GB2391757A

GB2391757A - Method of Interference Suppression in Mobile Communications.

Info

Publication number: GB2391757A
Application number: GB0220833A
Authority: GB
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2002-08-07
Filing date: 2002-08-07
Publication date: 2004-02-11
Also published as: GB0220833D0

Abstract

An interference suppression device (1) comprises a joint detection decision feedback equaliser (3) comprising a first wanted filter (8) and a first interferer filter (9), a joint decision device (12), a second wanted filter (14, 15) and a second interferer filter (13, 16); wherein a received signal is input (7) to the first filters, wherein a filtered wanted signal and a filtered interferer signal are input to the joint decision device; wherein each output of the joint decision device is filtered in the respective second filters and subtracted (10, 11) from the respective input signals to optimise the output of the joint decision device; and wherein an estimated interferer signal is output (17) for further processing. An interference suppression system incorporating the device and a method of suppressing interference are also provided.

Description

INTERFERENCE SUPPRESSION METHOD AND APPARATUS

This invention relates to a system for suppressing interference in a received signal. 5 In mobile radio systems where frequency re-use is employed to provide a system with high capacity across a wide geographical area, but using little of the radio spectrum, co-channel interference will limit the capacity of the system. The desired goal of such frequency re-use radio systems is to achieve frequency re-use in adjacent cells.

There are many approaches to reducing the impact of co-channel interference, for 10 example, power control or frequency hopping and generally several techniques are used in conjunction to achieve the desired performance. One technique is that of interference cancellation or suppression. This is a well known technique with systems employing direct sequence spread spectrum, but less so in non spread-spectrum systems. This is because the problem of separating the wanted signal, which usually has the same 15 modulation as the interferer signal, is made more difficult in non spread-spectrum signals. Previous work in this area has examined the use of a joint detection approach based upon a maximum likelihood sequence estimator (MLSE). This approach can achieve good performance, but the complexity can be very high, particularly when the modulation alphabet size is large.

20 Assuming that the channel impulse response pertaining to both the wanted and interfering signal paths can be estimated, the joint detection approach can be used with a single receive antenna, and so is suitable for a mobile receiver. In some systems the co-channel interference may use one of several modulation schemes, because some form of adaptive modulation method may be used. Consequently the interference 25 suppression receiver has to potentially detect the modulation on the interfering signal.

In accordance with a first aspect of the present invention, a method of suppressing interference in a received signal comprises applying a joint detection method to estimate a data sequence that modulates an interfering signal; wherein a wanted signal and the interfering signal are dispersed in tinge; the joint detection method 30 comprising applying the received signal to a first wanted signal filter and a first interferer signal filter; inputting the filtered signals to a joint decision device; filtering the output of the joint decision device in a second wanted signal filter and a second interferer signal filter; subtracting the second filtered outputs from the first filtered

outputs to optimise the wanted and interferer signals; and outputting an estimated interferer signal from the joint decision device.

Preferably, the method further comprises applying the received signal to a third interferer signal filter; inputting the third filtered signal to the joint decision device; and 5 filtering the output of the joint decision device in a fourth and subsequent interferer signal filters.

In accordance with a second aspect of the present invention, an interference suppression device, wherein the device comprises a joint detection decision feedback equaliser comprising a first wanted filter and a first interferer filter, a joint decision 10 device, a second wanted filter and a second interferer filter; wherein a received signal is input to the first filters, wherein a filtered wanted signal and a filtered interferer signal are input to the joint decision device; wherein each output of the joint decision device is filtered in the respective second filters and subtracted from the respective input signals to optimise the output of the joint decision device; and wherein an estimated interferer 15 signal is output for further processing.

Preferably, the device further comprises a third interferer signal filter to filter the received signal; and fourth and subsequent interferer signal filters to filter the output of the joint decision device.

Preferably, the first and third filters comprise feedforward filters.

20 Preferably, the second, fourth and subsequent filters comprise feedback filters.

In accordance with a third aspect of the present invention, an interference suppression system comprises a device according to the second aspect; a modulator; a subtracter; and a detector; wherein a received signal is input to the equaliser to derive an estimate of an interferer signal; wherein the estimated interferer signal is 25 remodulated and convolved in the modulator to reconstitute an interferer signal; wherein the interferer signal is subtracted from the received signal in the subtracter, and wherein the wanted signal is detected in the detector.

An example of an interference suppression device, system and method according to the present invention will now be described with reference to the accompanying 30 drawings in which: Figure 1 illustrates a system according to the present invention; and, Figure 2 illustrates in more detail an interference suppression device according to the present invention for use in the system of Fig. 1.

Figure I illustrates the system of the present invention for the situation where there is a single interferer signal present. A receiver 1 comprises an input 2 for a received signal and an interference suppression device comprising a joint detection 5 decision feedback equaliser (JD-DFE) 3 for estimating a wanted and an interfering signal from the received signal. A modulator 4 enables the estimated interferer signal output from the JD-DFE to be remodulated and convolved to reconstitute the received interfering signal.. A subtracter 5 is provided to subtract the reconstituted interferer signal from the received signal to derive a wanted signal plus noise and the wanted 10 signal is detected in a conventional detector 6.

In the invention interference is cancelled from the received signal by estimating the data sequence that modulates the interfering signal using a joint detection approach.

This joint detector is based upon the decision feedback equaliser and so can handle wanted and interfering signals that are dispersed in time. In Figs. I and 2, the channel 15 impulse responses applying to both the wanted and interfering signals are assumed to be estimated by some other means. An example is described in EP- B-0667683. The estimates of the wanted signal are discarded and the data symbol estimates of the interfering signal are re-modulated and convolved with an estimate of the interfering signals channel impulse response to produce an estimate of the received version of the 20 interfering signal. This signal is then subtracted from the received signal to produce an input for the conventional detector of the wanted signal.

Fig. 2 illustrates how a received signal is applied to the joint detection decision feedback equaliser 3 to produce estimates of both wanted and interfering signals. The received signal is input 7 to the equaliser 3. The received signal is filtered by a 25 feedforward wanted signal filter 8 and by a feedforward interferer signal filter 9 to derive a first estimate of wanted and interferer signals. These derived signals pass through respective subtracters 10, 1 into a joint decision device 12. Additionally feedback filtering stages 13, 14, 15, 16 are applied to the outputs of the joint decision device and subtracted from the filter wanted and interferer signals respectively to 30 optimise the estimated wanted and interferer signals output from the joint decision device. As the wanted signal estimate is not required here, it is discarded and only the estimated interferer signal 17 is output for further processing.

The received signal rk is modelled as:

D( , D'l' r = úh( )s( ) + I,h(l)s(l) +n i=o i=o Where the superscripts O and 1 are used to denote the wanted and interfering signals 5 respectively and h(m' denotes the channel impulse response. The first sum represents the wanted signal, the second represents the interferer signal and the received signal also includes a noise component nk. Two signals can be formed as inputs to the joint decision device 12, and these are given in the equations below: N -I N. N

O1 ( ) _ '.(o' d(n) (o) + (o) (1) sk -,Ci rk+; + i Sl-i,,e' St_j i=0 i=1 i=1 Nr-l Nh N -(1) _ (a) <,J(1) (1) (1) (0) sk,Cj rk+' +,U j Sk_; +,ej Sk i i=0 i=1 i=1 15 The filters C/m) 8, 9, d(m' 14, 16 and e(m' 13, 1S are obtained in the following manner using estimates of the impulse responses of the wanted and interfering signals h jm).

N -I i'4(mJC(m) _ h(m,.

i=0 q(m) =,h(m).h(m} +2i 1=0 D''- j d(m) =_ It C(m)h(m} 1-0 20 e( '=- c( ) h(l) 1=0 Di(' j e jl' = - I, C(l)ll( ) 1=0

The joint decision device 12 makes decisions on the symbols.s'' and s,"' using the following metric.

min [1 ' - h,< 'c<t, >st ' - h"co 's'i'| + |<'> - htictist-'' ht 'ct"s' |] The symbols of the interfering signal are then re-convolved with the impulse response of the interfering signal in modulator 4 and subtracted from the incoming received signal at subtracter 5. The detector 6 is then applied to detect the wanted signal. 10 One of the advantages of using the joint detection DEE 3 is that the complexity is relatively insensitive to the order of modulation, and it is possible to blindly detect the modulation type. With a joint MLSE approach as proposed in prior art systems the

complexity can become prohibitive for higher order modulations. One application of the joint detection DEE: 3 is in GSM systems where an adaptive modulation format 15 comprising GMSK (a binary modulation) and 8PSK modulation is used. The interfering signal could therefore be either of these two modulation formats and this would not be known at the receiver 1. By performing joint detection for both modulation formats and determining which interferer modulation leads to the lowest minimum mean square error blind detection of the modulation format can be performed 20 relatively easily.

Although, the example described is concerned with only a single interferer, the invention is equally suited to dealing with multiple interferer signals. The JDDFE is modified by providing an additional interferer feedforward filter between the input 7 and the joint decision device 12, together with multiple interferer feedback filters 25 between the output and input of the JDV.

Claims

1. A method of suppressing interference in a received signal, the method comprising applying a joint detection method to estimate a data sequence that 5 modulates an interfering signal; wherein a wanted signal and the interfering signal are dispersed in time; the joint detection method comprising applying the received signal to a first wanted signal filter and a first interferer signal filter; inputting the filtered signals to a joint decision device; filtering the output of the joint decision device in a second wanted signal filter and a second interferer signal filter; subtracting the second filtered 10 outputs from the first filtered outputs to optimise the wanted and interferer signals; and outputting an estimated interferer signal from the joint decision device.

2. A method according to claim 1, the method further comprising applying the received signal to a third interferer signal filter; inputting the third filtered signal to the 15 joint decision device; and filtering the output of the joint decision device in a fourth and subsequent interferer signal filters.

3. An interference suppression device, wherein the device comprises a joint detection decision feedback equaliser comprising a first wanted filter and a first 20 interferer filter, a joint decision device, a second wanted filter and a second interferer filter; wherein a received signal is input to the first filters, wherein a filtered wanted signal and a filtered interferer signal are input to the joint decision device; wherein each output of the joint decision device is filtered in the respective second filters and subtracted from the respective input signals to optimise the output of the joint decision 25 device; and wherein an estimated interferer signal is output for further processing.

4. A device according to claim 3, the device further comprising a third interferer signal filter to filter the received signal; and fourth and subsequent interferer signal filters to filter the output of the joint decision device.

5. A device according to claim 3 or claim 4, wherein the first and third filters comprise feedforvard filters.

(

6. A device according to claim 3 or claim 4, wherein the second, fourth and subsequent filters comprise feedback filters.

7. An interference suppression system, the system comprising a device according 5 to any of claims 3 to 6; a modulator; a subtracter; and a detector; wherein a received signal is input to the equaliser to derive an estimate of an interferer signal; wherein the estimated interferer signal is remodulated and convolved in the modulator to reconstitute an interferer signal; wherein the interferer signal is subtracted from the received signal in the subtracter, and wherein the wanted signal is detected in the.

10 detector.