WO2006066765A1

WO2006066765A1 - Selecting peak delay values for a rake receiver

Info

Publication number: WO2006066765A1
Application number: PCT/EP2005/013413
Authority: WO
Inventors: Xiaohui Wang
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2004-12-20
Filing date: 2005-12-12
Publication date: 2006-06-29

Abstract

Multipath components of transmitted data symbols are received with individual delays and processed by a RAKE unit having a number of fingers. A plurality of delay profiles are calculated, and delay values for peaks detected therein are determined. A number of peak delay values for at least one of the profiles are pre-selected, the number being larger than the number of fingers for the RAKE. The pre-selected values are ranked, so that a higher rank is given to peak delay values determined for more than one profile. A peak delay value for each finger of the RAKE unit is selected from the pre-selected and ranked values, such that the selected values are those having the highest ranks. Hereby fading true paths can be selected instead of strong noise peaks also in time critical processes, where filtering of the delay profiles is not possible.

Description

Selecting peak delay values for a RAKE receiver

Technical Field of the Invention The invention relates to a method of receiving digital data symbols sent from a transmitter through a transmission channel of a communications network, in which individual multipath components of a transmitted data symbol are received with individual delays, and wherein received signals are processed by a RAKE unit having a number of fingers. The invention further relates to a receiver for coded digital data symbols, and to a corresponding computer program and computer readable medium.

Description of Related Art

In wireless communications systems the physical channel between a trans- mitter and a receiver is typically formed by a radio link. As an example, the transmitter could be a base station, and the receiver could be a mobile station, or vice versa. In most cases the transmit antenna is not narrowly focused towards the receiver. This means that the transmitted signals may propagate over multiple paths. In addition to a possible direct path from the transmitter to the receiver, many other propagation paths caused by reflections from objects in the surroundings exist. 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 from various objects, such as buildings, moving vehicles or landscape details.

These different portions of the signal are a cause of interference in the receiver. Depending on the time resolution of the transmission system and the instantaneous phase relationship, portions with similar propagation distances combine at the receiver and form a distinct multipath component. The effect of the combining depends on the instantaneous relationship of the carrier wavelength and distance differences, and it may thus for a given multipath component be either enhancing or destructive. In case of destructive interference, the combining leads to significant decrease of the magnitude, or fad- ing, of the path gain for that path. Thus the gain of a true path may temporarily decrease considerably due to fading.

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

In these systems spreading and despreading are used. Data are transmitted from the transmitter side using a spread spectrum modulation technique wherein the data are scattered across a wide range of frequencies. Each channel is assigned a unique spreading code that is used to spread the data across the frequency range. The spreading code is a pseudo-random noise code and is composed of e.g. a binary sequence of 1's and O's, called "chips", which are distributed in a pseudo-random manner and have noise- like properties. The number of chips used to spread one data bit, i.e. chips/bit, may vary, and it depends, 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 with the same spreading code using 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 instant in time, which can be difficult due to the multipath effects mentioned above. The performance of a CDMA receiver is improved by using a RAKE receiver, where each multipath component is assigned a despreader whose reference copy of the spreading code is delayed equally 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 produce a symbol estimate. Thus the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths. To achieve the best possible signal-to-noise ratio (SNR) at the output of the RAKE combiner, the signal energy from as many physical paths as possible should be collected. The changing delays of all known multipath components should be tracked, and new paths should be discovered quickly after they appear. This is typically achieved by using a path searcher unit with an observation window shorter than the full search area. In a practical delay estimation system the path searcher unit is used periodically to re-scan the delay range with the purpose of detecting new paths.

To overcome the effect of fading and the fact that also noise peaks may occur in the received signal, the instantaneous delay profile may be filtered or averaged over several path searching events, i.e. typically over several frames of the incoming signal. Although the paths under fading may not appear as strong peaks in the instantaneous power profile, their filtered values over a number of path searching events should be well above the noise floor, and thus they will be detected by the path searcher. On the opposite, there may be some noise peaks in the instantaneous power profile, but their fil- tered values should be smothered out due to the nature of randomness.

There are, however, time-critical processes in the CDMA systems, to which the filtering technique is not applicable. To this category belongs paging, i.e. the process of seeking a user equipment or mobile phone from e.g. a base station. In order to save the power of the user equipment, paging is managed in a discontinuous way so that the radio frequency unit of the user equipment needs to wake up from sleeping only from time to time. During the short wake up period, the receiver has to find the delays of the paths and perform e.g. automatic frequency corrections and paging indicator detections. If a paging Indication is detected for a receiver, it will decode associated information. Since the time intervals between the radio frequency wake up events are rather long, the power profiles of the multipaths measured during individual wake up periods are typically not correlated due to their dynamic characteris- tics or frequency drift, and hence the delay profiles for individual wake up periods can not be filtered to avoid the effect of fading.

Thus during the delay detections only the instantaneous power profiles can be considered, so that real paths under fading may not be detected, while noise peaks may be interpreted as paths and used by the RAKE and channel estimators to do the paging indicator detections and information decoding. Such a loss of true paths and involvement of noise peaks lead to degradations of the paging performance.

Therefore, it is an object of the invention to provide a method of detecting multipath components, which can improve the ability of selecting the true path delays, even when they are under fading and suppressing strong noise peaks also in time critical processes, where filtering of the delay profiles over several frames is not possible. It will be understood, though, that the principles of the invention are also applicable to other situations, when filtering over several frames is possible but not desirable.

Summary According to the invention the object is achieved in that the method comprises the steps of calculating a plurality of delay profile from a respective plurality of sets of received pilot signals and for each of the plurality of delay profiles determining delay values for peaks detected in the delay profile; preselecting a number of peak delay values among the peak delay values de- termined for at least one of said plurality of delay profiles, said number of preselected delay values being larger than the number of fingers for said RAKE unit; ranking the pre-selected peak delay values in dependence of whether they are determined for more than one of said plurality of delay profiles, so that for at least some of the peak delay values a higher rank is given to peak delay values that are determined for more than one of said plurality of delay profiles than to peak delay values that are determined for only one of said plurality of delay profiles; and selecting from the pre-selected and ranked peak delay values a peak delay value for each finger of the RAKE unit, such that the selected peak delay values are those having the highest ranks.

By ranking peaks that can be found in more than one delay profile higher than those that only occur in one of the delay profiles, true paths under fading are upgraded while noise peaks that only occur randomly are downgraded. In this way it is possible to select true peaks for the RAKE, even during fading, while noise peaks are suppressed, which leads to better performance of the RAKE in time critical situations.

The step of calculating a plurality of delay profiles may further comprise the steps of calculating a first delay profile from a first set of received pilot signals and determining delay values for peaks detected in said first delay profile; and calculating a second delay profile from a second set of received pilot sig- nals, said second set of pilot signals being received after said first set of pilot signals, and determining delay values for peaks detected in said second delay profile.

Further, the pre-selected peak delay values may be pre-selected among peak delay values determined for one of said plurality of delay profiles.

When the method further comprises the steps of calculating said second delay profile with a higher resolution than said first delay profile; and preselecting peak delay values among the peak delay values determined for said second delay profile, the peak delay values can be determined more precisely, because they are based on the delay profile having the best resolution.

In some embodiments the step of ranking the pre-selected peak delay values further comprises the step of dividing the pre-selected peak delay values into a first group and a second group, wherein peak delay values in said first group are given higher ranks than peak delay values in said second group. The step of ranking the pre-selected peak delay values may then further comprise the step of ranking in each of said first and second groups the preselected peak delay values according to a signal level of the corresponding peaks, such that higher peaks are given higher ranks. In this way it is ensured that a higher peak is more likely to be selected for the RAKE than a lower peak.

The pre-selected peak delay values may be divided into groups in such a way that peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay val- ues that are determined for only one of said plurality of delay profiles are placed in said second group. This ensures that a peak determined for only one of the groups is always placed in the second group independent of its signal level. Alternatively, the pre-selected peak delay values may be divided into groups in such a way that the peak delay value representing the peak having the highest signal level is placed in said first group, and that for the remaining peak delay values, peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group. By always placing the strongest peak in the first group, this peak will always be delivered to the RAKE. This may be advantageous because it seems unlikely that the strongest peak should be a noise peak. If it is not determined for the other delay profile it more likely that there has been a sudden change in the transmission channel, and thus it would be better to include this strong peak.

As mentioned, the invention further relates to a receiver for digital data symbols sent from a transmitter through a transmission channel of a communications network, in which individual multipath components of a transmitted data symbol are received with individual delays, the receiver comprising a RAKE unit having a number of fingers for processing received signals. When the receiver is arranged to calculate a plurality of delay profiles from a respective plurality of sets of received pilot signals and for each of the plurality of delay profiles determine delay values for peaks detected in the delay profile; pre- select a number of peak delay values among the peak delay values determined for at least one of said plurality of delay profiles, said number of preselected delay values being larger than the number of fingers for said RAKE unit; rank the pre-selected peak delay values in dependence of whether they are determined for more than one of said plurality of delay profiles, so that for at least some of the peak delay values a higher rank is given to peak delay values that are determined for more than one of said plurality of delay profiles than to peak delay values that are determined for only one of said plurality of delay profiles; and select from the pre-selected and ranked peak delay val- ues a peak delay value for each finger of the RAKE unit, such that the selected peak delay values are those having the highest ranks, a receiver is provided, which can improve the ability of selecting the true path delays, even when they are under fading and suppressing strong noise peaks also in time critical processes, where filtering of the delay profiles over several frames is not possible.

The receiver may further be arranged to calculate the plurality of delay profiles by calculating a first delay profile from a first set of received pilot signals and determining delay values for peaks detected in said first delay profile; and calculating a second delay profile from a second set of received pilot signals, said second set of pilot signals being received after said first set of pilot signals, and determining delay values for peaks detected in said second delay profile.

Further, the receiver may be arranged to pre-select the pre-selected peak delay values among peak delay values determined for one of said plurality of delay profiles.

When the receiver is further arranged to calculate said second delay profile with a higher resolution than said first delay profile; and pre-select peak delay values among the peak delay values determined for said second delay profile, the peak delay values can be determined more precisely, because they are based on the delay profile having the best resolution. In some embodiments the receiver is further arranged to rank the preselected peak delay values by dividing the pre-selected peak delay values into a first group and a second group, wherein peak delay values in said first group are given higher ranks than peak delay values in said second group. The receiver may then further be arranged to rank the pre-selected peak delay values by ranking in each of said first and second groups the pre-selected peak delay values according to a signal level of the corresponding peaks, such that higher peaks are given higher ranks. In this way it is ensured that a higher peak is more likely to be selected for the RAKE than a lower peak.

The receiver may further be arranged to divide the pre-selected peak delay values into groups in such a way that peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group. This ensures that a peak determined for only one of the groups is always placed in the second group independent of its signal level. Alternatively, the receiver may be arranged to divide the pre-selected peak delay values into groups in such a way that the peak delay value representing the peak having the highest signal level is placed in said first group, and that for the remaining peak delay values, peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group. By always placing the strongest peak in the first group, this peak will always be delivered to the RAKE. This may be advantageous because it seems unlikely that the strongest peak should be a noise peak. If it is not determined for the other delay profile it more likely that there has been a sudden change in the transmission channel, and thus it would be bet- ter to include this strong peak.

In some embodiments, the receiver may be a WCDMA receiver. The invention also relates to a computer program and a computer readable medium with program code means for performing the method described above.

Brief Description of the Drawings

The invention will now be described more fully below with reference to the drawings, in which

figure 1 shows an example of multiple paths between a base station and a mobile station,

figure 2 shows a delay profile for the paths illustrated in figure 1 ,

figure 3 shows a sampled delay profile corresponding to the profile shown in figure 2,

figure 4 illustrates the use of a path searcher in combination with a RAKE receiver,

figure 5 shows a sampled delay profile with a low resolution and a subsequent sampled delay profile with a higher resolution,

figure 6 shows a flow chart illustrating an embodiment for selecting delay values according to the invention,

figure 7 shows vectors representing peaks from the delay profiles of figure 5, and

figure 8 shows an example of how a target vector is being formed according to the flow chart of figure 6. Detailed Description of Embodiments

Figure 1 shows a situation in which a base station 1 and a mobile station 2 of a wireless communications 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 objects in the surroundings cause a number of indirect paths to exist. Two such paths are shown in the figure. One indirect path 4 is reflected from a house 5, while another path 6 is caused by reflection from another building 7.

Since the part of a signal transmitted via one of the indirect paths 4 and 6 has to travel a longer distance to arrive at the mobile station 2, compared to the part of the signal travelling via the direct path 3, multiple instances of the same signal will be received by the mobile station 2 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 the time t may look as illus- trated in figure 2, which shows 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. Together these peaks form the impulse response of the channel. In figure 2 the peak P₃ received at the time t₃ corresponds to the direct path 3 in figure 1 , while the peaks P₄ and P₆ received at the times t₄ and t₆, respectively, correspond to the indirect paths 4 and 6 in figure 1. Thus, as an example, it is seen that the delay of the path 6 (corresponding to the peak P₆) is larger than the delay of the path 3 (corresponding to the peak P₃).

The delay profile shown in figure 2 is an instantaneous delay profile, and in such profiles noise peaks often occur in addition to the peaks representing the true peaks. Further, peaks representing true paths may momentarily fade, e.g. due to negative interference in the channel. Thus the true paths are not necessarily identical to the highest peaks in the instantaneous delay profile. However, the true paths typically have stable delay values for some time, while the noise peaks occur randomly. Therefore, the instantaneous delay profiles are normally filtered over a number of transmission frames in order to decrease the effect of the noise peaks.

Typically, the delay profile of the received signal will not be available as a continuous curve as the one illustrated in figure 2. Instead, it will consist of a number of sample values. This is illustrated in figure 3, which shows a sampled power delay profile corresponding to the continuous delay profile shown in figure 2. For each delay value τ_t (where / e [l,M], i.e. the full possible delay range), the corresponding power value g(τø is shown. In this case the available estimates of the power delay profile constitutes a contiguous sequence of equally spaced samples, τ_t = τ₀ + iΔτ.

The mobile station 2 and the base station 1 may be adapted for use in e.g. a Code Division Multiple Access (CDMA) system or a Wideband Code Division Multiple Access (WCDMA) system, and in that case the mobile station 2 may use a RAKE receiver, which is capable of identifying and tracking the various multipath signals for a given channel. In this way the energy or power of several multipath components can be utilized in the receiver. In the RAKE receiver each multipath component is assigned a despreader whose reference copy of the spreading code is delayed equally to the path delay of the corre- sponding multipath component. The outputs of the despreaders, i.e. the fingers of the RAKE receiver, are then coherently combined to produce a symbol estimate. Thus the RAKE receiver requires knowledge of the multipath delays and the values of the channel impulse response for all paths. The signal energy from as many physical paths as possible should be collected. This knowledge can be obtained from the delay profiles.

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

Since the structure of the propagation channel does not remain constant over time, the delays of existing paths change, old paths disappear and new paths appear. The changing delays of all known multipath components should be tracked, and new paths should be discovered quickly after they appear.

Therefore, infrequently activated limited-range path searchers are typically used for detecting new paths and, in some implementations, for re-detecting temporarily faded existing paths. This is illustrated in figure 4, in which a path searcher 11 is used in combination with a RAKE receiver 12.

The path searcher 11 is a device that periodically computes instantaneous impulse response estimates (complex or power) over a range of delays that is also called a path search window. The complex or power values for a given delay value may be estimated e.g. by correlating the received data for pilot symbols with an appropriately delayed copy of the spreading sequence. Since the path searcher 11 is mainly used only to detect the existence of paths, its output resolution may be lower than that required by the RAKE re- ceiver 12.

As mentioned, during normal reception of data the instantaneous delay profiles are typically filtered over a number of frames to reduce the effects of noise peaks and fading. However, in the CDMA systems there are also some time critical processes for which the filtering technique can not be used. Paging is an example of such a time critical process. Paging is the process where e.g. a base station seeks to get in contact with a passive user terminal. Most of the time the passive user terminal is in a sleep mode, and only from time to time it wakes up from the sleep mode to check if a paging signal for that terminal is present. During this short time the device has to find the delays of the paths in the transmission channel, perform e.g. automatic frequency corrections and paging indicator detections, and if a Paging Indicator (Pl) is actually detected for the device it will decode the information on the Paging Channel (PCH), which e.g. in a 3GPP network is transmitted on the Secondary Common Control Physical Channel (S-CCPCH). The available time for path detection during the wake-up period does not allow the use of filtering as described above, and since the time intervals between the wake- up events are rather long, the delay profiles measured during different wake- up periods are typically not sufficiently correlated to be used for filtering. In this situation the method described below can be used to reduce the effects of noise peaks and fading. It will be understood, though, that the embodiments of the invention are also applicable to other situations, when filtering is possible but not desirable.

The first step in estimating the true path delays during a wake-up period is to perform a coarse path search of an instantaneous delay profile over a delay range of e.g. 128 chips with a resolution of e.g. one chip. In the upper part of figure 5 is for illustrational purposes shown an instantaneous delay profile 21 with a delay range of 64 chips with a resolution of e.g. one chip. In this example the coarse path searcher (Search A) will find six peaks, i.e. PAL PA₂, PA₃, PA4, PA_S and PA_6> some of which may represent true paths, while others may represent noise peaks. The purpose of this coarse path search is to find the regions of the strongest paths, and the results are used to do the automatic frequency corrections and to perform a new and finer path search, i.e. a path search having a finer resolution, of a new instantaneous delay profile. In a practical implementation this finer path search could for example have a coverage of 32 chips with a resolution of a quarter chip. In the lower part of figure 5 a finer instantaneous delay profile 22 covering 16 chips with a resolution of a quarter chip and corresponding to the delay profile of the upper part of the figure is shown. The fine path search (Search B) now finds the six peaks psi, PB2, PB3, PB4, PBS and psβ-

Due to the better resolution of Search B this search gives a better estimation of the delay values, and thus it is better to use these values, or some of them, by the RAKE unit and the channel estimators for detecting the Paging Indicator and decoding the Paging Channel instead of those obtained from Search A. However, it can be seen by comparing the upper and lower part of figure 5 that peaks PBI , PB2, PB3 and pes correspond to peaks PAI, PA3, PA4 and PA_S, and thus these peaks can be expected to represent true paths, since they have at least been stable for the time between capturing the two delay profiles. On the other hand, peaks p_A2, PA_Θ, PB4 and p_Bβ only occur in one of the profiles, and thus they are more likely to be noise peaks. If for instance the three highest peaks from Search B were used for the RAKE, p_Bi, PB2 and PB4 would be selected with the consequence that the noise peak p_B4 would be included instead of PB₃ or PB_S that represent true paths, and such a loss of true paths and involvement of noise peaks lead to degradations of the paging performance.

One of the fundamental differences between true paths and noise peaks is that the former have stable delay values over time, while the latter occur ran- domly. Therefore the true delays are those which can be detected repeatedly or appear and last for some period of time, while the noise peaks appear and disappear from measurement to measurement. As mentioned, filtered delay profiles are not available for the time-critical events, but it is possible keep track of the delay values of the strong paths between the few calculated de- lay profiles, such as those shown as Search A and Search B in figure 5, and then re-arrange the significances of the paths in an instantaneous delay profile accordingly. Thus instead of using the highest peaks from Search B, these peaks can be re-arranged by assigning a higher priority to the peaks that are also known from Search A.

According to an illustrative embodiment, which is described below, the strongest path is always selected no matter whether it is a known path or an emerging path. Then a desired number of strong paths are chosen from the instantaneous power profile, i.e. Search B, as the candidates, and they are divided into two groups. The number of the delay candidates should be larger than the typical number of the multipaths or RAKE fingers, yet much smaller than the possible delay values of the path searchers so that as many true paths as possible, yet as few noise peaks as possible are involved in this process. The higher prioritised group contains the paths that were also known from Search A and it is placed closer to the strongest path, while the lower prioritised one consists of the emerging paths and is placed farther away. In each of the groups, the delays with higher power values have higher priority to be delivered to the RAKE etc. It can here be noted that comparing paths detected by the path searchers even with different resolutions is unambiguous in the way that one can identify a path as a known path if its delay to a previous one differs by less than or equal to a pre-defined value, e.g. +/- Vz chip.

A flowchart 100 of a possible implementation of this approach is shown in figure 6. An initial vector B contains the delay candidates directly from the instantaneous power profile at hand, i.e. Search B, and a target vector T is formed by the delay candidates with the desired order ready to be delivered to the RAKE etc.

First, in step 101 the peaks of the vector B are re-arranged according to their size so that the highest peaks are placed at the top of the re-arranged vector B¹. Figure 7 shows the original vectors A and B representing the peaks from Search A and Search B, respectively, and the re-arranged vector B¹. In step 102 the delay of the strongest peak is inserted as the first element in the target vector T, which has the same dimension as the initial vector B¹ of the candidate delays. In figure 8 it is illustrated that the strongest peak pei is inserted as the first element in the vector T.

The value of the next delay candidate from B' is then in steps 103 and 104 read and compared with vector A showing the delays from the previous search, i.e. Search A. If the peak is known from A it is inserted (in step 105) as the next element in the higher prioritized group of the target vector T, i.e. directly after the previously inserted elements in that group. If, however, the next peak is not known from A it is inserted (in step 106) as the last element of the target vector T, while the previously inserted elements in the lower prioritized group, if any, are shifted one step upwards. If there still remain can- didates (step 107), step 103 and the following steps are repeated until all elements of vector B¹ have been processed. Finally, in step 108 a number of the highest elements in the target vector T corresponding to the number of RAKE fingers, starting from the highest prioritized element are selected and delivered to the RAKE and the channel estimator.

Figure 8 shows as an example how the target vector T is being formed when the peaks from figure 5 are used. First, as mentioned above, the strongest peak PBI is inserted as the first element. The next peak in B' is PB₄, and since this peak is not known from A it is inserted as the last element in T. Peaks PB2 and p_B3, which are both known from A, are then inserted after p_Bi. Then P_B6, which is not known from A, is inserted as the last element in T, while ps₄ is shifted one step up. Finally pes is inserted after ps3, because it is also known from A. Vector T is now complete with elements p_Bi, PB2_> PB₃ and p_B5 in the higher prioritized group and elements p_B4 and p_Bβ in the lower prioritized group. The three highest elements PBI , P_B2 and p_B3 are then selected as the vector T to be delivered to the RAKE, because in this example the RAKE is assumed to have three fingers.

It is noted that step 101 is optional, since embodiments where no sorting is done are also possible, i.e. no vector B' is created. Instead, in each iteration the vector B may be searched for the largest peak not yet processed.

Above, vector A comprises the peaks detected during a first coarse path search, while vector B comprises the peaks detected during a subsequent finer path search, i.e. a search having a higher resolution. The target vector T is obtained from vector B by giving a higher priority to the peaks that are also known from vector A. Although this gives a more precise determination of the delay values because the peaks in vector B are determined with a higher resolution, it is of course also possible to obtain the target vector T from vector A by giving a higher priority to the peaks that are also known from vector B. The idea is that peaks that occur in both searches are given a higher priority than those occurring in only one of the two searches. In the embodiment described above, the delay of the strongest peak is inserted in step 102 as the first element in the target vector T, even if the peak should not be known from the first search, i.e. from vector A. This is done because it seems unlikely that the strongest peak should be a noise peak. If it is not known from the first search it is more likely that there has been a sudden change in the transmission channel, and thus it would be better to include this strong peak. However, step 102 may of course also be omitted, so that all peaks of vector B¹ are compared to vector A and placed in vector T in dependence of whether it is already known from vector A according to steps 103-106.

Further, instead of arranging the delay candidates in a target vector T they may be given a priority value in other ways according to the same principles, and then the delay candidates to be used by the RAKE can be selected based on the priority values.

It should also be noted that the embodiments described herein can easily be generalized without departing from the scope of the invention. For example, instead of pre-selecting only peak delay values from the peak delay values determined for one of the delay profiles, one might perform the pre-selection of peak delay values from the union of peak delay values determined for both the first and second delay profiles, and give a higher rank to those peak delay values that appear in both delay profiles. The invention could also be generalized to using three or more delay profiles (possibly with different resolution), where the pre-selection of peak delay values is made from the peak delay values determined for either one or several of the delay profiles. The ranking might then be generalized to involve more than two groups. For example, the highest ranked group could comprise those peak delay values that are determined for all of the involved delay profiles, the second highest ranked group those peak delay values that are determined for all but one of the involved delay profiles, and so on. Although a preferred embodiment of the present invention has been described and shown, the invention is not restricted to it, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.

Claims

P a t e n t c l a i m s :

1. A method of receiving digital data symbols sent from a transmitter through a transmission channel of a communications network, in which individual multipath components of a transmitted data symbol are received with individual delays, and wherein received signals are processed by a RAKE unit (12) having a number of fingers, c h a r a c t e r i z e d in that the method comprises the steps of: • calculating a plurality of delay profiles (21, 22) from a respective plurality of sets of received pilot signals, and for each of the plurality of delay profiles determining delay values for peaks detected in the delay profile;

• pre-selecting a number of peak delay values (p_Bi, PB2, PB₃, PB4_> PB_S and PBQ) among the peak delay values determined for at least one of said plurality of delay profiles, said number of pre-selected delay values being larger than the number of fingers for said RAKE unit (12);

• ranking the pre-selected peak delay values (PBI, PB2, PB₃, PB4, PB_S and p_Bβ) in dependence of whether they are determined for more than one of said plurality of delay profiles, so that for at least some of the peak delay values a higher rank is given to peak delay values that are determined for more than one of said plurality of delay profiles than to peak delay values that are determined for only one of said plurality of delay profiles; and • selecting from the pre-selected and ranked peak delay values a peak delay value for each finger of the RAKE unit (12), such that the selected peak delay values are those having the highest ranks.

2. A method according to claim 1, c h a r a c t e r i z e d in that the step of calculating a plurality of delay profiles further comprises the steps of: • calculating a first delay profile (21) from a first set of received pilot signals and determining delay values for peaks detected in said first delay profile; and

• calculating a second delay profile (22) from a second set of received pilot signals, said second set of pilot signals being received after said first set of pilot signals, and determining delay values for peaks detected in said second delay profile.

3. A method according to claim 1 or2, c h a r a c t e r i z e d in that the pre-selected peak delay values (PBI, PB2, PB₃, PB4, PB_S and psβ) are pre-selected among peak delay values determined for one of said plurality of delay profiles.

4. A method according to claim 2, c h a r a c t e r i z e d in that the method further comprises the steps of:

• calculating said second delay profile (22) with a higher resolution than said first delay profile (21); and

• pre-selecting peak delay values (p_B-ι, PB2, PB₃, PB4, PBS and p_Bβ) among the peak delay values determined for said second delay profile (22).

5. A method according to any one of claims 1 to4, c h a r a c t e r i z e d in that the step of ranking the pre-selected peak delay values (p_Bi, PB2, PB3, PB4, PBS and psβ) further comprises the step of dividing the pre- selected peak delay values into a first group and a second group, wherein peak delay values in said first group are given higher ranks than peak delay values in said second group.

6. A method according to claim 5, c h a r a c t e r i z e d in that the step of ranking the pre-selected peak delay values (p_Bi, PB2, PB3, PB4, PB_S and ps_δ) further comprises the step of ranking in each of said first and second groups the pre-selected peak delay values according to a signal level of the corresponding peaks, such that higher peaks are given higher ranks.

7. A method according to claim 5 or 6, c h a r a c t e r i z e d in that the pre-selected peak delay values (p_Bi, PB2, PB3, PB4, PBS and p_B6) are divided into groups in such a way that peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group.

8. A method according to claim 5 or 6, c h a r a c t e r i z e d in that the pre-selected peak delay values (psi, PB2, PB3, PB4, PB_S and psβ) are divided into groups in such a way that the peak delay value representing the peak having the highest signal level is placed in said first group, and that for the remaining peak delay values, peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group.

9. A receiver for digital data symbols sent from a transmitter through a transmission channel of a communications network, in which individual multipath components of a transmitted data symbol are received with individual delays, the receiver comprising a RAKE unit (12) having a number of fingers for processing received signals, c h a r a c t e r i z e d in that the receiver is arranged to:

• calculate a plurality of delay profiles (21 , 22) from a respective plurality of sets of received pilot signals, and for each of the plurality of delay profiles determine delay values for peaks detected in the delay profile;

• pre-select a number of peak delay values (PBI , PB2, PB3_> PB4, PB_S and PB₆) among the peak delay values determined for at least one of said plurality of delay profiles, said number of pre-selected delay values be- ing larger than the number of fingers for said RAKE unit;

• rank the pre-selected peak delay values (p_Bi, PB2, PB₃, PB4, PB_S and Pee) in dependence of whether they are determined for more than one of said plurality of delay profiles, so that for at least some of the peak delay values a higher rank is given to peak delay values that are determined for more than one of said plurality of delay profiles than to peak delay values that are determined for only one of said plurality of- delay profiles; and • select from the pre-selected and ranked peak delay values a peak delay value for each finger of the RAKE unit (12), such that the selected peak delay values are those having the highest ranks.

10. A receiver according to claim 9, c h a r a c t e r i z e d in that the receiver is further arranged to calculate the plurality of delay profiles by:

• calculating a first delay profile (21 ) from a first set of received pilot signals and determining delay values for peaks detected in said first delay profile; and

11. A receiver according to claim 9 or 10, c h a r a c t e r i z e d in that the receiver is further arranged to pre-select the pre-selected peak delay values (PBI, PB2, PB₃, PB4, PB_S and pee) among peak delay values determined for one of said plurality of delay profiles.

12. A receiver according to claim 10, c h a r a c t e r i z e d in that the receiver is further arranged to:

• calculate said second delay profile (22) with a higher resolution than said first delay profile (21); and

• pre-select peak delay values (p_Bi, PB2, PB3» PB4, PB5 and p_Bβ) among the peak delay values determined for said second delay profile (22).

13. A receiver according to any one of claims 9 to 12, c h a r a c t e r i z e d in that the receiver is further arranged to rank the preselected peak delay values (p_Bi, PB2, PB3, PB4, PB_S and psβ) by dividing the pre-selected peak delay values into a first group and a second group, wherein peak delay values in said first group are given higher ranks than peak delay values in said second group.

14. A receiver according to claim 13, c h a r a c t e r i z e d in that the receiver is further arranged to rank the pre-selected peak delay values (PBI, PB2. PB₃, PB4, PB5 and p_B6) by ranking in each of said first and second groups the pre-selected peak delay values according to a signal level of the corresponding peaks, such that higher peaks are given higher ranks.

15. A receiver according to claim 13 or 14, c h a r a c t e r i z e d in that the receiver is further arranged to divide the pre-selected peak delay values (p_Bi, PB2, PB3, PB4, PBS and PBΘ) into groups in such a way that peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group.

16. A receiver according to claim 13 or 14, c h a r a c t e r i z e d in that the receiver is further arranged to divide the pre-selected peak delay values (p_Bi, PB2, PB3, PB4. PBS and psβ) into groups in such a way that the peak delay value representing the peak having the highest signal level is placed in said first group, and that for the remaining peak delay values, peak delay values that are determined for more than one of said plurality of delay profiles are placed in said first group, while peak delay values that are determined for only one of said plurality of delay profiles are placed in said second group.

17. A receiver according to any one of claims 9 to 16, c h a r a c t e r i z e d in that the receiver is a WCDMA receiver.

18. A computer program comprising program code means for performing the steps of any one of the claims 1 to 8 when said computer program is run on a computer.

19. A computer readable medium having stored thereon program code means for performing the method of any one of the claims 1 to 8 when said program code means is run on a computer.