Abstract:
A method of updating a multipath search that has located one or more multipath component peaks in a received signal, the method comprising repeating the search using a delay spread window extending either back from the latest peak or forward from the earliest peak.

Description:
FIELD OF THE INVENTION 
     The invention relates to methods and apparatus for updating the search time window for multipath searches. 
     BACKGROUND OF THE INVENTION 
     In the mobile radio telecommunications field, it is widely understood that a wireless signal will often propagate between a transmitter and a receiver via several paths. The components arriving at the receiver via different paths are commonly referred to as multipath components and they may interfere with one another at the receiver leading to a degradation of the receiver&#39;s performance. 
     Various schemes exist for addressing the problem of multipath propagation. Often, these techniques rely on a determination of the relative time offsets between multipath components or, in other words, some measure of the difference in the propagation time for the signals travelling along the different paths. Often, the process of determining the existence of signal components arriving at a receiver via different paths and the time offsets between them is called a multipath search. Typically, the number of, and relative delays between, such components will vary over time, as will the amplitude and phase of each such component. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to provide a scheme for updating a multipath search. 
     According to one aspect, the invention provides a method of updating a multipath search that has located one or more multipath component peaks in a received signal by correlating the received signal against a known signal at various time offsets between the two signals, wherein the or each peak appears at a respective time offset and the method comprises performing an updating search for multipath component peaks in the signal by correlating the known signal against the received signal over a range of time offsets, wherein the range extends either in a first manner from the smallest time offset that produced a peak in the search to be updated in the direction of increasing time offset or in a second manner from the greatest time offset that produced a peak in the search to be updated in the direction of decreasing time offset. 
     The invention also consists in apparatus for updating a multipath search that has located one or more multipath component peaks in a received signal by correlating the received signal against a known signal at various time offsets between the two signals, wherein the or each peak appears at a respective time offset and the apparatus comprises processing means for performing an updating search for multipath component peaks in the signal by correlating the known signal against the received signal over a range of time offsets, wherein the range extends either in a first manner from the smallest time offset that produced a peak in the search to be updated in the direction of increasing time offset or in a second manner from the greatest time offset that produced a peak in the search to be updated in the direction of decreasing time offset. 
     According to another aspect, the invention provides a method of updating a multipath search that has located one or more multipath component peaks in a received signal, the method comprising repeating the search using a delay spread window extending either back from the latest peak or forward from the earliest peak. 
     The invention also consists in apparatus for updating a multipath search that has located one or more multipath component peaks in a received signal, the apparatus comprising processing means for repeating the search using a delay spread window extending either back from the latest peak or forward from the earliest peak. 
     In a preferred embodiment, several such updating searches are performed in series. Preferably, consecutive searches in the series are performed in opposite senses. That is to say, for any pair of updating searches in the series, one is performed in a manner which extends backwards from the latest known multipath component peak and the other is performed forwards from the earliest known multipath component peak. The extent of the search, in terms of delay spread, may be varied from search to search in the series. 
     The invention is preferably used in a participant of a wireless communications network, such as a mobile telephone. The invention may also be implemented as software for execution by a suitable data processing device, or as a combination of both hardware and software. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, an embodiment of the invention will now be described with reference to the accompanying figures, in which: 
         FIG. 1  is a block diagram of a mobile telephone; 
         FIG. 2  shows the result of a multipath search performed in the telephone of  FIG. 1 ; 
         FIG. 3  shows the result of a multipath search performed in the telephone of  FIG. 1 ; 
         FIG. 4  shows the result of a multipath search performed in the telephone of  FIG. 1 ; 
         FIG. 5  shows the result of a multipath search performed in the telephone of  FIG. 1 ; 
         FIG. 6  shows the result of a multipath search performed in the telephone of  FIG. 1 ; and 
         FIG. 7  shows the result of a multipath search performed in the telephone of  FIG. 1 ; 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows some basic elements of a mobile telephone  10  for a UMTS network.  FIG. 1  illustrates only the elements of the telephone  10  that are most useful for describing the invention; it will be apparent to the skilled person that the construction of the mobile telephone will be far more complex in practice. As shown in  FIG. 1 , the telephone  10  includes an antenna  12 , an RF section  14 , a rake receiver  16  a bit rate processor (BRP)  18  and a multipath searcher (AS)  20 . 
     The telephone  10  receives signals through antenna  12 . The received signals are downconverted by the RF section  14  to produce a baseband signal. The baseband signal is then passed to both the rake receiver  16  and the multipath searcher  20 . The multipath searcher  20  identifies the strongest group of multipath components in the baseband signal and allocates a finger of the rake receiver  16  to each of those components. The rake receiver  16  demodulates the allocated multipath components in parallel and combines the results in a time-aligned manner. The demodulated signal provided by the rake receiver  16  is then supplied to the BRP  18  for further processing. In the example of a voice call, the further processing performed by the BRP  18 , includes the extraction of speech information from the received signal and its ultimate conversion into an analogue signal for presentation to the user via a sound transducer. No further detail will be given about the operation of the RF section, the rake receiver  16  or the BRP  18  since the manner in which these units function will be readily understood by the skilled person. However, the operation of the MPS  20  will now be described in greater detail. 
     In the process of interacting with a basestation, the telephone  10  will monitor a signal sent out by a basestation in a common pilot channel (CPICH) of the UMTS network in which the telephone  10  is participating. The signal transmitted by the basestation in the CPICH is encoded using a scrambling code and that scrambling code is known to, or deduced by, the telephone  10 . The MPS  20  performs measurements on the signal acquired from the basestation in the CPICH in order to identify the multipath components arriving from the basestation. 
     The MPS  20  performs a set of correlation calculations on the CPICH signal acquired by the telephone  10  from the basestation. Each of the correlation calculations involves the correlation of the scrambling code with the CPICH signal from the basestation to produce a correlation value. However, each of the correlation calculations uses a different time offset between the scrambling code and the CPICH signal from the basestation. If one plots the correlation values against their time offset values, then the resulting curve will show a number of peaks, one peak for each of the multipath components of the signal sent by the basestation in the CPICH. (In practice, only the strongest multipath components will be discernible against the noise floor.) The heights of the peaks indicate the relative power levels of the multipath components that are present and the spacing between the peaks indicates the relative delays between the various multipath components. For this reason, it is usual to refer to such a plot as a power versus delay profile. 
     Upon first detecting CPICH transmissions from the basestation, the MPS  20  performs a set of coarse correlation calculations that are sufficient to provide a power versus delay profile in which the strongest multipath component peak can be discerned. Such a plot is shown in  FIG. 2 . The MPS  20  will repeat this coarse evaluation at intervals. Alternatively, coarse path position may also be obtained through other means such as correlation against synchronisation codes (e.g. for UMTS these are P-SCH, S-SCH). If, however, the telephone  10  starts to actively use the base station, e.g. as the result of a hand-over, then the MPS  20  performs a finer set of correlation calculations to produce a more detailed version of the power versus delay profile. 
     The UMTS standards presently state that multipath components must be monitored over at least a 20 μs window or delay spread of the power versus delay profile of a basestation that is actively being used. Therefore, the MPS  20  designates the location of the strongest peak found in the last iteration of the coarse search as the zero point on the delay axis and performs the correlation calculations that are needed to build up a detailed power versus delay profile over the interval 0 μs to 20 μs on the delay axis, as indicated by the arrow below the delay axis in  FIG. 2 . 
     In this example, the detailed search finds additional multipath components at delays of +9 μs and +17 μs on the delay axis as shown in  FIG. 3 . The MPS  20  is required to repeat the detailed 20 μs search periodically on the assumption that the multipath environment around the telephone  10  will change from time to time. However, at the next iteration of the detailed search, the 20 μs window is linked to the latest peak located in the previous search, in this case the peak at +17 μs, and extends in the direction of decreasing delay, as indicated by the arrow below the delay axis in  FIG. 4 . Thus, the search is now performed over the interval +17 μs to −3 μs on the delay axis. In this example, the new search finds the peaks at 0, +9 and +17 μs once more but also locates an additional peak at −2 μs, as indicated in  FIG. 5 . 
     When the time comes to perform the next iteration of the detailed search, the 20 μs window is linked to the earliest peak on the delay axis and extends forwards in the direction of increasing delay, as indicated by the arrow below the delay axis in  FIG. 6 . In this example, the new iteration of the search finds peaks at −2, 0 and +17 μs but the peak at +9 μs no longer exists. 
     The detailed 20 μs search is repeated periodically as long as the telephone  10  continues to actively use the basestation in question. Consecutive iterations of the search continue to extend in opposite senses, that is towards increasing delay or towards decreasing delay. Each iteration that extends in the direction of decreasing delay begins at the latest peak found by the previous iteration of the search and each iteration that extends in the direction of increasing delay begins at the earliest peak found by the previous iteration of the search. Thus, a 20 μs range of the power versus delay profile is monitored in a manner which places a relatively light burden on the data processing resources of the telephone in terms of the number of correlation calculations that need to be performed. The MPS  20  provides a particular benefit in the case where the search to be updated contains just a single multipath peak in that paths upto 20 μs away can be located without the penalty of having to search through a 40 μs time window in a single iteration of the search.