Abstract:
A mobile user receiver having a cancellation system for removing selected signals from a traffic signal prior to decoding includes a receiver having a system input for receiving communication signals from a transmitter over an air interface. The system input is supplied to a traffic signal cancellation system for canceling unwanted traffic signals. The system input is also supplied to a pilot signal cancellation system for removing a global pilot signal. The output of the pilot signal cancellation system is subtracted from the output of the traffic signal cancellation system to provide a cancellation system output free from unwanted traffic signals and the global pilot signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/266,408, filed Oct. 8, 2002, which is a continuation of U.S. patent application Ser. No. 09/175,174, filed Oct. 20, 1998, issued as U.S. Pat. No. 6,498,784 on Dec. 24, 2002, both of which are incorporated by reference as if fully set forth herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates generally to digital communications. More specifically, the invention relates to a system and method which cancels the global pilot signal and unwanted traffic signals from a received code division multiple access signal thereby removing them as interferers prior to decoding.  
         DESCRIPTION OF THE PRIOR ART  
         [0003]    Advanced communication technology today makes use of a communication technique in which data is transmitted with a broadened band by modulating the data to be transmitted with a pseudo-noise (pn) signal. The technology is known as digital spread spectrum or code division multiple access (CDMA). By transmitting a signal with a bandwidth much greater than the signal bandwidth, CDMA can transmit data without being affected by signal distortion or an interfering frequency in the transmission path.  
           [0004]    Shown in FIG. 1 is a simplified, single channel CDMA communication system. A data signal with a given bandwidth is mixed with a spreading code generated by a pn sequence generator producing a digital spread spectrum signal. The signal which carries data for a specific channel is known as a traffic signal. Upon reception, the data is reproduced after correlation with the same pn sequence used to transmit the data. Every other signal within the transmission bandwidth appears as noise to the signal being despread.  
           [0005]    For timing synchronization with a receiver, an unmodulated traffic signal known as a pilot signal is required for every transmitter. The pilot signal allows respective receivers to synchronize with a given transmitter, allowing despreading of a traffic signal at the receiver.  
           [0006]    In a typical communication system, a base station communicates with a plurality of individual subscribers fixed or mobile. The base station which transmits many signals, transmits a global pilot signal common to the plurality of users serviced by that particular base station at a higher power level. The global pilot is used for the initial acquisition of an individual user and for the user to obtain signal-estimates for coherent reception and for the combining of multipath components during reception. Similarly, in a reverse direction, each subscriber transmits a unique assigned pilot for communicating with the base station.  
           [0007]    Only by having a matching pn sequence can a signal be decoded, however, all signals act as noise and interference. The global pilot and traffic signals are noise to a traffic signal being despread. If the global pilot and all unwanted traffic signals could be removed prior to despreading a desired signal, much of the overall noise would be reduced, decreasing the bit error rate and in turn, improving the signal-to-noise ratio (SNR) of the despread signal.  
           [0008]    Some attempts have been made to subtract the pilot signal from the received signal based on the relative strength of the pilot signal at the receiver. However, the strength value is not an accurate characteristic for calculating interference due to the plurality of received signals with different time delays caused by reflections due to terrain. Multipath propagation makes power level estimates unreliable.  
           [0009]    There is a need to improve overall system performance by removing multiple noise contributors from a signal prior to decoding.  
         SUMMARY OF THE INVENTION  
         [0010]    A mobile user receiver having a cancellation system for removing selected signals from a traffic signal prior to decoding includes a receiver having a system input for receiving communication signals from a transmitter over an air interface. The system input is supplied to a traffic signal cancellation system for canceling unwanted traffic signals. The system input is also supplied to a pilot signal cancellation system for removing a global pilot signal. The output of the pilot signal cancellation system is subtracted from the output of the traffic signal cancellation system to provide a cancellation system output free from unwanted traffic signals and the global pilot signal.  
           [0011]    The present invention reduces the contributive noise effects of the global pilot signal and unwanted traffic signals transmitted in a spread spectrum communication system. The present invention effectively cancels the global pilot and unwanted traffic signal(s) from a desired traffic signal at a receiver prior to decoding. The resulting signal has an increased signal-to-noise ratio.  
           [0012]    Accordingly, it is an object of the present invention to provide a code division multiple access communication system receiver which reduces the contributive noise effects from the pilot and active, unwanted traffic signals.  
           [0013]    It is another object of the present invention to improve the desired traffic signal SNR by eliminating the noise effects of the global pilot and active traffic signals.  
           [0014]    Other objects and advantages of the system and method will become apparent to those skilled in the art of advanced telecommunications after reading the detailed description of the preferred embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a simplified block diagram of a prior art, CDMA communication system.  
         [0016]    [0016]FIG. 2A is a detailed block diagram of a B-CDMA™ communication system.  
         [0017]    [0017]FIG. 2B is a detailed system diagram of a complex number multiplier.  
         [0018]    [0018]FIG. 3A is a plot of an in-phase bit stream.  
         [0019]    [0019]FIG. 3B is a plot of a quadrature bit stream.  
         [0020]    [0020]FIG. 3C is a plot of a pseudo-noise (pn) bit sequence.  
         [0021]    [0021]FIG. 4 is a block diagram of a global pilot signal cancellation system according to the present invention.  
         [0022]    [0022]FIG. 5 is a block diagram of an unwanted traffic signal(s) cancellation system according to the present invention.  
         [0023]    [0023]FIG. 6 is a diagram of a received symbol p o  on the QPSK constellation showing a hard decision.  
         [0024]    [0024]FIG. 7 is a block diagram of a combined pilot and unwanted traffic signal cancellation system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.  
         [0026]    A B-CDMA™ communication system  17  as shown in FIG. 2 includes a transmitter  19  and a receiver  21 , which may reside in either a base station or a mobile user receiver. The transmitter  19  includes a signal processor  23  which encodes voice and nonvoice signals  25  into data at various bit rates.  
         [0027]    By way of background, two steps are involved in the generation of a transmitted signal in a multiple access environment. First, the input data which can be considered a bi-phase modulated signal is encoded using forward error-correcting coding (FEC)  27 . One signal is designated the in-phase channel I  33   x . The other signal is designated the quadrature channel Q  33   y . Bi-phase modulated I and Q signals are usually referred to as quadrature phase shift keying (QPSK).  
         [0028]    In the second step, the two bi-phase modulated data or symbols  33   x ,  33   y  are spread with a complex, pseudo-noise (pn) sequence  35 I,  35 Q using a complex number multiplier  39 . The operation of a complex number multiplier  39  is shown in FIG. 2B and is well understood in the art. The spreading operation can be represented as:  
         ( x+jy )×( I+jQ )=( xI−yQ )+ j ( xQ+yI )= a+jb.   Equation (1) 
         [0029]    A complex number is in the form a+jb, where a and b are real numbers and j 2 =−1. Referring back to FIG. 2 a , the resulting I  37   a  and Q  37   b  spread signals are combined  45   a ,  45   b  with other spread signals (channels) having different spreading codes, multiplied (mixed) with a carrier signal  43 , and transmitted  47 . The transmission  47  may contain a plurality of individual signals.  
         [0030]    The receiver  21  includes a demodulator  49   a ,  49   b  which mixes down the transmitted broadband signal  47  with the transmitting carrier  43  into an intermediate carrier frequency  51   a ,  51   b . A second down conversion reduces the signal to baseband. The QPSK signal  55   a ,  55   b  is then filtered  53  and mixed  56  with the locally generated complex pn sequence  35 I,  35 Q which matches the conjugate of the transmitted complex code. Only the original signals which were spread by the same code will be despread. All other signals will appear as noise to the receiver  21 . The data  57   x ,  57   y  is coupled to a signal processor  59  where FEC decoding is performed on the convolutionally encoded data.  
         [0031]    As shown in FIGS. 3A and 3B, a QPSK symbol consists of one bit each from both the in-phase (I) and quadrature (Q) signals. The bits may represent a quantized version of an analog sample or digital data. It can be seen that symbol duration t s  is equal to bit duration.  
         [0032]    The transmitted symbols are spread by multiplying the QPSK symbol stream by the complex pn sequence. Both the I and Q pn sequences are comprised of a bit stream generated at a much higher frequency, typically 100 to 200 times the symbol rate. One such pn sequence is shown in FIG. 3C. The complex pn sequence is mixed with the symbol bit stream producing the digital spread signal (as previously discussed). The components of the spread signal are known as chips having a much smaller duration t c .  
         [0033]    When the signal is received and demodulated, the baseband signal is at the chip level. When the I and Q components of the signal are despread using the conjugate of the pn sequence used during spreading, the signal returns to the symbol level.  
         [0034]    The embodiments of the present invention are shown in FIGS. 4, 5 and  7 . The global pilot signal cancellation system  61  embodiment is shown in FIG. 4. A received signal r is expressed as:  
           r=∝c   p   +βc   t   +n   Equation (2) 
         [0035]    where the received signal r is a complex number and is comprised of the pilot strength ∝ multiplied with the pilot code c p , summed with the traffic strength β multiplied with the traffic code c t , summed with random noise n. The noise n includes all received noise and interference including all other traffic signals. To cancel the global pilot signal from the received signal r, the system  61  must derive the signal strength of the pilot code ∝ where:  
         ∝≠β  Equation (3) 
         [0036]    since the global pilot is transmitted at a higher power level than a traffic signal.  
         [0037]    When the received signal r is summed over time, Equation (2) becomes:  
           Σr=∝Σc   p   +βΣc   t   +Σn.   Equation (4) 
         [0038]    Referring to FIG. 4, the received baseband signal r is input  63  into the pilot signal cancellation system  61  and into a pilot despreader  65  which despreads the pilot signal from the received signal r. First mixer  67  despreads the received signal r by multiplying with the complex conjugate c p * 69 of the pilot pn code used during spreading yielding:  
           Σrc   p   *   =∝Σc   p   c   p   *   +βΣc   t   c   p   *   +Σnc   p   * .  Equation (5) 
         [0039]    A complex conjugate is one of a pair of complex numbers with identical real parts and with imaginary parts differing only in sign.  
         [0040]    The despread pilot signal  71  is coupled to a first sum and dump processor  73  where it is summed over time. The first sum and dump  73  output O sd1  is:  
           O   sd1   =∝L+βΣc   t   c   p   *   +Σnc   p   *   Equation (6) 
         [0041]    where L is the product of the pilot spreading code c p  and the complex conjugate of the pilot spreading code c p   *  summed over L chips.  
         [0042]    The sum and dump  73  output O sd1  is coupled to a low pass filter  75 . The low pass filter  75  determines the mean value for each signal component. The mean value for pilot-traffic cross-correlation is zero and so is the mean value of the noise n. Therefore, after filtering  75 , the second and third terms in Equation (6) become zero. The low pass filter  75  output O lpf  over time is:  
         O lbf =∝L.  Equation (7) 
         [0043]    The low pass filter  75  output O lpf  is coupled to a processing means  77  to derive the pilot code strength ∝. The processing means  77  calculates ∝ by dividing the low pass filter  79  output O lpf  by L. Thus, the processing means  77  output O pm  is:  
         O pm =∝.  Equation (8) 
         [0044]    The pilot spreading code c p   *  complex conjugate generator  69  is coupled to a complex conjugate processor  79  yielding the pilot spreading code c p . The pilot spreading code c p  is input to a second mixer  81  and mixed with the output of a traffic spreading code C t   *  complex conjugate generator  83 . The resulting product from the second mixer  81  output is coupled to a second sum and dump processor  85 . The output O sd2  of the second sum and dump processor  85  is Γc p c t   *  and is combined with at a third mixer  87 . The third mixer  87  output  89  is ∝Γc p c t   * .  
         [0045]    The received signal r is also despread by traffic despreader  91 . The traffic despreader  91  despreads the received signal r by mixing the received signal r with the traffic code c t   *  complex conjugate generator  83  using a fourth mixer  93  yielding:  
           Σrc   t   *   =∝Σc   p   c   t   *   +βΣc   t   c   t   *   Σnc   t   * .  Equation (9) 
         [0046]    The traffic despreader  91  output  95  is coupled to a third sum and dump  97 . The third sum and dump  97  output O sd3  over time is:  
           O   sd3   =Σrc   t   *   =βL+∝Σc   p   c   t   *   +Σnc   t   *   Equation (10) 
         [0047]    where L is the product of the traffic spreading code c t  and the complex conjugate of the traffic spreading code c t   *  summed over L chips.  
         [0048]    The third sum and dump  97  output O sd3  is coupled to an adder  99  which subtracts the third mixer  87  output  89 . The adder  99  output O add  is:  
           O   add   =βL+∝Σc   p   c   t   *   +Σnc   t   *   −∝Σc   p   c   t   * . Equation (11) 
         [0049]    Thus, the pilot canceller  61  output O add  is equal to the received signal r minus the pilot signal simplified below:  
           O   add   =βL+Σnc   t   * .  Equation (12) 
         [0050]    The invention uses a similar approach to cancel unwanted traffic signal(s) from a desired traffic signal. While traffic signals are interference to other traffic signals just as the global pilot signal is, unwanted traffic signal cancellation differs from global pilot signal cancellation since a traffic signal is modulated by the data and is therefore dynamic in nature. A global pilot signal has a constant phase, whereas a traffic signal constantly changes phase due to data modulation.  
         [0051]    The traffic signal canceller system  101  embodiment is shown in FIG. 5. As above, a received signal r is input  103  to the system:  
           r=Ψdc   d   +βc   t   +n   Equation (13) 
         [0052]    where the received signal r is a complex number and is comprised of the traffic code signal strength η multiplied with the traffic signal data d and the traffic code c d  for the unwanted traffic signal to be canceled, summed with the desired traffic code strength β multiplied with the desired traffic code c t , summed with noise n. The noise n includes all received noise and interference including all other traffic signals and the global pilot signal. To cancel the unwanted traffic signal(s) from the received signal r, the system  101  must derive the signal strength of the unwanted traffic code Θ to be subtracted and estimate the data d, where:  
         Ψ≠d≠β.  Equation (14) 
         [0053]    When the received signal r is summed over time, Equation 13 can be expressed as:  
           Σr=ΨdΣc   d   +βΣc   t   +Σn.   Equation (15) 
         [0054]    Referring to FIG. 5, the received baseband signal r is input  103  into the desired traffic signal despreader  91  which despreads the desired traffic signal from the received signal r. Desired traffic signal mixer  93  mixes the received signal r with the complex conjugate c t   *  of the desired traffic pn code used during spreading. The despread traffic signal is coupled to a sum and dump processor  97  and summed over time. The sum and dump  97  output O sd3  is:  
           O   sd3   =Σrc   t   *   =βL+ΨdΣc   d   c   t   *   +Σnc   t   * .  Equation (16) 
         [0055]    The traffic signal canceller system  101  shown in FIG. 5 includes n unwanted traffic signal cancellers  115   1 - 115   n . An exemplary embodiment includes 10 (where n=10) unwanted traffic signal cancellers  115   1 - 115   10 .  
         [0056]    Each unwanted traffic signal canceller  115   1 - 115   n  comprises: an unwanted traffic signal despreader  139   1 - 139   n  that includes a first mixer  117   1 - 117   n  and an unwanted traffic signal code generator  119   1 - 119   n ; second  133   1 - 133   n  mixer, first  121   1 - 121   n  and second  123   1 - 123   n  sum and dump processors, a hard decision processor  125   1 - 125   n , a low pass filter  127   1 - 127   n , a processing means  129   1 - 129   n , third mixer  131   1 - 131   n , a conjugate processor  135   1 - 135   n , an adjustable amplifier  137   1 - 137   n , and a desired traffic signal code generator  83 .  
         [0057]    As above, the received signal r is input  103  into each unwanted traffic canceller  115   1 - 115   n . The unwanted traffic signal despreader  139   1 - 139   n  is coupled to the input  103  where the received signal r is mixed  117   1 - 117   n  with the complex conjugate c d1   * -c dn   *  of the traffic pn sequence for each respective unwanted signal. The despread  139   1 - 139   n  traffic signal is coupled to a first sum and dump processor  121   1 - 121   n  where it is summed over time. The first sum and dump  121   1 - 121   n  output O sd1n  is:  
           O   sd1n   =Σrc   dn   *   =ΨdL+βΣc   t   c   dn   *   +Σnc   dn   * .  Equation (17) 
         [0058]    where L is the product of the unwanted traffic signal spreading code c dn  and c dn   *  is the complex conjugate of the unwanted traffic signal spreading code.  
         [0059]    The first sum and dump  121   1 - 121   n  output O sd1n  is coupled to the hard decision processor  125   1 - 125   n . The hard decision processor  125   1 - 125   n  determines the phase shift Ø in the data due to modulation. The hard decision processor  125   1 - 125   n  also determines the QPSK constellation position d that is closest to the despread symbol value.  
         [0060]    As shown in FIG. 6, the hard decision processor  125   1 - 125   n  compares a received symbol p o  of a signal to the four QPSK constellation points x 1,1 , x −1,1 , x −1,−1 , x 1,−1 . It is necessary to examine each received symbol p o  due to corruption during transmission  47  by noise and distortion, whether multipath or radio frequency. The hard decision processor computes the four distances d 1 , d 2 , d 3 , d 4  to each quadrant from the received symbol p o  and chooses the shortest distance d 2  and assigns that symbol d location x −1,1 . The hard decision processor also derotates (rotates back) the original signal coordinate p o  by a phase amount Ø that is equal to the phase corresponding to the selected symbol location x −1,1 . The original symbol coordinate p o  is discarded.  
         [0061]    The hard decision processor  125   1 - 125   n  phase output Ø is coupled to a low pass filter  127   1 - 127   n . Over time, the low pass filter  127   1 - 127   n  determines the mean value for each signal component. The mean value of the traffic-to-traffic cross-correlation and also the mean value of the noise n are zero. Therefore, the low pass filter  127   1 - 127   n  output O lpfn  over time is:  
         O lpfn =ΨL.  Equation (18) 
         [0062]    The low pass filter  127   1 - 127   n  output O lpfn  is coupled to the processing means  129   1 - 129   n  to derive the unwanted traffic signal code strength Ø. The processing means  129   1 - 129   n  estimates Ø by dividing the filter  127   1 - 127   n  output O lpfn  by L.  
         [0063]    The other hard decision processor  125   1 - 125   n  output is data d. This is the data point d corresponding to the smallest of the distances d 1 , d 2 , d 3 , or d 4  as shown in FIG. 6. Third mixer  131   1 - 131   n  mixes the unwanted traffic signal strength Θ with each data value d.  
         [0064]    The unwanted traffic signal spreading code complex conjugate generator c d1   * -c dn   *  is coupled to the complex conjugate processor  135   1 - 135   n  yielding the unwanted traffic signal spreading code c d1 -c dn  and is input to the second mixer  133   1 - 133   n  and mixed with the output of desired traffic signal spreading code complex conjugate generator c t   * . The product is coupled to the second sum and dump processor  123   1 - 123   n . The second sum and dump processor  123   1 - 123   n  output O sd2n  is Γcd n c t   *  and is coupled to variable amplifier  137   1 - 137   n . Variable amplifier  137   1 - 137   n  amplifies the second sum and dump processor  123   1 - 123   n  output O sd2n  in accordance with the third mixer  131   1 - 131   n  output which is the determined gain.  
         [0065]    The variable amplifier  137   1 - 137   n  output  141   1 - 141   n  is coupled to an adder  143  which subtracts the output from each variable amplifier  137   1 - 137   n  from the output of the desired traffic signal despreader  115 . The output O is:  
           O=βL+ΨdΣc   d   c   t   *   +Σnc   t   *   −ΨdΣc   d   c   t   * .  Equation (19) 
         [0066]    The adder  143  output O (also the unwanted traffic canceller system  101  output) is equal to the received signal r minus the unwanted traffic signals simplified below:  
           O=βL+Σnc   t   *   Equation (20) 
         [0067]    where the noise n varies depending on the amount of traffic signals subtracted from the received signal.  
         [0068]    Another embodiment  145  canceling the global pilot signal and unwanted traffic signals is shown in FIG. 7. As previously discussed, the unwanted traffic cancellation system  101  includes the desired traffic signal despreader  91  and a plurality of unwanted traffic signal cancellers  115   1 - 115   n . The traffic cancellation system is coupled in parallel with the pilot cancellation system  61  previously described, but without a desired traffic signal despreader. A common input  147  is coupled to both systems  101 ,  61  with a common adder  149  which is coupled to the outputs O, O add  from both systems  101 ,  61 . The pilot and unwanted traffic signals are subtracted from the desired traffic signal yielding an output  151  free of interference contributions by the pilot and plurality of transmitted traffic signals.  
         [0069]    While specific embodiments of the present invention have been shown and described, many modifications and variations could be made by one skilled in the art without departing from the spirit and scope of the invention. The above description serves to illustrate and not limit the particular form in any way.