INTERFERENCE CANCELLATION SYSTEM IN A CDMA RECEIVER AND METHOD FOR CANCELLING INTERFERENCE

Abstract: An interference cancellation system in a CDMA receiver comprises a detector adapted to receive a plurality of receiver a plurality of receiver input signals and a decoder means for receiving the detector output signals. Further, by means of a feedback loop the output signals from the decoding means are re-applied to the detector which comprises combination means to combine the feedback signals and the received input signals to generate an interference-reduced output signal. The invention can be implemented by a multi-stage interference cancellation system wherein the stages are connected such that the decoder output signal of a previous stage is applied to the interference estimation means of the next stage. The invention also relates to corresponding methods for cancelling interference in CDMA communication systems.

Description: INTERFERENCE CANCELLATION SYSTEM IN A CDMA RECEIVER AND METHOD FOR CANCELLING INTERFERENCE The present invention relates to mobile communication systems, in particular to code division multiple access (CDMA) systems and is directed to an interference cancellation system in a CDMA receiver according to claims 1 and 19. Further, the present invention concerns an interference cancellation method in a CDMA receiver according to claims 11 and 23. The capacity of a CDMA system is mainly limited by the interference which each user experiences in his received signal. In order to increase the capacity within a cell of a CDMA communication system, interference has to be cancelled as far as possible. Furthermore, in systems with high interference, the spreading factor has to be raised in order to maintain a given quality of service (QoS). However, higher spreading factors directly translate to lower bit rates which are available for transmission of data. In the field of CDMA communication systems, the following methods have been used on the receiver side in order to mitigate the problem of interference. A known method is to jointly detect the received signals of a plurality of users. However, the joined detection method has the drawback in that it involves inversion of a matrix whose dimensions increase with the number of users. For example, for 64 users within a cell, a 64x64 matrix has to be inverted which involves extremely high complexity which results in excessive processing power and time. A further known method is a single-user detection which is one of the simplest detection schemes. It consists of detection of a single user neglecting all other signals present. Hence, for the single user case results are optimum but in a multiuser environment the performance, i. e. bit error rate etc., degrades significantly. IEEE Journal On Selected Areas In Communications, vol. 17, no. 12, December 1999, pages 2082 to 2089 "Iterative Multiuser Detection Using Antenna Arrays and FEC on Multipath Channels" describes a multiple-access communication receiver system with an iterative receiver that improves the initial estimates from an antenna array in a multi-path propagation environment. In section III of this paper, the structure of the iterative receiver is explained in more detail. To this end, the receiver, which is iterative, takes the output of the channel and performs a combination of soft interference cancellation and matched filtering. The interference canceller takes a priori probabilities on the channel symbols of every user which are then employed to construct a soft estimate of the symbol. The interference cancellation operation is then performed in the spread domain. There is, however, still the need for an interference cancellation system and method with less complexity and satisfactory performance in a multi-user environment. This object is solved by an interference cancellation system comprising the features of claims 1 and 19 as well as by a method having the steps as defined in claims 11 and 23. Preferred embodiments of the invention are subject to various dependent claims. In the following, embodiments of the invention are described in further detail with reference to the accompanying drawings in which: figure 1 illustrates the basic structure of a transmitter in a CDMA communication system; figure 2 illustrates the basic structure of a corresponding receiver in a CDMA communication system; figure 3 illustrates a first embodiment of an interference cancellation system according to the present invention; figure 4 shows a more detailed structure of the detector employed in the embodiment shown in figure 3, figure 5 shows a second embodiment of an interference cancellation system of the present invention in a multi-stage implementation, figure 6 shows an implementation of the present invention using turbo codes, and figure 7 shows a RAKE receiver in which an interference cancellation system of the present invention is implemented. Figure 1 shows the basic configuration of a transmitter 100 wherein each user's signal coming from a source 101 is encoded using an encoder 102 which employs an outer code, e.g. a forward error correction code (FEC) which can consist of block codes, convolutional codes or other codes. Subsequently, the encoded signals are spread using an inner code which can consist of pseudo-noise codes, orthogonal codes or other codes and combinations of these to facilitate multiple channel access. The inner code is typically different for each user. A spreading modulator 103 is used for modulating the output signal from encoder 102 and transmit same over a communication channel. Those skilled in the art will readily appreciate the detailed structure of a transmitter without the need of disclosing further details here. The basic structure of a receiver 200 is shown in figure 2. Here, the received input signal is demodulated in a demodulator 201 and despread in a despreader 202 using the corresponding inner code for each user. After despreading the signals, they are fed to a interference cancellation and decoding stage 210, the details of which are shown in figures 3 to 5. Consequently, the output signals from the interference cancellation and decoding stage 210 are obtained using information on the inner codes as well as information contained in the outer codes. By removing interference from the received input signal, system capacity as well as data rates within a cell can be increased. Figure 3 shows a first embodiment of the interference cancellation system according to the present invention. The interference cancellation and decoding stage 210 comprises a multi-user detector 220 having a first plurality of inputs 221 for receiving the receiver input signal of n users of a CDMA communication system. Furthermore, the detector comprises a plurality of second inputs 222 for receiving the output signals of a selection switch 260, the purpose of which will be described in more detail further below. The detector 220 comprises a plurality of outputs 223 for outputting the detector output signals. Further details of the detector 220 will be described with reference to figure 4. The detector outputs are connected with a log-likelihood calculator 230 which calculates the log-likelihood ratio of the applied signals according to the formula: - , p( = Q) Xy= i) Further, the log-likelihood calculator 230 is connected with its outputs to the inputs of a MAP decoder 240 which is a soft-input/soft-output decoder for decoding the data information in the received input signal. Further, the decoder 240 outputs an estimated log-likelihood ratio for all n users to input port B of the selection switch 260 using a feedback loop 250. Next, the detailed structure of the detector 220 is described with reference to figure 4. The plurality of estimated log-likelihood ratios M k is received at the second plurality of detector inputs 222 and applied to a scaling means 224 in order to form a tentative decision on the transmitted signal using the tanh function. Consequently, the signals which are input to the scaling means 224, ranging in their values between (- oo; + oo), are scaled to a signal which can be seen as a mean estimate of the log-likelihood ratio of the input signals, ranging between [- 1 ; +1]. Although the correct mathematical calculation of the scaling means 224 is U k = tanh (0,5 . 8k) simulations have proven that the following relationship is also advantageous: -1 for S < -2 U k = S 2 for -2 < Sk < +2 +1 for Sk> +2 The outputs of the scaling means 224 are connected with the corresponding inputs of a summing means 225 which adds the scaled signals to obtain an interference estimate which user k experiences from all other users. To this end, prior to summing, the signals applied to the input of the summing means are firstly scaled by their respective estimated amplitude and by the cross-correlation coefficient p ik between the signal of user i and the signal of user k. Consequently, the summing means 225 performs the following mathematical operation: wherein ξ represents the interference estimate, A, is the channel estimate, comprising information with respect to the attenuation as well as the phase rotation in the channel and Uj is the output signal from scaling means 224. Finally, the detector comprises combination means 226 to combine the received input signals r k and the output signals from the summing means 225. The output of the combination means 226 is provided at the plurality of detector outputs 223. Finally, the method of cancelling interference in the CDMA receiver described above will be explained in more detail. The received input signal of n users of the communication system are, during the first iteration, simultaneously provided to the first plurality of detector inputs 221 and through the selection switch 260 to the second plurality of detector inputs 222. In this case, selection switch 260 connects its input port A and conducts the signals applied thereto as output signals of the switch. Afterwards, meaning for each further iteration, the selection switch 260 selects input port B and outputs the signals applied thereto as output signals. In detector 220, the signals provided at the first and second plurality of inputs are logically combined in order to provide an improved signal y k for further processing. To attain this, the signals at the second plurality of detector inputs 222 are weighted included scaling in the scaling means 224 and taking into account the cross- correlation property of the inner code in the summing means 225 to obtain an estimate of the received interference. Subsequently, the interference is cancelled by simple subtraction of the weighted signals from the original received input signal. This is accomplished in summing means 226. Hence, the cancellation itself is a substraction of weighted signals from the originally received signal. Thus, it is very easy to implement. It should be noted that even when only applying one iteration, i.e. no usage of the outer code information provided by the decoder, an improvement over prior art systems is achieved. Since the interference cancellation removes more errors than it introduces, this process can be repeated over several iterations with subsequent estimates of the received interference being derived from the output of the MAP decoder. The output signals of detector 220 are applied through log-likelihood calculator 230 as mentioned above to MAP decoder 240. The soft output of the decoder provides on the one hand, infobits used for decoding the information data, and on the other hand, codebits which are fed back to selection switch 260 via feedback loop 250. It should be noted that the outer code employed by decoder 240 can be any scheme of forward error correction code although the need of soft decision output values at the decoder suggests the usage of convolutional codes or derivations of these, e.g. turbo codes. The decoder output signals which are fed back to input port B of selection switch 260 include the information contained in the outer code, i.e. the improved decoded signal for each user k. For this and each further iterations, the selection switch selects input port B and the signals which are applied to the second plurality of decoder inputs 222 correspond to the decoder output signals from the previous iteration. According to a variant of the interference cancellation system, not shown in the figures, instead of computing the cross-correlation matrix formed by cross-correlation coefficients p ik to form the interference estimate ς k , the process of spreading can also be applied to the signals output from scaling means 224. In this case, the tanh operation is performed on the despread signal and the tentative decision is then respread before summing over all the users within summing means 225. According to this variant, the output signals from summing means 225 are fed to combination means 226 to perform the interference cancellation on the received spread input signals r k . In other words, the received input signal r k is not despread, but despreading would have to be applied to the output signal from combination means 226 using the spreading code for user k. Also, for the initial estimate, the feedback signals have to be despread before entering scaling means 224. In this case, in the summing means 225 scaling is only done by the channel estimate A before summation. This variant is particularly important in dealing with asynchronous systems, that is, systems where user data does not arrive time-aligned at the receiver and/or when different users operate with different spreading factors i.e. in a multi-rate environment. A further variant to form a tentative decision in the receiver is, instead of usage of the tanh function any monotonous function that maps input values ranging from (- oo;oo) to output values ranging from [-1 ; +1], which could be simpler to implement but might not be the optimum in performance. In addition to spreading codes, a further scrambling code could be applied to the transmitted signal. In the receiver this would result either in the computation of the cross-correlation matrix of the combined spreading and scrambling codes or in the inclusion of descramblers before despreaders and rescramblers after respreaders. In actual radio environments, particular in mobile communication, the signals suffer from various forms of signal fading and multi-path propagation. To mitigate those effects, it is known to introduce interleavers in the transmitter and deinterleavers in the receiver and/or to include multi-path resolving structures in the receiver such as a RAKE receiver. These additional optional features can be included without problems in the embodiment of the invention described above. In fact, the introduction of different interleaver structures for each user promises improved performance, i.e. larger gains between the iterations. For a multi-path receiver, the cancelled signals can be used to perform the tasks of the RAKE receiver on the improved signal again. This will result in improved channel estimation, improved signal estimation and reduced interference between different RAKE fingers of the same user. With regard to figure 5, a second embodiment of the interference cancellation system will be described. Here, the interference canceller is embodied as a multistage system comprising a plurality of stages being similar structured. Moreover, those components which correspond with that of the first embodiment, namely the combining means 226 for performing the interference cancellation and the MAP decoder 240 have been designated with the same reference numerals. As shown in the figure in more detail, the initial stage 310 comprises a despreader 311 , an estimator 312 and a respreader 313 in sequence. Further, between the combining means 226 and the MAP decoder 240 a despeader 314 and a log- likelihood ratio calculator Σ (reference numeral 315) are connected in series. The second and third stages 320, 330 up until a N-th stage are structured identically to the initial stage except that the despreader 311 has been omitted. Moreover, the estimator 312 receives the output signal from the MAP decoder 240 as its input signal instead of the demodulated input signal from demodulator 201. From the final N-th stage, the decoder output is taken as the decoded information data. It should be noted that figure 5 is a simplified diagram in which each arrow represents a full vector of respective signals illustrated in figures 1 to 4. Also in this embodiment, it will be appreciated that the interference cancellation system removes more information errors than it introduces such that repetition of the interference cancellation process by using a plurality of stages increases the interference cancellation gain. Figure 6 shows an implementation of the present invention using turbo codes as mentioned above. In the upper half of the figure, a basic structure of a turbo decoder having blocks 401 to 406 is shown providing a decoder decision output 413. The lower half of the figure shows how the feedback signal 414 is calculated from results obtained by MAP decoders 402, 404. Turbo decoders are known in the art, such that detailed discussion of the decoding operation can be omitted and is understood by those skilled in the art. In brief, the signals obtained from the log-likelihood calculation stage 401 are fed to a first and second MAP decoder 402, 404 respectively. These signals represent systematic information that the MAP decoders respectively receive at their first inputs. At their second inputs, parity information is received and at their outputs infobit metric signals and codebit metric signals are provided. In more detail, in the case of decoder 402, infobit metric 1 is combined with the input signal of the decoder. The resulting signal is applied to the input of the second MAP decoder 404, where it is combined with the systematic information signals from the log-likelihood calculator 401. In an analogous manner, at the output of decoder 404 extrinsic metric signal is fed back to the input of the first MAP decoder 402, where it is combined with the systematic information signal from log-likelihood calculator 401. However, the signals of the second MAP decoder 404 undergo an interleaving operation (block 403), wherein the input signal is interleaved according to a predetermined scheme before being applied to the input the decoder. Further, an inverse π "1 operation (block 405) for the infobit metric signal at the decoder's output is effected. The outputted infobit metric signal is applied after the π "1 operation to a threshold decider block 406 which outputs either +1 or -1 depending on the sign of the input signal and which is subsequently used as a decoder decision signal 413. Basically, in turbo codes of rate 1/3, the first bit of each triple presents the systematic bit, the second bit is the parity 1 bit, and the third bit is the parity 2 bit. For the feedback, the systematic information from MAP decoder 404 is taken because this is generally improved compared to MAP decoder 402. However, since this systematic information is interleafed for MAP decoder 404, the interleaving process is inverted in stage 409 for calculation of systematic feedback. For parity 1 feedback, the parity 1 information derived from MAP decoder 402 is taken. For parity 2 feedback, the parity information from MAP decoder 404 is used. Further, the codebit metric signals 411 , 412 are obtained from MAP decoders 402, 404 and are ordered as a series of 2-tuples, the first bit representing systematic metric, the second being the parity metric. Therefore, in block 407, the signal 411 is split into the systematic metric and parity metric of MAP decoder 402. Likewise, in block 408, the signal 412 is split into systematic metric and parity metric of MAP decoder 404. These derived metric signals are multiplexed in block 410, so that each triple feedback information signals 414 follows the order of systematic/parity 1 /parity 2. Figure 7 shows a RAKE receiver to which the interference cancellation system of the present invention is applied. RAKE receivers are known in the art, such that a detailed explanation will be omitted as those in the art understand the basic structure thereof. Blocks 501 to 504 represent a first stage implementation and replace the demodulator 201 in conjunction with the despreader 311 in figure 5. The interference estimation block 505 is identical to the estimator 312 in figure 5. Further, the respreader 313 is replaced by blocks 506 and 507 representing a respreading operation and a channel replication respectively. Furthermore, the combining means 226 in figure 5 corresponds to a combining means 508. The despreader 314 in figure 5 is replaced by blocks 509 to 512. During operation, interference estimation block 505 receives its input from maximum- ratio combining means 504 and in block 507, a channel replication is formed from the estimate of the received signal including multi-path and fading characteristics of the radio channel. Therefore, information from the finger assignment block 502 is needed to determine the delay of the fingers. Likewise, information from channel estimation block 503 is needed to reproduce the fading characteristics of each path. The output of the maximum-ratio combining means 512 is used as an input to block 315 of the first stage. The outputs of the channel estimation block 511 and the finger assignment block 510 are fed to channel replication block 507 of the second stage. In subsequent stages, the following changes take effect compared to the first stage as described above. Blocks 501 to 504 are not implemented. In the despreader 507, the signals from previous stage blocks 510, 511 are used instead of the signals of blocks 502, 503. Finally, the input of block 505 uses the output of the MAP decoder 240 from the previous stage, if present. As an alternative, interference estimation block 505 can also comprise several different blocks to estimate the interference. For example, if pilot signals are transmitted, these are perfectly known to the receiver. Therefore, instead of forming an estimate from the received symbols for those pilot symbols, the original pilot symbols are used because the receiver can be 100% certain that these symbols were transmitted. Therefore, interference estimation of these symbols is most accurate.

Claims: Claims 1. An interference cancellation system in a CDMA receiver comprising: a detector (220), having a first (221 ) and a second plurality (222) of detector inputs, said first plurality of detector inputs (221 ) being adapted to receive a plurality of receiver input signals, each receiver input signal being assigned to one of a plurality of n users of a CDMA communication system, decoding means (240) for receiving detector output signals from a plurality of detector outputs (223) of said detector (220), a feedback loop (250) for applying decoding means output signals to the second plurality of detector inputs (222), said detector (220) comprising combination means (226) to combine the receiver input signals and the feedback signals which are fedback by the feedback loop (250) to form the detector output signals. 2. The interference cancellation system of claim 1 further comprising a selection switch (260) for selecting either the receiver input signals or the feedback signals for application to the second plurality of detector inputs (222) 3. The interference cancellation system of claim 1 or 2, further comprising a log- likelihood calculator (230) for calculating a log-likelihood ratio of the detector output signals. 4. The interference cancellation system according to one of claims 1 to 3, wherein the decoding means is a soft-input/soft-output decoder (240) which gives information on decoded infobits as well as information on codebits e.g. a MAP or a Bahl decoder. 5. The interference cancellation system of one of claims 1 to 4, further comprising scaling means (225) for scaling each of the feedback signals by a corresponding channel estimate. 6. The interference cancellation system according to claim 5, wherein said scaling means (225) further comprises cross-correlation calculation means for calculating the cross-correlation coefficient p ik between the feedback signals of user i (i=1...n) and user k (i≠k). 7. The interference cancellation system according to claim 5, further comprising: despreading means for despreading the feedback signals, and respreading means for respreading the despread feedback signals before scaling same. 8. The interference cancellation system according to one of claims 5 to 7, wherein said scaling means (225) further comprises summing means for summing for all n users except user k the scaled signals to obtain an interference estimate which user k experiences from all other users. 9. The interference cancellation system according to one of claims 1 to 8, further comprising conversion means (224) for obtaining an estimate of the feedback signals. 10. The interference cancellation system according to one of claims 1 to 9, wherein said combination means (226) subtracts the feedback signals from the receiver input signals to provide an estimated user k signal substantially without interference from all other users. 1. A method for interference cancellation in a CDMA receiver comprising the steps of: receiving a plurality of receiver input signals each being assigned to one of a plurality of n users of the CDMA system, applying the receiver input signals to a first plurality of detector inputs of a detector decoding the detector output signals to provide decoded bits of the receiver input signals, feeding the decoder output signals to a second plurality of detector inputs, and combining the receiver input signals at the first plurality of detector inputs and the feedback signals at the second plurality of detector inputs to provide detector output signals. 12. The method according to claim 11 , further comprising the steps of: applying the receiver input signals to the second plurality of detector inputs during a first iteration step, and applying the feedback signals to the second plurality of detector inputs during the second and each further iteration step. 13. The method according to claim 11 or 12, further comprising the step of calculating a log-likelihood ratio of the detector output signals. 14. The method according to one of claims 11 to 13, further comprising the steps of scaling each of the feedback signals by a corresponding channel estimate. 15. The method according to one of claims 11 to 14, further comprising the step of calculating the cross-correlation coefficient p ik between the feedback signals of user i (i=1...n) and user k (i≠k). 16. The method according to claim 14, further comprising the step of summing for all n users except user k the scaled signals to obtain an interference estimate which user k experiences from all other users. 17. The method according to one of claims 11 to 16, further comprising the step of conversion of the feedback signals to obtain an estimate thereof. 18. The method according to one of claim 11 to 17, wherein the combining step comprises subtracting the feedback signals from the receiver input signals to provide an estimated user k signal substantially without interference from all other users. 19. A multistage interference cancellation system in a CDMA receiver comprising a plurality of stages, each stage (310, 320, 330) comprising: an interference estimation means (312) for providing an interference estimation signal which each user k experiences from other users of a CDMA communication system, combining means (226) for combining a received input signal with the interference estimation signal to provide a resultant signal, and decoding means (240) being adapted to receive the resultant signal for generating a decoder output signal, wherein said stages (310, 320, 330) are connected such that the decoder output signal of a previous stage is applied to the interference estimation means of the next stage. 20. The multistage interference cancellation system according to claim 19, wherein the initial stage (310) further comprises: first despreading means (311) adapted to receive the input signal and to apply same to the interference estimation means (312), and respreading means (313) for respreading the interference estimation signal. 21. The multistage interference cancellation system according to claim 19 or 20, wherein each stage (310, 320, 330) further comprises second despreading means (314) for despreading the resultant signal from the combining means (226), and log-likelihood calculation means (315) for calculating the log-likelihood ratio and applying same to the decoding means (240). 22. The multistage interference cancellation system according to one of claims 19 to 21 , wherein the decoding means is a soft-input/soft-output decoder (240), e. g. a MAP or Bahl decoder. 23. A method of interference cancellation in a multistage CDMA receiver, comprising the following steps: estimating the interference which each user k experiences from other users of a CDMA communication system and providing an interference estimation signal, combining a received input signal with the interference estimation signal to provide a resultant signal, decoding the resultant signal and generating a decoder output signal, and estimating the interference in a next stage on the basis of the decoder output signal from the previous stage. 24. The method according to claim 23, further comprising the steps of: despreading the input signal before generating the interference estimation signal and respreading the interference estimation signal before combining same with the input signal. 25. The method according to claim 23 or 24, further comprising the steps of: despreading the resultant signal, and conversion of the despread resultant signal before decoding same.

Inventors: GLIGOREVIC SNJEZANA, GOLITSCHEK ALEXANDER, SORGER ULRICH, WIEBKE THOMAS, GLIGOREVIC, SNJEZANA, GOLITSCHEK, ALEXANDER, SORGER, ULRICH, WIEBKE, THOMAS
Applicants: GLIGOREVIC SNJEZANA, GOLITSCHEK ALEXANDER, MATSUSHITA ELECTRIC IND CO LTD, SORGER ULRICH, WIEBKE THOMAS, GLIGOREVIC, SNJEZANA, GOLITSCHEK, ALEXANDER, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., SORGER, ULRICH, WIEBKE, THOMAS
Application year: 2000
Main classification: H04B1
IPC codes: H04B1, H04J13