Abstract:
A method for a user equipment (UE) to establish a communication link comprising the steps of receiving an input communication signal at an initial search frequency, processing the input communication signal to retrieve a primary scrambling code, the retrieval of the primary scrambling code being a code decision, and adjusting the search frequency of the UE in response to the code decision.

Description:
[0001]    This application is a continuation of application Ser. No. 09/918,611, filed on Jul. 31, 2001; which claims priority from Provisional Application No. 60/271,642, filed on Feb. 27, 2001, all of which are incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The present invention generally relates to user equipment (UE) synchronization to a base station. More particularly the present invention relates to a cell search system which utilizes an improved initial cell search algorithm.  
           [0003]    Initial cell search algorithms are used to synchronize the UE to a base station. The UE accomplishes this procedure via a common downlink channel called the physical synchronization channel (PSCH). Referring to FIG. 2, the PSCH has a structure wherein the same primary synchronization code (PSC) is transmitted at the beginning of each slot, while a secondary synchronization code (SSC) is transmitted for each slot, resulting in fifteen (15) different SSCs. As those skilled in the art know, a frame that is fifteen (15) slots long can transmit fifteen (15) SSCs.  
           [0004]    The transmit order of the SSC depends on the primary scrambling code group number. As an example, in a five hundred and twelve (512) cell system, there are sixty four (64) groups. In each group, the patterns of the SSC and its cyclic shifts are different. As a result, there are five hundred and twelve (512) primary scrambling codes. Each cell, of a five hundred and twelve cell (512) system, is assigned a code such that no one code is used by more than one cell in a given reception area.  
           [0005]    Therefore the cell search synchronization systems determine the primary scrambling code of a cell utilizing an initial cell search algorithm. Common initial cell search algorithms utilize three (3) major algorithms: a step  1  algorithm detects the PSC and determines a chip offset; a step  2  algorithm uses the information given by step  1  and detects the slot offset and code group number; and a step  3  algorithm utilizes the information provided by the step  2  algorithm and detects the primary scrambling code. Unfortunately, each step algorithm has an inherent error associated with it. The error present in each of the steps is caused by the UE detection of noise associated with the received common downlink channel, which can result in a high number of false detections.  
           [0006]    Also, the common initial cell search algorithms can not handle a rejection by the upper layers of the wrong public land mobile network (PLMN). Since most algorithms detect the strongest cell in the common downlink channel, it is likely that each time the algorithm locates a cell the same PLMN will be associated with the cell. This results in a deadlock and ultimately an indication to the UE that there is no service.  
           [0007]    Accordingly, there exists a need for a system and method that reduces the number of false detections by the initial cell search algorithm and is able to overcome the deadlock associated with a rejection due to the wrong PLMN.  
         SUMMARY  
         [0008]    A method for a user equipment (UE) to establish a communication link comprising the steps of receiving an input communication signal at an initial search frequency, processing the input communication signal to retrieve a primary scrambling code, the retrieval of the primary scrambling code being a code decision, and adjusting the search frequency of the UE in response to the code decision.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is an illustration of the initial cell search system made in accordance with the preferred embodiment of the present invention.  
         [0010]    [0010]FIG. 2 is an illustration of the physical synchronization channel (PSCH).  
         [0011]    [0011]FIG. 3 is a block diagram of the step  1  module in accordance with the preferred embodiment of the present invention.  
         [0012]    [0012]FIG. 4 is a flow diagram of the step  1  module in accordance with the preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 5 is a block diagram of the step  2  module in accordance with the preferred embodiment of the present invention.  
         [0014]    [0014]FIG. 6 is a graphical illustration of the Fast Hadamard Transform (FHT) structure.  
         [0015]    [0015]FIG. 7 is an illustration of the input matrix structure in accordance with the preferred embodiment of the present invention.  
         [0016]    [0016]FIG. 8 is an illustration of the code group matrix structure in accordance with the preferred embodiment of the present invention.  
         [0017]    [0017]FIG. 9 is an illustration of the correlation matrix structure in accordance with the preferred embodiment of the present invention.  
         [0018]    [0018]FIGS. 10A and 10B show a flow diagram of the step  2  algorithm in accordance with the preferred embodiment of the present invention.  
         [0019]    [0019]FIG. 11 is a block diagram of the step  3  module in accordance with the preferred embodiment of the present invention.  
         [0020]    [0020]FIG. 12 is a block diagram of the step  3  correlator in accordance with the preferred embodiment of the present invention.  
         [0021]    [0021]FIGS. 13A and 13B are a flow diagram of the step  3  algorithm in accordance with the preferred embodiment of the present invention.  
         [0022]    [0022]FIGS. 14A and 14B show a flow diagram of the controller cell search decision logic in accordance with the preferred embodiment of the present invention.  
         [0023]    [0023]FIGS. 15A and 15B show a flow diagram of the controller window exclusion logic in accordance with the preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    The cell search synchronization system  10  in accordance with the preferred embodiment of the present invention is illustrated in FIG. 1. The system  10  comprises a step  1  module  12 , a step  2  module  14 , a step  3  module  16 , and a controller  18  to accomplish synchronization between a user equipment (UE) and a base station. In order to accomplish this synchronization, the UE, through the cell search synchronization system  10 , utilizes an initial cell search algorithm, to be disclosed hereinafter.  
         [0025]    The step  1  algorithm of the initial cell search algorithm is accomplished using the step  1  module  12 . Referring to FIG. 3, the step  1  module  12  comprises two Hierarchical Golay Correlators (HGC)  21 ,  22 , two absolute value modifiers (AVM)  23 ,  24 , a decision circuit  25 , a normalizer circuit  26 , a look up table  27 , a multiplier  28 , a splitter  19 , and a step  1  comparator  29 . The root raised cosine filter (RRCFIR)  1  shown is not a part of the step  1  module  12 , but are illustrated therein to provide a complete picture.  
         [0026]    The purpose of the step  1  module  12  is to find the strongest path over a frame worth of samples the UE has detected and determine the chip offset of the strongest path. The RRCFIR  1  coupled to the splitter  19  is a pulse shaped filter that samples the downlink communication signal from the base station at twice the chip rate and forwards the sample signal to the splitter  19 . The splitter  19  splits the sampled signal into its even and odd samples and passes them to HGCs  21 ,  22 .  
         [0027]    The HGCs  21 ,  22  are coupled to the AVMs  23 ,  24 , and the sample selector  34  of the step  2  module  14  (illustrated in FIG. 5), to be disclosed hereinafter. HGCs  21 ,  22  correlate the PSC of the input signal. As those skilled in the art know, the HGCs  21 ,  22  output the complex values of the even and odd samples of the input signal, respectively. The HGC  21 ,  22  outputs are forwarded to the AVMs  23 ,  24  and the sample selector  34 .  
         [0028]    The AVMs  23 ,  24 , coupled to the HGCs  21 ,  22  and the decision circuit  25 , determine the magnitudes of the HGCs  21 ,  22 , equation to generate the magnitudes is determined according to the following equation: 
           abs ( x )˜ max (| x   real   |, |x   imag |)+0.5* min (| x   real   |, |x   imag |)  Equation 1 
         [0029]    The use of the approximated absolute value in accordance with Equation 1 reduces the hardware required in this implementation and causes no significant performance degradation. Once the approximated absolute values have been determined by the AVMs  23 ,  24 , respectively, the modified even and odd samples are output to a decision circuit  25 .  
         [0030]    The decision circuit  25 , coupled to the AVMs  23 ,  24  and the controller  18 , determine the chip offset. The modified even and odd samples output from the AVMs  23 ,  24  are input into a MUX  8  within the decision circuit  25 , and combined into a single stream. This stream is a representation of the strength of the signal transmitted in one of the samples of each slot of each frame. As illustrated in FIG. 2, there are two thousand five hundred and sixty (2560) chips in each slot and fifteen (15) slots in each frame. Since the input signal is sampled at twice the chip rate, there are 5120 samples in each slot. Therefore, the decision circuit  25  determines the location of the PSC in the signal, chip offset, by sweeping through the 5120 accumulated samples at the end of each slot.  
         [0031]    The stream generated by the MUX is forwarded to an accumulator (not shown) within the decision circuit  25 . This accumulator has a five thousand one hundred and twenty (5120) sample long register which stores the accumulated sample value for each slot of every frame, and operates on the slot rate. The strength of the signal for each sample in a slot is added to the strength of the signal of each sample in every subsequent slot. As an example, the samples of slot  1  comprise the following signal strength values {1, 5, 3, 7}; the samples of slot  2  comprise the following signal strength values {2, 4, 8, 3}. Initially, the registers of the accumulator have the values {0, 0, 0, 0}. As each sample value from slot  1  is added to the registers of the accumulator, the register values change accordingly. For instance, when the first sample value of slot  1  is added to the first register value, the accumulator has the values {1, 0, 0, 0}; when the second sample value of slot  1  is added to the second register value, the accumulator has the values {1, 5, 0, 0} and so on. Once the last sample value of slot  1  is added to the accumulator, the first sample value of slot  2  is added to the first register of the accumulator, resulting in the accumulator having the values {3, 5, 3, 7}; when the second sample value of slot  2  is added to the second register value, the accumulator has the values {3, 9, 3, 7}. The preferred embodiment of the present invention, flushes the registers of the accumulator after five (5) frames have been accumulated, which is equivalent to seventy five (75) slots. The number of accumulated frames is counted by a step  1  counter (not shown) within the decision circuit  25 .  
         [0032]    A decision, determination of the chip offset, by the decision circuit  25  is generated at the end of each frame, fifteen (15) slots. The decision circuit  25  determines which register in the accumulator has the maximum accumulated sample value MAX and assigns an index to it. The index corresponds to the half chip location of the PSC signal for the base station with the strongest signal.  
         [0033]    Chip offset assignment is determined using the HGC offset value of 511. As those skilled in the art know, the output of the HGC are delayed by 256 chips. Therefore, when the decision circuit  25  assigns an index in the peak sample, the HGC offset value must be subtracted. Since the PSC is 256 chips long, 512 samples long, subtracting the HGC offset from the index equates to setting the chip offset to the beginning of the slot. If the index generated by the decision circuit  25  is greater than the HGC offset value of 511 then the chip offset is calculated in accordance with Equation 2 below: 
         chip_offset=INDEX−511  Equation 2 
         [0034]    If the index is less than the HGC offset value then the chip offset is calculated in accordance with Equation 3 below: 
         chip_offset=5120+INDEX−511  Equation 3 
         [0035]    As illustrated in FIG. 3, the decision circuit  25  also comprises a mask generator  5 , which is used to exclude a window around a rejected chip offset from detection by the decision circuit  25 . This mask generator  5 , therefore, prohibits the decision circuit  25  from utilizing an index associated with a rejected chip offset. The details of the mask generator  5  will be disclosed hereinafter.  
         [0036]    The calculated chip offset and the frame count step  1  counter are output to a controller  18 , to be disclosed hereinafter. The decision circuit  25  also outputs the maximum accumulated chip value MAX and the accumulated chip value OUTPUT for all registers.  
         [0037]    The accumulated chip value OUTPUT for all registers is output to a normalizer circuit  26 , where it is sampled at 20% the chip rate (one out of five), summed, and then normalized to 1024. The frame count step  1  counter is output to the lookup table  27  to determine the proper gain factor based on the number of frames accumulated. The output of the normalizer circuit  26  and the lookup table  27  are then multiplied by the multiplier  28 . The output of the multiplier  28  is considered the Noise Threshold and is forwarded to a step  1  comparator circuit  29 , to be compared to the maximum accumulated sample value MAX. If the maximum accumulated sample value MAX is greater than the Noise Threshold, the differential amplifier  29  outputs a high step  1  firm signal to the controller, indicating a good decision for step  1 , otherwise a low signal is output.  
         [0038]    As stated earlier, the chip offset and other outputs are determined at the end of every frame. Therefore, the reliability of the first decision is less than that of the second because the second decision is made over thirty slots instead of fifteen slots. The reliability increases as the number of slots accumulated increases. The highest reliable output is generated at the M 1 th frame, M 1  being an integer greater than or equal to one (1). The controller  18  resets the frame count step  1  counter and the accumulator registers at the end of every M 1 th frame. The performance results under different channel impairment show that five-frame integration is good enough to detect PSC. However, this integration can be changed to more or less frames.  
         [0039]    A flow diagram of the step  1  module is illustrated in FIG. 4. The UE detects the receipt of communications over the common downlink channel (step  401 ) and samples the signal at twice the chip rate generating even and odd samples (step  402 ). These even and odd samples are passed to the hierarchical Golay correlators (HGC)  21 ,  22  (step  403 ). The HGCs  21 ,  22  then forwards the outputs to the AVMs  23 ,  24  and sample selector  34  (step  404 ). The AVMs  23 ,  24  approximate the magnitudes of the even and odd outputs received from the HGCs  21 ,  22  (step  405 ) and forwards them to the decision circuit  25  (step  406 ). Upon receipt of the output magnitudes the decision circuit  25  combines the magnitudes (step  407 ), which represents the signal strength of the signal transmitted in one of the samples of each slot of each frame. The signal strength for each sample is accumulated for all slots within each frame (step  408 ). The decision circuit  25  then determines which sample in the frame has the maximum accumulated sample value (step  409 ) and assigns an index to it (step  410 ). Based on the index, a chip value is assigned to the index (step  411 ), known as the chip offset, and output to the controller  18  (step  412 ). A noise threshold value is then generated using the accumulated chip value for all samples and the frame count (step  413 ) and then compared to the maximum accumulated sample value (step  414 ), indicating a firm or tentative decision to the controller  18  (step  415 ).  
         [0040]    Referring back to FIG. 1, the outputs of the step  1  module  12 , the chip offset, step  1  firm, and step  1  counter, are forwarded to the controller  18 . The controller  18  forwards the chip offset to the step  2  module  14 . As stated above, the step  2  module  14  utilizes a step  2  algorithm which takes the chip offset output from step  1  and the HGC  21 ,  22  outputs and detects the slot offset and the code group number. The step  2  module  14  illustrated in FIG. 5, comprises a step  2  comparator  30 , a delay  32 , a sample selector  34 , a conjugator  36 , a complex multiplier  38 , a Fast Hadamard Transform (FHT)  33 , an envelope remover  31 , an input matrix generator  35 , an RS encoder  37 , and a step  2  decision circuit  39 .  
         [0041]    The purpose of the step  2  algorithm is to provide the step  3  algorithm with the scrambling code group number and the slot offset. The chip offset from the step  1  module  12  is sent from the controller  18  to a delay  32  of the step  2  module  14 . The chip offset is delayed for a frame through the delay  32  in order to allow the step  1  module to make a first decision. The delayed chip offset is then forwarded to the sample selector  34  which is coupled to the delay  32 , a conjugator  36  and the HGCs  21 ,  22  of the step  1  module  12 . Using the index determined by the decision circuit  25 , the sample selector  34  extracts the peak HGC  21 ,  22  outputs from the input signal, which are then conjugated by the conjugator  36  and output to the complex multiplier  38 .  
         [0042]    The same communication signal to the step  1  module  12  is input to an alignment circuit  15 , which aligns the input signal so that step  2  module  14  begins it search for the scrambling code group number and slot offset at the beginning of the slot. Once the signal is aligned, the alignment circuit  15  forwards it to the step  2  module  14 . Even though there are two thousand five hundred and sixty (2,560) chips in each slot, it should be apparent from FIG. 2 that the PSC is located within the first 256 chips of each slot. Since the chip offset has been determined by the step  1  module, the step  2  module determines the SSC using the location of the strongest PSC in the first 256 chips in each slot. As those skilled in the art know, when SSC codes are generated, an envelope sequence is applied to the rows of an Hadamard matrix in order to have some orthogonality between PSC and SSC codes. This envelope has to be removed before proceeding into the remaining portion of the step  2  algorithm. This envelope removal is accomplished by the envelope remover  31 .  
         [0043]    Once the envelope has been removed from the input signal, the signal is output from the envelope remover  31  to the FHT transform  33  coupled to the envelope remove  31  and multiplier  38 , which reduces the complexity of the pure Hadamard correlation operation. FIG. 6 is an illustration of the FHT structure. The output of the FHT transform  33  is multiplied by the conjugate of the peak HGC  21 ,  22  by the complex multiplier  38  coupled to the conjugator  36  and the FHT transform  33 . The use of the conjugate of the peak output from the HGCs  21 ,  22  provides a phase correction to the FHT output and transforms the one entry that corresponds to the transmitted SSC code onto the real axis.  
         [0044]    Once the FHT transform  33  output has been multiplied in the complex multiplier  38 , the real part of the FHT outputs are forwarded to the input matrix generator  35  by the multiplier  38 , which puts the FHT outputs into a real matrix of 15×16, called the input matrix. In the input matrix, there are fifteen (15) slots and in each slot sixteen (16) elements for a frame. The input matrix is updated per frame. The input matrix is then forwarded to the decision circuit  39  where a determination of the slot offset and code group number are made. The structure of the input matrix is illustrated in FIG. 7.  
         [0045]    A correlation matrix is generated within the step  2  decision circuit  39  using the input matrix  35  and a known code group matrix, which results in a 64×15 matrix. The correlation matrix is reset when the frame counter for the step  2  module reaches M 2 , similar to that disclosed in the step  1  module. In order to generate the correlation matrix, the decision circuit  39  steps through each of the elements of the code group matrix and the elements of the input matrix  35  in accordance with the equation 4 below: 
         corrMatrix[ i] [j ]+=inputMatrix [ k ] [code_group_matrix [ i] [k]]   Equation 4 
         [0046]    where j is an integer incremented from 0 to 14 by 1, that represents cyclic shifts performed on the identity matrix with respect to columns; i is an integer incremented from 0 to 63 by 1; and k is an integer incremented from 0 to 14 by 1. The structure of the code group matrix and the resulting correlation matrix are illustrated in FIGS. 8 and 9 respectively. Once the correlation matrix has been generated, the maximum entry is found by the decision circuit  39 . The corresponding row of the found maximum entry is the code group number and the column is the slot offset.  
         [0047]    Similar to the step  1  module  12 , if the max correlation MAX  2  is greater than the threshold, the comparator circuit  3   0  will output a high step  2  firm signal to the controller  18  indicating a firm decision, otherwise a low signal is output indicating a tentative decision. The threshold value is calculated using the mean magnitude value of the correlation matrix:  
               Th   =     k                   1   960          (       ∑     i   =   0     63            ∑     j   =   0     14          mag                   (     c   ij     )           )              
            k   =   5.12     ,       P   fa     =     10     -   4                   Equation                 5                               
 
         [0048]    where P FA  is the probability of false alarm. The step  2  module  14  outputs to the controller  18  the code group number, slot offset, step  2  firm, and step  2  counter.  
         [0049]    The flow diagram for the step  2  algorithm is illustrated in FIG. 10. The step  2  module receives the communication signal from the base station over the downlink channel (step  1001 ). An envelope sequence is removed from the communication signal (step  1002   a ) and output to an FHT transform  33 , (step  1003   a ). At the same time, the chip offset from the step  1  module  12  is input to a delay  32  in the step  2  module  14  (step  1002   b ) and forwarded to a sample selector  34 , which extracts the peak even or odd output generated by the HGCs  21 ,  22  of the step  1  module  12  based on the chip offset (step  1003   b ). The output of the FHT transformer  33  is then multiplied by the conjugate of the peak even or odd sample output from the sample selector  34  (step  1004 ) and transforms one entry of the FHT output that corresponds to the SSC code onto the real axis (step  1005 ). The real part of the FHT outputs for each slot in a frame are forwarded to the input matrix generator  35  (step  1006 ). The input matrix generator  35  then creates the input matrix (step  1007 ). The input matrix is then forwarded to the decision circuit  39  to determine the slot offset and code group number (step  1008 ). Utilizing the input matrix and known code group matrix, the decision circuit  39  generates a correlation matrix (step  1009 ). Once the correlation matrix has been generated, the decision circuit  39  locates the maximum entry in the correlation matrix (step  1010 ), for which the corresponding row of the found maximum entry is determined to be the code group number and the column is the slot offset. The code group number and the slot offset are then forwarded to the controller  18  (step  1011 ). A threshold value is then calculated using the mean magnitude value of the correlation matrix (step  1012 ) and compared to the max correlation (step  1013 ), forwarding an indication of a firm or tentative decision to the controller  18  (step  1014 ).  
         [0050]    The chip offset output from the step  1  module  12  and the slot offset and code group number output from the step  2  module, are forwarded by the controller  18  to the step  3  module  16 , which utilizes a step  3  algorithm for the purpose of determining which one of the primary scrambling codes is coming with the least probability of false alarm (P FA ) when the code group number is given. There are eight primary scrambling codes in each code group.  
         [0051]    The block diagram of the step  3  module  16  is illustrated in FIG. 11. Similar to the step  2  module  14 , the communication signal is input to a second alignment circuit  18  which aligns the output signal so that the step  3  module  16  begins its search for the scrambling code number at the beginning of the frame. Once the input signal has been aligned, the alignment circuit  18  forwards the input signal to the step  3  module  16 . The step  3  module comprises eight (8) scrambling code generators  40   1  . . .  40   8 , eight (8) correlator circuits  41   1  . . .  41   8 , a noise estimator circuit  42 , a step  3  decision circuit  44 , a decision support circuit  45 , a gain circuit  46 , and a comparator circuit  47 . The code group number generated by the step  2  module  14  is input to the eight (8) scrambling code generators  40   1  . . .  40   8  and scrambling codes are generated therefrom. The output of the scrambling code generators  40   1  . . .  40   8  is forwarded to the scrambling code correlators  41   1  . . .  41   8 , respectively.  
         [0052]    Along with the scrambling codes output from the scrambling code generators  40   1  . . .  40   8 , the communication signal, after processing by a realignment circuit  15  using the chip offset and slot offset output from the controller  18 , is input to the correlators  41   1  . . .  41   8 . The correlators  41   1  . . .  41   8  utilize non-coherent integration over a certain number of slots. Integration can be over multiple frames. The correlation is made coherently for each symbol that corresponds to the 256-chip data. The absolute value of the correlation results are accumulated over 10 * N symbols per frame, where N is the number of slots to be accumulated from the beginning of a frame. In a single slot there are ten 256-chip long data parts; therefore, ten 256-chip coherent correlation and ten accumulations are made per slot. FIG. 12 shows the details of a correlator  41   1 .  
         [0053]    After the correlators  41   1  . . .  41   8  generate the outputs, the maximum output and its index have to be found. The step  3  decision circuit  44  takes the outputs of the scrambled code correlators  41   1  . . .  41   8  determines the correlator  41   1  . . .  41   8  with the maximum output, and generates an index thereof. The index is the scrambling code number. The scrambling code number is then forwarded to the decision support circuit  45  and the controller  18 . The decision support circuit  45  observes the last M 3  decisions made by the decision circuit  44 . If a code repeats itself more than k repetitions out of M 3  inputs, then the code that has been repeated is the scrambling code number that is output from the decision support circuit  45  to the controller  18 . However, the output of the decision support circuit  45  is only utilized when there is no firm decision over the consecutive M 3  frames. Even though the decision support circuit is only illustrated in the step  3  module  16 , a decision support circuit  45  as disclosed in the step  3  module  16  can be utilized for both the step  1  and step  2  modules  12 ,  14  disclosed herein above.  
         [0054]    A firm decision is indicated when the determined maximum correlation value is greater than the calculated threshold value. The threshold value is calculated using the noise estimator circuit  42 , which is used for noise measurement, and a gain factor. The noise is determined by taking the magnitude of the difference between the successive common pilot symbols. This method of noise estimation eliminates any bias in the noise estimate due to orthogonal signal interference. The result of the noise estimator  42  is multiplied by the gain factor in the multiplier  46 , which is determined to be the threshold. When the determined maximum correlation is greater than the calculated threshold, the comparator  47  outputs a high step  3  firm signal indicating a firm decision, otherwise a low signal is generated indicating a tentative decision.  
         [0055]    The flow diagram of the step  3  algorithm is illustrated in FIG. 13. The code group number output from the step  2  module  14  is input to the step  3  module  16  scrambling code generators  40   1  . . .  40   8  (step  1301 ), which then generate scrambling codes therefrom (step  1302 ). The output of the scrambling code generators is then forwarded to the scrambling code correlators  41   1  . . .  41   8  (step  1303 ). Along with the scrambling codes output from the scrambling code generators  40   1  . . .  40   8 , the communication signal is correlated in the scrambling code correlators  41   1  . . .  41   8  (step  1304 ), which then generate ten 256 chip coherent correlations and ten non-coherent accumulations per time slot (step  1305 ). The accumulated results are forwarded to the step  3  decision circuit  44  (step  1306 ). The decision circuit  44  determines the correlator with the maximum output and generates an index thereof, which is the scrambling code number (step  1307 ). A threshold value is then calculated (step  1308 ) and compared to the maximum correlation value (step  1309 ). If the maximum correlation value is greater than the calculated threshold, the step  3  module  16  outputs a high step  3  firm signal (step  1310 ), which results in the decision circuit  44  outputting the scrambling code number to the controller  18  (step  1311 ). Otherwise, a low signal is output to the controller  18  (step  1312 ) and the scrambling code number is output to the decision support circuit  45  (step  1313 ). Since the decision support circuit  45  observes the last M 3  decisions made by the decision circuit  44 , a scrambling code number is output to the controller  18  when a scrambling code repeats itself k times out of M 3  inputs (step  1311 ).  
         [0056]    Referring back to FIG. 1, the controller  18  comprises a rejected chip offset buffer  9 , a rejected chip offset counter  11 , a rejected primary scrambling code vector buffer  13 , a rejected primary scrambling code counter  3 , a decision logic circuit  2  and a window exclusion logic circuit  6 . The controller  18  is used to make better decisions during the entire cell search algorithm in accordance with the preferred embodiment of the present invention.  
         [0057]    The flow diagram of the decision logic used by the controller  18  to determine the primary scrambling code for synchronization with the transmitting base station is illustrated in FIG. 14. The controller  18  receives the chip offset, the step  1  firm signal and the step  1  counter signal from the step  1  module  12  (step  1401 ). If the step  1  firm signal is high, the controller  18  forwards the firm chip offset to the step  2  module  14  (step  1402   a ), otherwise a tentative chip offset is forwarded (step  1402   b ). The step  2  module  14  generates the code group number, slot offset value, step  2  firm, and step  2  counter (step  1403 ). If the step  2  firm signal is high, the controller forwards the firm code group to the step  3  module (step  1404   a ). Otherwise, the controller  18  forwards a tentative code group to the step  3  module  16  (step  1404   b ) and if the step  2  counter is less than M 2 , the step  2  module  14  continues to generate the code group number (step  1403 ). If the step  2  counter is equal to M 2 , then the step  2  module  14  is reset (step  1407 ), which results in the step  2  module generating a code number and slot offset (step  1403 ). The step  3  module  16  then generates a scrambling code number and step  3  firm signal (step  1405 ) generated in step  1403 , receiving the slot offset and code group number. If the step  3  firm signal is high, then the decision logic circuit  2  determines that the scrambling code number is firm and ends the decision logic process. If the step  3  firm signal is low and the step  1  firm signal is high or the step  2  counter is less than M 2 , the step  2  module continues to generate a code group number (step  1403 ). Otherwise, the step  2  module receives a reset signal from controller  18  and resets the step  2  counter to 0 (step  1407 ). This procedure continues until the decision output by the step  3  module  16  is firm.  
         [0058]    Due to a possible initial frequency error in the VCO, excess loss of signal correlation may occur. Therefore, the VCO is frequency stepped in order to control the maximum possible frequency error between the UE and the cell. Upon initialization of the UE, the controller  18  initializes the cell search frequency using the frequency synthesizer  20 . Referring to FIG. 1, the frequency synthesizer  20  comprises an adaptive frequency circuit (AFC)  4  and a voltage controlled oscillator (VCO)  7  or numerically controlled oscillator (NCO). The AFC  4 , coupled to the controller  18  and the VCO  7 , comprises a frequency allocation table (FAT) and a frequency step table (FST).  
         [0059]    When the controller  18  is initialized, the AFC  4  sets the frequency using the first frequency in the FAT and the offset value from the FST. This initial frequency is the frequency used by the controller  18  to conduct the cell search. The FST is a table of step frequencies, or offset frequencies, for example {0, 2, −2, 4, −4, 6, −6 . . . N, −N} which are used to offset the frequency in use by the controller  18 . The FAT includes a plurality of predetermined frequencies for which the controller  18 , or a level 1 controller (not shown) utilize to locate and synchronize the UE to the base station. For purposes of this disclosure, the plurality of frequencies listed are defined as F 0 , F 1 , F 2  . . . F N  in the FAT and the offset frequencies in the FST are defined as SF 0 , SF 1 , −SF 1 , SF 2  −SF 2  . . . SF N , −SF N . Accordingly, when the controller is initialized, the offset frequency is SFO and the frequency &gt;F 0 . The AFC  4  combines the two values F 0 +SF 0 , and forwards the resulting frequency value to the VCO or NCO  7 , which maintains the UE frequency at this forwarded frequency.  
         [0060]    The controller  18  performs the decision logic disclosed above. If after X number of frames the output step  3  firm does not go high, the controller signals the AFC  4  to step  2  the next offset in the FST, for example, SF 1 . The AFC  4  then combines the new offset frequency with the frequency of the FAT, F 0 +SF 1 , and outputs the resulting frequency to the VCO or NCO  7  to maintain the UE at this frequency.  
         [0061]    The controller  18  continues to step through the offset frequencies in the FST until a high signal is detected from the step  3  module  16 , indicating a firm detection or until all offset frequencies have been tried by the controller  18 . Once all of the offset frequencies have been tried, the AFC  4  resets the FST offset frequency to SF 0 , steps to the next frequency in the FAT, F 1  and combines the two values, F 1 +SF 0 , for output to the VCO or NCO  7 . The VCO or NCO  7  then regulates the UE frequency to this new resulting frequency and the controller  18  then performs the decision logic until a high signal is detected from the step  3  module  16 . This process of stepping through the FST and then stepping to the next FAT frequency is continued until a high signal is output by the step  3  module  16 . Once this event occurs the detection of a scrambling code, the AFC  4  locks the FST offset value at its current position, not to be readjusted until the controller  18  is initialized.  
         [0062]    As those skilled in the art know, most service providers in a communication system have a different public land mobile network (PLMN). The UE utilizes the detected PLMN to determine whether or not the service provider provides service in the UE&#39;s location. The controller  18  utilizes a window exclusion logic within the window exclusion logic circuit  6  for overcoming a rejection due to the wrong PLMN. Since detecting the HGC  21 ,  22  output at peak value always gives the same PLMN, the controller  18  utilizes the window exclusion logic to overcome this deadlock. The window exclusion logic circuit is coupled to the decision logic circuit  2 , rejected chip offset vector buffer  9 , a rejected chip offset counter  11 , a rejected primary scrambling code vector buffer  13 , and a rejected primary scrambling code counter  3 . The window exclusion logic circuit  6  checks the primary scrambling code output from the step  3  module against the rejected primary scrambling codes stored in the rejected primary scrambling code vector buffer  13 . If the primary scrambling code output from the step  3  module is found in the buffer  13 , or the wrong PLMN is detected, the window exclusion logic circuit  6  rejects the code and initializes the decision logic circuit again. Each time a primary scrambling code is rejected, the chip offset that was generated by the step  1  module is stored in the rejected chip offset vector buffer  9  and used by the mask generator  5 . The mask generator  5  of the decision circuit  25  within the step  1  module  12  uses the values stored in the rejected chip offset vector buffer  9  and rejected chip offset counter  11  from the controller  18  to determine which chips in each slot to exclude in the window. The exclusion of the detected primary scrambling codes and chip offsets are made only within a single frequency band. The buffers and counters are reset when there is an acknowledgment by the base station or new frequency band is used by the level 1 controller.  
         [0063]    In order to adjust the frequency band used by the controller  18  during the window exclusion logic process, the layer  1  controller signals the AFC  4  to step to the next frequency in the FAT. Since the offset frequency of the FST is set, the AFC combines the new frequency with the set offset frequency. The VCO or NCO  7  is then adjusted to maintain this combined frequency.  
         [0064]    A flow diagram of the window exclusion logic utilized by the controller is illustrated in FIG. 15. The controller  18  runs the cell search decision logic and finds a primary scrambling code (step  1501 ). The primary scrambling code is passed to the upper layers (step  1502 ) which store the frequency and the primary scrambling code index (step  1503 ). If the PLMN is correct for the particular service provider, the UE is synchronized to the base station, and the process is terminated (step  1504 ). If the PLMN is incorrect and there is a frequency remaining in the FAT of the AGC  4 , the AGC  4  steps to the next frequency in the FAT and the controller  18  changes the frequency, stores the primary scrambling code in the vector buffer  13 , and resets the cell search algorithm (step  1505 ). It should be noted that the failure condition monitors either the counter buffers  3 ,  11 , or a timer to determine whether a failed condition occurs. A failed condition indicates that synchronization will not occur under the current conditions (e.g. frequency). If there is no frequency left within the FAT, the controller  18  begins to the sweep the frequencies with the stored primary scrambling code (step  1506 ). The controller  18  then sets the first frequency and passes the rejected primary scrambling code to the initial cell search with window exclusion method (step  1507 ). The controller  18  resets the initial cell search with window exclusion method and also resets the failure condition (step  1508 ). The rejected primary scrambling code is pushed into the rejected primary scrambling code vector buffer  13  and the rejected primary scrambling code counter is incremented (step  1509 ). The cell search decision logic is run and a primary scrambling code and chip offset are found (step  1510 ). If the primary scrambling code is stored in the rejected primary scrambling code vector buffer  13 , then the chip offset is pushed into the rejected chip offset vector buffer  9  and the rejected chip offset counter  11  is incremented (step  1511 ). The cell search decision logic is again run excluding a window around the rejected chip offset (step  1512 ). If the primary scrambling code generated by this cell search decision logic is again stored in the rejected primary scrambling code vector buffer, then the detected chip offset is pushed onto the rejected chip offset vector buffer and the rejected chip offset counter is incremented (step  1511 ) and the cell search decision logic excluding a window of value rejected chip offset is run again (step  1512 ). Steps  1511  and  1512  continue until the detected primary code is not in the list at which point the primary scrambling code is forwarded to the upper layers to await an acknowledgment by the base station (step  1513 ). If there is a failure condition and there is no frequency left, the controller  18  indicates that no service is available (step  1517 ) and the process is terminated. If there was a failure and there was a frequency remaining in the bandwidth, the controller  18  sets a new frequency and passes the rejected primary scrambling code for that frequency (step  1516 ). The controller  18  then resets the initial cell search with window excluding method and the failure condition monitor (step  1508 ). The controller  18  then continues the initial cell search with window exclusion method as disclosed above. If there is no failed condition and the PLMN is correct, the controller  18  indicates that the UE is synchronized to the base station upon receipt of the acknowledgment (step  1518 ), and the process is terminated. If the PLMN is incorrect, the rejected primary scrambling code is pushed into the rejected primary scrambling code vector buffer  13  and the rejected primary scrambling code counter  3  is incremented (step  1515 ). The cell search decision logic is run again excluding a window around the previously rejected chip offset value (step  1512 ). This procedure continues until the controller indicates that no service is available or an acknowledgment from a base station is received.