GB2336506A

GB2336506A - Method of Reducing Co-channel Interference

Info

Publication number: GB2336506A
Application number: GB9807681A
Authority: GB
Inventors: Rorie Julian Turnbull O'neill; Rene Pihl Jepsen; Reza Karimi; Behzad Mohebbi
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1998-04-14
Filing date: 1998-04-14
Publication date: 1999-10-20
Anticipated expiration: 2018-04-14
Also published as: DE19915584A1; GB9807681D0; FR2777402B1; GB2336506B; DE19915584B4; FR2777402A1

Abstract

A method for reducing co-channel, interference in a radio communication system with very low dispersion by introducing artificial dispersion allowing a receiver to differentiate between a wanted signal and an interfering signal. Specifically, the invention is applicable to cellular mobile communication systems such as GSM.

Description

2336506 METHOD AND SYSTEM FOR INTERFERENCE REDUCTION

Field of the Invention

The present invention relates to a method of facilitating interference reduction in a radio communications system, in particular for co-channel interference in a cellular mobile communication system.

Background of the Invention

Interference reduction and multi-user detection methods have received considerable attention in research for Code Division Multiple Access fflDMA) based cellular applications. For Time Division Multiple Access (TDAIA) systems, such as the Global System for Mobile communication (GSM), relatively little research has been carried out. This is in part because the problem is more difficult in that signal separation is possible, for example using a joint Maximum Likelihood Sequence Estimation (MLSE) equaliser in the receiver. Such a joint detection receiver uses the fact that co-channel interference is deterministic in nature once the impulse response of the channel for each interferer (and the wanted signal) are known.

However, even with such information, effective separation requires the impulse responses for each of the signals to be significantly different. The degree of cross-correlation sets the fundamental performance of the receiver.

For some propagation conditions, such as for example in a small cell indoor environment, the channel will not necessarily introduce significant variation in the propagation transfer function of the wanted signal and interferences. Differentiation will therefore not be possible.

Summary of the Present Invention

The present invention seeks provide interference reduction in a radio communication system, in particular for co-channel signals in a cellular mobile communication system.

2 According to an aspect of the present invention, there is provided a method of interference reduction in a radio communications system including the steps of. introducing into a first signal an artificial dispersion thereby generating a first artificially dispersed signal; and differentiating between the first signal and a second signal in response to the artificial dispersion.

Preferably the artificial dispersion is introduced into each co-channel signal. The artificial dispersion may specifically be a spectral signature or a time delay between a first and second version of the signal. The radio io communication system is can typically be a GSM system or similar TDMA system.

According to a different aspect of the invention, there is provided a system for interference reduction in a radio communications system comprising: means for introducing into a first signal an artificial dispersion thereby generating a first artificially dispersed signal; and means for differentiating between the first signal and a second signal in response to the artificial dispersion.

A Description of the Drawing

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawing, in which the sole Figure is a schematic diagram of an embodiment of co-channel interference suppression system.

Description of the Preferred Embodiment

The preferred embodiment of the invention is in cellular mobile communication system such as GSM although the invention is by no means limited to such a system. The description furthermore considers Time Division Multiple Access to be used.

In most cellular systems the propagation from a transmitter to a receiver varies with time and geographical location. Typically many different signal components are received from the transmitted signal due to many different rays of the signal reaching the receiver. This gives rise to dispersion. For very low levels of dispersion where all rays are received at approximately the same time, dispersion is typically ignored. For larger dispersion, where the rays travel along multiple paths with significantly different path lengths, the 3 dispersion will give rise to separate multipath signal components being reached at the receiver. For optimum reception these signal components must be combined in the receiver.

If interference reduction techniques are applied it is required for the receiver to be able to differentiate between a wanted signal and an interfering signal. It is known in the art how this can achieved for high levels of dispersion but reliable differentiation for low levels of dispersion is problematic. An example of an algorithm for interference reduction differentiating between signals io depending on the dispersion can be found in "Co-channel interference canceling receiver for TDNIA mobile systemJ, Ranta, Hottinen & Honkasalo, Proceedings of MEE Int. Conf. on Communications, (ICC), Seattle, 1995.

The preferred embodiment of the current invention is directed to a narrowband application wherein the dispersion can be very low, such as for example in a an indoor environment with very small cells. Extra robustness in co-channel interference for such environments can be achieved by interference reducing systems such as for example joint detection ( ref.: "Cochannel interference canceling receiver for TDNIA mobile systemJ, Ranta, Hottinen & Honkasalo, Proceedings of IEEE Int. Conf. on Communications, (ICC), Seattle, 1995). However, as previously mentioned this requires differentiation which is problematic for low dispersion.

The described embodiment addresses the problem of little or virtually no natural dispersion during propagation of the signals. It seeks to provide sufficient dispersion in order to be able to separate a wanted signal from an interferer. The described embodiment is directed towards separation and interference reduction for co-channel signals.

In particular, the described embodiment introduces into co-channels signals, at the point of transmission, an artificial dispersion.

As depicted in the Figure, the method introduces into each signal a predetermined difference which can be used to extract that signal from the co- channel signals at the receiver. The difference can be any suitable characteristic, such as a particular spectral signature or a time delay between transmitted versions of the signal.

For example, each co-channel transmitter can transmit two signal components corresponding to the original signal and a time delayed version of this. The time delay between the two signal components would be different in 4 different transmitters allowing the receiver to differentiate between signals on this basis. Additionally, the delayed signal can have different gain and phase values for different transmitters thereby allowing differentiation between the signals for identical time delays.

The example shown in the Figure provides signals from three base stations to one mobile with artificial dispersion or differences being introduced into each signal for subsequent extraction. The mobile 103 transmits to the base station 101 through a channel with a one tap transfer function 109. The two co- lo channel interferers 105, 107 also transmits through a one tap transfer function but artificial dispersion is introduced resulting in the signals received at the base station corresponding to two tap transfer functions 111, 113.

For optimum signal separation, the differences introduced into the various cochannel signals are independent of one another.

The artificial dispersion patterns can be repeated throughout the network as required. Reuse is possible provided that the reuse distance exceeds the frequency reuse distance. Reuse of patterns is also possible between neighbouring cells with different frequencies.

It is important to note that this technique can be used with transmission from a single antenna, although it can be used in addition to transmission diversity if required.

It will be obvious the person skilled in the art that the method is equally applicable in the uplink and downlink directions.

Apparatus for carrying out the above-described method will be readily apparent to the skilled person'.

Claims

1. A method of interference reduction in a radio communications system including the steps of.

introducing into a first signal an artificial dispersion thereby generating a first artificially dispersed signal; and differentiating between the first signal and a second signal in response to the artificial dispersion.

io

2. A method according to claim 1, wherein artificial dispersion is introduced into each co-channel signal.

3. A method according to claim 1 or 2, wherein the artificial dispersion is a spectral signature.

4. A method according to claim 1 or 2 wherein the artificial dispersion is introduced by transmitting at least a first and second version of the first signal with a time delay between them.

5. A method according to claim 1 or 2 wherein the first artificially dispersed signal is transmitted from only one antenna.

6. A method according to any of the claims 1 to 5 wherein joint detection of a plurality of signals is performed in the receiver.

7. A method according to any of the claims 1 to 6 wherein the radio communication system is a cellular mobile communication system.

8. A method according to claim 7 wherein the cellular mobile 30 communication system uses Time Division Multiple Access with one a maximum of one user allocated in each time slot for each cell.

9. A method according to claim 7 wherein the cellular mobile communication system is a GSM system.

10. A system for interference reduction in a radio communications system comprising:

6 means for introducing into a first signal an artificial dispersion thereby generating a first artificially dispersed signal; and means for differentiating between the first signal and a second signal in response to the artificial dispersion.

11. A system according to claim 10, wherein artificial dispersion is introduced into each co-channel signal.

12. A system according to claim 10 or 11, wherein the artificial dispersion 10 is a spectral signature.

13. A system according to claim 10 or 11 wherein the artificial dispersion is introduced by transmitting at least a first and second version of the first signal with a time delay between them.

14. A system according to claim 10 or 11 wherein the first artificially dispersed signal is transmitted from only one antenna.

15. A system according to any of the claims 10 to 15 wherein joint detection 20 of a plurality of signals is performed in the receiver.

16. A system according to any of the claims 10 to 16 wherein the radio communication system is a cellular mobile communication system.

17. A system according to claim 16 wherein the cellular mobile communication system uses Time Division Multiple Access with one a maximum of one user allocated in each time slot for each cell.

18. A system according to claim 16 wherein the cellular mobile 30 communication system is a GSM system.

19. A method of interference reduction in a radio communications system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

20. A system for interference reduction in a radio communications system substantially as hereinbefore described with reference to the accompanying drawing.