GB2370469A

GB2370469A - Improvements in CDMA receivers

Info

Publication number: GB2370469A
Application number: GB0021997A
Authority: GB
Inventors: Hafedh Trigui
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2000-09-07
Filing date: 2000-09-07
Publication date: 2002-06-26
Anticipated expiration: 2020-09-07
Also published as: GB2396788B; GB0021997D0; GB0406605D0; GB2370469B; GB2396788A

Abstract

In a CDMA receiver having a Rake receiver and a plurality of Interference Estimation Units, the channel impulse response of a channel is made use of to determine a cancellation control factor; this can alleviate the need for off-line testing and empirical determination of suitable values and give better results.

Description

IMPROVEMENTS IN CDMA RECEIVERS

The present invention relates to improvements in code division multiplexed (CDMA) receivers, in particular wideband CDMA (WCDMA) receivers. References to CDMA in this specification are not restricted to present implementation of CDMA systems but are intended to include equivalents, developments or derivatives of code division multiplexed systems.

In WCDMA networks, the data rate for one user can be increased by reducing its spreading factor. This technique is referred as multirate transmission or variable spreading factor (VSF) technique. Alternatively, the spreading factor (SF) can be fixed and the data rate increased by allocating several parallel spreading codes, i. e. , data channels for the same service. This is referred as a multicode technique. The receiver is generally based on the conventional RAKE receiver.

In a multipath environment and for small spreading factors, inter-path interference (IPI) may result from the imperfect spreading sequence autocorrelations. Then, the multipath components (despreader outputs) are correlated and significant performance loss may result from the use of RAKE receivers. Although the IPI can be reduced in a multicode technique with high SF, the performance of the RAKE receiver is limited by the inter-user interference (IUI) since each data channel acts as a separate user in a CDMA system. Since the interference (IPI and/or IUI) in a classical CDMA system is ignored, this limits capacity even if a tight power control is used. A theoretically more efficient way to detect different users in the reverse link is based on multi-user detection. The maximum likelihood (ML) receiver, which is the optimal (non-linear) receiver for Gaussian noise statistics, requires the knowledge of the channel parameters of all the users. The implementation of such a receiver is very complex in practice and may require short scrambling codes and further preliminary operations such as pre-whitening in the asynchronous case. As a more practical receiver, the Multistage Parallel Interference Canceler (PIC) appears a promising candidate for the front end of a base station due to its limited additional complication with respect to the conventional Rake receiver. Unlike the originally proposed multi

user detector (see S. Verdu,"Minimum Probabitity of Error for Asynchronous Gaussian Multiple Access Channels", IEEE Trans. Information Theory, vol. 32, pp. 85-96, Jan 1986), the PIC receiver does not require (one symbol duration) short scrambling codes or a pre-whitener in the asynchronous transmission case (reverse link). However, the PIC requires that the data symbols and the channel coefficients of all the users are estimated in order to create the multi-access interference estimate. As is well known, the efficient implementation of the PIC applies the concept of the interference estimation unit (lEU) as a general building block. Figure 1 shows the case of three stages and three users. At each stage, an Interference Estimation Unit (lEU) estimates the symbols transmitted by one user (Figure 2). The number of IEUs in each stage is the number of processed users. As the number of the stages increases, the estimation quality of the information sequences (of the considered users) and the residual error (ambient noise plus probably other low-rate users) should be improved. It is generally assumed that the performance of the conventional RAKE receiver is acceptable for low rate users and that the interference cancellation is necessary for the high rate users only. In the early cancellation stages, the reliability of symbol decisions is worse than at later stages and the cancellation of wrong estimates will probably add interference rather than remove it. It was proposed (example in N. Correal et al.,"Improved CDMA Performance through Bias Reduction for Parallel Interference Cancellation", Proc. Global Conference, Dec. 1997) to multiply the hard-decision (the projection shown in Figure 2 was simply a sign function) estimates by an empirical cancellation control factor a in order to cancel a fraction of the interference estimate rather than the full estimate if the detected symbols are thought to be unreliable. Initially it was proposed that a is simply set to a value that gives reasonable results.

N. Ishii et al.,"Multi-user Space Time Interference Cancellation System for CDMA Mobile Communications", Proc. VTC'99 conference, Sept. 1999, noted that the optimal empirically determined value Of a depends on the load of the system (number of users). This value seemed to be smaller when the system becomes overloaded. It has been proposed that the cancellation control factor be chosen from among a discrete set of values on the basis of bit error rate (BER) performance.

EP-A-954 112, the entire disclosure of which is herein incorporated by reference, discloses an arrangement in which the cancellation control factor is a step function of the measured signal to interference ratio (SIR). The SIR is compared with two thresholds to determine into which of three ranges the SIR falls. Depending on the result of this comparison one of three predetermined values is used as cancellation control factor. Whilst advantageous over earlier

systems, this technique requires an off-line trial-and-error test to configure the system and the system may not function optimally under varying loads.

Preferred embodiments of the present invention aim to provide a technique which does not necessarily rely on an off-line empirical determination of coefficients.

In a first aspect the present invention provides a method of determining a cancellation control factor (a) associated with a reception channel for use in an interference estimation unit (lEU) of a CDMA receiver, the method comprising obtaining a measure of the channel impulse response of the reception channel ; and determining the cancellation control factor based on the measure of the channel impulse response of the reception channel.

It has been appreciated pursuant to the present invention that a suitable cancellation control factor can be determined based on a measure of the channel impulse response of the reception channel. Previously this was not considered a relevant factor and only an ad hoc empirical value for a was used instead.

Hence the technique pursuant to the present invention may not require an off-line trial-and-error test to be carried out, and it may yield more accurate results than the conventional technique.

The cancellation control factor can then be used in the CDMA receiver in the estimation of the expectation value (yak) of an emitted data symbol in a sequence.

The estimation is then based on a received signal and the cancellation control factor.

It will be appreciated that in order to estimate the expectation value of the emitted data symbol a cancellation control factor need not explicitly be determined. Instead, the expectation value of the emitted data symbol can be estimated directly based on the received signal and the channel impulse response. Hence, in a closely related aspect the present invention provides a method of estimating in a COMA receiver the expectation value (âk) of an emitted data symbol in a sequence, the method comprising receiving a signal which is based on the sequence of emitted data symbols, via a channel which may be subject to noise;

obtaining a measure of the channel impulse response of the reception channel; and estimating the expectation value of the emitted data symbol based on the received signal and the measure of the channel impulse response.

However, in many applications a cancellation control factor will be determined.

Hence, preferably, estimating the expectation value of the emitted data symbol comprises forming a received symbol based on the received signal and weighting the received symbol by a cancellation control factor which is based on the measure of the channel impulse response.

Preferably, the cancellation control factor is also based on a measure of the signal to interference-plus-noise ratio or the signal to noise ratio.

The cancellation control factor may be substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.

For ease of implementation, the cancellation control factor can, for certain values of the received signal, be expressed as a constant, i. e. a zero order approximation; such an approximation can yield a useful correction. Hence, suitably, the cancellation control factor is substantially constant over a range of values of the received signal.

The present invention can be applied to a CDMA receiver which includes a plurality of interference cancellation stages, wherein each stage comprises a plurality of interference estimation units. The cancellation control factor may be determined at the first stage and remain constant for at least one (preferably for all) later stage (s); a constant value for several stages may be effective. In this case the cancellation control factor is preferably communicated from the first stage to the at least one later stage.

Alternatively, the cancellation control factor may be redetermined at at least one later stage (optionally redetermined at each stage).

An approximation may also be used for estimating the expectation value of the emitted data symbol. Hence, preferably, estimating the expectation value (yak) of

the emitted data symbol comprises forming a received symbol (Zk) based on the received signal, the expectation value of the emitted data symbol preferably being a substantially linear function of the received symbol over a first range of values of the received symbol.

Preferably, the expectation value of the emitted data symbol is substantially constant over a second range of values of the received data symbol.

Instead of using a linear approximation to estimate the expectance of the sequence of emitted data symbols a more accurate calculation can be performed.

Hence, preferably, estimating the expectation value of the emitted data symbol comprises forming a received data symbol based on the received signal, and estimating the expectation value substantially as a function varying substantially as the hyperbolic tangent of a function of the product of the channel impulse response and the received data symbol.

Preferably, the expectation value is estimated as

ca a =t) =-ov

wherein âk is the expectation value of the emitted data symbol, E (x I y) is the expectation value of x given y, ak is the emitted (binary) data symbol, Zk is the received symbol, c is the channel impulse response, Og is the absolute value of the binary data signal ak, and Ov is the square root of the variance of the noise.

Again, employing a piecewise approximation, the expectation value may be estimated as

wherein âk is the expectation value of the emitted binary data symbol, a is a cancellation control factor determined based on the channel impulse response, Zk is the received data symbol, and Og is the absolute value of the binary sequence ak, Preferably, the cancellation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.

The present invention also provides processing means for estimating a cancellation control factor (a) associated with a reception channel for use in an interference estimation unit (lEU) of a CDMA receiver, the processing means comprising means for obtaining a measure of the channel impulse response of the reception channel ; and means for determining the cancellation control factor based on the measure of the channel impulse response of the reception channel.

As used herein, the term"processing means"encompasses (but is not limited thereto) hardware for carrying out the specified functions, software (whether stored on a storage medium or loaded onto suitable hardware) for instructing suitable hardware to perform the specified functions and any equivalents.

Preferably, the processing means further comprises means for estimating the expectation value (âk) of an emitted data symbol of a sequence based on a received signal and the cancellation control factor.

In a further aspect the present invention provides processing means for estimating in a CDMA receiver the expectation value (âk) of an emitted data symbol in a sequence, the processing means comprising

means for receiving a signal which is based on the sequence of emitted data symbols, via a channel which may be subject to noise ; means for obtaining a measure of the channel impulse response of the reception channel ; and means for estimating the expectation value of the emitted data symbol based on the received signal and the measure of the channel impulse response.

The apparatus may include means for estimating path information; this information is preferably stored and transferred to all the interference estimation units.

The apparatus may also include means for transforming the received chip rate signal into symbol rate signals wherein the number of symbol rate signals is equal to the number of paths detected in the received chip rate signal. The apparatus may further include means for combining the symbol rate signals into one signal. The apparatus will normally include means for obtaining a measure of the signal to interference-plus-noise ratio.

Preferably, the means for estimating the expectation value of the emitted data symbol comprises means for forming a received symbol based on the received signal and means for weighting the received symbol by a cancellation control factor which is based on the measure of the channel impulse response.

Preferably, the cancellation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.

As an approximation, the cancellation control factor is preferably substantially constant over a range of values of the received signal.

Preferably, the means for estimating the expectation value (âk) of the emitted data symbol comprises means for forming a received symbol (Zk) based on the received signal, the means for estimating the expectation value (âk) being arranged to estimate the expectation value of the emitted data symbol in

substantially linear dependency upon the received symbol over a first range of values of the received symbol. Preferably, the means for estimating the expectation value (yak) is arranged to estimate the expectation value of the emitted data symbol substantially as a constant over a second range of values of the received symbol.

Preferably, the means for estimating the expectation value of the emitted data symbol comprises means for forming a received data symbol based on the received signal, wherein the means for estimating the expectation value is arranged to estimate the expectation value substantially as a function varying substantially as the hyperbolic tangent of a function of the product of the channel impulse response and the received data symbol.

Preferably, the means for estimating the expectation value is arranged to estimate the expectation value as

\ co ov I Jc'A* o 2

wherein âk is the expectation value of the emitted data symbol, E (x I y) is the expectation value of x given y, ak is the emitted data symbol, Zk is the received symbol, c is the channel impulse response, Og is the absolute value of ak, and Oy is the square root of the variance of the noise.

Employing a piecewise approximation, the means for estimating the expectation value is preferably arranged to estimate the expectation value as

0 = o ^ ex ov ! z,) > } a g (ss | kl a

wherein as is the expectation value of the emitted data symbol, a is a cancellation control factor determined based on the channel impulse response, Zk is the received data symbol, and oa is the absolute value of ak.

The present invention also provides a CDMA receiver including a plurality of interference cancellation stages, each stage comprising a plurality of interference estimation units, the interference estimation units of the first stage comprising the above processing means, and means for communicating respective cancellation control factors to at least one later stage. This may reduce complexity.

Alternatively, each interference estimation unit may comprise the above processing means.

In a preferred implementation, the invention provides a method of obtaining a binary data symbol from a received signal comprising: receiving a chip rate signal ; despreading the chip rate signal to obtain pilot symbol rate signals ; Rake combining the pilot symbol rate signals to form a received signal ; estimating the channel impulse response; estimating SINR ; determining a cancellation control factor using the estimates of channel impulse response and SINR ; and determining the expectance of a binary data symbol using the cancellation control factor.

In a further aspect the present invention provides a computer program comprising instructions for carrying out the above method (s).

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows the structure of a multistage parallel interference canceler ; and Fig. 2 shows functions of an Interference Estimation Unit (lEU).

Fig. 3 shows a block diagram of structure in accordance with the present invention; and Fig. 4 shows a flow diagram illustrating a method in accordance with the present invention.

The invention is particularly advantageous when applied to demodulating binary data signals as the processing is simplified and the embodiment to be described is concerned with such a case. The techniques disclosed and the principle of making use of channel impulse response may however be applied to other applications Referring to Figure 1, it is assumed that at the output of the Rake combiner of the CDMA receiver the discrete time (real-valued) signal ==c. ak +Vk is received,

where c is a scalar (memoryless) channel impulse response, a E {-a +a} is the deterministic unknown emitted sequence and Vk is a Gaussian white noise with zero mean and variance Ou 2. Considering the unconstrained (infinite-alphabet) problem of the detection of ak based on the received signal Zkl the optimal Maximum a Posteriori (MAP) solution is given

x-x oa = lak I = 1, t/) =e is the hyperbolic tangent function and E (x I Y) ex +e x

is the conditional expectation value of X given y. The detected symbol is

allowed to take any value in the interval [-aa, oJ depending on the noise component Vk in the signal zk. When the value of the signal Zk approaches zero, the probability of obtaining wrong hard decisions increases which may degrade dramatically the following processing (final detection or future iterations). It A would be preferable in these kind of situations to use the value oak rather than or < ign ?k). In practice an approximation to the non-linear hyperbolic tangent function, for example a piecewise linear approximation corresponding to clipped (saturated) soft decisions, may give satisfactory results. Thus,

'4 < COT, 2 a Uv 2 o' () ! > cer.

c wherein (Ta is known and the coefficient ;. is nothing else than the ratio of the v c'o signal to interference-plus-noise ratio SINR--- -and the channel at the v v

output of the Rake combiner. Then the two quantities (SINR and c) can be estimated at the first stage of the PIC by the known pilot symbols. Now, we

0-. r-fm have appreciated that the factor c--"t-- (in Figure 2) is not only a cry c

function of SINR as was suspected previously but also of the Rake combining method. As an example, this may be purely spatial Minimum Mean Squares Error (MMSE). The two forms currently used in most systems and well known by those skilled in the art are Received Signal Strength Indicator (RSSI) and Interference Signal Strength Indicator (ISSI) combining methods. Thus, the segment-projection block introduced in the interference estimation unit (Figure 2)

< , V < L a may be approximated by the function ' crasE > -a

The factor a is a positive real number and it is not necessarily constrained as had previously been suggested to be less than one.

The structure of an embodiment of a present invention will now be described with reference to Figures 1 to 3. Figures 1 and 2 are largely conventional and will not be described in detail. It is noted, however that there are a plurality of IEUs, typically one for each user, and an advantage of the embodiment at least is that the cancellation control factor may be determined independently for each user. Although in Figs. 1 and 2 the arrangement shows the cancellation control factor being communicated to a multiplying unit, it will be appreciated that these functions can be integrated. Referring to Figure 3 a processor 10 is connected to receiver antenna 20 via despreader 22 and Rake combiner 24, channel impulse estimator 30, signal to interference plus noise ratio estimator 40 and interference estimation unit 50. The despreader 22 receives path information from path information store 26. Whilst in the figure the processor, receiver antenna, channel impulse estimator and signal to interference plus noise ratio estimator are shown as separate units, it will appreciated that some or all of their functions may be implemented on a single chip and further that they may form part of the interference estimation unit. However, for ease of illustration they will be described separately. Channel impulse estimator 30, connected to processor 10, estimates the channel impulse response c based on the received pilot signal received by receiver antenna 20. Likewise, signal to interference plus noise ratio estimator 40, connected to processor 10, estimates the signal to interference plus noise ratio based on the received pilot signal. Processor 10 determines the cancellation control factor a on the basis of the estimated channel impulse response c and the signal to interference plus noise ratio, for example as the square root of the ratio of the signal to interference plus noise ratio and the channel impulse response. Processor 10 then communicates the cancellation control factor a to interference estimation unit 50 where it is used to determine the expectance of a sequence of emitted data symbols based on a sequence of formed received data symbols.

Certain components may be shared among channels ; for example the path information store may supply path information to many EUs. Although estimation of SINR and channel impulse response is in this embodiment carried out independently for each IEU, a common processor may perform the calculations for several or all users.

Figure 4 shows a flow diagram of steps performed to determine the expectance of a sequence of emitted data symbols based on a sequence of estimated data symbols. Initially, a chip rate signal (which contains at least 4 samples per chip) is received (100). Next, pilot symbol rate signals are obtained by despreading the previous signal on the basis of the knowledge of the timing information and the spreading sequence of the user of interest (110). Then the signal Zk is formed by Rake combining the previous signals (120). The channel impulse response c is estimated (130) and the signal to interference plus noise ratio SINR is estimated (140) based on the received pilot signal. The cancellation control factor a is then determined based on the channel impulse response c and the signal to interference plus noise ratio (150). Thereafter, the expectance âk of the sequence of emitted data symbols is determined based on the sequence of estimated data symbols Zk and the cancellation control factor a (160).

Whilst Figure 4 shows a separate step 150 of determining the cancellation control factor a it will be appreciated that the expectance âk can be determined without determining a cancellation control factor a.

Certain advantages offered by at least preferred embodiments (it is noted that embodiments which do not necessarily possess all advantages may still retain certain benefits of the invention) are listed below : - 1) The interference cancellation control factor, introduced intuitively for convergence purposes, is chosen to be the square-root of the tangent of a non linear function (hyperbolic tangent function for the Gaussian interference case) at the origin.

2) The interference cancellation control factor appears as a result of an optimization problem and not an empirical design parameter to the interference cancellation structure. A segment projection bloc was included in the lEU (Figure 2) to take into account the fact that the detected information symbols are not

constrained to be the discontinuous values but they are allowed to take any value in the segment.

3) The computation of this factor uses the signal to interference-plus-noise ratio (SINR) and the channel at the output of the Rake combiner. Since the SINR is usually estimated for the power control mechanism, the only addition to a normal structure is a scalar channel estimate.

4) The channel estimate can be estimated at the first stage and remains constant for (some or typically all) the later stages. Alternatively, it can be recalculated for each stage.

5) The tedious unrealistic off-line trial-and-error test may be completely avoided in this proposal.

6) Unlike previous proposals, we find that the interference cancellation control factor does not depend only on SINR but also on the (1 D or 2D) Rake combining method.

7) The interference cancellation control factor is user-dependent. This dependence is typically implicit.

While the present invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made to the invention without departing from its scope as defined by the appended claims.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Claims

Claims 1. A method of determining a cancellation control factor (a) associated with a reception channel for use in an interference estimation unit (lEU) of a CDMA receiver, the method comprising obtaining a measure of the channel impulse response of the reception channel ; and determining the cancellation control factor based on the measure of the channel impulse response of the reception channel.
2. A method according to Claim 1, further comprising estimating the expectation value (âk) of an emitted data symbol in a sequence based on a received signal and the cancellation control factor.
3. A method of estimating in a CDMA receiver the expectation value (âk) of an emitted data symbol in a sequence, the method comprising receiving a signal which is based on the sequence of emitted data symbols, via a channel which may be subject to noise; obtaining a measure of the channel impulse response of the reception channel ; and estimating the expectation value of the emitted data symbol based on the received signal and the measure of the channel impulse response.
4. A method according to Claim 3, wherein estimating the expectation value of the emitted data symbol comprises forming a received symbol based on the received signal and weighting the received symbol by a cancellation control factor which is based on the measure of the channel impulse response.
5. A method according to any of Claims 1,2 or 4, wherein the cancellation control factor is also based on a measure of the signal to interference-plus-noise ratio or the signal to noise ratio.
6. A method according to any of Claims 1,2 or 4, wherein the cancellation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.
7. A method according to any of Claims 1,2, 4 or 5, wherein the cancellation control factor is substantially constant over a range of values of the received signal.
8. A method according to any of Claims 1,2 or 4 to 7, wherein the CDMA receiver includes a plurality of interference cancellation stages, each stage comprising a plurality of interference estimation units, wherein the cancellation control factor is determined at the first stage and remains constant for at least one later stage.
9. A method according to Claim 8, wherein the cancellation control factor remains constant for all further stages.
10. A method according to any of Claims 1,2 or 4 to 7, wherein the CDMA receiver includes a plurality of interference cancellation stages, each stage comprising a plurality of interference estimation units, the method further comprising redetermining the cancellation control factor at at least one later stage.
11. A method according to Claim 2 or 3, wherein estimating the expectation value (âk) of the emitted data symbol comprises forming a received symbol (Zk) based on the received signal, and wherein the expectation value of the emitted data symbol is a substantially linear function of the received symbol over a first range of values of the received symbol.
12. A method according to Claim 11, wherein the expectation value of the emitted data symbol is substantially constant over a second range of values of the received data symbol.
13. A method according to any preceding claim, wherein the cancellation control factor is calculated independently for a plurality of interference estimation units each corresponding to respective users.
14. A method according to Claim 13, wherein the expectation value is estimated as

c (3 CO ==0. tanh (JV

wherein âk is the expectation value of the emitted data symbol, E (x I y) is the expectation value of x given y, ak is the emitted data symbol, Zk is the received symbol, c is the channel impulse response, oa is the absolute value of ak, and Oy is the square root of the variance of the noise.
15. A method according to Claim 12 comprising estimating the expectation value as

a t a*Z Vjz zea raj a k a (oV) a a

wherein âk is the expectation value of the emitted data symbol, a is a cancellation control factor determined based on the channel impulse response, Zk is the received data symbol, and Oa is the absolute value of ak.
16. A method according to Claim 15, wherein the cancellation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.
17. Processing means for estimating a cancellation control factor (a) associated with a reception channel for use in an interference estimation unit (lEU) of a CDMA receiver, the processing means comprising means for obtaining a measure of the channel impulse response of the

reception channel ; and means for determining the cancellation control factor based on the measure of the channel impulse response of the reception channel.
18. Processing means according to Claim 17, further comprising means for estimating the expectation value (âk) of an emitted data symbol of a sequence based on a received signal and the cancellation control factor.
19. Processing means for estimating in a CDMA receiver the expectation value (a) of an emitted data symbol in a sequence, the processing means comprising means for receiving a signal which is based on the sequence of emitted data symbols, via a channel which may be subject to noise; means for obtaining a measure of the channel impulse response of the reception channel ; and means for estimating the expectation value of the emitted data symbol based on the received signal and the measure of the channel impulse response.
20. Processing means according to Claim 13, wherein the means for estimating the expectation value of the emitted data symbol comprises means for forming a received symbol based on the received signal and means for weighting the received symbol by a cancellation control factor which is based on the measure of the channel impulse response.
21. Processing means according to any of Claims 17, 18 or 20, wherein the cancellation control factor is also based on a measure of the signal to interference-plus-noise ratio or the signal to noise ratio.
22. Processing means according to any of Claims 17,18 or 20, wherein the cancellation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.
23. Processing means according to any of Claims 17, 18,20 or 21, wherein the cancellation control factor is substantially constant over a range of values of the received signal.
24. Processing means according to Claim 18 or 19 or any of Claims 20 to 23 as dependent thereon, wherein the means for estimating the expectation value (âk) of the emitted data symbol comprises means for forming a received symbol (Zk) based on the received signal, the means for estimating the expectation value (yak) being arranged to estimate the expectation value of the emitted data symbol in substantially linear dependency upon the received symbol over a first range of values of the received symbol.
25. Processing means according to Claim 24, wherein the means for estimating the expectation value (âk) is arranged to estimate the expectation value of the emitted data symbol substantially as a constant over a second range of values of the received symbol.
26. Processing means according to Claim 18 or 19, wherein the means for estimating the expectation value of the emitted data symbol comprises means for forming a received data symbol based on the received signal, and wherein the means for estimating the expectation value is arranged to estimate the expectation value substantially as a function varying substantially as the hyperbolic tangent of a function of the product of the channel impulse response and the received data symbol.
27. Processing means according to Claim 26, wherein the means for estimating the expectation value is arranged to estimate the expectation value

as

c (Y CO d (a=oatanh- (3v

wherein âk is the expectation value of the emitted data symbol, E (x I y) is the expectation value of x given y, ak is the emitted data symbol, Zk is the received symbol, c is the channel impulse response, oa is the absolute value of ak, and

Ov is the square root of the variance of the noise.
28. Processing means according to Claim 25, wherein the means for estimating the expectation value is arranged to estimate the expectation value as

a f a*z,, vjzj < -a a a k) (IasignlVlZkl > (X

wherein yak is the expectation value of the emitted data symbol, a is a cancellation control factor determined based on the channel impulse response, Zk is the received data symbol, and oa is the absolute value of ak.
29. Process ; il-$-U meal-is ac% ^, % ^Jl% AA ; I ll t^f +L ^W % r. 2Q., Wkmrm ; r% tke farttdallat ; or% 29. Processing means according to Ciaim 28, wherein the cancei'ation control factor is substantially a function of the square root of a ratio of the signal to interference-plus-noise ratio and the channel impulse response.
30. A CDMA receiver including a plurality of interference cancellation stages, each stage comprising a plurality of interference estimation units, the interference estimation units of the first stage comprising processing means according to any of Claims 17, 18 or 20 or any of Claims 21 to 29 as dependent thereon, and means for communicating respective cancellation control factors to at least one later stage.
31. A CDMA receiver including a plurality of interference cancellation stages, each stage comprising a plurality of interference estimation units, each interference estimation unit comprising processing means according to any of Claims 17 to 29.
32. A method of determining a cancellation control factor (a) associated with a reception channel for use in an interference estimation unit (lEU) of a

CDMA receiver, the method comprising : obtaining a measure of the channel impulse response of the reception channel ; obtaining a measure of the SINR of the reception channel ; and determining the cancellation control factor based on the measure of the channel impulse response and the measure of the SINR of the reception channel.
33. A method according to any of Claims 1 to 16 or 32 for use in demodulating a binary data signal.
34. A method of obtaining a binary data symbol from a received signal comprising: receiving a chip rate signal ; despreading the chip rate signal to obtain pilot symbol rate signals ; Rake combining the pilot symbol rate signals to form a received signal ; estimating the channel impulse response; estimating SINR ; determining a cancellation control factor using the estimates of channel impulse response and SINR ; and determining the expectance of a binary data symbol using the cancellation control factor.
35. A computer program comprising instructions for carrying out the method according to any of Claims 1 to 16 or 32 to 35.
36. A method, processing means, a CDMA receiver or a computer program, substantially as herein described.