GB2385749A

GB2385749A - Beamforming and space time encoding in communications system.

Info

Publication number: GB2385749A
Application number: GB0204428A
Authority: GB
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2002-02-26
Filing date: 2002-02-26
Publication date: 2003-08-27
Also published as: GB0204428D0

Abstract

A transmitter, 16 and a receiver, 19, where the number of antennas at the receiver, 5, 6, is greater than the plurality of antennas 7,8 at the transmitter. The transmitter uses a space-time encoder and the receiver uses a beamforming network for each antenna and a space time decoder to decode the outputs of the beam forming networks. The system for a mobile communications network thus avoids the need to interleave the data and redundancy symbols, beam forming also increases the quality of the received signal.

Description

COMMUNICATION SYSTEM AND METHOD This invention relates to a communication system and method.

Communication systems, for example mobile phone networks commonly generate redundancy symbols during a data encoding stage which can be used to correct errors in a subsequent received signal. However, the effect of introducing redundancy in this way is to increase the transmission rate of the system with consequent expansion of the bandwidth for a given signal to noise ratio, which is not desired.

Another approach to error correction for communication over fading channels with the antennas separated sufficiently so that the fading processes applying to the different transmission paths between transmitter and receiver is different. means that. the data and redundancy symbols are spread over time, but where a fade wipes out part of the data and redundancy symbols, that part is likely to still be present in another channel, so the error correction can restore the data at the receiver. There is no need to interleave the data and redundancy symbols for transmission.

In high capacity radio systems where frequencies are required to be re-used nearby, the quality of the received decoded signal may be reduced by the effect of cochannel interference.

In accordance with the present invention, a communication system comprises a transmitter and a receiver, each comprising a plurality of antennas; wherein the number of antennas at the receiver is greater than or equal to the number of antennas at the transmitter; wherein the transmitter further comprises a space-time encoder; and wherein the receiver further comprises a beamforming network for each antenna and a space-time decoder to decode the outputs of the beamforming networks.

In accordance with a second aspect of the present invention, a method of communication comprises encoding a data stream and redundancy symbols in a spacetime encoder and transmitting the stream and symbols from a plurality of antennas; receiving signals representing the data stream and redundancy symbols at a plurality of antennas; processing the signals received at each antenna in respective beamforming networks; and decoding the processed signals in a space-time decoder.

The present invention avoids the need to interleave the data and redundancy symbols, or increase bandwidth and transmission rate by using a space-time

encoder/decoder arrangement. It further improves the quality of the received signal by beamforming the signals received at each antenna.

An example of a communication system and method according to the present invention will now be described with reference to the accompanying drawings in which :- Figure 1 illustrates a transmitter for a system according to the present invention; Figure 2 illustrates a receiver for a system according to the present invention; and, Figure 3 illustrates an example of a system according to the present invention, in more detail.

Use of multiple antennas at both the transmitter and the receiver has been shown to provide significant capacity gains in channels where a large number of scatterers are present such that the received signal at the antenna array is spatially decorrelated. One of the transmission methods recently considered where multiple transmit antennas and multiple receive antennas are employed is known as space-time coding. Essentially space-time coding is a form of error correction coding where the redundancy symbols are transmitted via a different spatial path, so the symbol rate of the transmission does not increase. In the example in Fig. 1, a 1/nit rate encoder 1 is shown whose output is multiplexed in multiplexer 2 and modulated 3,4 then input to NT transmit antennas 5, 6.

At the receiver (not shown), NR receive antennas are used, where NR A..

Consequently there are NTN possible transmission paths between the transmitter and receiver. The space-time decoder must take account of this at the receiver. Each individual transmission path can be characterised by its own impulse response and to represent the complete channel a matrix representation is used. If hnT, nR represents the channel impulse response between transmit antenna nT and receive antenna nR then the channel is represented by the following matrix:

With transmitted signals s,,, the received signals are given by the following :

Where (D denotes convolution and r,, s,, n, are row vectors containing samples of the received signal, the transmitted signal and noise respectively.

A maximum likelihood receiver for such a signal structure would seek the information sequence a that minimises the following quantity:

In practice when the space-time code is formed using a convolutional code (or in some special cases a block code) the Viterbi algorithm would be used to effect a more efficient implementation of the detection algorithm.

The space-time coding technique has particular potential for use in high capacity communications systems and in such systems a high degree of spatial re-use of frequencies would be required. Consequently, high levels of co-channel interference might be expected. Although good performance can be obtained from space-time coding, high levels of co-channel interference will cause performance to degrade significantly. One technique to suppress co-channel interference while maintaining the benefits of space-time encoding is beamforming.

This use of beamforming to suppress co-channel interference and retain the advantages of space-time encoding is illustrated by the receiver 19 of Fig. 2. Two

receiver antennas 7,8 receive signals and demodulate these in respective RF demodulators 9,10. The beamforming networks 11,12 for each antenna 7,8 receive the signals from both antennas and beamforming weights are calculated 13,14, then the output of the beamforming networks is input to the space-time decoder 15.

Fig. 3 illustrates an example of a communication system according to the present invention. Multiple signals are effectively generated by the space-time encoding process at the transmitter 16 and multiple receive antennas 7,8 are required to form the metric used in the space-time decoding operation 15, consequently when beamforming is introduced to specifically suppress co-channel interference, multiple beamforming networks 11,12 are required. By using different training sequences 17,18 (normally embedded within the transmission for synchronisation purposes) it is possible to determine a set of weights to maximise the signal from each of the transmit antennas at each of the receive antennas. The outputs of the beamforming networks are then applied to the conventional space-time decoding process. If there are the same number of receive antenna elements 7,8 as transmit antenna elements 5,6 (and assuming a 1/nitrate code), then the antenna array has insufficient degrees of freedom to suppress both the signals from the other antenna elements and any interfering signals, however a performance benefit does result if the beamforming network is trained to maximise the signal to noise plus interference ratio of one particular transmitted signal. If the antenna array has more receive antennas than there are transmit antennas then the beamforming network would have sufficient degrees of freedom to suppress interfering signals and better performance will result.

If w (. ~) are a set of beamforming weights applying to each of the NT sets produces the following signals:

The signals are then applied to the space-time decoding operation, consequently a maximum likelihood decoder would minimise the following quantity with respect to the information sequence â :

Where #n,m is the channel impulse response viewed at the output of the beamforming network between the output of the m-th beamforming network and transmit element n.

The weights of the beamforming network are obtained by minimising the following quantity:

This results in the following system of equations:

ro Where R is used to represent the matrix formed by : . - !.

Claims

CLAIMS 1. A communication system, the system comprising a transmitter and a receiver, each comprising a plurality of antennas; wherein the number of antennas at the receiver is greater than or equal to the number of antennas at the transmitter; wherein the transmitter further comprises a space-time encoder; and wherein the receiver further comprises a beamforming network for each antenna and a space-time decoder to decode the outputs of the beamforming networks.
2. A method of communication, the method comprising encoding a data stream and redundancy symbols in a space-time encoder and transmitting the stream and symbols from a plurality of antennas; receiving signals representing the data stream and redundancy symbols at a plurality of antennas; processing the signals received at each antenna in respective beamforming networks; and decoding the processed signals in a space-time decoder.