GB2478601A

GB2478601A - OFDM channel estimation via antenna-subcarrier assignment and channel correlation matrix

Info

Publication number: GB2478601A
Application number: GB201004163A
Authority: GB
Inventors: Cheran Malsri Vithanage
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2010-03-12
Filing date: 2010-03-12
Publication date: 2011-09-14
Anticipated expiration: 2030-03-12
Also published as: GB201004163D0; GB2478601B

Abstract

At an OFDM receiver 300 of subcarriers assigned to transmitting antennas 201-n (see fig. 2), antenna-to-subcarrier assignment (ASTA) information (I1, I2, ..., In) is fed into a channel estimator 306 which constructs a frequency correlation matrix according to equation 2. This approximates resultant channelRf as zero for uncorrelated channels from different antennas (ie. for a#b) and asRa,f = Ef(ffH) for channels from the same antenna (ie. a= b), where E is the expectation value, H is the Hermitean transpose andfis a channel fading coefficient vector based on the Channel Impulse Responsehand the partial Fourier matrixΘ. These fading coefficients may be estimated according to a minimum mean squared error (MMSE) method as in equation 4. The ASTA information may be transmitted via a separate control channel or via a differentially encoded preamble.

Description

WIRELESS COMMUNICATIONS METHOD AND APPARATUS

This invention relates to a method of estimating channel characteristics of a channel in a wireless orthogonal frequency division multiplexed (OFDM) transmission system using multiple transmit antennas. In particular, this invention relates to a method of estimating transmission characteristics of channels used for subcarriers in an OFDM transmission system.

OFDM is a frequency division multiplexing scheme where a signal is divided into a plurality of data streams and each data stream is carried on its own subcarrier. By dividing the data over multiple subcarriers each subcarrier has a transmission rate which is less than the transmission rate would be for a single carrier. OFDM effectively converts a rapidly-modulated wideband signal into a plurality of narrowband signals.

The subcarriers are transmitted at orthogonal frequencies which are spaced from each other such that cross-talk between the channels is eliminated.

The reduced data rate of each subcarrier has particular advantages where channel conditions are significant. One example of the effect of channel conditions on the transmission is where the transmitted signal suffers from multipath. Multipath 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.

By splitting the signal into multiple subcarrier signals the reduced data rate of each subcarrier reduces potential degradation of the signal quality due to the multipath effect.

To facilitate coherent detection at the receiver, knowledge of the channel characteristics is desirable. To estimate the channel characteristics, a preamble can be transmitted from the transmitter to the receiver. A preamble is predetermined symbols which are known both at the transmitter and the receiver and which are transmitted in the subcarriers of the OFDM transmission. Since the preamble is known both at the transmitter and the receiver then any distortion of the preamble due to channel effects can be determined at the receiver and in this manner the channel characteristics can be estimated.

This invention relates to OFDM transmissions with multiple transmit antennas, as shown schematically in Figure 1. Figure 1 shows an OFDM wireless system 100 comprising an OFDM wireless transmitter 200 and an OFDM wireless receiver 300.

The OFDM wireless transmitter 200 has T transmit antennas, of which the first antenna 201-1 and the 1th antenna 201-n1 are shown. Signals transmitted by the antennas of the transmitter 200 propagate through a transmission channel 400 to the receiver 300.

The transmission system of the present invention is such that at any one time only a single antenna is active for each transmitted subcarrier. This arrangement is shown in Figure 2, where for each of the 16 subcarriers only one transmit antenna is active at any one time. Such systems are attractive due to the diversity benefits they offer and have been considered in, for example, C. M. Vithanage and S. C. J. Parker and M. Sandell, "Antenna Selection with Phase Precoding for High Performance UWB Communication with Legacy WiMedia multi-band OFDM devices", Proc. IEEE International Conference on Communications, pages 3938-3942, 2008 and in H. Zhang and R. U. Nabar, "Transmit antenna selection in MIMO-OFDM systems: bulk versus per-tone selection", Proc. IEEE International Conference on Communications, pages 4371-4375, 2008.

Conventionally, the antenna to select for each subcarrier is selected to maximise a meaningful metric, such as the resultant received signal-to-noise ratio (SNR). As demonstrated in Vithanage et at, one consequence of such transmissions is that the length of the channel impulse response (CIR) as seen by the receiver increases dramatically. Thus, conventional OFDM channel estimation methods, which expect a certain maximum channel length, cannot be utilised to obtain good channel estimations in such a case.

One solution proposed in Vithanage et al is that when the transmitter knows the channels to the receiver exactly, the phases of the transmit signals are precoded such that the resultant CIRs, as seen by the receiver, are of reduced length. For this scheme to work, the transmitter needs to know the channels (in terms of complex coefficients) on each of the subcarriers. Thus it necessitates either the feedback of channel state information (CSI) from the receiver to the transmitter or the use of calibration hardware at the transmitter (when time division duplexing is being utilised), so that the transmitter can glean the channels to the receiver from the reverse channels, by exploiting the reciprocity in wireless channels. Both of these options have drawbacks. The first requires the transmission of analogue control information, which reduces the bandwidth for data transmission and the second increases the hardware costs.

C. M. Vithanage and S. C. J. Parker and M. Sandell, "Antenna Selection with Phase Precoding for High Performance UWB Communication with Legacy WiMedia multi--band OFDM devices", Proc. IEEE International Conference on Communications, pages 3938-3942, 2008 discloses a transceiver design for improving the robustness and mean throughput when communicating with ultra wide band devices. Multiple antennas are used in conjunction with per subcarrier antenna selection based on channel state information at the transmitter.

C. Vithanage and M. Sandell and J. Coon and Y. Wang, "Precoding in OFDM-based multi-antenna ultra-wideband systems", IEEE Communications Magazine, 47(1):41-47, 2009 discloses a system for performing precoding in OFDM-based multi-antenna ultra wide band networks.

M. Sandell and J. P. Coon, "Per-subcarrier antenna selection with power constraints in OFDM systems", IEEE Transactions on Wireless Communications, 8(2):673-677, 2009 discloses a scheme that allocates an equal number of subcarriers to all antennas in a multi-antenna OFDM system, by using integer optimization.

H. Zhang and R. U. Nabar, "Transmit antenna selection in MIMO-OFDM systems: bulk versus per-tone selection", Proc. IEEE International Conference on Communications, pages 4371-4375, 2008 discloses a diversity gain analysis for transmit antenna selection in a multiple input, multiple output OFDM system with linear receivers.

J. J. van de Beek, 0. Edfors, M. Sandell, S. K. Wilson and P. 0. Borjesson, "On channel estimation in OFDM systems", Proc. IEEE Vehicular Technology conference, pages 815-819, 1995 discloses a method of channel estimation based on time-domain channel statistics, using minimum mean squared error (MMSE) and least square (LS) estimators.

C. J. Ahn, S. Takahashi and H. Hirada, "Differential modulated pilot symbol assisted adaptive OFDM for reducing the MLI", IEEE Tencon Conference, 2004 discloses a wireless system where a base station is in control of the modulation level of each subcarrier. The overhead necessary for conveying the modulation level information, which is transmitted as a data symbol, is reduced by using differential modulated pilot symbols.

W. Nat-n and Y. H. Lee, "Preamble-based cell identification for cellu'ar OFDM systems," IEEE Transactions on wireless communications, vol. 7(12), December 2008, discloses preamble-based cell identification (CID) schemes for OFDM systems which include the optimal schemes based on the Bayesian and the maximum likelihood (ML) approaches, a suboptimal scheme that is a simplification of the ML scheme, and a differential decoding-based scheme that does not require any channel information.

The differential decoding-based scheme performs like the suboptimal scheme for most practical channels of interest and outperforms existing schemes, yet it is simpler to implement than the others.

WO-A-2006062308 discloses a cell search device using a preamble for a downlink of a cellular system using an orthogonal frequency division multiplexing access (OFDMA) scheme.

EP-A-1438800 discloses a method for combining pilot symbols and Transmit Parameter Signalling (TPS) channels within an OFDM frame. The method uses Differential Space-Time Block Coding to encode a fast signalling message at an OFDM transmitter. At an OFDM receiver, the encoded fast signalling message can be decoded using differential feedback to recover information about the channel responses that would normally be carried by pilot symbols. In wireless data transmission employing adaptive modulation and coding, an instantaneous channel quality measurement, independent of the origin of interference for example, neibouring-celI interference, white thermal noise, or residual Doppler shift is provided. Using the correlation between a signal which has been symbol de-mapped, and one which has also been soft decoded and re-encoded, a channel quality indicator is produced.

EP-A-1290845 discloses a method for synchronising OFDM symbols during radio transmissions, said method being used to facilitate strong and efficient frame and frequency synchronisation. Additional pilots are added to the OFDM symbols by the transmitter in order to form pilot pairs. A sequence is modulated on said pilot pairs and then extracted by the receiver in order to produce a measure for each OFDM symbol by comparing the extracted sequence and a stored sequence. The OFDM symbol with the largest measure is recognised as being the first OFDM symbol in a frame. The extracted sequence and the stored sequence are compared by means of cross correlation, said cross correlation providing the measure. The integer frequency error can thus also be determined.

US-A-2007/0217552 discloses a system including a differential demodulation module and a correlation module. The differential demodulation module differentially demodulates modulated signals to generate differentially demodulated signals. The correlation modules correlate the differentially demodulated signals with derived preamble sequences and generates correlation values.

The content of each of the above documents are incorporated in their entirety herein by reference.

According to a first aspect of the invention there is provided a method of estimating channel characteristics of a channel in a wireless orthogonal frequency division multiplexed, OFDM, transmission system having a plurality of subcarriers which are transmitted by a plurality of transmit antennas, a, to a receiver, the channel being made up of individual channels from each transmit antenna to the receiver, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the method comprising: determining a channel frequency correlation matrix, R01, of the individual channels from each antenna to the receiver; evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: 1 0;nEIa andmElb witha!=b (Rj)nm 1(Ri) ;n,mEI0 where I, is the subcarrier assignment for antenna a.

The method may further comprise the step of estimating the frequency domain channel fading coefficients based on the channel frequency correlation matrices, such that the mean squared error, MSE, in the estimation is minimised given by: MUSE = RJXH(XRJXH +NOIN)'y where X=>Xa; n7. is the number of transmit antennas; X0 is the an NxN diagonal matrix having the signals transmitted by antenna a along its diagonal; N is the number of subcarriers; H is the Hermitian transpose; N0 is the noise variance; and y is an Nxl vector representing the received signal.

The estimation of the frequency domain channel fading coefficients for a subcarrier n may be based on a subset {n-U,n-U+1,...,n+U-1,n+U} of the N subcarriers, where U is an integer, and is given by: f,MMSE r1X11By where X=>X0; n7. is the number of transmit antennas; X is a (2U+1)x(2U+1) diagonal matrix having the signals transmitted by antenna a on the subset {n -U, n -U + 1,..., n + u-i, n + u} of the subcarriers along its diagonal; B is a (2U +1)x (2u + 1) matrix representing B = XRJX'1 + N0I2+1; R1 is the frequency correlations of the fading coefficients on the set of subcarriers {n-U,n-U+1,...,n+U-1,n+U}; isthe U+1 st row of R1;and y is a (2U+1)xl vector representing the received signal on the set of subcarriers {n-U,n-U+1,...,n+U-1,n+U}.

The antennas may be geographically dispersed and may also be associated with a plurality of transmitters.

The antenna to subscriber assignment may be generated at the receiver and transmitted from the receiver to the or each transmitter or at the or a said transmitter and transmitted from the or the said transmitter to the receiver. The antenna to subscriber assignment may be transmitted via a control channel.

The antenna to subscriber assignment may be transmitted to the receiver by: differentially coding the antenna to subscriber assignment into a preamble; transmitting the preamble from the or the said transmitter to the receiver; receiving the coded preamble at the receiver; and decoding the received differentially coded preamble to undertake a first derivation of the antenna to subscriber assignment.

The differential coding may be performed in the time domain.

The differential coding may be performed by: generating a first preamble; and generating a second preamble, wherein the second preamble is a modified version of the first preamble, the modification being sed on the antenna to subcarrier assignment.

The differential coding may be performed in the frequency domain.

The differential coding may be performed by: assigning the subcarriers into pairs of a first and second subcarrier, each pair of subcarriers being allocated to the same antenna; and generating a preamble based on the pairs of subcarriers, wherein the second subcarrier in each pair is a modification of the first subcarrier in that pair, the modification being based on the allocated antenna.

The method may further comprise undertaking a second derivation of the antenna to subscriber assignment at the receiver based on the ATSA estimate generated by the first derivation.

According to a second aspect of the invention there is provided an orthogonal frequency division multiplexed, OFDM, transmission system, comprising: a plurality of transmit antennas, a, associated with one or more transmitter, for transmitting an OFDM transmission over a channel using a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment; a receiver for receiving the OFDM transmission, the channel being made up of individual channels from each transmit antenna to the receiver, the receiver comprising: means for determining a channel frequency correlation matrix, Raj of the individual channels from each antenna to the receiver; means for evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: I \ 1 0;nEIa andmElb witha!=b f)n,m = ;n,m E where T is the subcarrier assignment for antenna a.

According to a further aspect of the invention there is provided a receiver for receiving an orthogonal frequency division multiplexed, OFDM, transmission transmitted over a channel from a plurality of transmit antennas, a, the channel being made up of individual channels from each transmit antenna to the receiver and the transmission having a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the receiver comprising: means for determining a channel frequency correlation matrix, R0, of the individual channels from each antenna to the receiver; means for evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: , f 0;nElOandmelbwitha!=b -l(,i)n,m;n,m E where I, is the subcarrier assignment for antenna a.

According to a second 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.

This invention * enables near-MMSE optimal estimation of channels after per-subcarrier antenna selection, and * enables implementation without a separate transfer of control information from transmitter to receiver, to aid the above channel estimation In another aspect of the invention, the explicit transmission of the antenna to subcarrier assignment (ATSA) control information is avoided by differentially coding the ATSA information on the preamble sequences.

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 further described by way of example with reference to the accompanying drawings in which: Figure 1 is schematic representation of an orthogonal frequency domain multiplexed (OFDM) wireless system according to an embodiment of the invention; Figure 2 is a representation of a subcarrier allocation in a two antennas system according to an embodiment of the invention; Figure 3 is a block diagram of a transmitter of the system of Figure 1; Figure 4 is a block diagram of a receiver of the system of Figure 1; Figure 5 is a flow chart showing the overall method for estimating frequency domain channel fading coefficients according to an embodiment of the invention; Figure 6 is a flow chart showing a procedure for channel estimation with differentially coded antenna to subcarrier assignment information in accordance with an embodiment of the invention; Figure 7 is a representation of differential coding in time; Figure 8 is a representation of differential coding in frequency; Figure 9 is a flow diagram of a procedure for repeated detection of ATSA information to improve channel estimation; Figure 10 is a block diagram of a transmitter of the system of Figure 1 where the ATSA is differentially coded; Figure 11 is a block diagram of a receiver of the system of Figure 1 where the ATSA is differentially coded; Figure 12 is a graph showing channel estimation results with and without ATSA information; Figure 13 is a graph showing MSE performance with differential coding in the time domain; Figure 14 is a graph showing MSE performance with differential coding in the frequency domain; and Figure 15 is a graph showing channel estimation results with and without ATSA information.

Figure 1 is a schematic representation of an orthogonal frequency domain multiplexed (OFDM) wireless transmission system according to an embodiment of the invention.

The system 100 comprises an OFDM wireless transmitter 200 having T transmit antennas and an OFOM wireless receiver 300 have a receive antenna 309. Wireless communication takes place between the transmitter 200 and the receiver 300 via a channel 400.

Figure 3 is a block diagram of the transmitter 200 of the OFDM system 100 shown in Figure 1 A frame of data symbols, in this case a preamble stream for determining the channel characteristics, are received by an antenna to subcarrier assignment module 202. The antenna to subcarrier assignment module 202 allocates each subcarrier to an antenna such that at any one time only a single antenna is active for each transmitted subcarrier, as shown in Figure 2.

The symbols are passed from the antenna to subcarrier assignment module 202 to the assigned transmitter path and are serial-to-parallel converted in a serial to parallel converter module 203. The inverse fast Fourier transform (IFFT) of the result is taken in a multicarrier modulator 204. The output of the multicarrier modulator 204 is input to a guard interval and parallel to serial converter module 205 where a guard interval is added and the result is parallel-to-serial converted. The resultant baseband signal is converted to an analogue signal in a digita' to analogue converter (DAC) 206 and thereafter upconverted to the transmission frequency in an RF upconversion and transmission module 207 and then transmitted via the respective n transmit antennas 201.

The receiver 300 shown in Figure 4 comprises an antenna 309 for receiving wireless transmissions from the transmitter 200. The signal detected by the antenna 309 is fed into an RF downconversion module 301 which downconverts the signal to baseband.

The baseband signal is passed to an analogue to digital converter 302 and converted to a digital signal. The digital signal is fed into a guard interval removal and parallel to serial converter module 303 where the guard interval, which was inserted at transmission, is removed and the resulting signal is serial-to-paraHel converted. The output is passed to a multicarrier demodulator 304 where it is fast Fourier transformed and the output is passed to parallel-to-serial converter 305 where it is parallel-to-serial converted. The output of the parallel to serial converter 305 comprises a channel corrupted version of the transmitted frame of data and preamble symbols.

The output of the parallel to serial converter 305 is passed to a channel estimator 306, where the antenna to subscriber assignment information is used to estimate the channel 400 between the baseband of the transmitter 200 and the baseband of the receiver 300, as is explained below. The ATSA could have been fed forward by the transmitter 200 as described above and as illustrated by the thick line at the top of Figure 4. Transmitting the ATSA information from the transmitter 200 to the receiver 300 can waste bandwidth resources which could otherwise be used for data transmission. Such overheads can especially be a drain on resources when the underlying channels change often, causing different antenna-to-subcarrier assignments. Alternatively, when the ATSA information is embedded in the preamble as described below, the receiver 300 itself can calculate the ATSA information by running the algorithm described below.

The thick lines in Figures 3 and 4 denote transfer of control parameters between the two communicating devices 200, 300. In practice, these control signals themselves need to be multiplexed with the information bearing signals and transmitted and received via the antennas 210, 309 of the respective transmitter 200 and receiver 300.

The skilled person will understand that the transmitter 200 and receiver 300 may also respectively comprise receiver and transmitter portions, i.e., both of the transmitter 200 and receiver 300 shown may form part of two transceivers. The skilled person will also understand that on the transmitter side of the system of Figure 3 there may be a plurality of transmitters 200.

Suppose orthogonal frequency division multiplexed (OFDM) modulations are utilised with N subcarriers and suppose that on each subcarrier only a single transmit antenna is allowed to transmit, i.e., per-subcarrier transmit antenna selection is employed.

Such systems are described in the two Vithanage et at referenced documents and in Zhang etal.

Now suppose that the channel impulse response (CIR) from antenna a E {i,..., n,. } to h-h h h the receiver is a \ a,1' a,2 a.LJ, where L is the largest spread in the individual CIRs. For simplicity, the channels from different antennas are assumed to be uncorrelated, although the following can be generalised to account for antenna correlations.

Taking the partial Fourier matrix 0 to be the NxL matrix with the (ri,I)th element (for -.21r(n-IXI-1) nrl,2,...,N; I=1,2,...,L) being e N, the frequency domain channel fading coefficients from antenna a are given by a = Oha. Suppose the nth element of a ia,), which denotes the frequency domain channel fading coefficient in the nth subcarrier from ath antenna.

In a typical system employing per-subcarrier antenna selection as described above, for the nth subcarrier, the transmit antenna is selected to maximise the resultant received signal power, f2 Other practical constraints might lead to a different antenna to subcarrier assignment, for example as described in Sandell et al. Suppose a set of subcarriers, I {i,,...,w} is selected for each antenna a and that all the subcarriers are utilised, i.e., (J'a = {1,2,...,N} the union of all sets of subcarriers for each antenna is the set of subcarriers. Since only one antenna is allowed to transmit on each subcarrier, then a' =0 for a!=b, i.e., for a pair of different antennas, the set of subcarriers for each antenna do not intersect.

Now suppose X8 is an N N diagonal matrix representing the transmit signals from antenna a on its diagonal. Specifically, the (n,n)th element denotes the signal transmitted on subcarrier n (forn=1,2,...,N), which is zero if a l.e, if there is a transmission on a subcarrier n for a particular antenna then the diagonal value on column n of row n of the matrix Xe is the transmitted signal and if there is no transmission on subcarrier n for a particular antenna then the diagonal value on column n of row n of the matrix Xe is zero.

Due to the properties of OFDM transmissions, which allow for a convenient inter-symbol interference-free frequency domain signal analysis, the received signals on the set of subcarriers can be expressed by an N vector y: y=X0f+w.

where w represents the complex Gaussian additive noise realisation, which is modelled to have a zero mean and covariance matrix NOIN, where N is the N x N identity matrix. It is assumed that each non-zero element of X9 is of unit magnitude and we take the system signal-to-noise ratio (SNR) as._L. The punctured nature of Xe (i.e., that a signal is only transmitted on some of the subcarriers) enables the above signal model to be re-expressed as yXf+w (1) where X=Xa and nth element of the frequency domain channel fading coefficient vector f is ia,, when r E 1a i.e., when there is a transmission on a subcarrier n for that antenna.

The frequency correlations within the resultant frequency domain channel fading coefficient vector f are now considered First, suppose the individual CIRs, ha have time domain correlation matrices: R0, _Eh(haha) where Eh denotes expectation value and H is the Hermitian or conjugate transpose.

Thus, the individual frequency channel correlations are given by the matrices R01 = and substituting a Oha into this equation gives: R1 = ®Ra1OH Suppose Rf =i(ff"), which constitutes the correlations of interest. Consider its (n,m)th element, En1.m). If a!=b, this is simply zero due to the assumption that the channels from different antennas are uncorrelated. If a=b, then computation of *a,n,rn) is not a trivial task. In fact, this is currently not known in closed form to the best of the inventor's knowledge. This invention proposed the use of the (n,m)th element of Ra,f to approximate the unknown quantity. Thus the required frequency correlation matrix can be approximated as: 1 0;nEI andmElb witha!=b (R) =!i a (2) I n.m;n,m E The point to note here is that when the receiver knows the frequency correlations of the channels from each transmit antenna (i.e., R01 for a=i,2 flT) then given the antenna4o-subcarrier assignment (ATSA) produced by a per-subcarrier antenna selection (i.e. II,12,...,Ifl) it is able to derive the frequency correlations within the resultant channel (i.e. R1) using the described approximation.

In order to perform the channel estimation, a preamble is transmitted from each antenna to the receiver. That is the whole transmit sequence contained in x, which is termed a preamble, is known to the receiver. The received signals due to the transmission of such preambles are commonly utilised by receivers to obtain channel estimates. Such estimates could be that of the channel impulse responses or the frequency domain fading coefficient.

When per-subcarrier transmit antenna selection is employed with the antenna to subcarrier assignment (ATSA) given by the sets a for a =1,2 from the received signal model of equation (1), when the receiver does not have the ATSA information, it can only perform a simple least squares (LS) estimation of the channel to obtain: f=XHy (3) This is termed the per-tone channel estimation in Vithanage, Parker and Sandell.

When the receiver has the ATSA information a different estimation technique can be performed. The receiver may have the ATSA information if, for example, it was generated at the receiver and transmitted on to the transmitter in the first place, or if the ATSA information was passed from the transmitter to the receiver via a control channel.

When the receiver has the ATSA information then the receiver is able to compute the frequency correlations of the channels due to per-subcarrier antenna selection, as described in the previous section. Thus it can go beyond a simple least squares channel estimation and obtain a minimum mean squared error (MMSE) estimation as demonstrated at page 391 of S. M. Kay, Fundamentals of statistical signal processing: Estimation theory", Prentic Hall, 1993, the contents of which are incorporated herein by reference: MMSE =RX'(XRX' +NOIN)1y (4) Due to the use of the approximate correlations matrix, the above estimator is in turn an approximate MMSE estimator. However, numerical investigations have revealed that its suboptimality is very small. Therefore, the above will be termed the MMSE estimator below.

Figure 5 shows the steps involved in performing an MMSE optimal channel estimation in systems with per-subcarrier antenna selection, according to an embodiment of the invention where the ATSA information is transmitted to the receiver.

The skilled person willI understand that the channel estimation procedure mentioned above can be applied to co-operative communications systems where the individual transmitters are placed in different physical locations, but co-operate to transmit information in a manner such that only a single transmitter is occupying a given subcarrier.

If we denote the n th row of R1 as rf,,,, then the estimation of the fading coefficient on the n th subcarrier represented by the equation (4) can be written as: J'PI,MMSE r1X'1 (XRIX'+ N014' Y (5) One thing to note from equation (5) is that since the vector y is of length N, then the matrix B is of dimensions N x N. For a large number of subcarriers, N, B becomes a large matrix and the computation of its inverse can be burdensome.

To reduce the computational complexity of evaluating fMMSE' the following approach can be used. When estimating the fading on a subcarrier n, a reduced set of subcarriers around the position n of the received signals is considered. For example, the received signals on subcarriers {n-U,n--U+1,...,n+U--1,n+U} can be considered, where U is an ineteger such that the number of subcarriers in the reduced set is (2U+1). When such a reduced set is considered, since the vector y is of much shorter length than the number of subcarriers N, the corresponding matrix B can be inverted with much less of a computational burden.

As stated above, in order to perform the MMSE estimation the receiver requires knowledge of the ATSA. The ATSA information can be fed forward by the transmitter or, when the ATSA information is embedded in the preamble as described below, the receiver 300 itself can calculate the ATSA information by running the following algorithm, which gives a procedure for avoiding the separate transfer of ATSA information to the receiver.

The elements of the transmit preamble x are taken to be of unit amplitude. From S. M. Kay referenced above, it can be seen that the mean squared error (MSE) performances of least squares (LS) or minimum mean squared error (MMSE) estimations are indifferent to the phases of the elements of the preamble sequence, as long as they are perfectly known at the receiver. Thus, in a sense these phases are free parameters which can be utilised to transfer some control information (such as ATSA information), without adversely affecting the channel estimation at the receiver.

This is the principle behind this part of the invention.

According to an embodiment of the invention, ATSA information is coded inside the preambles. Since at the point of preamble reception, the receiver 300 does not have any estimate of the channel, then a coherent detection of any embedded information is not possible. Therefore according to the invention the ATSA information is differentially coded inside the preamble sequences, using the technique described in Ahn et al referred to above.

Two methods of differential coding are described below and the overall process suggested by the invention is summarised in Figure 6. In step S610, a preamble sequence is received through the channel 400 and in step S620 the receiver 200 first recovers the ATSA information via a differential detection. In step S630 the ATSA information is used to derive the expected channel frequency correlation matrix from equation (2) above. This enables the receiver 300 to perform an MMSE type estimation in step S640.

In one embodiment of the invention, the differential coding is performed in the time domain. After assigning the subcarriers to the appropriate antennas, two preamble sequences are generated to aid the channel estimation at the receiver. Suppose they are X1 =diag(x1) and X2 =diag(x2). Here, x1 and x2 are length N vectors and diag(x) denotes a matrix with x as the diagonal. The ATSA information can be differentially coded within these sequences. Suppose the vector z(z1,z2,...,zw)T represents the ATSA information in that z, = a if the n th subcarrier is active on antenna a. Consider an alphabet S_-{s1,s2,...,s} given, for example, by Sa We select the first preamble, x1 to be a sequence of the same symbol, say s1. The second preamble x2 is selected such that its nth elements is s1s0 if the ath antenna is signalling on the nth subcarrier. This scheme is shown in Figure 7 for a 16 subcarrier system with 3 transmit antennas.

Now suppose the channels remain the same during the transmission of the two preambles (this condition is satisfied in many channels due to the underlying time coherence). Even if the channels do change between the two preambles, the change is likely to be small and hence the same procedure described below applies. Then the received signals can be expressed as: y1 =X1f+w1 (6) y2=X2f+w2. (7) Let y and Y2,n denote the received signals at the nth subcarrier and also let w1,, and w2 be the corresponding noise realisations, corresponding to the two preambles.

Consider the ratio: S1S/, +W2fl (8) s1f+w1 In the absence of noise (i.e. when w = w2,, = 0), r is equal to Sa, which represents the antenna being utilised at the nth subcarrier. Thus, as long as the operating SNR is sufficiently large and hence the noise term is insignificant and the channel fading in the two preambles are close to each other (i.e., f,, does not change significantly when going from one preamble to the other -this is to be expected since most practical channels do not change a lot from one OFDM symbol to the other), this procedure enables the receiver to identify the transmit antennas being utilised for each subcarrier.

With noisy received signals (due to the additive noise components or due to slight changes in the channel fading coefficients), one would need to map the ratio r, to the nearest symbol in the alphabet s{s1,s2,...,s} to detect the actual underlying symbol. This is similar to the conventional detection of phase-shift keyed symbols.

After such an identification stage, the receiver can build up the transmitted preambles X and X2 knowing how the transmitter would map ATSA to preambles. Furthermore it can also derive the expected frequency correlation matrix Rf using the ATSA information from equation (2). This information then enables the receiver to perform the MMSE channel estimation described above, based on the detected ATSA information.

In another embodiment of the invention, differential coding is performed in the frequency domain, i.e. across the subcarriers. The principle behind differential coding is to allow the receiver 300 to detect the transmitted information without an explicit channel estimation. This is possible when the channel fading coefficients remain approximately the same in the dimension along which the differential coding is to be performed. In a conventional single antenna transmission scheme without any per-subcarrier selection process, the channel fading in consecutive subcarriers are similar due to the frequency coherence of the channels. In other words, due to inherent smoothness of practical wireless channels, the fading cannot change arbitrarily when going from one subcarrier to another neighbouring subcarrier. However, when a per-subcarrier antenna selection is being performed, this coherence is distorted whenever a switch occurs from one transmit antenna to another, when considered across the subcarriers, as discussed in Vithanage, Sartdell, Coon and Wang referenced above.

Differential coding across the subcarriers in the frequency domain is applied as follows.

The switching of the transmit antenna is only allowed on the odd subcarrier indices (e.g. on the 1st, 3 rd, 5th, and so on), as shown in Figure 8, where the subcarriers are arranged into pairs. This ensures that the channel coherence between two subcarriers is preserved. The differential coding is then applied across such sets of two subcarriers as illustrated in Figure 8. Firstly, the same symbol, say s is set to be transmitted on subcarriers with odd indices. Suppose n is an odd subcarrier index and also suppose that the ath transmit antenna is signalling in the subcarriers with indices n and (n÷1) and the signal S1Sa is transmitted on the (ri-i-1)st subcarrier. Now, take w,,, y and f,, to represent the additive noise realisation, received signal and, the frequency domain channel fading coefficient on subcarrier i-i, respectively. This gives: (9) y,,+ =s1s0f,,÷1 +w,,1. (10) Consider the ratio: r = y,, = s1s0f1 + w,,.1 " y,, s1f+w In the absence of noise (i.e. when w,, = w1 = 0), and when the channel fades on subcarriers n and (n+i) are equal, r., is equal to Sa, which represents the antenna being utilised at the nth and (n+1)st subcarriers. Thus, as long as the operating SNR is sufficiently large and the channel fades are close to each other on the two subcarriers considered for the differential coding, this procedure enables the receiver to identify the transmit antennas being utilised at each subcarrier. Note that with noisy received signals (due to the additive noise components or due to slight changes in the channel fading coefficients), one would need to map the ratio r, to the nearest symbol in the alphabet S = {s1,s2,...,s} to detect the actual underlying symbol. This is similar to the conventional detection of phase-shift keyed symbols. After such an identification stage, the receiver can build up the transmitted preamble x, knowing how the transmitter would map ATSA to preambles. Furthermore it can also derive the expected frequency correlation matrix Rf using the ATSA information from equation (2). This information enables the receiver to perform the MMSE channel estimation described above, based on the detected ATSA information.

According to the procedure described above, after the differential detection of the antenna-to-subcarrier assignment information, the ATSA information is used to obtain an explicit channel estimation on each of the subcarriers. One can improve upon the ATSA detection using these channel estimates, as shown in Figure 9. After the steps S610 to S640 of Figure 6 have been undertaken, coherent detection of the ATSA is performed in step S650, based on the channel estimates already generated. In step S660, the frequency correlation matrix is built up based on the first derivation of the ATSA information. In step S670 a second derivation of channel estimates is performed based on the revised frequency correlation matrix, to produce an improved channel estimate.

Figure 10 is a block diagram of a transmitter 200a where the ATSA is differentially coded, according to an embodiment of the invention. The transmitter 200a has the same structure as the transmitter 200 shown in Figure 3, with the addition of a block 208 to differentially code the ATSA on the preamble sequences.

Figure 11 is a block diagram of a receiver 300a where the ATSA is differentially decoded, according to an embodiment of the invention. The receiver 300a has the same structure as the receiver 300 shown if Figure 3, with the addition of a block 307 to differentially decode the ATSA from the preamble sequences.

Conventional systems with per-subcarrier transmit antenna selection need to contend with per-tone channel estimation or try to obtain channel estimates at the transmitter to enable phase precoding to reduce the channel lengths perceived by the receiver.

The proposed methods perform significantly better than per-tone channel estimation due to the exploitation of antenna-to-subcarrier assignment information. Furthermore these control information need not be explicitly transmitted, thus does not increase the control information overhead of the system.

The new methods of channel estimation are demonstrated by computer simulations. A system with N=128, L=4 and nT =2 is used. The per-subcarrier transmit antenna selection is based on the magnitudes of the frequency domain channel coefficients as described above. Different channel estimates are compared based on the mean squared error (MSE) they result in.

Figure 12 shows the MSE performances achievable with and without ATSA information. The MMSE channel estimator embodying the invention significantly outperforms the conventional per-tone estimator (i.e., the LS estimator). Note that it is assumed that the receiver knows the channel correlations from the individual transmit antennas perfectly. In practice, such information can be estimated well since the channel statistics usually change much slower compared to the rate at which data is transferred. This knowledge is used with the ATSA information to build up the correlations of the channels after per-subcarrier antenna selection.

The transmission of ATSA information by differential coding in the time domain is shown in the graph of Figure 13. As can be seen from the graph, when the SNR is high, the ATSA estimation scheme with differential coding performs identically to a scheme where the ATSA is separately conveyed to the receiver. Furthermore, suprisingly even at low SNRs, the proposed procedure is able to significantly improve the channel estimation performance over a simple LS estimation. Similar observations can be made from Figure 14, which shows the effect of differential coding in the frequency domain, i.e., across subcarriers.

Figure 15 illustrates the performance of the method of Figure 9 of refining the ATSA detection from the channel estimates. For this particular example, the resultant improvement was small and only manifested in low SNRs of around 0 to 10dB.

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

Claims

CLAIMS: 1. A method of estimating channel characteristics of a channel in a wireless orthogonal frequency division multiplexed, OFDM, transmission system having a plurality of subcarriers which are transmitted by a plurality of transmit antennas, a, to a receiver, the channel being made up of individual channels from each transmit antenna to the receiver, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the method comprising: determining a channel frequency correlation matrix, R01, of the individual channels from each antenna to the receiver; evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: r o;nEIQandmEl:b witha!=b R) =a \ I n,m;n,m E where I, is the subcarrier assignment for antenna a.
2. A method as claimed in claim 1, further comprising the step of estimating the frequency domain channel fading coefficients based on the channei frequency correlation matrices, such that the mean squared error, MSE, in the estimation is minimised given by: MMSE =R1X'(XR1X" +NQIN)y where X_>2Xa; n is the number of transmit antennas; X0 is the an NxN diagonal matrix having the signals transmitted by antenna a along its diagonal; N is the number of subcarriers; H is the Hermitian transpose; N0 is the noise variance; and y is an Nxl vector representing the received signal.
3. A method as claimed in claim 2, wherein the estimation of the frequency domain channel fading coefficients for a subcarrier n is based on a subset {n-U,n-U+1,...,n+U--1,n+U} of the N suboarriers, where U is an integer, and is given by: = where X=>X0; n2. is the number of transmit antennas; X0 is a (2U-i-1)x(2U+1) diagonal matrix having the signals transmitted by antenna a on the subset {n-U,n-U+1,...,n+U-1,n+U} of the subcarriers along its diagonal; B is a (2U +1)x(2U +1) matrix representing B = XRJXH + N0I2+1; R is the frequency correlations of the fading coefficients on the set of subcarriers {n-U,n-U+1,...,n+U--1,n+u}; r1 is the U+1 st row of R1; and y is a (2U+1)xl vector representing the received signal on the set of subcarriers {n-U,n-U+1....,n-i-U-1,n.-i_U}.
4. A method as claimed in claim 1, 2 or 3, wherein the antennas are geographically dispersed.
5. A method as claimed in claim 4, wherein the geographically dispersed antennas are associated with a plurality of transmitters.
6. A method as claimed in any one of the preceding claims, wherein the antenna to subscriber assignment is generated at the receiver and transmitted from the receiver to the or each transmitter.
7. A method as claimed in any one of claims 1 to 6, wherein the antenna to subscriber assignment is generated at the or a said transmitter and transmitted from the or the said transmitter to the receiver.
8. A method as claimed in claim 4 or 5, wherein the antenna to subscriber assignment is transmitted via a control channel.
9. A method as claimed in claim 7, wherein the antenna to subscriber assignment is transmitted to the receiver by: differentially coding the antenna to subscriber assignment into a preamble; transmitting the preamble from the or the said transmitter to the receiver; receiving the coded preamble at the receiver; and decoding the received differentially coded preamble to undertake a first derivation of the antenna to subscriber assignment.
10. A method as claimed in claim 9, wherein the differential coding is performed in the time domain.
11. A method as claimed in claim 10, wherein the differential coding is performed by: generating a first preamble; and generating a second preamble, wherein the second preamble is a modified version of the first preamble, the modification being based on the antenna to subcarrier assignment.
12. A method as claimed in claim 9, wherein the differential coding is performed in the frequency domain.
13. A method as claimed in claim 12, wherein the differential coding is performed by: assigning the subcarriers into pairs of a first and second subcarrier, each pair of subcarriers being allocated to the same antenna; and generating a preamble based on the pairs of subcarriers, wherein the second subcarrier in each pair is a modification of the first subcarrier in that pair, the modification being based on the allocated antenna.
14. A method as claimed in any one of claims 9 to 13, further comprising undertaking a second derivation of the antenna to subscriber assignment at the receiver based on the ATSA estimate generated by the first derivation.
15. An orthogonal frequency division multiplexed, OFDM, transmission system, comprising: a plurality of transmit antennas, a, associated with one or more transmitter, for transmitting an OFDM transmission over a channel using a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment; a receiver for receiving the OFDM transmission, the channel being made up of individual channels from each transmit antenna to the receiver, the receiver comprising: means for determining a channel frequency correlation matrix, R01, of the individual channels from each antenna to the receiver; means for evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: 1 0;nEI0andmEIwirha!=b (Rj)nm l(Raj)nm;n,m E where a is the subcarrier assignment for antenna a.
16. A receiver for receiving an orthogonal frequency division multiplexed, OFDM, transmission transmitted over a channel from a plurality of transmit antennas, a, the channel being made up of individual channels from each transmit antenna to the receiver and the transmission having a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the receiver comprising: means for determining a channel frequency correlation matrix, R01, of the individual channels from each antenna to the receiver; means for evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: \ [ 0;nEl0andmElb witha!=b R) i \.n,m Raj)nm;n,m E I where I is the subcarrier assignment for antenna a.
17. A carrier medium carrying computer readable code for controlling a microprocessor to carry out the method of any one of claims 1 to 14.Amendments to the claims have been filed as follows CLAIMS: 1. A method of estimating channel characteristics of a channel in a wireless orthogonal frequency division multiplexed, OFDM, transmission system having a plurality of subcarriers which are transmitted by a plurality of transmit antennas, a, to a receiver, the channel being made up of individual channels from each transmit antenna to the receiver, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the method comprising: determining a channel frequency correlation matrix, Raj of the individual channels from each antenna to the receiver, where Raj = a fra1), a 0ha ha is the channel impulse response from antenna a and 0 is the partial Fourier matrix; evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: ( \ 1 0;nelaandmElbwjtha!=b Ri)n,m 1(Raj) ;n,mE a where I is the subcarrier assignment for antenna a.2. A method as claimed in claim 1, further comprising the step of estimating the frequency domain channel fading coefficients based on the channel frequency * * correlation matrices, such that the mean squared error, MSE, in the estimation is *:*::* minimised given by: MMSE = RJXH(XRJXn +NQIN)'y *q*. nr where X=Xa * a=I n. is the number of transmit antennas; Xa is the an NxN diagonal matrix having the signals transmitted by antenna a along its diagonal; N is the number of subcarriers; H is the Hermitian transpose; N0 is the noise variance; and y is an Nxl vector representing the received signal.3. A method as claimed in claim 2, wherein the estimation of the frequency domain channel fading coefficients for a subcarrier n is based on a subset {n-U,n-U+1,...,n+U-1,n+U} of the N subcarriers, where U is an integer, and is given by: ffl,MMSE = r1XBy where X=>X; n.j. is the number of transmit antennas; Xa is a (2U+1)x(2U+1) diagonal matrix having the signals transmitted by antenna a on the subset {n-U,n -U -i--1,...,n + U -i,n + u} of the subcarriers along its diagonal; B is a (2u +1)x(2U +1) matrix representing B XRJX11 +N0I2+1; R is the frequency correlations of the fading coefficients on the set of subcarriers {n-U,n-U--i-1 n+U-1,n+U}; ** r1 is the U +1 St row of R; and y is a (2U+1)xl vector representing the received signal on the set of subcarriers {n-Un---U+1,...,n-i-U-1,n-+.U}.4. A method as claimed in claim 1, 2 or 3, wherein the antennas are geographically dispersed.* 5. A method as claimed in claim 4, wherein the geographically dispersed antennas are associated with a plurality of transmitters.6. A method as claimed in any one of the preceding claims, wherein the antenna to subscriber assignment is generated at the receiver and transmitted from the receiver to the or each transmitter.7. A method as claimed in any one of claims 1 to 6, wherein the antenna to subscriber assignment is generated at the or a said transmitter and transmitted from the or the said transmitter to the receiver.8. A method as claimed in claim 4 or 5, wherein the antenna to subscriber assignment is transmitted via a control channel.9. A method as claimed in claim 7, wherein the antenna to subscriber assignment is transmitted to the receiver by: differentially coding the antenna to subscriber assignment into a preamble; transmitting the preamble from the or the said transmitter to the receiver; receiving the coded preamble at the receiver; and decoding the received differentially coded preamble to undertake a first derivation of the antenna to subscriber assignment.10. A method as claimed in claim 9, wherein the differential coding is performed in the time domain.11. A method as claimed in claim 10, wherein the differential coding is performed by: generating a first preamble; and s... generating a second preamble, wherein the second preamble is a modified * version of the first preamble, the modification being based on the antenna to subcarrier :5p S..* S assignment. * I * 5** 12. A method as claimed in claim 9, wherein the differential coding is performed in the frequency domain.I* 13. A method as claimed in claim 12, wherein the differential coding is performed I..* 30 by: assigning the subcarriers into pairs of a first and second subcarrier, each pair of subcarriers being allocated to the same antenna; and generating a preamble based on the pairs of subcarriers, wherein the second subcarrier in each pair is a modification of the first subcarrier in that pair, the modification being based on the allocated antenna.14. A method as claimed in any one of claims 9 to 13, further comprising undertaking a second derivation of the antenna to subscriber assignment at the receiver based on the ATSA estimate generated by the first derivation.15. An orthogonal frequency division multiplexed, OFDM, transmission system, comprising: a plurality of transmit antennas, a, associated with one or more transmitter, for transmitting an OFDM transmission over a channel using a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment; a receiver for receiving the OFDM transmission, the channel being made up of individual channels from each transmit antenna to the receiver, the receiver comprising: means for determining a channel frequency correlation matrix, Raj of the individual channels from each antenna to the receiver, where = rafrafah2) = ha is the channel impulse response from antenna a and 0 is the partial Fourier matrix; means for evaluating, based on the antenna to subcarrier assignment and the *. 20 channel frequency correlation matrices, the correlation characteristics of the resultant *I..channel based on the channel frequency correlation matrix: * a 1 0;nElaandnzElbwitha!=b (Rj)n,m 1j)nm;n,rnE I where a is the subcarrier assignment for antenna a.16. A receiver for receiving an orthogonal frequency division multiplexed, OFDM, *S*...* transmission transmitted over a channel from a plurality of transmit antennas, a, the channel being made up of individual channels from each transmit antenna to the receiver and the transmission having a plurality of subcarriers, each subcarrier being transmitted by a single transmit antenna at any one time in accordance with an antenna to subcarrier assignment, the receiver comprising: means for determining a channel frequency correlation matrix, Raj of the individual channels from each antenna to the receiver, where R0 = Oh1 ha is the channel impulse response from antenna a and 0 is the partial Fourier matrix; means for evaluating, based on the antenna to subcarrier assignment and the channel frequency correlation matrices, the correlation characteristics of the resultant channel based on the channel frequency correlation matrix: 1 0;nE I and m Tb with a!= b R1 n,m 1(Rai) ;n,rn Ta where I is the subcarrier assignment for antenna a.17. A carrier medium carrying computer readable code for controlling a microprocessor to carry out the method of any one of claims 1 to 14. S... * S *.*S* S. *.S * S * S. * . * * S. *S* * S *S..SS..... * .