HK1116308A - Selecting delay values for a rake receiver - Google Patents

Description

Selecting delay values for a RAKE receiver

Technical Field

The invention relates to a method of receiving digital data symbols transmitted from a transmitter over a transmission channel of a communication network, wherein individual multipath components of the transmitted data symbols are received with individual delays, and wherein the received signal is processed by a RAKE unit having a plurality of fingers (fingers), the method comprising the steps of: calculating a delay profile from a set of received pilot signals; and determining a delay value for a peak detected in the delay profile. The invention also relates to a receiver of encoded digital data symbols as well as a corresponding computer program and computer readable medium.

Background

In wireless communication systems, the physical channel between a transmitter and a receiver is typically formed by a radio link. As an example, the transmitter may be a base station and the receiver may be a mobile station, or vice versa. In most cases, the transmit antenna is not finely aligned with the receiver. This means that the transmitted signal may propagate through multiple paths. In addition to the possible direct path from the transmitter to the receiver, there are many other propagation paths due to reflections from surrounding objects. Thus, the receiver may receive multiple instances of the same signal at different times (i.e., with different delays) because different portions of the signal are reflected by various objects such as buildings, moving vehicles, or topographical features.

These different parts of the signal are the cause of interference in the receiver. Depending on the time resolution and instantaneous phase relationship of the transmission system, the portions with similar propagation distances combine at the receiver to form unique multipath components. The effect of this combination depends on the instantaneous relationship of the carrier wavelength and the range difference, and thus may be either additive or destructive for a given multipath component. In the case of destructive interference, the combination results in a significant reduction in the magnitude of, or attenuation of, the path gain of the path. The gain of the real path may temporarily be significantly reduced due to attenuation.

Many transmission systems attempt to reduce the effects of multipath propagation and fading using a receiver that combines the data symbol energy from all multipath components. In Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) systems, so-called RAKE receivers may be employed to utilize the energy of different received portions of a signal in the receiver.

In these systems, spreading and despreading are used. Data is transmitted from the transmitter side using a spread spectrum modulation technique that spreads the data over a wide frequency range. Each channel is assigned a unique spreading code for spreading the data over the frequency range. The spreading code is a pseudo-random noise code and consists of binary sequences of, for example, 1's and 0's (called "chips"), which are distributed in a pseudo-random manner and have noise-like properties. The number of chips (i.e., chips/bit) used to spread a data bit may vary and is dependent, at least in part, on the data rate of the channel and the chip rate of the system.

In the receiver, the received signal must be despread and demodulated using the same spreading code at the same chip rate to recover the transmitted data. Furthermore, the timing of the demodulation must be synchronized, i.e. the despreading code must be applied to the received signal at the correct moment, which may be difficult due to the above mentioned multipath effects. The performance of a CDMA receiver is improved by employing a RAKE receiver in which each multipath component is assigned a despreader whose reference copy of the spreading code is delayed to be equal to the path delay of the corresponding multipath component. The outputs of the despreaders (i.e., the fingers of the RAKE receiver) are then coherently combined to generate symbol estimates.

The RAKE receiver therefore requires knowledge of the values of the channel impulse responses and the multipath delays for all paths. In order to obtain the best possible signal-to-noise ratio (SNR) in the output of the RAKE combiner, the signal energy should be collected from as many physical paths as possible. The varying delays of all known multipath components should be tracked and new paths should be discovered quickly after they occur. This is typically achieved using a path searcher unit with a shorter viewing window than the full search area. In a practical delay estimation system, in order to detect a new path, a path searcher is periodically used to rescan the delay range.

The performance of a CDMA receiver depends heavily on the quality of the multipath delay detection unit. If the detected multipath delays deviate from the correct values, the transmission power carried by these paths is at least partially lost and the noise level increases, degrading the performance of the receiver. A common way to accurately find the multipath delay is to accumulate the power profile of the received pilot signal over a sufficiently long time and then filter the delay profile over many radio frames to mitigate the effects of fading. The power profile thus obtained is fairly stable and the detected delays are then passed to the RAKE and channel estimator for further demodulation of the user data.

One of the basic requirements of such accurate delay detection is obviously that the time for accumulating pilot symbols and filtering the power profile is long enough. However, in CDMA systems there are time critical processes that cannot guarantee this, and for these processes filtering techniques are therefore not applicable. Paging is in the category of the process of finding a user device or mobile phone, for example, from a base station. In order to conserve power in the subscriber device, paging is managed in a discontinuous manner so that only the radio frequency unit of the subscriber device needs to be awoken from sleep. During a short wake-up period, the receiver must find the delay of the path and perform, for example, automatic frequency correction and page indicator detection. If a paging indication is detected for the receiver, it will decode the relevant information. In this case, the detection time is critical, and thus the delay detection is usually rather coarse, which may result in e.g. missing paging indications and decoding errors of the paging message if no further measures are taken.

Although for normal non-time critical cases (such as demodulating information in connected mode) the path search is performed by repeatedly calculating and filtering delay profiles to obtain the delays of stationary peaks averaged over time, for time critical events, the delay values of the channel estimator and RAKE must be determined from the instantaneous delay profiles, since long time filtered profiles are temporarily not available.

Even for non-time critical cases, where the delays derived from the filtered delay profiles are averaged to the optimum values, these delays may deviate from the optimum delays by several time slots, degrading the performance of the receiver during these periods. However, the overall degradation is very limited, especially after deinterleaving and decoding.

Conversely, the problem with time critical events is more severe because the degradation of receiver performance due to inaccurate delay detection cannot be averaged out or corrected over many time slots or frames. Thus, for example, for page indication detection, only a few page indication symbols are accumulated to determine whether a page is present. If it fails, the receiver will miss the paging indication and get an error in the paging message.

It is therefore an object of the present invention to provide a method of detecting multipath components that improves the ability to select the correct and accurate path delay also in time critical processes where the delay profile cannot be filtered over multiple frames. It should be understood, however, that the principles of the present invention are also applicable to other situations where filtering over multiple frames is feasible but not desirable.

Disclosure of Invention

According to the invention, this object is achieved in that the method further comprises the steps of: pre-selecting a plurality of peak delay values among the peak delay values determined for the delay profile, the pre-selected peak delay values representing a largest peak detected in the delay profile; calculating, for each of the preselected peak delay values, a signal-to-interference ratio of delay values in a section around the preselected peak delay value; selecting a delay value having the highest signal-to-interference ratio in each interval; and providing the selected delay values to the RAKE unit and assigning each selected delay value to one finger of the RAKE unit.

By monitoring the signal-to-interference ratio values around the delays from the path searcher and then relocating the fingers of the RAKE according to the highest signal-to-interference ratio values, the performance of the receiver is significantly improved in time critical situations such as paging, since fairly accurate delay values can be provided in a short time after waking up the receiver. At the same time, the required computational resources can still be kept low, since the signal-to-interference ratio values only need to be calculated for a few delay values around each peak.

When the method further comprises the step of rearranging the selected delay values to satisfy a minimum separation constraint, it is ensured that different RAKE fingers actually track different multipath delay signals.

In one embodiment, the method further comprises the steps of: processing the received signal to obtain pilot symbols and user data symbols simultaneously; and using the pilot symbols in calculating the signal-to-interference ratio. By processing the pilot signal and the user data signal simultaneously, a fast solution is achieved, since the user data symbols are ready as soon as they are needed. The method may then further comprise the steps of: calculating a channel estimate for each of said selected delay values; providing user data symbols corresponding to the selected delay value; and combining the channel estimates with the provided user data symbols. Alternatively, the method may further comprise the steps of: calculating a channel estimate for each of said selected delay values; setting channel estimates for the remaining delay values to zero; providing user data symbols corresponding to all delay values; and combining the channel estimates with the provided user data symbols.

In another embodiment, the method further comprises the steps of: storing the received signal; processing the received signal to obtain pilot symbols; using the pilot symbols in calculating the signal-to-interference ratio; processing the stored signal to obtain user data symbols corresponding to the selected delay value; calculating a channel estimate for each of said selected delay values; and combining the channel estimates with the provided user data symbols. By storing the received signal and then processing the user data signal only for the selected delays after selecting the delay values for the RAKE fingers, computational resources can be saved.

As mentioned, the invention also relates to a receiver for receiving digital data symbols transmitted from a transmitter over a transmission channel of a communication network, wherein individual multipath components of the transmitted data symbols are received with individual delays, the receiver comprising a RAKE unit having a plurality of fingers for processing the received signal, and the receiver being arranged to: calculating a delay profile from a set of received pilot signals; and determining a delay value for a peak detected in the delay profile. The receiver is further arranged to: pre-selecting a plurality of peak delay values among the peak delay values determined for the delay profile, the pre-selected peak delay values representing a largest peak detected in the delay profile; calculating, for each of the preselected peak delay values, a signal-to-interference ratio of delay values in a section around the preselected peak delay value; selecting a delay value having the highest signal-to-interference ratio in each interval; and providing the selected delay values to the RAKE unit and assigning each selected delay value to one finger of the RAKE unit, a receiver is provided that improves the ability to select the correct and accurate path delays also in time critical processes where the delay profile cannot be filtered over multiple frames.

When the receiver is further arranged to rearrange the selected delay values to meet a minimum separation constraint, it is ensured that different RAKE fingers are actually able to track different multipath delay signals.

In one embodiment, the receiver is further arranged to: processing the received signal to obtain pilot symbols and user data symbols simultaneously; and using the pilot symbols in calculating the signal-to-interference ratio. By processing the pilot signal and the user data signal simultaneously, a fast solution is achieved, since the user data symbols are ready as soon as they are needed. The receiver may then be further arranged to: calculating a channel estimate for each of said selected delay values; providing user data symbols corresponding to the selected delay value; and combining the channel estimates with the provided user data symbols. Alternatively, the receiver may then be further arranged to: calculating a channel estimate for each of said selected delay values; setting channel estimates for the remaining delay values to zero; providing user data symbols corresponding to all delay values; and combining the channel estimates with the provided user data symbols.

In another embodiment, the receiver is further arranged to: storing the received signal; processing the received signal to obtain pilot symbols; using the pilot symbols in calculating the signal-to-interference ratio; processing the stored signal to obtain user data symbols corresponding to the selected delay value; calculating a channel estimate for each of said selected delay values; and combining the channel estimates with the provided user data symbols. By storing the received signal and then processing the user data signal only for the selected delays after selecting the delay values for the RAKE fingers, computational resources can be saved.

In some embodiments, the receiver may be a WCDMA receiver.

The invention also relates to a computer program and a computer readable medium having program code means for performing the above method.

Drawings

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which:

fig. 1 shows an example of multiple paths between a base station and a mobile station;

FIG. 2 shows a power delay profile of the path illustrated in FIG. 1;

FIG. 3 shows a sampled delay profile corresponding to the profile shown in FIG. 2;

fig. 4 shows a schematic diagram of a RAKE receiver;

FIG. 5 illustrates a sampling delay profile with low resolution and the calculation of signal-to-interference ratios around peaks in the delay profile;

fig. 6 shows a schematic diagram of a RAKE receiver comprising a relocation unit;

FIG. 7 shows a flow chart illustrating the principle of delayed relocation; and

fig. 8 shows a flow chart illustrating the calculation of the signal-to-interference ratio.

Detailed Description

Fig. 1 shows a case where a base station 1 and a mobile station 2 of a wireless communication system communicate with each other. As an example, a signal transmitted from the base station 1 is received by the mobile station 2. However, the transmitted signal travels along multiple paths from the base station to the mobile station. In this case, there is a direct and unobstructed propagation path 3, but in addition to this direct path, reflections from surrounding objects may result in multiple indirect paths. Two such paths are shown in the figure. One indirect path 4 is reflected from a house 5 and the other path 6 is caused by a reflection from another building 7.

Since the portion of the signal transmitted via one of the indirect paths 4 and 6 must travel a longer distance to reach the mobile station 2 than the portion of the signal traveling via the direct path 3, the mobile station 2 will receive multiple instances of the same signal at different times (i.e., with different delays).

Thus, if a pilot signal is transmitted from the base station 1, the power P received at the mobile station 2 as a function of time t may look as shown in fig. 2, fig. 2 showing an example of a power delay profile. The power delay profile shows all signals received at the mobile station, including noise and interference signals. However, only the peaks in the power delay profile correspond to the multipath components of the transmitted signal. These peaks together form the impulse response of the channel. In FIG. 2, at time t₃Received peak P₃Corresponding to the direct path 3 in fig. 1, at time t₄And t₆Received peak P₄And P₆Corresponding to the indirect paths 4 and 6, respectively, in fig. 1. Thus, as an example, one can see path 6 (corresponding to peak P)₆) Is greater than path 3 (corresponding to peak P)₃) The delay of (2).

The delay profile shown in fig. 2 is an instantaneous delay profile, and in such a profile, a noise peak generally occurs in addition to a peak representing a true peak. Furthermore, the peaks representing the real paths may be temporarily attenuated by negative interference, e.g. in the channel. The true path is not necessarily equivalent to the highest peak in the instantaneous delay profile. However, the real path usually has a stable delay value within a certain time, and the noise peak appears randomly. In addition, the delay of the real path may also differ slightly between slots or frames. Therefore, the instantaneous delay profile is typically filtered over multiple transmission frames to reduce the effects of noise peaks and obtain accurate and stable delay values.

In general, the delay profile of the received signal cannot be obtained as a continuous curve as illustrated in fig. 2. Instead, the delay profile will include a plurality of sample values. This is illustrated in fig. 3, which fig. 3 shows a sampled power delay profile corresponding to the continuous delay profile shown in fig. 2. For each delay value τ_i(where i ∈ [1, M)]I.e. the full possible delay range), the corresponding power value g (τ) is shown_i). In this case, a usable estimate of the power delay profileForming a continuous sequence of equally spaced samples, tau_i＝τ₀+iΔτ。

The mobile station 2 and the base station 1 may be adapted for use in, for example, a Code Division Multiple Access (CDMA) or Wideband Code Division Multiple Access (WCDMA) system, and in that case the mobile station 2 may use a RAKE receiver that is capable of identifying and tracking various multipath signals for a given channel. In this way, the energy or power of the multiple multipath components can be utilized in the receiver. In a RAKE receiver, each multipath component is assigned a despreader whose reference copy of the spreading code is delayed to be equal to the path delay of the corresponding multipath component. The outputs of the despreaders (i.e., the fingers of the RAKE receiver) are then coherently combined to generate symbol estimates. The RAKE receiver therefore requires knowledge of the values of the channel impulse responses and the multipath delays for all paths. Signal energy should be collected from as many physical paths as possible. This knowledge can be obtained from the delay profile.

Although reference is made herein to a RAKE receiver in a mobile station, it should be noted that the algorithm described below may be used in any CDMA receiver, i.e. in a mobile station or a base station, and the transmission direction may be uplink or downlink.

Since the structure of the propagation channel does not remain constant over time, the delays of the existing paths change, the old paths disappear and new paths appear. The varying delays of all known multipath components should be tracked and new paths should be discovered quickly after they occur. Thus, a limited-range path searcher, which is activated infrequently, is typically used to detect new paths, and in some implementations to re-detect temporarily attenuated existing paths. This is illustrated in fig. 4, which shows a schematic diagram of a RAKE receiver.

In the receiver, the received spread data signal is supplied to a path searcher 11 and a RAKE unit 12. The path searcher 11 is a device that periodically calculates instantaneous impulse response estimates (complex numbers or powers) over a range of delays, also referred to as a path search window. The complex or power value for a given delay value may be estimated, for example, by correlating the received data of the pilot symbols with appropriately delayed copies of the spreading sequence. Since the path searcher 11 is mainly used only to detect the presence of paths, its output resolution may be lower than that required by the RAKE unit 12. The detected path delays (i.e., the delays representing the peaks in the delay profile) are then transmitted to the RAKE unit 12 and the channel estimator 13.

The received signal is then despread in RAKE unit 12, wherein a RAKE finger is assigned to each reported delay and each RAKE finger gives a complex despread data symbol. In the channel estimator 13, channel estimates for the respective paths are calculated based on the despread data symbols provided by the RAKE unit 12 and the detected delays provided by the path searcher 11. In combiner 15, the despread data symbols provided by RAKE unit 12 are multiplied by the conjugate channel estimate (provided by conjugate unit 14) and the result is used for further decoding in decoder 16.

In connected mode, i.e. when the receiver is already in the process of receiving a data signal, the usual way to find the multipath delay accurately is to accumulate the power profile of the pilot over a sufficiently long time and then filter the delay profile over many radio frames so that the effect of the attenuation is mitigated and a stable peak averaged over time is obtained.

As mentioned, during normal reception of data, the instantaneous delay profile is typically filtered over multiple frames to ensure stable and correct values. However, in CDMA systems, there are also some time critical processes for which filtering techniques cannot be employed. Paging is an example of such a time critical process. Paging is the process by which, for example, a base station seeks to get in contact with a passive user terminal. Most of the time, a passive user terminal is in a sleep mode and only sometimes wakes up from the sleep mode to check whether there is a paging signal for the terminal. During this short time, the device has to find the path delay in the transport channel, e.g. perform automatic frequency correction and paging indicator detection, and if a Paging Indicator (PI) for the device is actually detected, it decodes the information related to the Paging Channel (PCH), which is transmitted on the secondary common control physical channel (S-CCPCH), e.g. in a 3GPP network. The time available for path detection during the wake-up period does not allow for the filtering as described above, and since the time interval between wake-up events is rather long, the delay profiles measured during different wake-up periods are typically not sufficiently correlated to be available for filtering. In this case, the following method can be employed to ensure a stable and correct value. It should be understood that embodiments of the present invention are also applicable to other situations where filtering is possible but not desirable.

The first step in estimating the true path delay during the wake-up period is to perform a coarse path search on the instantaneous delay profile over a delay range of, for example, 128 chips at, for example, a one chip resolution. The upper part of fig. 5 shows for illustration purposes an instantaneous delay profile 21 with a resolution of e.g. one chip and a delay range of 64 chips. In this example, the coarse path searcher finds six peaks, P_A1、P_A2、P_A3、P_A4、P_A5And P_A6. The purpose of this coarse path search is to find the area of the strongest path, although the result is not very accurate, the delay coming out of the path searcher is usually not far from the true value due to the low resolution and the fact that the delay profile has not been filtered yet. Thus, since the RAKE and channel estimators actually require precise delays to get the highest power and lowest noise for each path, the delays provided by this first coarse path search can be improved by searching for delays corresponding to the highest power and lowest noise for each path around the delays provided by this first coarse path search (e.g., slot-based), and then repositioning the RAKE fingers and channel estimator accordingly. This may be achieved by calculating a signal-to-interference ratio (SIR) for the delay values within the cell around the respective delay provided by the coarse path search. In fig. 6, this is performed in the relocation unit 17. Since the SIR values are only calculated for a few delay values around the found peak, the computational resources required for SIR calculation are rather limited, which makes thatEnabling the solution to be implemented also in receivers having only limited processing power.

According to one embodiment, the RAKE fingers are therefore not located to the delays directly from the path searcher. Instead, the SIR values around the first few strongest delays (e.g. one quarter chip to both sides of the strong path, or three quarters of a chip to both sides of the strong path) are monitored and the delay with the highest SIR value is then selected as the delay candidate for that path. This is shown in the lower part of fig. 5, and fig. 5 shows the peak intensity at the four strongest peaks (i.e., P in the upper part of the figure)_A1、P_A3、P_A4And P_A5) The surrounding calculated SIR values. It can be seen that in this example, the peak P is_A1The surrounding highest SIR is found at a delay value of one quarter chip above the delay initially provided by the coarse path search, and for the peak P_A3The initially provided value is actually the value with the highest SIR value. For peak P_A4The highest SIR value is found at a delay value of one half chip above the initially provided delay, for peak P_A5Found at a delay value of one quarter chip below the original delay.

A flow chart 100 illustrating the principle of the delayed relocation performed in the relocation unit 17 is shown in fig. 7. First, in step 101, a delay is received from the path searcher 11. In step 102, SIR values around the delay of the strongest path are calculated and monitored. How to calculate the SIR value will be described later with reference to the flowchart of fig. 8. Next, in step 103, the delay having the highest SIR value of the SIR values just calculated is selected and stored as the delay representing the path. The path just processed is then masked out (mask out), i.e. removed from the group of paths to be examined, in step 104, and it is determined whether more paths need to be considered in step 105. If more paths need to be considered, steps 102 to 105 are repeated for the currently strongest path. In the example illustrated in fig. 5, four strongest paths are considered, which corresponds to a RAKE unit with four fingers. When a sufficient number of paths have been processed, the saved delays may be rearranged in step 106 to meet a minimum spacing constraint that requires a certain minimum distance between adjacent paths. This constraint ensures that different RAKE fingers actually track different multipath delayed signals. In the example of fig. 5, all paths have satisfied the constraint. The resulting delays are then used in the RAKE fingers and channel estimator to demodulate the user data in step 107. The location of the RAKE delays can be determined on a slot-by-slot basis to enable the receiver to more efficiently utilize the transmitted power in a dynamic manner.

As mentioned, the flow chart 200 in fig. 8 shows how the SIR value is calculated in step 102 of fig. 7. First, in step 201, channel estimates with the pilot pattern removed are read for all pilot symbols in a slot. It should be noted that for each delay from the coarse path search and its neighbors, the channel estimate is obtained by multiplying the despread pilot symbols from the RAKE by the complex conjugate of the corresponding transmitted pilot symbols to remove the pilot pattern. Subsequently, in step 202, the average of the channel estimates is calculated for each delay, and then the sum of the squares of the real and imaginary parts is calculated to obtain the estimated power. In step 203, the square of the variance of the channel estimate is calculated for each delay to obtain the estimated interference. Finally, in step 204, the SIR for each slot is calculated for each delay as the ratio of the estimated power to the estimated interference.

The use of the resulting delays in the RAKE unit and channel estimator is different from the normal connected mode, where the delays to be used in a given slot can be calculated from previously received data. In the present case, these delays may need to be used to demodulate data received in the same time slot. Several different ways of doing this will be described below. The method chosen in a particular case may depend on the division between hardware and software in the implementation of the receiver. In the following implementation it will be appreciated that the channelization codes and pilot patterns are known in the receiver and the delays are the delays from the path searcher and their neighbors, so for example for the common pilot channel (CPICH), the RAKE can be operated directly after the coarse path search to get the SIR values and channel estimates for all possible delay candidates. The best delay value is then selected from the fine SIR profile and used to combine with the user data on the Dedicated Physical Channel (DPCH). On the other hand, the path searcher, which is a discrete unit and does not require RAKE, calculates the power profile for all sample points within the delay window to find the delays of the strong paths.

In one implementation, user data and a pilot signal from a common pilot channel (CPICH) are despread simultaneously, the CPICH symbols are used to calculate SIR values, and then the despread symbols and corresponding channel estimates are passed to a combiner only for delays located as described above.

An alternative is to pass the despread symbols for all delays from the path searcher and their neighbors to the combiner, but to set the channel estimates for the undetermined delays to zero for the combiner, thereby effectively excluding them from the combined symbols.

In a different implementation it is also possible to record the received signal while calculating SIR values using CPICH symbols, then despread only for the located delays and estimate the channel coefficients for these delays for the combiner.

Although embodiments of the invention have been described and shown, the invention is not restricted thereto but can also be implemented in other ways within the scope of the subject-matter defined in the following claims.

Claims (15)

1. A method of receiving digital data symbols transmitted from a transmitter over a transmission channel of a communication network, wherein individual multipath components of the transmitted data symbols are received at separate delays, and wherein the received signal is processed by a RAKE unit (12) having a plurality of fingers, the method comprising the steps of:

-calculating a delay profile (21) from a set of received pilot signals; and

-determining delay values for peaks detected in the delay profile, characterized in that the method further comprises the steps of:

preselecting a plurality of peak delay values (P) among the peak delay values determined for the delay profile_A1、P_A3、P_A4、P_A5) The preselected peak delay value represents a maximum peak detected in the delay profile;

for each of said preselected peak delay values (P)_A1，P_A3，P_A4、P_A5) Calculating a signal-to-interference ratio of delay values in an interval around the preselected peak delay value;

selecting the delay value with the highest signal-to-interference ratio in each interval; and

-providing the selected delay values to the RAKE unit (12) and assigning each selected delay value to one finger of the RAKE unit (12).

2. The method according to claim 1, characterized in that the method further comprises the steps of: the selected delay values are rearranged to satisfy a minimum spacing constraint.

3. Method according to claim 1 or 2, characterized in that the method further comprises the steps of:

processing the received signal to obtain pilot symbols and user data symbols simultaneously; and

using the pilot symbols in calculating the signal-to-interference ratio.

4. A method according to claim 3, characterized in that the method further comprises the steps of:

computing a channel estimate for each of said selected delay values;

providing user data symbols corresponding to the selected delay value; and

-combining said channel estimates with said provided user data symbols.

5. A method according to claim 3, characterized in that the method further comprises the steps of:

computing a channel estimate for each of said selected delay values;

setting the channel estimates for the remaining delay values to zero;

providing user data symbols corresponding to all delay values; and

-combining said channel estimates with said provided user data symbols.

6. Method according to claim 1 or 2, characterized in that the method further comprises the steps of:

storing the received signal;

processing the received signal to obtain pilot symbols;

using the pilot symbols in calculating the signal-to-interference ratio;

processing the stored signal to obtain user data symbols corresponding to the selected delay value;

computing a channel estimate for each of said selected delay values; and

-combining said channel estimates with said provided user data symbols.

7. A receiver for receiving digital data symbols transmitted from a transmitter over a transmission channel of a communication network, wherein individual multipath components of the transmitted data symbols are received with individual delays, the receiver comprising a RAKE unit (12) having a plurality of fingers for processing the received signal, and the receiver being arranged to:

-calculating a delay profile (21) from a set of received pilot signals; and

-determining delay values for peaks detected in the delay profile, characterized in that the receiver is further arranged to:

preselecting a plurality of peak delay values (P) among the peak delay values determined for the delay profile_A1、P_A3、P_A4、P_A5) The preselected peak delay value represents a maximum peak detected in the delay profile;

for each of said preselected peak delay values (P)_A1，P_A3，P_A4、P_A5) Calculating a signal-to-interference ratio of delay values in an interval around the preselected peak delay value;

selecting the delay value with the highest signal-to-interference ratio in each interval; and

-providing the selected delay values to the RAKE unit (12) and assigning each selected delay value to one finger of the RAKE unit (12).

8. A receiver according to claim 7, characterised in that the receiver is further arranged to rearrange the selected delay values to meet a minimum separation constraint.

9. The receiver according to claim 7 or 8, characterized in that the receiver is further arranged to:

processing the received signal to obtain pilot symbols and user data symbols simultaneously; and

using the pilot symbols in calculating the signal-to-interference ratio.

10. The receiver of claim 9, characterized in that the receiver is further arranged to:

computing a channel estimate for each of said selected delay values;

providing user data symbols corresponding to the selected delay value; and

-combining said channel estimates with said provided user data symbols.

11. The receiver of claim 9, characterized in that the receiver is further arranged to:

computing a channel estimate for each of said selected delay values;

setting the channel estimates for the remaining delay values to zero;

providing user data symbols corresponding to all delay values; and

-combining said channel estimates with said provided user data symbols.

12. Receiver according to claim 7 or 8, characterized in that the receiver is further arranged to:

storing the received signal;

processing the received signal to obtain pilot symbols;

using the pilot symbols in calculating the signal-to-interference ratio;

processing the stored signal to obtain user data symbols corresponding to the selected delay value;

computing a channel estimate for each of said selected delay values; and

-combining said channel estimates with said provided user data symbols.

13. Receiver according to any of claims 7 to 12, characterized in that the receiver is a WCDMA receiver.

14. A computer program comprising program code means for performing the steps of any one of claims 1 to 6 when said computer program is run on a computer.

15. A computer readable medium having stored thereon program code means for performing the method of any one of claims 1 to 6 when said program code means are run on a computer.