FR2777402A1 - METHOD AND SYSTEM FOR REDUCING INTERFERENCE - Google Patents

Abstract

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

Description

METHOD AND SYSTEM FOR REDUCING INTERFERENCE

  Field of the Invention The present invention relates to a method for facilitating the reduction of interference in a radiocommunication system, in particular interference between channels in a communication system.

mobile cellular communication.

  BACKGROUND OF THE INVENTION The reduction of interference and the methods of detection of multiple users have been the subject of particular attention in the search for cellular applications based on multiple access by code difference (AMDC). Research on time division multiple access (TDMA) systems such as the mobile special group (GSM) is relatively unimportant. This is partly explained by the fact that the problem is more difficult since signal separation is possible, for example by using a maximum likelihood sequence estimation (ESVM) equalizer in the receiver. Such a combined detection receiver uses the fact that the interference between the channels is deterministic in nature once the impulse response of the channel is known for each disruptor and

the desired signal).

  However, even with such information, effective separation requires that the impulse responses for each of the signals be significantly different. The degree of cross-correlation determines the fundamental performance of the receiver. Under certain propagation conditions, for example in an interior environment with small cells, the pathway will not necessarily introduce significant variation in the transfer function:

  propagation of the desired signal and interference.

  Differentiation will therefore not be possible.

  Summary of the present invention The object of the present invention is to reduce interference in a radiocommunication system, in particular for multiplex signals in a mobile cellular communication system. According to one aspect of the present invention there is provided a method for reducing interference in a radiocommunication system comprising the following steps: introducing an artificial dispersion into a first signal thereby generating a first artificially dispersed signal; and differentiate between the first signal and a second

  signal in response to artificial dispersion.

  Preferably, the artificial dispersion is introduced into each of the multiplex signals. The artificial dispersion can be specifically a spectral signature or a time delay between a first and a second version of the signal. The radiocommunication system can typically be a system

GSM or similar TDMA system.

  According to a different aspect of the invention, there is proposed a system for reducing interference in a radiocommunication system comprising: means for introducing an artificial dispersion into a first signal, thereby generating a first artificially dispersed signal; and a means for differentiating between the first signal and a second

  signal in response to artificial dispersion.

Description of the figure

  An embodiment of the present invention is described below, by way of illustration only, with reference to the single accompanying drawing which is a schematic diagram of an embodiment of a system for suppressing

interference between channels.

  Description of the preferred embodiment

  The preferred embodiment of the invention applies to a mobile cellular communication system such as GSM, although the invention is not in

  no way limited to such a system. The description

  further consider the use of multiple access to

time distribution.

  In most cellular systems, the spread from transmitter to receiver varies with time and geographic location. Typically, many different signal components are received from the transmitted signal due to the many different radii of the signal reaching the receiver. This causes dispersion. For very low dispersion levels in which all the rays are received approximately at the same time, the dispersion is typically ignored. For a wider dispersion, in which the rays travel along multiple paths with substantially different path lengths, the dispersion causes components of separate multipath signals obtained at the receiver. For optimal reception, these signal components must be

combined in the receiver.

  If interference reduction techniques are applied, the receiver must be able to differentiate between a desired signal and a disturbing signal. It is known in the art how this can be achieved for high levels of dispersion but reliable differentiation for low levels of dispersion is problematic. An example of an interference reduction algorithm for differentiating between dispersion dependent signals can be found in "Co-channel interference canceiling receiver for TDMA mobile systems", Ranta, Hottinen & Honkasalo, review of the 'IEEE Int. Conf. we

  Communications (ICC), Seattle, 1995.

  The preferred embodiment of the present invention is intended for a narrow band application in which the dispersion may be very low, for example in an indoor environment with very small cells. For such environments one can obtain a higher robustness of the interferences between the channels thanks to interference reduction systems such as combined detection (see: "Co-channel interference canceiling receiver for TDMA

  mobile systems ", Ranta, Hottinen & Honkasalo, account-

  from the IEEE Int. Conf. on Communications (ICC), Seattle, 1995. However, as noted above, this requires differentiation which is

  problematic for low dispersion.

  The embodiment described addresses the problem of a natural dispersion which is weak or practically zero during the propagation of the signals. Its purpose is to provide sufficient dispersion to separate a desired signal from a disturbance. The embodiment described is intended for the separation of and the reduction of interference in the signals.

multiplex.

  In particular, the described embodiment introduced in the multiplex signals, at the point of

  transmission, an artificial dispersion.

  As described in the figure, the method introduces into each signal a predetermined difference which can be used to extract this signal from the multiplex signals at the receiver. The difference can be any suitable characteristic, such as a particular spectral signature or a

  delay between transmitted versions of the signal.

  For example, each multiplex transmitter can transmit two signal components corresponding to the

  original signal and a time-delayed version of it.

  This time delay between the two signal components would be different in different transmitters, thus allowing the receiver to differentiate the signals, on this basis. In addition, the delayed signal can have different gain and phase values for different transmitters, thus allowing differentiation between signals for different

identical timers.

  The example in the figure provides signals from three base stations to a mobile with artificial dispersion or differences introduced into each signal for further extraction. The mobile 103 transmits to the base station 101 via a channel with a transfer function to an outlet 109. The two disturbers between the channels 105, 107 also transmit via a transfer function to an outlet but an artificial dispersion is introduced, which means that the signals received at the base station correspond to the two outlet transfer functions

111, 113.

  To obtain optimal signal separation, the differences introduced into the different multiplex signals must be independent of each other. Artificial dispersion models can be repeated throughout the network as needed. Reuse is possible provided that the reuse distance exceeds the frequency reuse distance. The reuse of models is also possible between neighboring cells having

different frequencies.

  It should be noted that this technique can be used with transmission from a single antenna, although it can be used if necessary in addition

of transmission diversity.

  For the specialist in the art it is obvious that the method is also applicable in the senses

rising and falling.

  A device for carrying out the process will appear from

obvious to the specialist.

Claims (20)

Claims   1. A method of reducing interference in a radiocommunication system comprising the following steps: introducing an artificial dispersion into a first signal, thereby generating a first artificially dispersed signal; and differentiating between the first signal and a second signal in response to the artificial dispersion. 2 The method of claim 1, wherein the artificial dispersion is introduced into each multiplex signals.   3. Method according to claim 1 or 2, wherein the artificial dispersion is a spectral signature. 4. Method according to claim 1 or 2, in which the artificial dispersion is introduced by the transmission of at least a first and a second version of the first signal with a time delay between them. 5. Method according to claim 1 or 2, wherein the first artificially dispersed signal is transmitted from a single antenna.   6. Method according to any one of   claims 1 to 5, wherein the combined detection   of a plurality of signals is carried out in the receiver. 7. Method according to any one of   claims 1 to 6, wherein the system of   radiocommunication is a communication system mobile cell.   8. The method of claim 7, wherein the mobile cellular communication system uses time division multiple access with at most one user assigned to each time slot for each cell. 9. The method of claim 7, wherein the mobile cellular communication system is a GSM system.   10. A system for reducing interference in a radiocommunication system comprising: means for introducing an artificial dispersion into a first signal, 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. The system as claimed in claim 10, in which the artificial dispersion is introduced into each multiplex signal.   12. The system of claim 10 or 11, wherein the artificial dispersion is a spectral signature. 13. The system of claim 10 or 11, wherein the artificial dispersion is introduced for the transmission of at least a first and a second version of the first signal with a time delay between them. 14. The system of claim 10 or 11, wherein the first artificially dispersed signal is transmitted from a single antenna.   15. System according to any one of   claims 10 to 15, wherein the detection   combined of a plurality of signals is carried out in the receiver.   16. System according to any one of   claims 10 to 16, wherein the system of   radiocommunication is a communication system mobile cell.   17. The system as claimed in claim 16, in which the mobile cellular communication system uses time division multiple access with a maximum of one user assigned to each tranche of time for each cell.   18. The system of claim 16, wherein the mobile cellular communication system is a GSM system.   19. Method for reducing interference in a radiocommunication system substantially as described: above with reference to and as illustrated in the attached drawing.   20. Interference reduction system in a radiocommunication system substantially as described   above with reference to the accompanying drawing.