Abstract:
An apparatus for finely synchronizing code signals with an encoded received signal that includes:  
     (a) a sampling device for sampling the received signal at regular sampling intervals;  
     (b) a pulse shaper for shaping the sampled received signal pulses in order to output a first and second pulse-shaped sample in dependence on an autocorrelation function;  
     (c) a buffer for buffering the two samples;  
     (d) a code signal generator for generating the code signal;  
     (e) correlation devices for correlating the generated code with the two buffered samples to form two correlation values; and  
     (f) an interpolation device for forming an interpolation value as a function of the two pulse-shaped samples and the deviation between the two correlation values.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of copending International application No. PCT/DE00/03859, filed Nov. 2, 2000 which designated the United States and which was not published in English.  
         BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0002]    The invention relates to an apparatus for finely synchronizing code signals with a coded received signal, in which locally generated code signals are brought into complete phase correspondence with the coded received signal.  
           [0003]    [0003]FIG. 1 shows a prior art apparatus for finely synchronizing code signals with a coded received signal. A coded received signal passes via a signal input E to a sampling device  100  for sampling at a specific sampling frequency f A . The sampled signal is fed to a pulse shaper  102  that has an autocorrelation function. Two consecutive samples pulse-shaped by the pulse shaper  102  are fed to three correlators K 1 , K 2 , K 3 . The correlators K 1 , K 2 , K 3  are multipliers. The correlation devices K 1 , K 2 , K 3  are connected respectively to code generators G 1 , G 2 , G 3 . The output correlation values of the correlators K 2 , K 3  are subtracted by a subtractor S to form a difference signal or deviation signal ε. The deviation signal ε passes via a loop filter  104  to a voltage-controlled oscillator VCO that drives the generators G 1 , G 2 , G 3 . The correlators K 2 , K 3  and the subtractor S form a phase deviation detector for generating a phase deviation signal ε. The phase detector, the loop filter  104 , the voltage-controlled oscillator VCO, and the generators G 2 , G 3  for the correlators K 2 , K 3  form a DLL circuit (DDL: Delay Locked Loop). The generator G 1  forms the difference code for the correlator K 1 , which correlates the received coded input signal with the reference code. The output signal of the correlator K 1  is output for the purpose of further data processing such as descrambling and dechannelization and/or despreading.  
           [0004]    The code generator G 2  forms a code C(t+T C /2) leading by a specific time phase, and the code generator G 3  forms a code C(t−T C /2) lagging by the same time phase. The voltage-controlled oscillator VCO drives the generators G 2 , G 3  in such a way that the deviation signal ε is minimized. Because of this correction, the phase of the locally generated code is brought into complete correspondence with the phase of the coded received signal, or is synchronized.  
           [0005]    The disadvantage in the case of the apparatus shown in FIG. 1 for fine synchronization consists in that a total of three code generators G 1 , G 2 , G 3  are required to generate phase-shifted code signals. The outlay on circuitry in the prior art apparatus for fine synchronization is therefore very high.  
         SUMMARY OF THE INVENTION  
         [0006]    It is accordingly an object of the invention to provide an apparatus for finely synchronizing a code signal with a coded received signal which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.  
           [0007]    In particular, it is an object of the invention to create an apparatus for finely synchronizing a code signal with a coded received signal that requires only a code signal generator for generating the local code signal.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for finely synchronizing a code signal with a coded received signal that includes: a sampling device for sampling the received signal at uniform sampling intervals, a pulse shaper for shaping the sampled received signal pulses in order to output a first and second pulse-shaped sample in dependence on an autocorrelation function; a buffer for buffering the two samples, a code signal generator for generating the code signal, a correlation device for correlating the generated code with the two buffered samples to form two correlation values, and an interpolation device for forming an interpolation value as a function of the two pulse-shaped samples and as a function of the deviation between the two correlation values.  
           [0009]    In accordance with an added feature of the invention, the correlation devices are preferably multipliers.  
           [0010]    In accordance with an additional feature of the invention, the code signal generator generates a scrambling code.  
           [0011]    In accordance with another feature of the invention, the code signal generator generates a despreading code.  
           [0012]    In accordance with a further feature of the invention, the correlation devices are preferably respectively connected downstream of integrators.  
           [0013]    In accordance with a further added feature of the invention, a subtractor is provided for subtracting the correlation values in order to form a deviation signal.  
           [0014]    In accordance with a further additional feature of the invention, a digital FIR (Finite Impulse Response) loop filter is preferably connected downstream of the subtractor.  
           [0015]    In accordance with another added feature of the invention, the sampling interval is exactly half the chip duration T C .  
           [0016]    In accordance with another additional feature of the invention, the interpolation device is a linear TVI interpolator.  
           [0017]    In accordance with yet an added feature of the invention, the interpolation device is a quadratic TVI interpolator.  
           [0018]    In accordance with yet an additional feature of the invention, the interpolation device has a deviation factor calculating unit for calculating a deviation factor as a function of the deviation signal.  
           [0019]    In accordance with yet another feature of the invention, the pulse shaper is an RRC filter.  
           [0020]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0021]    Although the invention is illustrated and described herein as embodied in an apparatus for finely synchronizing code signals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
       
    
    
       [0022]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 shows a prior art fine synchronization apparatus;  
         [0024]    [0024]FIG. 2 shows an inventive apparatus for finely synchronizing a code signal with a coded received signal;  
         [0025]    [0025]FIG. 3 shows another embodiment of the apparatus for finely synchronizing a code signal with a coded received signal; and  
         [0026]    [0026]FIGS. 4 a ,  4   b , and  4   c  show the formation of the two pulse-shaped samples in dependence on the autocorrelation function of the pulse shaper.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 2 thereof, there is shown an inventive apparatus for fine synchronization of a code signal with a coded received signal. The apparatus has a signal input  1  for receiving the coded received signal. The coded received signal is fed via a line  2  to a sampling device  3  that samples the coded received signal at a specific sampling frequency f sample . The sampling frequency is a function of the chip duration T C :  
           f   sample =2 /T   C    
         [0028]    The sampled coded received signal is fed via a line  4  to a pulse shaper  5  for shaping the sampled received signal pulses. The pulse shaper  5  is required in order to minimize the intersymbol interference of the received signal. The pulse shaper  5  is preferably an RRC (Root Raised Cosine) filter with a specific normalized autocorrelation function ACF.  
         [0029]    The autocorrelation function is prescribed by:  
             ϕ     g                 g            (   t   )       =         cos        (     π                 r                   t     T   c         )         1   -       (     2      τ                   t     T   c         )     2            sin                   c        (       π                 t       T   c       )           ,     0   ≤   r   ≤   1     ,                         
 
         [0030]    where T C , is the chip duration.  
         [0031]    The output of the pulse shaper  5  is connected to a buffer  7  via a line  6 . The buffer  7  has a first memory area  7   a  for a first leading pulse-shaped sample, and a second memory area  7   b  for a subsequent pulse-shaped sample. The buffer  7  is controlled by a control unit  9  via a control line  8 . The control unit  9  drives the buffer  7  such that in each case the first sample is always buffered in the memory area  7   a  and the second sample is always buffered in the memory area  7   b.  The buffered previous sample s(t+Δ) is fed to a first correlation device  12  via a line  10  and a branching line  11 . The subsequent sample s(t−Δ) buffered in the memory area  7   b  of the buffer  7  passes via a line  13  and a branching line  14  to a second correlation device  15 . The correlation devices  12 ,  15  are preferably multiplier devices. A code signal generator  16  generates a code signal that is output, via a line  17 , to the second correlation device  15  and to the first correlation device  12 .  
         [0032]    The two correlation devices  12 ,  15  receive the same code signal from a single code signal generator  16 . The first correlation value, generated by the first correlation device  12 , is applied to a subtractor  20  via a line  18 . The second correlation value, generated by the second correlation device  15 , is applied to the subtractor  20  via a line  19 . The subtractor  20  subtracts the two correlation values present on the lines  18 ,  19  to form a difference signal or deviation signal ε that is output to a loop filter  22  via an output line  21  of the subtractor  20 . The loop filter  22  is a digital FIR filter. The filtered deviation signal is fed via a line  23  to an interpolation device  24 . The interpolation device  24  receives the first sample s(t+Δ) via a line  25  and the subsequent sample s(t−Δ) via a line  26 , and outputs the generated interpolation value via a line  27  for the purpose of further data processing of the received signal. The interpolation device  24  is preferably a linear or quadratic TVI (Time Variant Interpolator) interpolator.  
         [0033]    In a preferred embodiment, the interpolation device  24  includes a deviation factor calculating unit  100  for calculating a deviation factor N as a function of the filtered deviation signal ε.  
         [0034]    The interpolation device  24  generates the interpolation value ŝ from the two pulse-shaped samples s and the deviation factor N in accordance with the following equation:  
           s   ^     =       N                 x            s        (     t   +   Δ     )       -     s        (     t   -   Δ     )         4       +     s        (     t   -   Δ     )           ,                         
 
         [0035]    where Δ=T  C /4, and thus is a quarter of the chip duration.  
         [0036]    The circuitry requirement in the fine synchronization apparatus shown in FIG. 2 is relatively low, since only a code signal generator  16  is provided for generating the local code signal.  
         [0037]    [0037]FIG. 3 shows a particularly preferred embodiment of the inventive fine synchronization apparatus. Identical reference symbols in this case denote components identical to those in FIG. 2. In the case of the fine synchronization apparatus shown in FIG. 3, a plurality of different local code signals are synchronized with the coded received signal. The code signal generator  16   a  generates a scrambling code that is output via the line  17   a  to the multipliers  12   a,    15   a  in order to decrypt the samples. The decrypted samples are fed via lines  28 ,  29  to the downstream multipliers  12   b,    15   b.  The code signal generator  16   b  generates a despreading code and/or analyzing code. The despreading code is fed via the line  17   b  to the two multipliers  12   b,    15   b,  which multiply the despreading code with the decrypted samples present on the lines  28 ,  29 .  
         [0038]    The decrypted and despread samples are fed via lines  30 ,  31  to the downstream multipliers  12   c,    15   c.  The multipliers  12   c,    15   c  receive, via the line  17   c,  pilot symbols for multiplying with the decrypted and despread samples present on the lines  30 ,  31 . The output of the multipliers  12   c,    15   c  are connected via lines  32 ,  33  to integrators  34 ,  35  that carry out time integration over a specific period that is preferably half the chip duration T C . The integrated signals pass via lines  36 ,  37  to signal squaring elements  38 ,  39 . The outputs of the signal squaring elements  38 ,  39  are connected to the subtractor  20  via the lines  18 ,  19 . The sampling signals s(t−Δ) and s(t−Δ) illustrated in FIGS. 2, 3 are complex signals.  
         [0039]    [0039]FIGS. 4 a - 4   c  show various sampling situations using the autocorrelation function of the sampling pulse shaper  5 . In this case, Δ=T C /4 is therefore a quarter of the chip duration.  
         [0040]    [0040]FIG. 4 a  shows the ideal case of correctly timed sampling. In the case of accurately timed sampling, the level of the first sample of s(t+Δ) and of the second sample s(t−Δ) is of exactly the same height, and so the deviation signal ε is 0. The interpolation device  24  calculates the interpolation estimate ŝ from the two samples for the purpose of further data processing.  
         [0041]    [0041]FIG. 4 b  shows the situation in the case of sampling that is too late, where the preceding sample s(t+Δ) has a smaller value than the subsequent sample s(t−Δ). The interpolation device  24  calculates the interpolation estimate ŝ once again from the two samples and the deviation ε between the correlation values formed from samples.  
         [0042]    [0042]FIG. 4 c  shows the reverse situation, in which sampling is performed too early. Here, the first sample s(t+Δ) is greater than the subsequent sample s(t−Δ). The interpolation device  24  once again calculates the interpolation estimate ŝ as a function of the two samples and of the deviation between the correlation values formed from the samples.  
         [0043]    By contrast with the prior art, in the case of the inventive apparatus, it is not phase-shifted codes that are used for correlation, but time-shifted sampled data that lead or lag by T C /4 by comparison with an estimated optimum sampling instant. It is possible in this way to provide a single clock source as the time reference signal inside the fine synchronization apparatus, and thus a single chip clock signal for the entire receiver. The optimum sampling time is therefore determined with a very low outlay on circuitry. Since the sampling rate is half of the chip duration, a resolution of T c /8 is required in order to determine and compensate time deviations of this order of magnitude.