Near-optimal low-complexity decoding of space-time codes for fixed wireless applications

Abstract

An improved multi-antenna receiver is realized for detecting signals transmitted by a multi-antenna transmitter by summing signals received at the plurality of receiver antennas after multiplying each by a respective constant. The summed signal is applied to a maximum likelihood detector. The respective constants, λj, where j is an index designating a particular receiver antenna, are determined by evaluating the largest eigenvector of the matrix A, where Λ is a vector containing values λj, and A is a matrix containing elements αij, which is the transfer function between the ith transmitter antenna to the jth receiver antenna. The αij terms are determined in the receiver in conventional ways.

Description

BACKGROUND

This invention relates to wireless systems and, more particularly, to systems having more than one antenna at the receiver and at the transmitter.

Physical constraints as well as narrow bandwidth, co-channel interference, adjacent channel interference, propagation loss and multi-path fading limit the capacity of cellular systems. These are severe impairments, which liken the wireless channel to a narrow pipe that impedes the flow of data. Nevertheless, interest in providing high speed wireless data services is rapidly increasing. Current cellular standards such as IS-136 can only provide data rates up to 9.6 kbps, using 30 kHz narrowband channels. In order to provide wideband services, such as multimedia, video conferencing, simultaneous voice and data, etc., it is desirable to have data rates in the range of 64-144 kbps.

Transmission schemes for multiple antenna systems may be part of a solution to the problem of the currently available low data rates. Such schemes were first proposed in papers by Wittneben, and by Seshadri and Winters, where the problem was addressed in the context of signal processing.

One prior art arrangement having a single transmitter antenna and multiple receiver antennas is shown in FIG. 1. Each of the receiver antennas receives the transmitted signal via a slightly different channel, where each channel i is characterized by transfer function α_i. Using an approach known as “Maximum Ratio Combining,” the prior art approach to detection contemplates multiplying each received signal that had been influenced by α_i, by the complex conjugate signal, α_i*, summed, and then processed.

In U.S. Pat. No. 6,115,427 titled “Method and Apparatus for Data Transmission Using Space-Time Codes and Multiple Transmit Antennas,” filed on May 6, 1997, a coding perspective was adopted to propose space-time coding using multiple transmit and receive antennas. Space-time coding integrates channel coding, modulation, and multiple transmit antennas to achieve higher data rates, while simultaneously providing diversity that combats fading. It may be demonstrated that adding channel coding provides significant gains over the schemes of Wittneben and Seshadri and Winters. In said co-pending application, space-time codes were designed for transmission using 2-4 transmit antennas. These codes perform extremely well in slowly varying fading environments (such as indoor transmission media). The codes have user bandwidth efficiencies of up to 4 bits/sec/Hz which are about 3-4 times the efficiency of current systems. Indeed, it can be shown that the designed codes are optimal in terms of the trade-off between diversity advantage, transmission rate, decoding complexity and constellation size.

It can also be shown that as the number of antennas is increased, the gain increases in a manner that is not unlike a multi-element antenna that is tuned to, say, a particular direction. Unfortunately, however, when maximum likelihood detection is employed at the receiver, the decoding complexity increases when the number of transmit and receive antennas is increased. It would be advantageous to allow a slightly sub-optimal detection approach that substantially reduces the receiver's computation burden.

SUMMARY

Such an approach is achieved with a receiver arrangement where signals received at a plurality of antennas are each multiplied by a respective constant and then summed prior to being applied to a maximum likelihood detector. The respective constants, λ_j, where j is an index designating a particular receiver antenna, are derived from a processor that determines the largest eigenvector of the matrix A, where Λ is a vector containing the values λ_j, and A is a matrix containing elements α_ij, which is the transfer function between the i^th transmitter antenna to the j^th receiver antenna. The α_ij terms are determined in the receiver in conventional ways.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a block diagram of prior Maximal Ratio Combining detection; and

FIG. 2 presents a block diagram of an embodiment including a transmitter having a plurality of antennas, and a receiver having a plurality of antennas coupled to an efficient detection structure.

DETAILED DESCRIPTION

FIG. 2 presents a block diagram of a receiver in accord with an embodiment of the invention. It includes a transmitter 10 that has an n plurality of transmitting antenna 1, 2, 3, 4, and a receiver 20 that has an m plurality of receiver antennas 21, 22, 23, 24. The signals received by the receiver's antennas are multiplied in elements 25, 26, 27, and 28, and summed in adder 30. More specifically, the received signal of antenna j is multiplied by a value, λ_j, and summed. The collection of factors λ_j can be viewed as a vector Λ. The outputs of the receiver antennas are also applied to processor 40 which, employing conventional techniques, determines the transfer functions α_ij for i=1, 2, 3, . . . , n and j=1, 2, 3, . . . , m. These transfer functions can be evaluated, for example, through the use of training sequences that are sent by the different transmitter antennas, one antenna at a time.

The evaluated α_ij signals of processor 40 are applied to processor 45 in FIG. 2 where the multiplier signals λ_j, j=1, 2, 3, . . . , m are computed. Processor 45 also evaluates a set of combined transfer function Values γ_i, i=1, 2, 3, . . . , n (which are described in more detail below). Signals γ_i of processor 45 and the output signal of adder 30 are applied to detector 50 which detects the transmitted symbols in accordance with calculations disclosed below.

It is assumed that the symbols transmitted by the antennas of transmitter 10 have been encoded in blocks of L time frames, and that fading is constant within a frame. A codeword comprises all of the symbols transmitted within a frame, and it corresponds, therefore, toc₁¹c₁²c₁³ . . . c₁⁴c₂¹c₂²c₂³ . . . c₂⁴c₃¹c₃²c₃³ . . . c₃⁴ . . . c_m¹c_m²c_m³ . . . c_m⁴,  (1)where the superscript designates the transmitter's antennas and the subscript designates the time of transmission (or position within a frame).

From the standpoint of a single transmitting antenna, e.g., antenna 1, the signal that is received from antenna 1 in response to a transmitted symbol c_i¹ at time interval t is:

$\begin{array}{cc}\begin{array}{c}{R}_t=c_t^1\left({\alpha }_{11}{\lambda }_1+{\alpha }_{12}{\lambda }_2+{\alpha }_{13}{\lambda }_3+\dots +{\alpha }_{1m}{\lambda }_m\right)\\ =c_t^1\sum _{j=1}^m{\lambda }_j{\alpha }_{1j}\\ =c_t^1{\gamma }_1\end{array}& \left(2\right)\end{array}$ (when noise is ignored). If each λ_j value is set to α*_1j, (where α*_1j is the complex conjugate of α_1j) then the received signal would simply be

$\begin{array}{cc}{R}_t=c_t^1\sum _{j=1}^m{{\alpha }_{1j}}^2& \left(3\right)\end{array}$ yielding a constructive addition.

Of course, the values of λ_j cannot be set to match α*_ij and concurrently to match the values of α_ij where i≠1; and therein lies the difficulty. When all n of the transmitting antennas are considered, then the received signal is

$\begin{array}{cc}{R}_t=\sum _{j=1}^n\left(c_t^i\sum _{j=1}^m{\lambda }_j{\alpha }_{\mathrm{ij}}\right)=\sum _{i=1}^n{c}_t^i{\gamma }_i& \left(4\right)\end{array}$

In accordance with the present disclosure, the objective is to maximize

$\sum _{i=1}^n{{\gamma }_i}^2$ because by doing so, signal R_t contains as much information about c_tⁱ, i=1, 2, 3, . . . n as is possible. However, it can be easily shown that if a matrix A is constructed such that

$\begin{array}{cc}A=\sum _{i-1}^n{\left({\Omega }_i^{*}\right)}^T{\Omega }_i,\mathrm{where}{\Omega }_i=\left({\alpha }_{i1},{\alpha }_{i2},{\alpha }_{i3}{\mathrm{\dots \alpha }}_{\mathrm{im}}\right),\mathrm{then}& \left(5\right)\\ \sum _{i=1}^n{{\gamma }_i}^2=\Lambda {A\left({\Lambda }^{*}\right)}^T.& \left(6\right)\end{array}$

The receiver, thus, has to maximize ΛA(Λ*)^T, subject to the constraint ∥Λ∥²=1. The solution to this problem is to choose Λ to be the eigenvector of A which corresponds to the maximum eigenvalue of A. Accordingly, processor 45 develops the matrix A from the values of α_ij, finds the eigenvalues of A in a conventional manner, selects the maximum eigenvalue of A, and creates the vector Λ. Once Λ is known, processor 45 develops signals γ_i for i=1, 2, 3, . . . , n, where

${\gamma }_i=\sum _{j=1}^m{\lambda }_j{\alpha }_{\mathrm{ij}}$ and applies them to detector 50. Finally, detector 50 minimizes the metric

$\sum _{t=1}^L{R_t-\sum _{i=1}^n{c}_t^i{\gamma }_i}^2$ from amongst all possible codewords in a conventional manner. As can be seen, this approach reduces the complexity of decoding by almost a factor of m.

FIG. 2 depicts separate multipliers to multiply received signals by multiplication factors λ_i, and it depicts separate blocks for elements 30, 40, 45, and 50. It should be understood, however, that different embodiments are also possible. For example, it is quite conventional to incorporate all of the above-mentioned elements in a single special purpose processor, or in a single stored program controlled processor (or a small number of processors). Other modifications and improvements may also be incorporated, without departing from the spirit and scope of the invention, which is defined in the following claims.

Claims

1. A method for operating a receiver with an n plurality of antennas, the method comprising: obtaining n signals at the n plurality of antennas, transmitted from m antennas of a transmitter;developing, by a processor associated with the receiver, a sum signal that corresponds to the addition of the n signals that are each pre-multiplied by a respective factor λj, where j is an index integer specifying that factor λj multiplies the signal received from antenna j of the n plurality of antennas,developing, by the processor, values for transfer functions αij, where i is an index that references the transmitter antennas, and j is an index that references the antennas,wherein the factors λj are developed from the transfer functions αij; anddetecting, by the processor, symbols transmitted by the m transmitter antennas embedded in the sum signal,wherein the factors λj are components of a vector Λ, where the vector Λ is an eigenvalue of ΛA(Λ*)T, and where A is a matrix containing the elements αij.

2. The method of claim 1, wherein detecting, by the processor, symbols transmitted by the m transmitter antennas embedded in the sum signal comprises, comparing the sum signal to a signal corresponding to symbols ci transmitted by transmitter antenna i of said m transmitter antennas multiplied by corresponding factors γi.

3. The method of claim 2, wherein comparing the sum signal to a signal corresponding to symbols ci transmitted by transmitter antenna i of said m transmitter antennas multiplied by corresponding factors γi comprises, comparing the sum signal to a signal corresponding to symbols ci transmitted by transmitter antenna i of said m transmitter antennas multiplied by corresponding factors γi, wherein the corresponding factor γi is related to said factors λj for j=1, 2, 3, . . . , m, and αij.

4. The method of claim 2, wherein detecting, by the processor, symbols transmitted by the m transmitter embedded in the sum signal comprises, minimizing a metric

5. A method for operating a signal processing apparatus for use in a wireless receiver, wherein the wireless receiver forms part of a wireless system having a wireless transmitter employing multiple transmitting antennas, and wherein the wireless receiver includes at least two receiving antennas for receiving signals transmitted from the multiple transmitting antennas, the method comprising: receiving, at an input section of the signal processing apparatus, multiple signals provided by the at least two receiving antennas;developing, at a processing section of the signal processing apparatus, multiplying values from transfer function values that are associated with the multiple transmitting antennas and the at least two receiving antennas;developing, at the processing section of the signal processing apparatus, a matrix from the transfer function values;finding, at the processing section of the signal processing apparatus, eigenvalues of the matrix;determining a maximum eigenvalue of the eigenvalues;creating, at the processing section of the signal processing apparatus, an eigenvector of the matrix which corresponds to the maximum eigenvalue of the matrix;generating, at the processing section of the signal processing apparatus, a subset of the set of all values of the received encoded symbols from the eigenvector;multiplying, at a multiplying section of the signal processing apparatus, the received signals by the multiplying values to produce multiplied received signals; andsumming, at a summing section of the signal processing apparatus, the multiplied received signals.

6. The method of claim 5, wherein receiving, at the input section, multiple signals provided by the at least two receiving antennas comprises, receiving from the multiple transmitting antennas, encoded symbols in blocks of multiple time frames and, wherein a codeword comprises all encoded symbols transmitted within a time frame.

7. A method for operating a system for processing wireless data, the method comprising: receiving, at an m number of receiving antennas associated with a system processor, a wireless signal, wherein the wireless signal represents multiple codewords; andprocessing, at the system processor under a less than optimal computational process, the wireless signal to determine the codewords, wherein a number of computations is reduced by approximately a factor of m, at an increase in less than a factor of m in frame error probability from an optimal computational process, and wherein the optimal computational process computes all codewords.

8. The method of claim 7, wherein the wireless signal received at the m number of receiving antennas is transmitted by multiple transmitting antennas and is encoded under a space-time modulation scheme.

9. The method of claim 7 further comprising computing, at the system processor, eigenvectors based on the m number of receiving antennas.

10. The method of claim 7, wherein the wireless signal received at the m number of receiving antennas is transmitted by multiple transmitting antennas that transmit encoded symbols in blocks of multiple time frames and, wherein a codeword comprises all encoded symbols transmitted within a time frame.

11. The method of claim 1, wherein the values for transfer functions αij are developed based at least in part upon the n signals obtained at the n plurality of antennas.

12. The method of claim 5, wherein the transfer function values are developed based at least in part on the received multiple signals.