Abstract:
In a method for synchronization of a receiver to a transmitter which periodically transmits a sequence which is known in the receiver, a subset of possible synchronization times is determined in a first selection process by repeated correlation of the received signal with the known sequence and comparison of the correlation responses, as calculated for the correlations, with a threshold value. The threshold value is in this case varied adaptively as a function of at least one parameter which is measured during the correlations.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT/DE03/02680, filed Aug. 8, 2003 which was not published in English, which claims the benefit of the priority date of German Patent Application No. DE 102 41 690.7, filed on Sep. 9, 2002, the contents of which both are herein incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method and an apparatus for synchronization of a receiver to a transmitter, with the transmitter periodically transmitting a sequence which is known in the receiver. 
     BACKGROUND OF THE INVENTION 
     In many areas of communications technology and in particular in mobile radio systems, a receiver must be synchronized to a transmitter before a connection is set up. For this purpose, the transmitter generally periodically transmits a specific sequence, which is known in the receiver and is also referred to as a sync word. This sequence is searched for in the incoming data stream in the receiver. When the sequence is found in the incoming data stream, the sought synchronization time is obtained from the timing of the detected sequence. 
     The data stream is typically subdivided in the transmitter into so-called slots (time slots) with a fixed number of bits (which are also referred to as chips when the spreading code method is used). The sync word which must be identified by means of a suitable method in the receiver is located at the start of a slot such as this. The expression slot synchronization is used in this case. 
       FIG. 1  shows a known procedure for determination of the slot start by means of a matched filter (search filter). The sampling time period is annotated T, while k denotes the time index (that is to say the discrete time). The sample values of the in-phase and quadrature components of the baseband signal x(kT) are supplied as an input signal  1  to an amplitude control means AGC (Automatic Gain Control)  2 . The signal  3  emitted from the AGC  2  is passed to a low-pass filter TP  4 . A signal  6  with a real amplitude is produced by magnitude formation (addition of the squares of the signal components) in a unit  5 . This signal  6  is filtered by a matched filter  7 . The impulse response of this filter  7  corresponds to the complex-conjugate sync word reflected in the time domain. The matched filter  7  produces a result g(kT) for each sample value x(kT). The maximum value occurs, in a similar way to that in the case of a correlator, at the filter output when the slot start has been found (minus the latency of the filter  7 ). The filtered signal g(kT) for all the sampling time kT, for example k=1, . . . , 2560, . . . , 5120, provided that 5120 samples are taken within one slot, is thus referred to as the correlation response  8 . 
     A search for the maximum is carried out in a unit  9  within an observation interval, which corresponds to the slot duration, in order to detect correlation peaks in the correlation response  8 . The maximum U max  and the associated time T max  are passed to an evaluation unit  10 . The evaluation unit  10  compares the detected maximum U max  with a specific, previously defined threshold value. The evaluation result  11  is signalled to a decision maker  12 . If the threshold value has been exceeded, the decision maker  12  decides that a sync word has been detected at the time T max . 
     Since the mobile radio channel is not a static channel, it is not sufficient to carry out the method described above for only a single slot. A number of slots typically have to be processed in order to make an error-free decision on the slot boundaries. In consequence, intermediate results of the correlation responses  8  must be stored over a number of slot intervals and must be accumulated in the evaluation unit  10  before comparison with the threshold value. The threshold value level determines the yield and confidence of the method. The higher the preset threshold value in the evaluation unit  10  the fewer sync words are detected but, furthermore, the lower is the probability of an incorrect decision based on a sync word being simulated by interference. 
     The slot length in the UMTS (Universal Mobile Telecommunication System) Standard is 2560 chips (an in-phase and quadrature component in each case). An oversampling factor of 2 is normally used. Up to 10240 sample values therefore have to be stored, based on a typical resolution of 8 bits per slot. 
     A multistage evaluation method may be used in order to reduce the amount of memory. The principle of multistage evaluation is based on the idea of a subset of possible times for the slot start in different (mobile radio) cells being determined first of all during a initial selection process, for each slot. Only the times included in this subset are processed further and are possible candidates for the slot start of one or more cells. The slot start of each cell is then determined in a second selection step, which is more accurate and has a higher resolution than the first selection step. 
     With regard to the example of the UMTS standard, it is possible, by way of example, to provide for the 5120 correlation values emitted from the matched filter for each slot (for one signal component with double oversampling) to be restricted to a subset of, for example, 1280 correlation values. This initial selection process can be carried out with relatively little effort, but itself reduces the possible synchronization times by a factor of 4. The second selection step requires considerably greater accuracy (since only that time index at which the slot start is located must be determined from the 1280 time indices), but it need be carried out only for the preselected 1280 time indices k. 
     One difficulty with a multistage method such as this is that the two selection steps (initial selection and final selection) must be matched to one another as accurately as possible. In practice, the maximum size of the subset which can be processed in the second selection step is limited by hardware requirements (memory size, clocking etc). If the number of possible time indices in the initial selection process is greater than this subset, it is not possible to take account of all the preselected time indices in the second selection step. This results in the loss of information which was obtained in the first selection step. If, on the other hand, the number of time indices calculated in the first selection step is considerably less than the size of the subset (for example 1280 correlation values), the capacity of the second selection step is not utilized. Furthermore, during the initial selection process, there is a risk of this preselection process being excessively strict bearing in mind the low accuracy, thus likewise reducing the amount of information available for the second selection step. It is therefore important for the initial selection process to be carried out so as to just exhaust the capacity of the subsequent second selection step. 
     SUMMARY OF THE INVENTION 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The invention is directed to a method for synchronizing a receiver to a transmitter, which ensures that a number of preselected possible synchronization times obtained in a first selection step is matched to a predetermined capacity of a second selection step. A further aim of the invention is to provide an apparatus for synchronizing a receiver to a transmitter, which allows capacity matching between a first and a second selection step. 
     According to one embodiment of the present invention, a set of possible synchronization times for one or more cells is determined in a first selection step by repeated correlation of the signal received by the receiver with the known sequence and comparison of the correlation responses, as calculated for the correlations, with a threshold value. In a second selection step, the actual synchronization time of one cell or the actual synchronization times of two or more cells is or are determined from the set of possible synchronization times. 
     In this exemplary embodiment, the threshold value is adaptively varied in the first selection step as a function of at least one parameter which is measured during the correlations. The threshold value is varied in such a way that the set of two or more possible synchronization times determined in the first selection step always comprises virtually the same number of possible synchronization times, with the number being predetermined. This guarantees that the second selection step is always based on an essentially constant number of possible synchronization times, thus resulting in optimum matching between the first and the second selection step. 
     The first selection step preferably comprises the following: (a1) comparison of the correlation values of a correlation response with the threshold value; (a2) notation of the times at which the correlation values of the correlation response exceed the threshold value; (a3) measurement of the at least one parameter; (a4) setting of the threshold value as a function of the parameter for the next correlation process; (a5) repetition of steps (a1) to (a4) for further correlation responses; and (a6) determination of the set of possible synchronization times as those times at which the adaptive threshold value has been exceeded a predetermined number of times. 
     It should be mentioned that the set of possible synchronization times, can be fixed by presetting the number of times the threshold value must be exceeded in order to include the appropriate synchronization time in the set. However, this definition is too coarse on its own (that is to say without the threshold value adaptation according to the invention). For example, a situation may occur in which a (fixed) threshold value is exceeded m times for a given number of correlations (Ncor) for a comparatively large number of synchronization times (for example considerably greater than 1280) and where the same threshold value is exceeded m+1 times only for a comparatively small number of possible synchronization times (for example considerably less than 1280). Both criteria (the threshold value being exceeded m times and the threshold value being exceeded m+1 times) are then unsuitable for production of a set of possible synchronization times whose size is about 1280. Effective capacity matching between the first selection step and the second selection step is achieved by the adaptive threshold value adjustment from one correlation process to the next in accordance with the present invention. 
     The at least one measured parameter which is used for adaptive adjustment of the threshold value is preferably characteristic of the statistics of the correlation response. In this situation, a first advantageous method variant is characterized in that a first parameter is a measure of the standard deviation of the correlation response. A second advantageous method variant is characterized in that a second parameter indicates the number of times at which the correlation values of the correlation response exceed the current threshold value in the individual matched filter runs. Taking account of the statistical properties of a correlation response for the adjustment of the threshold value for the evaluation of the next correlation response ensures effective control of the number of possible synchronization times selected in the first selection step. 
     One advantageous procedure for adjusting the threshold value from one correlation process to the next is characterized in that the threshold value is increased by a first threshold value offset when the number of times at which the correlation values of the correlation response exceed the threshold value is greater than a first predetermined number. Further, the threshold value is reduced by a second threshold value offset when the number of times at which the correlation values of the correlation response exceed the threshold value is less than a second predetermined number, and in that, for example, the first threshold value offset and/or the second threshold value offset increase as the distribution width and standard deviation of the correlation values of the correlation response rises. This means that the threshold value correction becomes greater as the distribution of the correlation response becomes flatter. In contrast, if the distribution width and standard deviation of the correlation response are small, only minor threshold value corrections are carried out from one correlation process to the next. 
     One exemplary embodiment of the invention is characterized in that the correlations in the first selection step are evaluated on the basis of hard-decided (that is to say dual) signal values. This considerably reduces the computation complexity, although a relatively high degree of inaccuracy must be expected owing to the minimal resolution of the signal values (as is desired of the 2-stage selection method used according to the invention). 
     The second selection step may include any desired further processing of the possible synchronization times determined in the first selection step. According to one embodiment variant of the invention, the second synchronization step likewise includes a correlation step. This means that the signal obtained at the possible synchronization times determined in the first synchronization step are once again correlated with the known sequence (sync word) with higher accuracy. According to one method variant, the correlations in the second selection step are evaluated on the basis of soft-decided signal values (that is to say a correlation result for a specific hypothetical synchronization time has a high resolution of, for example, 16 bits and is also stored with this resolution for subsequent evaluation). This results in the increased accuracy required in the second selection step. 
     The method according to the invention for slot synchronization of mobile stations is preferably used in the UMTS Standard. According to the UMTS Specification 3GPP TS 25.211 V4.4.0 (2002-03), a sequence comprising 256 chips is transmitted at each slot start in a first synchronization channel pSCH (Primary Synchronization Channel). This sequence is used to determine the slot start in the method according to the invention. 
     According to another embodiment of the invention, an apparatus for synchronization of a receiver to a transmitter has a means for repeated correlation of the signal received in the receiver with the known sequence, with correlation values of a correlation response in each case being produced. The apparatus further comprises a means for comparison of the correlation response with a threshold value, a means for adaptive variation of the threshold value as a function of at least one measured parameter, and a first selection means for determination of possible synchronization times by selection of those times at which the adaptive threshold value has been exceeded a predetermined number of times. 
     In addition, the apparatus includes a memory for storage of the possible synchronization times, and a second synchronization means which uses the set of possible synchronization times to determine the actual synchronization time of one or more cells. The means for adaptive variation of the threshold value varies the threshold value as a function of the at least one measured parameter in such a way that the total number of possible synchronization times corresponds to a predetermined number. The adaptive variation of the threshold value between successive correlations means that the number of possible synchronization times selected by the first selection means is matched to the capacity of the second selection step. 
     One alternative embodiment of the invention is characterized by a means for evaluation of the correlation response and for production of the at least one parameter, with the parameter being characteristic of the statistical properties of the correlation response. Evaluation of the statistical properties of the correlation responses allows good threshold value matching to be achieved for the stated purposes. 
     The means for evaluation of the correlation response and for production of the at least one parameter as well as the means for adaptive variation of the threshold value are, in one example, hard-wired hardware circuits. This means that the successive threshold value matching during successive correlations can be carried out independently of a processor, that is to say it does not demand any computation power from a processor. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in the following text using one exemplary embodiment and with reference to the drawing in which: 
         FIG. 1  is a prior art circuit diagram illustrating a system for synchronizing a receiver to a transmitter; 
         FIG. 2  is a block diagram illustrating a circuit architecture for synchronizing a receiver to a transmitter, according to one exemplary embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating a unit configured to perform initial selection of possible synchronization times from  FIG. 2 ; 
         FIG. 4  is a circuit diagram illustrating a control unit for adaptive threshold value adjustment according to one embodiment of the invention; and 
         FIG. 5  is a circuit diagram illustrating a unit for determination of the statistics obtained in the first selection step according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to  FIG. 2 , a circuit according to the invention for slot synchronization has a matched filter MF, to which complex sample values of a synchronization signal which is transmitted via the first synchronization channel pSCH are supplied via an input  100 . Assuming oversampling by a factor of 2, this represents 5120 sample values for the in-phase component and 5120 sample values for the quadrature component for each slot. Via an input  101 , the match filter MF receives filter coefficients which are produced from a read only memory FC (filter coefficients). The filter coefficients are dependent on the sync word (synchronization sequence) which is transmitted via the synchronization channel pSCH. The filter coefficients are chosen such that the impulse response of the matched filter MF corresponds to the complex-conjugate sync word reflected in time. 
     The matched filter MF produces a result for the real part and for the imaginary part of each sample value. The imaginary part of the filtered signal is emitted via a data link  102 , and the real part of the filtered signal is emitted via a data link  103 . 
     These filtered signals are then squared in the squarers SQRi and SQRr, respectively. The squared signal components are then added sample value by sample value with the correct timings in an adder AD. A signal which is referred to in the following text as the correlation response is emitted for one slot period at one output  104  of the adder AD. The correlation response comprises 5120 correlation values for each slot. This is determined coherently, that is to say taking account of the amplitude and phase of the synchronization signal. 
     The correlation response is passed via a data link  105  to a first selection unit S 1  and to a second selection unit S 2 . The outputs of the two selection units S 1 , S 2  are connected to a result memory RES-RAM, which is a volatile memory and can store 128 values. Furthermore, the two selection units S 1  and S 2  are connected via respective data links  107  and  106  to two further volatile memories TEMP-RAM and MASK-RAM. The memory TEMP-RAM in the exemplary embodiment explained here has a memory size of 1280×16 bits, and the memory size of the memory MASK-RAM is, by way of example, 320×16 bits. As indicated by the shadow lines in  FIG. 2 , all of the volatile memories RES-RAM, TEMP-RAM and MASK-RAM may be duplicated. 
     The first selection unit S 1  comprises a preselection unit PSU, a statistics unit SS (Slot Statistics) and a calculation unit MC for determination of variables which are stored in the memory MASK-RAM. The second selection unit S 2  comprises an accumulator ACC and a peak detector PD. 
     The circuit illustrated in  FIG. 2  operates as follows: 
     The preselection unit PSU is supplied with the correlation responses successively via the data link  105  at an input  108 . Ncor denotes the number of correlation responses which should be evaluated for an initial selection process. The index n indexes the correlation responses, that is to say n=1, 2, . . . , Ncor. 
     During evaluation of a correlation response n in the unit PSU, each correlation value of this correlation response is compared with a threshold value T(n). The index k indexes the correlation values of a correlation response, and corresponds to the number of sample values for one slot, that is to say in the example explained here, k=1, 2, . . . , 5120. 
     The threshold value T(1) for the first correlation response (n=1) is signaled to the unit PSU via a digital signal processor DSP which is not illustrated. The initial threshold value is calculated in the DSP as a function of RSSI (field strength) measurements and of the AGC setting, as well as on the basis of noise measurements, and is signaled to the unit PSU before the evaluation of the first correlation response n=1. 
     The preselection unit PSU compares each correlation value (relating to the index k) of the first correlation response (n=1) with the threshold value T(1). A count z 1 (k)=1 is set for those correlation values which are above the threshold value T(1). 
     The accumulated counts z n (k) of all the previous correlation responses including the current correlation response n relating to all the sample values (k) for a slot are each administered in the form of 4-bit words in the memory TEMP-RAM. The memory TEMP-RAM is therefore partitioned in the form 5120×4 bits (in general: the number of correlation values×the count word length). During the evaluation of the n=1 subsequent correlation responses where n=2, 3, . . . , Ncor these counts are incremented on a sample-value related basis whenever the k-th correlation values of these correlation responses are in each case greater than the associated threshold values T(n). This will be described in more detail in the following text. 
     The correlation responses n=1, 2, . . . , Ncor are also passed via the data link  109  to the statistics unit SS. The task of the statistics unit SS is to evaluate statistical properties of the current correlation response n, and then to emit a control signal (via the control line  110 ), which is passed to the unit PSU and, if appropriate, indicates a change in the threshold value T(n+1). The changed threshold value T(n+1) is then used for evaluation of the next correlation response n+1 in the unit PSU. Once again, the individual correlation values (5120 of them) are then successively compared with the new threshold value T(n+1). The count z n+1 (k) which is administered in the memory TEMP-RAM is incremented for each sample index k, provided that the threshold value T(n+1) is not exceeded. 
     After the processing of the Ncor correlation responses, the memory TEMP-RAM thus contains the number of times that the threshold value has been exceeded Z Ncor (k) for each time index k. After Ncor correlations, Z NCor (k) can thus assume the value Ncor as a maximum. 
     The memory contents of TEMP-RAM are then evaluated. Which of the counts Z Ncor (k) (index k) and how many of them have exceeded a target count (ztar) are then determined. The target count ztar is calculated by the DSP in advance on the basis of signal strength and noise measurements. It is of course, also dependent on Ncor. 
     If the adaptive threshold value matching process, which will be described in more detail in the following text, was successful, then the number of times the threshold value was exceeded with respect to the target count ztar is, quite accurately, 1280. In general terms, this corresponds highly accurately to the predetermined size of the subset of possible synchronization times intended for further data processing. 
     The memory TEMP-RAM is evaluated by the calculation unit MC. The calculation unit MC checks for each sample index k whether the associated count Z Ncor (k) satisfies the condition Z Ncor (k)&gt;ztar. A flag is set to the value 1 in the memory MASK-RAM for each sample index k for which this condition is satisfied. The memory MASK-RAM is thus used as a 1-bit memory for 5120 entries (in general: the number of correlation values×1). 
     This therefore identifies the subset (sought in the first selection step) of possible candidates for the synchronization time of one or more cells. We will return later to the further data processing based on this subset as determined in the first selection step. 
       FIG. 3  shows a circuit example of the design of the unit PSU. The unit  108  is connected to a first input  201  of a comparator COMP. The current threshold value T(n) is applied to the second input  202  of the comparator COMP. 
     If the correlation value entered via the input  108  is greater than T(n), the comparator COMP emits an activation signal  203 . The activation signal  203  is passed to a control input  204  of an incrementer INC. A buffer store  206  is connected to the incrementing input  205  of the incrementer INC. The buffer store  206  contains a count z n−1 (k) which has previously been read from the memory TEMP-RAM. When an activation signal  203  is present, the count z n−1 (k) is incremented by the incrementer INC. Initially, it is assumed that the incremented count z n (k) is less than or equal to 15, that is to say there are no problems in storing it as a 4-bit word in the memory TEMP-RAM. In this situation, the value 0 is applied via a multiplexer MUX to the subtraction input  208  of a subtractor SUB. The incremented count z n (k) is received by the subtractor SUB via a data link  207  and is passed on without being changed via a data link  209  to a saturation stage SAT which is designed for the value range (0, . . . , 15). Furthermore, the incremented count z n (k) is passed to a comparator  210 , which checks whether z n (k) assumes the value 15. 
     The incremented count z n (k) emitted from the saturation stage SAT is written to the memory TEMP-RAM via the data link  107 . The comparator  210  as well as a control stage  211  connected downstream from the comparator prevent the memory TEMP-RAM from overflowing in the situation where the count z n (k) has reached its maximum value 15. If the result of the comparison process carried out in the comparator  210  for at least one incremented count for a correlation response n is positive, the control stage  211  switches the multiplexer MUX to the value 1 at the start of the evaluation of the next correlation response n+1. This results in all of the counts z n+1 (k) which have been supplied to the substractor SUB being decremented by the value 1 irrespective of whether or not they have been incremented in the unit INC. This ensures that the value emitted at the output of the substractor SUB is never greater than 15. The saturation stage SAT in this case prevents negative counts from being produced. 
     In this case, the threshold value T(n) is matched for successive correlations n=1, 2, . . . , Ncor as follows: 
     The counts z n (k) are passed via the data link  212  to the statistics unit SS throughout the entire evaluation of the correlation response n. Furthermore, the statistics unit SS receives further information via the data links  213  (correlation values),  214  (current threshold value T(n)) and  215  (activation signal and threshold value decision). Once the correlation response n has been processed in the unit PSU, the statistics unit SS signals via the control line  110  to the unit PSU the amount by which the previous threshold value T(n) should be changed for the evaluation of the next correlation response n+1 (new threshold value: T(n+1)). 
     One simple option for threshold value adaptation is explained in  FIG. 3  with reference to the units ACC 1 , a multiplexer  216 , a further multiplexer MUX 1  and a 4-value memory  217 . The multiplexer  216  chooses the value 0 for n=1, so that the accumulator ACC 1  emits to the comparator COMP the initial threshold value T(1) supplied from the DSP. The multiplexer  216  is connected to the output of the multiplexer MUX 1  for processing all the other correlation responses. The multiplexer MUX 1  has four inputs which receive the values c, −c, 1, −1. c is a positive integer greater than 1, which, for example, was calculated in advance by simulations. The control signal  110  emitted from the statistics unit SS now selects one of the four following options:
 
 T ( n+ 1)= T ( n )+1
 
 T ( n+ 1)= T ( n )−1
 
 T ( n+ 1)= T ( n )+ c 
 
 T ( n+ 1)= T ( n )− c 
 
       FIG. 4  shows a first section SS_ 1  of the statistics unit SS. The circuit section SS_ 1  controls the adaptive threshold value adjustment. 
     An adder AD 1  and a register RE 1  are connected in the form of an accumulator. The register RE 1  has a reset input  301 , by means of which it is reset to the value 0 before the start of evaluation of a correlation response. The threshold value decisions are passed to the adder AD 1  via the input  215 . At the end of processing of a correlation response, a value cand(n) is produced at the output  302  of the register RE 1 , which indicates how many positive threshold value decisions have been made in the unit PSU during the processing of the n-th correlation response:
 
 c and( n )= d ( k )
 
     In this case, d(k)=1 indicates a positive threshold value decision, and d(k)=0 a negative threshold value decision. 
     A further distribution measure (“standard deviation”) for the correlation values of the n-th correlation response is calculated by means of a substractor SU 1 , a selection stage SEL, an accumulator which comprises a register RE 2  and an adder AD 2  and an optional scaling stage SCA. The subtractor SU 1  forms the difference between this correlation value and the threshold value T(n) for each correlation value relating to the sample k. The selection stage SEL ensures that the differences are passed on only when they are positive, that is to say when the correlation value is greater than the threshold value. These positive differences are added up in the accumulator AD 2 , RE 2  over the sample index k=1, . . . , 5120. After any scaling which may also need to be carried out by the unit SCA (the scaling is ignored in the present example), the further statistical measure sttdev(n) for the distribution width (“standard deviation”) of the correlation values of the correlation response n is produced.
 
sttdev( n )=(correlation value( k )− T ( n )),
 
considering only positive expressions (correlation value (k)−T(n)).
 
     The register RE 2  is reset via a reset line  304 . 
     The unit SCA is connected to a control circuit CON via a data line  303 . The control circuit CON uses the information cand(n) and sttdev(n) to decide how the threshold value T(n+1) should be changed for the next correlation response n+1. The algorithm that is provided for this purpose is specified in the following text. In this case, a and b are fixed parameter values which have been calculated in advance by simulation. 
     
       
         
           
             
               
                 
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                 ⁡ 
                 
                   ( 
                   n 
                   ) 
                 
               
               + 
               1 
             
           
         
       
     
     The new threshold value T(n+1) is emitted on the basis of the above case decision. The control line  305  presets the required parameters. The unit PSU illustrated in  FIG. 3  is then controlled in the already described manner via the control line  110 . 
     It is evident that the correlation response is evaluated in the statistics unit SS on the basis of two characteristic variables, specifically a characteristic variable (sttdev(n)) for assessment of the “sharpness” of correlation peaks, and a variable (cand(n)) for assessment of the “yield”. This makes it possible to decide whether a high yield is due to one or more correlation peaks having been found, or is due to interference influences. 
     The performance of the method according to the invention is also governed by suitable choice of the parameters a, b and c. In particular, the desired number of preselected possible synchronization times (in this case 1280) can be adjusted by the choice of the parameter b. 
     The circuit section SS_ 2  illustrated in  FIG. 5  can be integrated in the statistics unit SS in order to check the initial selection step. After the evaluation of all of the correlation responses, the circuit section SS_ 2  is supplied via the data link  107  with all of the final counts Z Ncor (k), k=1, . . . , 5120, from the memory TEMP-RAM. The circuit section SS_ 2  is activated via the control line  400  which is connected to a switch GATE. 
     The circuit section SS_ 2  has four comparators COMP 1 , COMP 2 , COMP 3 , COMP 4 , four count memories THRES 1 , THRES 2 , THRES 3 , THRES 4 , four accumulators AC 1 , AC 2 , AC 3 , AC 4  and four registers R 1 , R 2 , R 3 , R 4 . The circuit section SS_ 2  allocates the counts Z Ncor (k) determined over Ncor correlation responses to different classes. For example, the values stored in the count memories THRES 1 , THRES 2 , THRES 3 , THRES 4  may be the numbers 1, 2, 3, 4. The comparators COMP 1 , COMP 2 , COMP 3  and COMP 4  always emit a value 1 at their outputs  401 ,  402 ,  403 ,  404  whenever the value z Ncor (k) is equal to or greater than the value stored in the respective count memory THRES 1 , THRES 2 , THRES 3 , THRES 4 . The comparator outputs  401 ,  402 ,  403 ,  404  are accumulated in the respective accumulators AC 1 , AC 2 , AC 3 , AC 4 , and are stored in the registers R 1 , R 2 , R 3 , R 4 . 
     In consequence, the register R 1  contains the number of counts which are equal to or greater than the value stored in the first count memory THRES 1 , the register R 2  contains the number of counts which are equal to or greater than the value stored in the count memory THRES 2 , etc. It is now assumed that the target count ztar (for example ztar=3), which has already been defined by the DSP, is stored in the count memory THRES 3 . In this case, the register R 3  contains the number of counts z Ncor (k) greater than 3. This number is in the region of 1280, provided that the threshold value matching has operated. The values stored in the registers R 1 , R 2 , R 3  and R 4  can be read out and checked by the DSP (not shown) via an output  405 . If the threshold value matching fails, it may be necessary to change the parameter a, b or c. Between 2 and 60 correlation responses are typically processed in the described manner for the first selection step. 
     The 1280 possible synchronization times found in the first selection step are then further restricted in the second selection step. In the exemplary embodiment described here, only a further selection of possible synchronization times to produce a total of 128 remaining candidates is envisaged initially. A first option is to carry out the second selection step without carrying out any further correlation processes, solely on the basis of the data in the memory MASK-RAM. This is possible only when the quality of the first selection step (despite the poor capability to differentiate between its results) is sufficiently good that the best 128 synchronization times can be selected on the basis of the result class allocation process described above, and be stored in the memory RES-RAM. 
     In most cases, the second selection step includes another correlation process, which is more accurate and is carried out exclusively on the basis of the possible synchronization times found in the initial selection step. This means that only correlation values at the times stored in the memory MASK-RAM are calculated and processed further here. 
     At least part of the second selection step is carried out in the unit S 2 , which accesses the memory MASK-RAM via the data link  106  and accumulates correlation values in the accumulator ACC at the preselected times. By way of example, the correlation values are likewise calculated in the unit MF. The correlation values are in this case determined with high accuracy (16 bits) and are temporarily stored in the memory TEMP-RAM for further selection. In contrast to the first selection step, no yes/no decision is made here, with the full resolution of the accumulated correlation values being utilized in this case. The peak detector PD then selects from the accumulated correlation values those 128 accumulated correlation values with the maximum peak height. These are stored in the memory RES-RAM. In the course of further processing steps (frame synchronization, code identification), the possible times are further restricted to the sought synchronization time, or it is possible to provide for the sought synchronization time to be determined (as the accumulated correlation value with the maximum peak height) in the unit S 2  itself, solely with the aid of the peak detector PD. 
     While the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.