-ib7) 54 b -b -Fib -ib k2 4 6 8 Note that the real and imaginary part of each element {s) is composed of the sum of two bipolar quantities, not four as is the case for embodiment 1. Thus the PAPR of the corresponding channel symbol is reduced.

The 2A channel symbols are presented in pairs to the block diversity encoder (15), in this illustrative embodiment an Alamouti space time block encoder. Operation then proceeds as in embodiment I. Embodiment 3, with improved channel time diversity In the presence of time varying fading, diversity can be increased and performance improved in accordance with FIGURE 4. In this illustrative embodiment the bit encoder encodes blocks of bits (16) containing 8 bits, as in the first embodiment, and four uncoded channel symbols (19) are generated for transmission on two antennas as in the first embodiment:-I-ti1 -sI s Ft21 -2 As a further measure to introduce diversity a time delay (54) is introduced between the transmission of the two pairs of encoded channel symbols. Thus the first pairs of encoded channel symbols (4 1,42,45,46) on each antenna (11,12) are transmitted in successive symbol periods (40): = -The next pairs of symbols (43,44,47,48) are transmitted on each antenna in successive symbol periods (40) after a time gap (54):

-S -S -54

The time gap may introduce a delay over a plurality of symbol periods, and preferably is larger than the time coherence of the fading channel, so that to some extent it dc-correlates the channel response.

Note that this embodiment introduces a time delay 54) in transmission and reception, leading to extra memory buffering requirements. In some circumstances that might be undesirable and is overcome by following embodiments.

Embodiment 4, using antenna diversity Where additional antennas are available, diversity is achieved without resorting to introducing a transmission delay (54) by a further embodiment in accordance with FIGURES. In this case, the first pairs of symbols (41,42,45,46), denoted as inprevious embodiments by [t1' t2']T are transmitted on a first and second antenna (11.12), while the second pairs of symbo Is (43,44,47,48), denoted as in previous embodiments Ft1' t2']T, rather than being transmitted on the first and second antenna are transmitted on a third and fourth antenna (60,61). Transmission takes place in consecutive symbol periods (40). In symbol periods where antenna I and 2 transmit, antenna 3 and 4 have symbol periods in which they do not transmit any radio signals within the channel bandwidth, so called transmit gaps (63), and vice versa. Deeorrelation between the channel response of successive transmissions is achieved by spatial separation of antennas.

Embodiment 5, using frequency diversity For systems where a number of independent frequency carriers are available, such as those employing OFDM, diversity can be increased using frequency domain techniques.

In accordance with FIGURE 7, and using the same notation as previously to donate the first and second pair of symbols, Ft1 2 1T and It1" 2 "F are transmitted simultaneously in time. The first pairs of symbols (41,42, 45, 46) Ft1' 2 1T transmitted on the first and second antennas (11,12) are transmitted in frequency carriers (71)which are preferably contiguous in frequency and similarly the second pairs of symbols Ft1' t2lT. The carriers used to transmit the first pairs of symbols [t1' t2']T and the second pairs [t1 t2"]T are widely separated, preferably by a larger frequency gap (72) than the channel coherence bandwidth. Thus the two symbols comprising t1' are transmitted on frequency carriers with similar fading characteristics, but different from ". The same frequency carriers are used for symbols t2' and symbols t2".

This embodiment introduces diversity with needing to impose a time delay, however preferably the pairs of frequency carriers are separated by a sufficiently wide bandwidth (72) to provide some dc-correlation between the channel response at the respective pairs of frequency carriers.

Embodiment 6, carrier orthogonality and antenna diversity An illustrative embodiment according to Figure 8 applies to systems employing independent carriers such OFDM, and where two additional antennas (60,6 1) are available.

Here diversity is increased without the requirement for transmission on pairs of frequency carriers which are widely separated in frequency, and without the requirement for time delay sincc all symbols arc transmittcd simultaneously in time. Symbols It1 t2 are transmitted on a first pair of antennas (11,12), while [t1' 2 "V arc transmitted on a second pair of antennas (60,61). The first pair of antennas (11,12) each use frequency carriers (71) which are preferably contiguous in frequency, and the second pair of antennas (60,61) use carriers (73) which are preferably contiguous in frequency. On carrier frequency used by one pair of antennas, the other pair does not transmit, i.e. the other pair has a transmission gap (63) on that frequency. There are no special requirements regarding the amount of frequency separation between the two pairs of frequency carriers (71, 73) used by the first (11,12) and second (60,6 1) pair of antennas, other than that they are non overlapping in frequency. In particular there is no requirement for a frequency gap between them.

Embodiment 7, transmitting at less than unity rate Where data rate requirements permit, fewer than 2 bits can be transmitted per symbol. In the following 71h exemplary embodiment, referring to FIGURE 9, one bit is transmitted per symbol on average, i.e. a rate of 1⁄4 that of embodiment 1, spread over four symbol periods by the bit encoder (13). The transmit procedure follows that of the first embodiment, but since there arc only 4 bits {b1, , , b4 in a block of bits (16) rather than the 8 of embodiment 1, not all Walsh spreading codes are used. The spreader (14) chooses spreading codes to use alternately from [w1... } (i.e. from real codes and rumling down the Hadamard matrix) and [w3, w7,...) (i.e. from with imaginary codes and running up the Hadamard matrix): {Wt)W2JW7,W8} This balances the number of real and imaginary codes, since tfwi, w2} are real and [w8, w7} imaginary, which is advantageous to reduce the peak to average power ratio (PAPR), and cnsurcs as many codcs as possible have zero cross-correlation. Thus thcrc arc four spread sequences (17) 2 B3,84 Tppj led to the symbol encoder. The operation of the symbol encoder (18) and block diversity encoder (15) proceeds as in embodiment 1. The extension to other rates, and also to the methods of the second embodiment, is straightforward for the skilled person with the number of real and imaginary codes either balanced or differing by 1.

Rates above that of two bits per symbol can be supported by supplying blocks of bits (16) cncodcd according to an M-ary rcal modulation schcme, such as 4-PAM, at the cost of orthogonality between transmitted bits. Thus each element of the block of bits would correspond to two or more information bits.

With these methods dynamic data rate transmission can be efficiently supported, provided that the receiver is able to determine the transmission rate. The skilled person will be aware of techniques that enable this.

Embodiment 8: not using spatial diversity The principles may also be advantageously applied to arrangemcnts with onc transmit antenna. In this embodiment the operation of the block diversity encoder (15) is omitted, and the uncoded channel symbols are transmitted directly. The advantage of diversity provided by the application of Walsh spreading over several transmission opportunities is however maintained.

In this embodiment transmission of channel symbols need not take place in pairs.

However it may be advantageous to mainta.in the same timing or frequency relationship in order to allow identical time or frequency arrangements with systems that have antenna diversity, and thus group transmission opportunities in pairs.

Receiver Embodiment 1 for Walsh Hadamard block coding with STBC This receiver embodiment decodes transmissions that are sent in accordance with transmitter embodiment 1, and is illustrated in FIGURE 1.

During a given channel symbol transmission opportunity, encoded channel symbols [i S2] are sent by the transmitter (10), transmitted on different antennas (11, 12). The channel response including the effects of the transmit chain, the air-link and the receive chain (25) may be modelled by a complex multiplicative distortion composed of a magnitude response and phase response. The channel response h1, h2 between the transmit antennas (11,12) and a receive antenna 21 are denoted:-= a1e°1 It2 = Where a, B take real values. The resulting contributions to the received baseband signals are:-11 = h1s1 r2 = h2s2 The receiver for the Alamouti block diversity code is well known, and is denoted by dc-combiner (23) which produces dc-combined signals corresponding to the transmitted pair of symbols, from observed pairs of received signals Fri 121. The de-s2 i combined symbols output from the dc-combiner and submitted to bit decoder (24) are computed by: cj=h1r1+h2 11* = -h1 12* where h, 112 are estimates of the channel response provided by channel estimator (22).

Channel estimation can be performed by conventional techniques such estimating the channel from transmitted reference symbols.

The bit dccodcr (24) estimates the transmitted bits. The de-combincd symbols[ §1 arc is scaled by the expression: (II + which the estimated power of the diversity channel responses k1 112, provided by channel estimator (22), raised to a fixed real constant -p. This produces scaled symbol estimates: k I =(hfl+Ihfl) [s 52] A value of p 0.8 is preferably used, which provides a compromise between estimator noise and bias.

Two operations of the Alamouti decoder on successive symbol pairs and the scaling operation generates four scaled estimates for the transmitted symbols:- [s;, ;, ;, } The transmitted block of bits (16) is estimated using the following expression, in which the scaled estimates (} for the transmitted symbols are dc-spread using the complex conjugate of the set of spreading codes used in the transmitter, and the sign of the real part of the resulting despreading operations is taken as the estimate of the transmitted Mock of bits: S2 1 1 1 1 * 1 1 -1 1 -1 03 1 1 -1 -1 i =Sign Re 1 -1 -1 1 2 b5 I t I -1 -1 1 -L -i -i 54 57 -i -i b3 Here the conjugation operator applied to a matrix denotes element-wise conjugation.

The person skilled in the art will readily adapt the principles of this receiver to cater for other transmitter embodiments according to this invention.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described and the scope of the invention is defined by the appended claims.

For example, the receiver is can use multiple antennas, although the illustrative embodiments have only one receive antenna. The skilled person would readily adapt well known antenna diversity receive techniques, for example switched antenna receive diversity.

In the illustrative embodiments antenna diversity is used as one method of providing channel diversity, however the method is not limited to systems using wireless radio transmission, and a system where signals are coupled onto a transmission media at multiple points, such as capacitiye, inductive, electrical, magnetic, optical or acoustic coupling onto a transmission medium would also embody the use of different channels.

Polarization diversity can be used in place of or in addition to antenna diversity.

The present invention can be applied in mobile telephony networks and other forms of wireless communication systems where transmission may be power limited such as wireless broadband and digital terrestrial broadcast networks. The diversity principles can be applied to wired powerline communication systems with multiple phase coupling.

Thc invcntion may takc thc form of a computcr program containing one or morc sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims sct out below.

Claims (14)

1. Claims 1. A transmission method in which symbols arc transmitted over a plurality of transmit channels, the method comprising the following steps: -applying a Walsh spreading code to each bit in a block of bits (16) to produce a set of spread sequences (17); -summing the spread sequences (17) element by element to create a block of uncoded channel symbols (19); -transforming blocks ofuncodcd channels symbols using a block diversity coder (15) into a block of encoded channel symbols (41-48); and -transmitting the encoded channel symbols.
2. 2. A method according to claim 1, wherein the block diversity coder is an Alamouti diversity encoder.
3. 3. A method according to claim 1 or 2, wherein the spreading code for a given bit corresponds to a row of a Hadamard matrix multiplcd by I or /
4. 4. A method according to any one of the preceding claims, wherein the set of Walsh spreading codes includes N purely real codes of length iv and N purely imaginary codes of length N.
5. 5. A method according to any one of the preceding claims, wherein two different sets ofN bits in a block of2N bits arc spread using the same set of N spreading codes, and the spread sequences corresponding to the two sets of bits are summed to alternate uncoded channel symbols.
6. 6. A method according to any one of the preceding claims, wherein transmission of pairs of encoded channel symbols within the same block of encoded channel symbols (41- 48) are separated by a time gap (54) of at least a plurality of symbol periods.
7. 7. A method according to any one of the preceding claims, wherein transmission of pairs of encoded channel symbols within the same block of encoded channel symbols (41- 48) are separated bya frequency gap (74) of at least apluralityof carriers.
8. 8. A method according to any one of the preceding claims, wherein transmission of one pair of encoded channel symbols within the same block of encoded channel symbols (41-48) are take place on one set of antennas (11,12) and transmission of another pair of encoded channel symbols takes place on a different set of antennas (60,6 1).
9. 9. A method according to any one of the preceding claims, wherein transmission is at less than unity rate, and the number of real and imaginary Walsh codes in the set of Walsh spreading codes uscd to spread thc block of bits is equal or differs by one.
10. 10. A reception method in which symbols from a dc-combiner (23) are scaled by a constant power of the summed diversity channel responses provided by a channel estiniator (22) to produce scaled estimates for the transmitted symbols, and in which the scaled estimates for the transmitted symbols are dc-spread using the complex conjugate of the set of spreading codes used in the transmitter, and the sign of the real part of the resulting de-spreading operations is output as the estimate of the transmitted block of bits.
11. 11. Apparatus for transmitting symbols over a plurality of transmit channels, the apparatus compnsing: a bit spreader for applying a Walsh spreading code to each bit in a block of bits to produce a set of spread sequences; a symbol encoder for summing the set of spread sequences element by element to create a block of uncoded channel symbols; a block diversity coder for transforming blocks of uncoded channel symbols into a block of encoded channel symbols; and a transmitter for transmitting the encoder channel symbols.
12. 12. Apparatus for receiving a signal transmitted over a plurality oftransmit channels, the apparatus comprising: a channel estimator for estimating the channel response of the transmit channels; a decombiner responsive to the channel estimator and the received signals for producing the combined signals; and a bit decoder responsive to the channel estimates and decombined signals for outputting an estimate of the transmitted block by dispreading scaled estimates for the transmitted symbol using the complex conjugate of the set of spreading codes used in the transmitter and taking the sign of the real part of the resulting dispreading operation to output as the estimate.
13. 13. Transmitting according to claim 11 and an apparatus for receiving according to claim 12.
14. 14. A computer program comprising program code means that, when executed by a computer system instructs the computer to perform the steps of: -applying a Walsh spreading code to each bit in a block of bits (16) to produce a set of spread sequences (17); -summing the spread sequences (17) clement by clement to create a block of uncoded channel symbols (19); and -transforming blocks of uncoded channels symbols using a block diversity coder (15) into a block of encoded channel symbols (41-48).