Abstract:
Method and receiver for processing a signal in a wireless communication system in which the signal comprises a sequence of chips. The signal is receive data at least one rake finger and sampled. There is a time spacing t 1  between successive samples less than the time spacing t c  between successive chips in the signal. Channel conditions on the channel are estimated and based on estimated channel conditions by the following steps: monitoring timing of the signal on one of the at least one rake finger to determine a time difference between the timing of the signal on the one of the at least one rake finger and the timing of the generation of the samples, the determined time difference being a multiple of t 2 , where t 2 &lt;t 1 ; aligning the timing of the generation of the samples with the timing of the signal on the one of the at least one rake finger to within a timing range t 2 .

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Great Britain Application No. GB 0916913.7 filed on Sep. 25, 2009, entitled “A METHOD AND SYSTEM FOR PROCESSING A SIGNAL.” The above application is commonly assigned with the present application and is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    This invention relates to a method and system for processing a signal, in particular in a wireless communication system. 
       BACKGROUND 
       [0003]    In wireless communication systems, such as a 3GPP Wide Band Code Division Multiple Access (WCDMA) system (3GPP TS 25.211, “Technical Specification Group Radio Access Network; Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, December 2005) a rake receiver is used to receive signals transmitted over a wireless channel. A rake receiver typically performs maximum ratio combining of the received signal by combining the received signals of different paths on the channel proportionally to the strength of each path. The rake receiver assumes that the channel has a limited number of paths and assigns a rake finger to each of the paths. Each rake finger implements the operation of descrambling and/or despreading the received signal where the timing at which the sequence of the signal is taken corresponds to the channel path delay to which the rake finger is assigned. The signal output from the rake receiver can be passed to a decoder for decoding the signal. 
         [0004]    The signal is typically sampled at a sampling rate 1/T, where T is the time spacing between samples. The sampling rate is generally chosen to be small in order to save power and memory usage in the receiver. However, according to the Nyquist theorem, the signal should be sampled at a sampling rate which is at least two times the bandwidth of the signal in order for recovery of all of the data in the signal to be possible. This means that a signal is typically sampled at a rate of approximately two times the bandwidth of the signal. The sampled signal is typically passed to an interpolation filter which samples the signal with a sampling rate of 
         [0000]    
       
         
           
             
               
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         [0000]    where N I  is an oversampling factor used by the interpolation filter. 
         [0005]    With reference to  FIG. 1   a  there is shown the timing of the signal samples which are input to the interpolation filter  106  having a time spacing between the samples of T. The timing of the samples taken by the interpolation filter is generally not perfectly aligned with the timing of the incoming signals.  FIG. 1   b  shows the timing of the samples which are output from the interpolation filter having a time spacing between the samples of 
         [0000]    
       
         
           
             
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         [0000]    The interpolation filter can only correct for timing errors of more than 
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         [0000]    This means that, due to the finite sampling rate of the interpolation filter, timing errors of less than 
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         [0000]    are not corrected by the interpolation filter. 
         [0006]    Increasing the oversampling factor of the interpolation filter allows the interpolation filter to correct smaller timing errors. Therefore the precision at which the path delays can be matched improves as the oversampling factor is increased. The value of the path delay can be matched by the interpolation filter with a precision of 
         [0000]    
       
         
           
             
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         [0000]    as would be apparent to one skilled in the art. It is therefore advantageous in terms of correcting for smaller timing errors to increase the oversampling factor used in the interpolation filter. However, increasing the oversampling factor used in the interpolation filter increases the complexity of the system. Increasing the complexity of the system can be problematic in that the performance of the system can be adversely affected. 
       SUMMARY 
       [0007]    In a first aspect of the present invention there is provided an embodiment of a method of processing a signal in a wireless communication system, the signal comprising a sequence of chips, the method comprising: receiving the signal at least one rake finger; sampling the received signal to generate samples of the signal, wherein there is a time spacing t 1  between successive samples which is less than the time spacing t c  between successive chips in the signal; estimating channel conditions on the channel; and based on the estimated channel conditions, selectively performing the steps of: monitoring the timing of the signal on one of the at least one rake finger to determine a time difference between the timing of the signal on the one of the at least one rake finger and the timing of the generation of the samples, the determined time difference being a multiple of t 2 , where t 2  is less than t 1 ; and using the determined time difference to align the timing of the generation of the samples with the timing of the signal on the one of the at least one rake finger to within a timing range t 2 . 
         [0008]    In a second aspect of the present invention there is provided an embodiment of a system for processing a signal in a wireless communication system, the signal comprising a sequence of chips, the system comprising: at least one rake finger for receiving the signal; means for sampling the received signal to generate samples of the signal, there being a time spacing t 1  between successive samples which is less than the time spacing t c  between successive chips in the signal; means for monitoring the timing of the signal on one of the at least one rake finger to determine a time difference between the timing of the signal on said one of the at least one rake finger and the timing of the generation of the samples, the determined time difference being a multiple of t 2 , where t 2  is less than t 1 ; means for using the determined time difference to align the timing of the generation of the samples with the timing of the signal on the one of the at least one rake finger to within a timing range t 2 ; means for estimating channel conditions on the channel; and means for determining, based on the estimated channel conditions, whether the timing of the generation of the samples is to be aligned with the timing of the signal on said one of the at least one rake finger to within a timing range t 2 . 
         [0009]    In a third aspect of the present invention there is provided an embodiment of a method of processing a signal in a wireless communication system, the signal comprising a sequence of chips, the method comprising: receiving the signal at a rake finger; sampling the received signal to generate samples of the signal, wherein there is a time spacing t 1  between successive samples which is less than the time spacing t c  between successive chips in the signal; estimating channel conditions on the channel; and based on the estimated channel conditions, adaptively switching in fine resolution timing correction method steps comprising: monitoring the timing of the signal on the rake finger to determine the timing of the signal on the rake finger to an accuracy of 
         [0000]    
       
         
           
             
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         [0000]    where t 2  is less than t 1 ; and aligning the timing of the generation of the samples with the timing of the signal on the rake finger to within a timing range t 2 . 
         [0010]    The introduction of higher order modulation in order to achieve a high throughput rate puts severe constraints on receiver performance. In particular, the inventors have realized that for the 3GPP WCDMA system, in its Release 7 where QAM and MIMO modes have been introduced, achieving high precision timing resolution becomes particularly crucial to attain the highest performance. The inventors have implemented a new approach that achieves arbitrarily high time precision without increasing complexity or having any impact on existing receiver structures. 
         [0011]    In some embodiments, a timing precision is achieved on the strongest rake finger corresponding to a grid with a fine timing resolution of 
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         [0000]    where N p  is a phase coefficient of the interpolation filter described in more detail below and takes a value greater than 1. In this way the timing of the strongest finger is found with a finer resolution than that provided by the interpolation filter alone (which can provide a timing resolution of 
         [0000]    
       
         
           
             
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         [0012]    Finger time tracking is used to decide where to place the strongest finger on a grid with time spacing 
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         [0000]    Then the allocation of the remaining fingers is decided with respect to the strongest finger based on a grid provided by the interpolation filter with time spacing 
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         [0013]    The fine timing correction of the strongest finger may be achieved digitally by using different filter coefficients corresponding to different phases for the sampling of the signal by the interpolation filter, each phase corresponding to a time shift multiple of 
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         [0000]    Alternatively, the timing adjustment may be made to the received signal before it reaches the interpolation filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the following drawings in which: 
           [0015]      FIG. 1   a  is a representation of the timing of incoming signal samples; 
           [0016]      FIG. 1   b  is a representation of the timing of samples generated by the interpolation filter; 
           [0017]      FIG. 2  is a schematic diagram of a system according to a preferred embodiment; 
           [0018]      FIG. 3  is a flow chart of a process of processing a signal according to an embodiment; 
           [0019]      FIG. 4  is a schematic diagram of a system according to an alternative embodiment; 
           [0020]      FIG. 5  is a representation of time shifts corresponding to different phase coefficients of the interpolation filter of an embodiment; 
           [0021]      FIG. 6  is a graph showing the results of a first simulation; and 
           [0022]      FIG. 7  is a graph showing results of a second simulation. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments. 
         [0024]    With reference to  FIG. 2  there is now described a receiver according to a preferred embodiment. The system  200  comprises a baseband chip  202  and a Radio Frequency (RF) chip  204 . The baseband chip  202  comprises an interpolation filter  206 , a rake receiver  208 , a finger tracker block  210 , a filter coefficient table block  212  and a controller  216 . An output of the radio frequency chip  204  is connected to a first input of the interpolation filter  206 . An output of the interpolation filter  206  is connected to a first input of the rake receiver  208  and also connected to an input of the finger tracker block  210 . A first output of the finger tracker block  210  is connected to a second input of the rake receiver  208 . A second output of the finger tracker block  210  is connected to an input of the filter coefficients table block  212 . An output of the filter coefficients table block  212  is connected to a second input of the interpolation filter  206 . An output of the rake receiver block  208  passes to a decoder. The output of the RF chip  204  is connected to an input of the controller  216 . An output of the controller  216  is connected to a third input of the interpolation filter  206 . 
         [0025]    The components shown in  FIG. 1  are implemented in software in the embodiment herein described, but the components could alternatively be implemented in hardware or firmware. The baseband chip  202  of the embodiment operates in the digital domain. The output of the RF chip  204  is passed through an analog to digital converter (not shown) before reaching the interpolation filter  206  and the controller  216 . 
         [0026]    In operation and with reference to  FIG. 3 , in step S 302  a signal is received wirelessly at the system  200  using a receiver (not shown in  FIG. 2 ). The signal is received over a channel which has at least one path. The received signal is passed to the RF chip  204 . The chip rate of the signal is larger than the symbol rate, such that one symbol in the signal is represented by multiple chips. 
         [0027]    As described above, the signal is typically sampled at a sampling rate 1/T which is approximately twice the bandwidth of the signal. The sampled signal is passed to the interpolation filter  206  from the RF chip  204 . The interpolation filter  206  has an oversampling factor N I , such that in step S 304  the interpolation filter samples the signal with a sampling rate of 
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         [0028]    In step S 306  the samples output from the interpolation filter  206  are input to the rake receiver block  208  which performs maximum ratio combining of the received signal as is known in the art. The path delays can be matched with a precision of 
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         [0000]    due to the sampling rate of the interpolation filter  206 . 
         [0029]    In step S 310  the channel conditions are estimated and in step S 312  the estimated channel conditions are used to determine whether timing errors of less than 
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         [0000]    will significantly degrade the quality of the signal when it is decoded. These steps are described below in greater detail. If timing errors of less than 
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         [0000]    will significantly degrade the quality of the signal then in step S 312  it is determined that fine timing correction is required and the method passes to step S 314 . However, if in step S 312  it is determined that fine timing correction is not required then there is no need to perform fine timing correction and so steps S 314  to S 320  are not performed to save unnecessary usage of processing resources. The method continues with step S 302  such that more of the signal is received and sampled with the current timing precision provided by the interpolation filter  206  of 
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         [0030]    The output of the interpolation filter  206  passes into an input of the finger tracker block  210 . Where in step S 312  it is determined that further timing correction is required, in step S 314  the finger tracker block  210  identifies the strongest finger, i.e. the path of the signal which has the greatest received power. In step S 316 , the finger tracker block  210  uses the samples from the interpolation filter  206  having a time spacing of 
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         [0000]    to track (i.e., monitor) the channel path delays, for example by using an early-late tracking algorithm. In this way, the extent to which the samples output from the interpolation filter  206  are aligned with the timing of the signal can be determined. An error in the matching between the samples output from the interpolation filter  206  and the timing of the signal leads to inter-symbol interference when the samples are descrambled and despread in the rake receiver block  208 . Such inter-symbol interference will deteriorate the quality of the decoded signal. The timing of the received signal (i.e., the time between transmission and receipt of the signal) is not known exactly at the receiver. The timing of the signal will depend upon the channel. It can be important to match the timing of the channel paths to the timing of the signal to reduce the inter-symbol interference in the decoded signal. As illustrated in the examples shown in Table 1, the degradation of the signal to interference ratio is more severe for higher timing errors. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Signal to Interference Ratio for different timing 
               
               
                 errors (T c : chip period) in AWGN channel conditions. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Timing Error 
                 
                   
                     
                       
                         
                           T 
                           c 
                         
                         64 
                       
                     
                   
                 
                 
                   
                     
                       
                         
                           T 
                           c 
                         
                         32 
                       
                     
                   
                 
                 
                   
                     
                       
                         
                           T 
                           c 
                         
                         16 
                       
                     
                   
                 
               
               
                   
                   
               
               
                   
                 Signal to Interference 
                 32.5 
                 26.5 
                 20.4 
               
               
                   
                 Ratio (dB) 
               
               
                   
                   
               
             
          
         
       
     
         [0031]    Table 1 shows results for a channel in which the timing error is a limiting factor for the signal to noise ratio of the decoded signal. In such channel conditions, reducing the timing error increases the signal to noise ratio. However, for channels in which the timing error is not a limiting factor for the signal to noise ratio of the decoded signal, the signal to noise ratio is limited by another factor (such as the dispersion of a multi-path channel) and so reducing the timing error does not increase the signal to noise ratio of the decoded signal. 
         [0032]    The inventors have realized that the consequence of timing error is particularly noticeable with the highest user categories that were introduced in the 3GPP Release 7 for the support of 64-QAM constellation and the MIMO (Multiple-Input-Multiple-Output) mode. In order to achieve high throughput rates, these features use higher order modulation than that used in most other features that have been used previously. This specific system has sixteen orthogonal code sequences in the signal which makes it particularly sensitive to timing errors. Reducing the timing errors to an acceptable level in this specific system by increasing the oversampling factor alone may cause a problem in terms of complexity. 
         [0033]    Although increasing the oversampling factor reduces timing errors, it does not always improve the performance of the system. As described above in relation to Table 1, reducing timing errors improves the performance of the system when the timing errors are the limiting factor for the performance of the system. However, when other factors are limiting the performance of the system reducing timing errors might not improve the performance of the system. Indeed, higher timing precision (i.e., higher oversampling factor) improves performance (i.e., improves the quality of the decoded signal) only when the quality of the signal itself allows it. In multi-path channels, the inter-path interference generated by the channel itself may be the dominant factor, such that the need for high timing precision is reduced. The strongest channel path (strongest rake finger) generates the most interference due to timing error and hence improving the timing precision of the strongest rake finger gives the greatest performance improvement. 
         [0034]    The single path channel when the channel has only one path is the case that shows the highest sensitivity to timing errors. In this case, the single path of the channel is treated as the strongest path. Therefore using a low complexity technique that achieves higher precision on the timing of the strongest finger (path) allows the recovery of the loss due to insufficient timing precision in both multi-path and single path channels. 
         [0035]    Embodiments of the present invention provide a timing correction mechanism that reduces the timing errors without increasing the oversampling factor. Furthermore, the timing correction mechanism is adaptive in the sense that it is used only when it is required, thereby reducing unnecessary use of computing power. 
         [0036]    In order to determine when the timing correction mechanism is required in step S 312 , the received signal is passed to the controller  216  which is shown to be on the baseband chip  202  in  FIG. 2 , but the controller  216  may alternatively be located somewhere other than on the baseband chip  202  in the system  200 . Furthermore,  FIG. 2  shows the received signal passing to the controller  216  from the RF chip  204 . In other embodiments, the signal may be passed to the controller  216  from the output of the interpolation filter  206 . The controller  216  estimates the channel conditions to determine whether a more precise timing correction is required than that provided by the oversampling in the interpolation filter  206 , which in the best case scenario can prevent timing errors of 
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         [0000]    or more, but cannot prevent timing errors of less than 
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         [0037]    For example the determination made by the controller  216  may be dependent upon the dispersion of the channel. Where there is a dispersion on the channel greater than a threshold level, reducing the timing errors will not improve the performance of the system, so a more precise timing correction is not required. As another example, the signal to noise ratio (SNR) of the signal may be used to determine whether the more precise timing correction mechanism is required. Where the SNR of the signal is below a threshold level, the noise of the channel may be limiting the performance of the system such that a more precise timing correction is not required. 
         [0038]    The strongest rake finger is identified in step S 314  as described above. A fine timing resolution is used for the timing allocation of the strongest finger such that the timing of the signal on the strongest finger is found with a fine resolution of 
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         [0000]    where N p  is a phase number coefficient used in the interpolation filter  206  as described below and is greater than 1. This means that the timing of the signal on the strongest rake finger is found to a greater precision than is given by the samples output by the interpolation filter  206 . 
         [0039]    Relative to the fine timing resolution used for allocating the strongest finger, the coarser timing resolution of 
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         [0000]    provided by the interpolation filter  206  is used for the timing allocation of the remaining rake fingers. 
         [0040]    In this way the timing of the signal on the strongest finger is mapped to a grid with a fine resolution of 
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         [0000]    whilst the timing of any remaining rake fingers are mapped (with respect to the strongest rake finger) to a grid with a course resolution of 
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         [0041]    The fine timing resolution 
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         [0000]    can be applied to the strongest finger at relatively low cost in complexity. In the embodiment described above with reference to  FIGS. 1 and 3 , the fine timing resolution can be applied digitally on the baseband chip  202 . This is achieved in step S 318  in which a signal is sent from the finger tracker block  210  to the filter coefficient table block  212  thereby effecting a change in the filter coefficients. In particular, the phase of the interpolation filter  206  is altered in accordance with the fine timing resolution of the strongest finger as found in the finger tracker block  210 . The alteration of the phase of the interpolation filter  206  is described in more detail below. In step S 320  the interpolation filter  206  applies the altered filter coefficients such that the timing of the generation of the samples in the interpolation filter  206  is aligned with the timing of the signal on the strongest finger to within a timing range of 
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         [0042]    In an alternative embodiment, as shown in  FIG. 4 , rather than altering the timing of the generation of the samples in the interpolation filter  206 , the timing of the signal sent to the interpolation filter  206  from the RF chip  204  may be altered such that the timing of the generation of the samples in the interpolation filter  206  is aligned with the timing of the signal on the strongest finger to within a timing range of 
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         [0000]    In this alternative embodiment a signal is sent from the finger tracker block  210  to the RF chip  204  instructing the RF chip  204  to alter the timing of the signal accordingly. 
         [0043]    We return to the embodiment shown in  FIGS. 1 and 3  in which the filter coefficients are altered to effect the fine timing resolution mechanism. The interpolation filter  206  has a phase which is governed by the phase coefficient N p  in the filter coefficients table block  212 . The phase of the interpolation filter  206  corresponds to a time shift multiple of 
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         [0000]    which is used to determine when to sample the signal. For example filter phase f p (where p=0, . . . , N p -1) corresponds to a time shift 
         [0000]    
       
         
           
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         [0000]    with respect to phase reference f 0 . By altering the phase of the interpolation filter  206  the time spacing between the samples generated by the interpolation filter  206  is unchanged (i.e., it remains at 
         [0000]    
       
         
           
             
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         [0000]    but the timing of each of the samples is shifted by an amount 
         [0000]    
       
         
           
             
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         [0000]    such that the timing of the sampling of the interpolation filter  206  can be adjusted with a resolution of 
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         [0044]    A minimum of N p  filter phases are needed to achieve a timing resolution of 
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         [0000]    In this case the time shifts corresponding to the different phases form a uniform grid spanning a time interval 
         [0000]    
       
         
           
             
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         [0000]    This is shown in  FIG. 5  which shows the timing corresponding to the different phases (f 0  to f N     p-1   ) of the interpolation filter  206 . The phase coefficients f p  of the interpolation filter can be generated by interpolating the reference interpolation filter coefficients f 0  at time shifts 
         [0000]    
       
         
           
             p 
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                     p 
                     = 
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                       N 
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                     1 
                   
                 
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         [0045]    The strongest rake finger is identified. In the case of a single path channel, the strongest rake finger corresponds to the one path of the single path channel. In the case of a multi-path channel, the strongest rake finger corresponds to the path with the highest received signal power. The signal on the strongest rake finger is monitored in order to determine the timing of the signal on that rake finger. The timing of the strongest finger is found to a resolution of 
         [0000]    
       
         
           
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         [0000]    (which is a finer resolution than that provided by the interpolation filter of 
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         [0046]    The finger tracking on the strongest finger determines the phase used by the interpolation filter  206 , thereby allowing the output of the interpolation filter  206  to capture the strongest finger timing with an error less than or equal to 
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                     N 
                     P 
                   
                 
               
               . 
             
           
         
       
     
         [0000]    The tracking of the remaining of the fingers has no effect on which phase of the interpolation filter  206  is used. 
         [0047]    The tracking of the strongest finger is fine enough to allow a fine timing resolution of 
         [0000]    
       
         
           
             
               T 
               
                 
                   N 
                   I 
                 
                  
                 
                   N 
                   P 
                 
               
             
             . 
           
         
       
     
         [0000]    However, the tracking of the remaining fingers is only good enough to allow a coarse resolution of 
         [0000]    
       
         
           
             
               T 
               
                 N 
                 I 
               
             
             . 
           
         
       
     
         [0048]    As an example, we denote by (p,t m ) a representation of the tracked timing of finger m, which corresponds to a timing estimate, {circumflex over (τ)} p , where 
         [0000]    
       
         
           
             
               
                 
                   τ 
                   ^ 
                 
                 p 
               
               = 
               
                 
                   
                     
                       t 
                       m 
                     
                      
                     
                       T 
                       
                         N 
                         I 
                       
                     
                   
                   + 
                   
                     p 
                      
                     
                       T 
                       
                         
                           N 
                           I 
                         
                          
                         
                           N 
                           P 
                         
                       
                     
                   
                 
                 = 
                 
                   
                     ( 
                     
                       
                         t 
                         m 
                       
                       + 
                       
                         p 
                         
                           N 
                           P 
                         
                       
                     
                     ) 
                   
                    
                   
                     T 
                     
                       N 
                       I 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where p is the associated phase of the interpolation filter  206  and t m  is the delay in number of samples at the output of the interpolation filter  206 . 
         [0049]    Only the strongest finger tracking controls the phase p. Assuming that the finger tracking decides to update the strongest finger timing (p,t s ) by 
         [0000]    
       
         
           
             
               d 
                
               
                 T 
                 
                   
                     N 
                     I 
                   
                    
                   
                     N 
                     P 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    then the new timing corresponds to (p′,t s ′): 
         [0000]    
       
         
           
             { 
             
               
                 
                   
                     
                       
                         p 
                         ′ 
                       
                       = 
                       
                         
                           ( 
                           
                             p 
                             + 
                             d 
                           
                           ) 
                         
                          
                         MODULO 
                          
                         
                             
                         
                          
                         
                           N 
                           P 
                         
                       
                     
                   
                 
                 
                   
                     
                       
                         
                           t 
                           s 
                           ′ 
                         
                         = 
                         
                           
                             t 
                             s 
                           
                           + 
                           
                             
                               ( 
                               
                                 p 
                                 + 
                                 d 
                                 - 
                                 
                                   p 
                                   ′ 
                                 
                               
                               ) 
                             
                              
                             DIV 
                              
                             
                                 
                             
                              
                             
                               N 
                               P 
                               ′ 
                             
                           
                         
                       
                       , 
                     
                   
                 
               
                 
             
           
         
       
     
         [0000]    where MODULO denotes the modulo operation and DIV the integer division operation. 
         [0050]    The remaining fingers should be tracked normally, ignoring the use of different phases of the interpolation filter  206 . An optional modification can be added to improve tracking of the remaining fingers when the new phase coming from the strongest finger p′ and the old one p are too far apart. In this case the timing of the remaining fingers can be updated in the following way: 
         [0000]    
       
         
           
             { 
             
               
                 
                   
                     
                       if 
                        
                       
                           
                       
                        
                       
                         ( 
                         
                           
                             p 
                             - 
                             
                               p 
                               ′ 
                             
                           
                           &gt; 
                           
                             
                               N 
                               P 
                             
                             2 
                           
                         
                         ) 
                       
                     
                   
                   
                     
                       
                         t 
                         m 
                         ′ 
                       
                       = 
                       
                         
                           t 
                           m 
                         
                         + 
                         1 
                       
                     
                   
                 
                 
                   
                     
                       if 
                        
                       
                           
                       
                        
                       
                         ( 
                         
                           
                             p 
                             - 
                             
                               p 
                               ′ 
                             
                           
                           &lt; 
                           
                             - 
                             
                               
                                 N 
                                 P 
                               
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                   
                     
                       
                         t 
                         m 
                         ′ 
                       
                       = 
                       
                         
                           t 
                           m 
                         
                         - 
                         1. 
                       
                     
                   
                 
               
                 
             
           
         
       
     
         [0051]    Although the invention is proposed to enhance the performance of a rake receiver, it is also advantageous to use it in conjunction with a chip spaced equalizer. Indeed, in this case failure to sample the signal with the right timing can transform a flat channel with single path impulse response into frequency selective channel with multi-path response which makes equalization much more difficult. The same principle proposed here for the tracking of the strongest finger can be used in the case where an equalizer is used. 
         [0052]    To evaluate the effectiveness of the solution system simulations have been carried out, and the results are shown in  FIGS. 6 and 7 . The simulations measure the BLock Error Rate (BLER) performance for a fixed transport block size transmitted on the High Speed Physical Downlink Shared Channel (HS-PDSCH). Simulations assume an Additive White Gaussian Noise (AWGN) channel and a Rake receiver structure.  FIGS. 6 and 7  show the performance for Category 14 transport block size 32264 with 64-QAM modulation [3GPP TS 25.321, “Technical Specification Group Radio Access Network; Medium Access Control (MAC) protocol specification”, September 2008, Annex A]. In  FIGS. 6 and 7  performance is shown in terms of BLER versus Geometry. The Geometry is the ratio of the useful received signal power to the noise plus inter-cell interference power. An interpolation filter with oversampling factor N 1 =8 is used in the two cases.  FIG. 6  relies on only the interpolation filter for timing, so the phase of the interpolation filter is not altered as described above, and so the timing of the samples taken by the interpolation filter are accurate only to within 
         [0000]    
       
         
           
             ± 
             
               
                 T 
                 
                   2 
                    
                   
                     N 
                     I 
                   
                 
               
               . 
             
           
         
       
     
         [0000]      FIG. 7  shows the performance when the proposed multi-phase interpolation filter approach is used with an eight possible phases for the interpolation filter (i.e., N p =8). 
         [0053]    Results are shown for different values of timing error introduced between the transmitter and the receiver. Timing errors of 0, 
         [0000]    
       
         
           
             
               
                 T 
                 c 
               
               64 
             
             , 
             
               
                 
                   T 
                   c 
                 
                 32 
               
                
               
                   
               
                
               and 
                
               
                   
               
                
               
                 
                   T 
                   c 
                 
                 16 
               
             
           
         
       
     
         [0000]    are shown in  FIGS. 6 and 7 . The timing errors that are introduced 
         [0000]    
       
         
           
             ( 
             
               
                 
                   T 
                   c 
                 
                 64 
               
               , 
               
                 
                   
                     T 
                     c 
                   
                   32 
                 
                  
                 
                     
                 
                  
                 and 
                  
                 
                     
                 
                  
                 
                   
                     T 
                     c 
                   
                   16 
                 
               
             
             ) 
           
         
       
     
         [0000]    are all less than or equal to 
         [0000]    
       
         
           
             
               
                 T 
                 c 
               
               
                 2 
                  
                 
                   N 
                   I 
                 
               
             
             , 
           
         
       
     
         [0000]    therefore they cannot be corrected by the interpolation filter which takes samples approximately every 
         [0000]    
       
         
           
             
               
                 T 
                 c 
               
               
                 N 
                 I 
               
             
             , 
           
         
       
     
         [0000]    so can only correct for timing errors of at least 
         [0000]    
       
         
           
             ± 
             
               
                 
                   T 
                   c 
                 
                 
                   2 
                    
                   
                     N 
                     I 
                   
                 
               
               . 
             
           
         
       
     
         [0000]      FIG. 6  shows the impact of the introduced timing error. It can be seen that the increasing timing errors have a dramatic effect on performance and lead to an error floor even in the case of infinite geometry (no noise). 
         [0054]      FIG. 7  on the other hand shows that using the proposed approach for increasing timing resolution allows the recovery of all the loss due to timing errors. This is possible because the increased timing resolution allows for the correction of timing errors of at least 
         [0000]    
       
         
           
             
               ± 
               
                 
                   T 
                   c 
                 
                 
                   2 
                    
                   
                     N 
                     I 
                   
                    
                   
                     N 
                     P 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    which in the example shown in  FIG. 7  (for N I =8 and N p =8) is 
         [0000]    
       
         
           
             ± 
             
               
                 
                   T 
                   c 
                 
                 128 
               
               . 
             
           
         
       
     
         [0055]    There has been described a method and system for determining whether timing errors caused by the finite sampling period of the interpolation filter  206  are a significant factor in reducing the quality of the signal. If these timing errors (of the order of 
         [0000]    
       
         
           
             
               ± 
               
                 
                   T 
                   c 
                 
                 
                   2 
                    
                   
                     N 
                     I 
                   
                 
               
             
             ) 
           
         
       
     
         [0000]    are not limiting the quality of the signal (i.e., if other factors are limiting the quality of the signal), then it is not necessary to reduce the timing errors further. In that case the fine timing of the signal as described above to find a phase coefficient to apply to the interpolation filter is advantageously not performed. 
         [0056]    However, when the timing errors of the interpolation filter are limiting the quality of the signal then the method described above is implemented to determine fine timing resolution by aligning the sampling of the interpolation filter to the timing of the signal to an accuracy of within 
         [0000]    
       
         
           
             ± 
             
               
                 
                   T 
                   c 
                 
                 
                   2 
                    
                   
                     N 
                     I 
                   
                    
                   
                     N 
                     P 
                   
                 
               
               . 
             
           
         
       
     
         [0000]    This will improve performance and the quality of the signal that is decoded in the system. This is especially valid in the case of a single path channel, where small timing errors can lead to substantial inter-symbol interference. 
         [0057]    Since the sampling rate of the interpolation filter  206  is not increased, there is no additional cost in terms of complexity and no impact on the architecture of existing systems. 
         [0058]    By setting the number of different phases used in the interpolation filter, the precision of the timing correction can be set accordingly. The present invention is particularly attractive for embodiments in which reprogrammable interpolation filter coefficients are used, such that the phase coefficient for the interpolation filter  206  can be altered in accordance with the current channel conditions to thereby set the precision of the timing correction dynamically. The present invention is particularly useful where the interpolation filter  206  is implemented as a software block, although it would be possible to implement the interpolation filter as a hardware block. 
         [0059]    The baseband chip  202  preferably operates in the digital domain. However, it is possible in other, less preferred embodiments for the baseband chip  202  to operate in the analog domain. 
         [0060]    While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appendant claims.