Abstract:
A wireless communication receiver performs robust cell searching, excluding interference due to UL transmissions from other UE, by qualifying the output of a matched filter with a metric indicative of the momentary signal-to-noise ratio (SNR). The momentary SNR metric is derived over the same amount of samples as the length of the matched filter. By discarding filter outputs during low momentary SNR, synchronization interference from UL transmissions is avoided. The momentary SNR metric and filter outputs are efficiently calculated, with only a few states and operations, compared to a conventional tapped delay line filter implementation. A limited list of cell candidates is populated, with information on correlation, timing, cell identity within cell group, and SNR metric for the K strongest candidates with respect to the matched filter correlation values. This list is used for later cell search stages, where a secondary synchronization channel is decoded.

Description:
TECHNICAL FIELD 
     The present invention relates generally to wireless communication systems, and in particular to a method and apparatus for robust cell detection by a receiver. 
     BACKGROUND 
     Many types of cellular wireless communication systems are known in the art. When a mobile station, or user equipment (UE), powers up, it is unaware of which cell, or even system, in which it is operating. The UE initiates a search of predetermined frequencies, searching for known synchronization signals. Once the synchronization signals are found and decoded, the UE searches for a control channel and downloads system- and cell-specific information. 
     The Universal Mobile Telecommunications System (UMTS) is a third-generation (3G) wireless communication technology. The UMTS access network is the UMTS Terrestrial Radio Access Network (UTRAN). Long Term Evolution (LTE) is an evolution of UMTS. The access network in LTE is called Evolved UTRAN (E-UTRAN). E-UTRAN will operate over a very wide span of operating bandwidths and carrier frequencies, in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) systems. The LTE air interface utilizes Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single-Carrier FDMA (SC-FDMA) in the uplink. LTE systems are anticipated to operate from micro cells up to macro cells with 100 km cell range. The OFDMA radio access technology adapts well to a variety of different propagation conditions, as will be required to handle the different radio conditions that may occur in LTE systems. 
     To facilitate the cell searching function necessary for mobile UE, two synchronization signals are transmitted periodically every 5 ms in a cell: a primary synchronization signal (P-SCH) and a secondary (S-SCH). The synchronization signals carry information on the physical layer cell identity, a unique (in a wide geographical area) identity of the cell. 
     Three versions are defined for the P-SCH, one for each of three cell identities within one out of 504 groups of cells. The three versions of P-SCH are common to all cell groups. Since there are only three versions, the straight-forward, prior art approach to P-SCH detection is to conduct matched filtering over at least 5 ms of received samples for each of the P-SCH versions, in order to identify correlation peaks that may reveal synchronization signals from one or more cells. Once a cell candidate has been found, the location of S-SCH can be hypothesized based on the position of P-SCH. Hypothesized, since the position of S-SCH differs depending on duplex mode (FDD/TDD) and cyclic prefix length. 
     The matched filtering approach is straight-forward to apply in case of FDD, where uplink (UL) and downlink (DL) transmissions occur on different radio channels—that is, UL and DL are separated in frequency. The case of TDD, however, presents a greater challenge, as the same radio channel is used for UL and DL. Whether a subframe is allocated for UL or DL depends on how TDD cells on that particular carrier frequency are configured.  FIG. 5  depicts a TDD frame structure (the figure is taken from FIG. 4.2-1 of 3GPP Technical Specification 36.211, “3 rd  Generation Partnership project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” V8.4.0, Section 6.11). A subframe in the TDD frame may be allocated to either UL or DL traffic. P-SCH is transmitted in positions marked DwPTS. Table 1 depicts the UL/DL configuration options (taken from Table 4.2-2 of the same specification), wherein U denotes UL traffic, D denotes DL traffic, and S is switched (both UL and DL). 
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Uplink-downlink allocation on subframe basis for different configurations in TDD 
               
             
          
           
               
                 Uplink-downlink 
                 Downlink-to-Uplink 
                 Subframe number 
               
             
          
           
               
                 configuration 
                 Switch-point periodicity 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
               
                 0 
                 5 ms 
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 S 
                 U 
                 U 
                 U 
               
               
                 1 
                 5 ms 
                 D 
                 S 
                 U 
                 U 
                 D 
                 D 
                 S 
                 U 
                 U 
                 D 
               
               
                 2 
                 5 ms 
                 D 
                 S 
                 U 
                 D 
                 D 
                 D 
                 S 
                 U 
                 D 
                 D 
               
               
                 3 
                 10 ms  
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 4 
                 10 ms  
                 D 
                 S 
                 U 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 5 
                 10 ms  
                 D 
                 S 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 6 
                 5 ms 
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 S 
                 U 
                 U 
                 D 
               
               
                   
               
             
          
         
       
     
     The first time a TDD carrier is visited, during initial cell search or inter-frequency cell search, the synchronization (timing) used on that particular carrier is unknown. When the UE is searching for cells, it must search over the full 5 ms interval. Some of the subframes over which the search is conducted will be allocated for UL, and others for DL. If a nearby UE transmits on its UL, this may cause interference at a receiver that is orders of magnitude larger than the signal transmitted from the network (i.e., a base station, known in E-UTRAN as an evolved Node B (eNB)). 
     A further complication in initial cell search is that since no signaling is available, the UE is unaware of which duplex mode to expect on a particular carrier frequency. Table 2 depicts the frequency bands that are supported in E-UTRAN, along with duplex mode(s) used in each particular band (the figure is taken from Table 5-2.1 of 3GPP Technical Specification 36.101, “3 rd  Generation Partnership project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception (Release 8),” V8.3.0, Section 5.2). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 E-UTRAN frequency bands and supported duplex modes 
               
             
          
           
               
                   
                 Uplink (UL) 
                 Downlink (DL) 
                   
               
               
                   
                 eNode B receive 
                 eNode B transmit 
               
               
                 E-UTRA 
                 UE transmit 
                 UE receive 
                 Duplex 
               
               
                 Band 
                 F UL     —     low -F UL     —     high   
                 F DL     —     low -F DL     —     high   
                 Mode 
               
               
                   
               
             
          
           
               
                 1 
                 1920 MHz-1980 MHz 
                 2110 MHz-2170 MHz 
                 FDD 
               
               
                 2 
                 1850 MHz-1910 MHz 
                 1930 MHz-1990 MHz 
                 FDD 
               
               
                 3 
                 1710 MHz-1785 MHz 
                 1805 MHz-1880 MHz 
                 FDD 
               
               
                 4 
                 1710 MHz-1755 MHz 
                 2110 MHz-2155 MHz 
                 FDD 
               
               
                 5 
                 824 MHz-849 MHz 
                 869 MHz-894 MHz 
                 FDD 
               
               
                 6 
                 830 MHz-840 MHz 
                 875 MHz-885 MHz 
                 FDD 
               
               
                 7 
                 2500 MHz-2570 MHz 
                 2620 MHz-2690 MHz 
                 FDD 
               
               
                 8 
                 880 MHz-915 MHz 
                 925 MHz-960 MHz 
                 FDD 
               
               
                 9 
                 1749.9 MHz-1784.9 MHz 
                 1844.9 MHz-1879.9 
                 FDD 
               
               
                   
                   
                 MHz 
               
               
                 10 
                 1710 MHz-1770 MHz 
                 2110 MHz-2170 MHz 
                 FDD 
               
               
                 11 
                 1427.9 MHz-1452.9 MHz 
                 1475.9 MHz-1500.9 
                 FDD 
               
               
                   
                   
                 MHz 
               
               
                 12 
                 698 MHz-716 MHz 
                 728 MHz-746 MHz 
                 FDD 
               
               
                 13 
                 777 MHz-787 MHz 
                 746 MHz-756 MHz 
                 FDD 
               
               
                 14 
                 788 MHz-798 MHz 
                 758 MHz-768 MHz 
                 FDD 
               
               
                 . . . 
               
               
                 17 
                 704 MHz-716 MHz 
                 734 MHz-746 MHz 
                 FDD 
               
               
                 . . . 
               
               
                 33 
                 1900 MHz-1920 MHz 
                 1900 MHz-1920 MHz 
                 TDD 
               
               
                 34 
                 2010 MHz-2025 MHz 
                 2010 MHz-2025 MHz 
                 TDD 
               
               
                 35 
                 1850 MHz-1910 MHz 
                 1850 MHz-1910 MHz 
                 TDD 
               
               
                 36 
                 1930 MHz-1990 MHz 
                 1930 MHz-1990 MHz 
                 TDD 
               
               
                 37 
                 1910 MHz-1930 MHz 
                 1910 MHz-1930 MHz 
                 TDD 
               
               
                 38 
                 2570 MHz-2620 MHz 
                 2570 MHz-2620 MHz 
                 TDD 
               
               
                 39 
                 1880 MHz-1920 MHz 
                 1880 MHz-1920 MHz 
                 TDD 
               
               
                 40 
                 2300 MHz-2400 MHz 
                 2300 MHz-2400 MHz 
                 TDD 
               
               
                   
               
             
          
         
       
     
     Note that some bands overlap, e.g., band 2 and 36, meaning that carrier frequencies within 1930-1990 MHz can be used for both FDD and TDD (although not in the same geographical area). Countermeasures against UL interference will be required by the UE whenever TDD cannot be ruled out. Otherwise, the UE may mistake other UEs for eNBs, and the cell search will fail or take a very long time. 
     One method of UL interference mitigation during initial cell search on a TDD carrier (initial or inter-frequency) is to estimate the interference in the filter output, and scale any correlation peak down by the interference (i.e., the correlation peak value is divided by the interference estimate). The interference is estimated in an interval spanning about half a subframe on each side of the analyzed time instant. This approach may require high complexity in terms of memory and/or computational demands, depending on implementation. For a computationally efficient implementation, large buffers are necessary to keep track of the samples entering and leaving a window for interference level estimation. 
     Furthermore, this approach assumes a substantial number of samples will be present, in addition to the interval over which the cell search is conducted. In initial cell search this is not a problem since the UE is not mandated to find a cell within a particular time span, and hence the UE can collect as many samples as needed. However, when conducting inter-frequency cell search the first time on a TDD carrier, the timing (synchronization) will not be known, so measures to deal with UL interference are required. At the same time, there are only a limited number of samples available in the 6 ms transmission gap. Radio frequency switching will consume parts of this gap, and in the standard, it is only assumed that an efficient gap of 5 ms will be available. Therefore, it is not possible to use a large number of samples in addition to the ones to be examined, since most of the samples are needed in the search for cells. 
     SUMMARY 
     According to one or more embodiments of the present invention, UL transmissions from other UE are excluded during cell searching by qualifying the output of a matched filter with a metric indicative of the momentary signal-to-noise ratio (SNR). A momentary SNR is one derived over the same amount of samples as the length of the matched filter. By discarding filter outputs during low momentary SNR, synchronization interference from UL transmissions is avoided. The momentary SNR metric and filter outputs are efficiently calculated, with only a few states and operations, compared to a conventional tapped delay line filter implementation. A limited list of cell candidates is populated, with information on correlation, timing, cell identity within cell group, and SNR metric for the K strongest candidates with respect to the matched filter correlation values. This list is used for later cell search stages, where a secondary synchronization channel is decoded. In one embodiment, the list is protected from being overrun by multi-path components of the same synchronization signal. 
     One embodiment relates to a method of wireless communication network cell detection. Synchronization signals, each including a cell identification, are received from the wireless network. A correlation metric and a momentary signal to noise ratio (SNR) metric of the received signals are determined by correlating the received synchronization signals with one or more known synchronization patterns. A list of cell candidates having the highest correlations, that also exceed a predetermined threshold SNR metric, is maintained. 
     Another embodiment relates to a cell identifier for a receiver in user equipment (UE) operative in a wireless communication network. The cell identifier includes a matched filter operative to receive digitized samples of received synchronization signals, and operative to output a correlation metric indicative of the correlation between the signal samples and one or more known synchronization signals, and a signal to noise ratio (SNR) metric. The cell identifier further includes a candidate selection unit operative to receive the correlation metric and SNR metric, and operative to maintain a list of cell candidates, each candidate comprising the correlation and SNR metrics, a cell identification, and a sample time. The controller is operative to add a cell candidate to the list only if the associated SNR metric exceeds a predetermined threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a receiver according to an embodiment of the present invention. 
         FIG. 2  is a functional block diagram of the matched filter of the receiver of  FIG. 1 . 
         FIG. 3  is a flow diagram of a method of network cell detection according to an embodiment of the present invention. 
         FIG. 4  is a diagram depicting wireless communication cell interference. 
         FIG. 5  is a diagram of a TDD frame structure, according to an industry standard. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are described herein in the context of cell search and P-SCH detection in LTE. However, the invention is not limited to that technology. 
       FIG. 1  depicts a wireless communication system receiver  10 . The receiver  10  includes an antenna  12 , front end processing circuit  14 , analog to digital converter (ADC)  16 , subcarrier filter  18 , matched filter  20 , and candidate selection function  22 , and stores a cell candidate list  24  in memory (not shown). The receiver  10  receives, e.g., OFDM signals at one or more antennas  12 , and front end processes the signals at  14 . As known in the art, front end processing may include low noise amplification, bandpass filtering, downconversion (in one or more steps) to baseband frequency, and separation of in-phase (I) and quadrature (Q) signal components. The I and Q analog component signals are digitized by the ADC  16 , yielding sample streams I(n) and Q(n) for successive sampling instances n. 
     Signal samples are then filtered by the subcarrier filter  18  to pass only the subcarriers that are of interest for the P-SCH detection and candidate selection. Details of the design of the subcarrier filter  18 , the sampling rate, and similar parameters may be readily determined, for a particular application, by those of skill in the art, and hence are not further elucidated herein. 
     The filtered, complex-valued signal stream x(n) (containing P-SCH) is subjected to matched filtering, and the absolute value of the output is squared, forming the correlation metric ρ(n) for that particular sample and set of filter coefficients. At the same time, a ratio η(n) is formed, which is related to the momentary SNR. 
     Cell candidates are selected in the candidate selection function  22  based on their momentary SNR and correlation metrics, and added to a limited cell candidate list  24 . As used herein, the term “limited” cell candidate list means the list has a finite number of entries. A list  24  entry may include, e.g., an identifier (ID), the correlation metric ρ(n), the momentary SNR metric η(n), and the sample time n. In the case of severe interference, such as due to UL transmissions from nearby UEs in the case of TDD, the momentary SNR metric value will be small. Correlation metrics associated with low momentary SNR levels are discarded. 
     In one embodiment, the duplicative effects of multi-path propagation are minimized by temporal windowing in the candidate selection function  22 . In multi-path propagation, the same radio signal from the transmitter will arrive at the UE antenna  12  at multiple instants, due to different propagation path lengths as the signal bounces off of, e.g., buildings, terrain, and the like. In case of a small cell candidate list  24 , potentially the whole list  24  may be filled with candidates originating from the same cell due to multi-path propagation. This is avoided in the candidate selection function  22  by defining a temporal window of M sample intervals from the latest addition to the cell candidate list. If a new candidate should be added to the list, based on its correlation and SNR metrics, and it falls within the window, the candidate selection function  22  updates the latest addition to the cell candidate list  24 , rather than adding a new candidate. The value M may be chosen as the number of chips corresponding to the (worst case) delay spread expected in the system, which typically is in the order of the length of the cyclic prefix for OFDM. 
     The selection process is based on dynamic thresholds—comparing the correlation metric associated with each candidate cell identification (with a sufficiently high momentary SNR metric) to those previously selected for inclusion in the list. This means that after processing the whole cell search time interval, the K largest candidates are reported, regardless of the absolute values of their correlation metrics. In this manner, fixed absolute thresholds, which often are sub-optimized, are avoided. 
     Mathematically, let  x (n)=[x(n) x(n−1) . . . x(n−N+1)] T  denote a vector of the last N complex-valued samples received in a receiver  10  at sampling time n, and further  h   0 =[h 0  h 1  . . . h N−1 ] T  a time-domain representation of length N of P-SCH. In the present context it is not important to which of the three cell identities within the cell group the P-SCH filter  20  belongs. 
     Assume that  h   0  is unitary such that  h   0   H   h   0 =1 (this is just a matter of scaling). Then  h   0  can be considered a unitary basis vector in C N , and it follows that N−1 other unitary basis vectors  h   i , i=1, 2, . . . , N−1 in C N  can be found that all fulfill: 
     
       
         
           
             
               
                 
                   
                     
                       
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     The exact structure of those unitary basis vectors is not important; only that the property of Equation (1) is true. The additional N−1 basis vectors are only important for the theoretical motivation described below. 
     Using the above definitions, the matched filtering of  x (n) can then be expressed as:
 
 y ( n )= h   0   H     x   ( n )  (2)
 
where y(n), the filter  20  output, is a complex valued scalar.
 
     The energy contained in the filtered signal is:
 
ρ( n )=| y ( n )| 2   = x     H ( n )   h     0     h     0   H     x   ( n )
 
     The signal prior to filtering can be described in the unitary basis vectors, of which  h   0  is the P-SCH filter in use: 
     
       
         
           
             
               
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     The energy of the signal prior to filtering may then be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     It can easily be shown that the ratio of the signal energy captured by the matched filter  20  to the total energy over the same interval is related to the momentary SNR: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Note that in this discussion, it has been assumed that corner cases are avoided, e.g. when η(n)=0 or η=1. Those cases can easily be trapped and handled accordingly. 
     By forming the ratio η(n), relative information on the quality of the signal is obtained. This information can be used for discriminating large absolute correlation metrics ρ(n) resulting from interference (e.g., UL transmissions from other UEs in TDD) from those resulting from actual received signals. 
     Some interference will be captured by the matched filter  20 . In the case that the interference can be modeled as complex-valued additive white Gaussian noise, the SNR becomes approximately inversely proportional to the filter length N when no signal is present. Furthermore, since it is the momentary SNR that is utilized, the estimates will fluctuate when no signal is present. This has to be considered when using a threshold for discriminating interference from actual signals. However, the SNR levels in the case of noise-only will not depend on the actual variance of the noise, so even if the variance of the interference is increased several orders of magnitude, the momentary SNR metric will still hover around the same level. 
       FIG. 2  depicts an efficient realization of the matched filter  20  that does not require the norm of the filter  h  to be unity. Prior to filtering, all states are cleared to zero, and the initial N−1 values are ignored as transient output. The signal sample stream x(n) is delayed by delay elements  26 , and tapped off at multipliers  28 , which are set to predetermined values. The multiplier  28  outputs are summed by adder  30 , and the square of the absolute value determined at function  32 . Element  34  is a multiplier that scales, or normalizes, the absolute magnitude filter output, and outputs the correlation metric ρ(n) for sample time n. The scaling prevents the outputs from different implementations of filter  18  (for different cell IDs) generating differently scaled correlation metrics, which would have the effect of favoring some cell IDs over others. In parallel, an accumulator  38  comprising two functions for squared absolute values  40 , and adder with three inputs, and a state register (functioning as a delay element)  44 , accumulates the energy contained in the samples currently subjected to the matched filter  20 . The divider  36  calculates and outputs the SNR metric η(n) according to Equation (3). 
     The time-domain representation of the matched filter  20  for P-SCH for each of the three cell identities within the group shall be used. However, the actual design of the filter  24  is flexible. For example, in one embodiment, a low-pass filter may be convolved with the matched filter  24  in order to suppress interference from adjacent carriers outside those carrying P-SCH information. In general, those of skill in the art may tailor the filters to achieve better performance under different conditions, given the teachings of the present disclosure. 
       FIG. 3  depicts a method  50  for cell candidate selection according to one embodiment of the present invention. The method  50  begins at block  52 , when the candidate selection function  22  receives a new correlation metric ρ(n) and momentary SNR metric η(n) for sample time n. Initially, the candidate selection function  22  checks whether that particular time n is masked (block  54 ). Masking can be applied for timings of cells that have already been found in a previous cell search. This prevents already known cells from occupying entries in the cell candidate list, at the expense of weaker cells that have not yet been identified. If the time n is masked, the rest of the method  50  is skipped, n is incremented (block  68 ), and new correlation and momentary SNR metrics are received (block  52 ). 
     If the time n is not masked (block  54 ), the candidate selection function  22  checks whether the momentary SNR metric exceeds a threshold T (block  56 ). The SNR metric threshold is only needed when visiting a TDD carrier the first time, before the cell synchronization has been established. This includes both Public Land Mobile Network (PLMN) scan and inter-frequency cell search. Only the strongest cell is of primary interest until the synchronization has been established. Referring to  FIG. 4 , depicting a typical cell coverage pattern  70 , the worst location from a cell interference perspective is in the center, where all three cells  74  have similar strength. If only considering the received power from either of the eNBs  72 , the SNR would be approximately −3 dB. However, the momentary SNR is determined on a sample basis, and additional interference will result from overlapping multi-path components. Furthermore, depending on the characteristics of the subcarrier filter  18  (see  FIG. 1 ), carriers adjacent to those carrying synchronization signals may contribute additional interference. 
     The relation between η(n) and the momentary SNR is given in Equations (3) and (4). As a non-limiting example, if it is required that the SNR should be larger than −10 dB for a peak to be included in the P-SCH detection, then from Equation (3), 
             T   =         10       -   10     /   10         1   +     10       -   10     /   10           .           
Referring again to  FIG. 3 , if the momentary SNR metric η(n) is below T (block  56 ), the rest of the method  50  is skipped, n is incremented (block  68 ), and new correlation and momentary SNR metrics are received (block  52 ).
 
     If the momentary SNR metric η(n) exceeds T (block  56 ), the candidate selection function  22  compares the correlation metric ρ(n) against a dynamic threshold ρ′ corresponding to the correlation metric for the weakest cell candidate currently included in the list  24  (block  58 ). If the correlation metric ρ(n) is below ρ′ (block  58 ), the rest of the method  50  is skipped, n is incremented (block  68 ), and new correlation and momentary SNR metrics are received (block  52 ). 
     If the correlation metric ρ(n) exceeds ρ′ (block  58 ), the candidate selection function  22  determines whether a new candidate was added to the list  24  within the last M time instants, as determined from current time n and the stored time m of the last addition to the list  24  (block  60 ). If not, the candidate selection function  22  replaces the entry in the cell candidate list  24  having the lowest correlation value, with the current (time n) cell candidate (block  62 ). The parameters m, ρ′, and ρ″ (described below) are updated (block  62 ). In particular, m=n, ρ″=ρ(n), the cell candidate list is sorted with respect to the candidates&#39; correlation metrics ρ, and the parameter ρ′ is set to the correlation metric for the weakest cell candidate. The sample time n is incremented (block  68 ), and new correlation and momentary SNR metrics are received (block  52 ). 
     If a candidate was added to the list  24  within the last M time instants (block  60 ), then the candidate selection function  22  compares the correlation metric ρ(n) to the correlation metric value for the most recently added or updated list  24  entry, denoted ρ″ (block  64 ). If ρ(n) exceeds ρ″, then the most recently added or updated entry is updated to the parameters for the current candidate (i.e., correlation metric ρ(n), time n, cell (filter) identity, and momentary SNR metric η(n)), and the value of the most recently added peak is updated: ρ″=ρ(n) (block  66 ). This ensures that entries in the cell candidate list  24  are not displaced by multi-path copies of the signal giving rise to candidate n, but rather, the list entry corresponding to the same signal (within M time intervals) is updated by the current peak, if it is stronger. In this manner, the maximum correlation out of a plurality of multi-path signal copies is captured, without displacing other entries from the cell candidate list  24 . The sample time n is then incremented (block  68 ), and new correlation and momentary SNR metrics are received (block  52 ). 
     It is evident from  FIG. 3  that most steps of the method  50  can be implemented in parallel, using comparators and a few registers. Each comparator outputs true or false—hence with some additional logic, one can deduce whether an entry shall be added or updated, or whether the algorithm shall go to the next sample. 
     Although conceptually the cell candidate list  24  is to be sorted, in practice it is sufficient, in one embodiment, to keep track of two cell candidate registers: one having the smallest correlation metric, and the one that was added most recently. This, too, can be handled with low complexity and in parallel using comparators and a few registers. Accordingly, the method  50  may be implemented with low complexity, consuming little silicon area or battery power. 
     In some embodiments, complexity may be decreased even further, depending on how the algorithms are to be deployed and how much meta-information is to be used in subsequent cell search stages. For example, if the metrics η(n) will not be used once the cell candidate list  24  has been formed, or the ratio can be calculated at low cost at a later stage, then the divider  36  in the matched filter  20 , can be removed. Instead, the filter  20  may output ρ(n) and  x   H (n) x (n). In this case, the decision η(n)&gt;T? ( FIG. 3 , block  56 ) is replaced by ρ(n)&gt;T· x   H (n) x (n)? This means that a division is replaced by a multiplication, which is generally a lower-complexity operation. Furthermore, in this case,  x   H (n) x (n) may be stored in the cell candidate list  24  instead of η(n). Other optimizations will be apparent to those of skill in the art, given the present disclosure, all of which fall within the scope of the present invention, as defined by the claims. 
     The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.