Abstract:
A communication system including an automatic control (AGC) circuit, a receiver, an analog to digital converter (ADC) and an insertion phase variation compensation module. The AGC circuit receives and amplifies communication signals. The gain of the AGC circuit is continuously adjusted. The AGC circuit outputs an amplified signal to the receiver which, in turn, outputs an analog complex signal to the ADC. The ADC outputs a digital complex signal to an insertion phase variation compensation module which counteracts the effects of phase offsets introduced into the communication signal due to the continuous gain adjustments associated with the AGC circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application claims priority from U.S. provisional application No. 60/476,471, filed Jun. 6, 2003, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention generally relates to wireless communication systems. More particularly, the present invention relates to digital signal processing (DSP) techniques used to compensate for phase variations associated with automatic gain control (AGC) adjustments.  
       BACKGROUND  
       [0003]     In a conventional phase-sensitive communication system, a receiver uses automatic gain control (AGC) to automatically adjust gain as a function of the amplitude of a radio frequency (RF) and/or intermediate frequency (IF) communication signal. A real valued gain factor generated by the AGC is applied to the communication signal. In the analog domain, the amplitude of the communication signal is maintained within a predefined signal amplitude range and is then converted to a digital signal by an analog to digital converter (ADC), which also limits the signal amplitude range. The objective of the AGC is to maintain a constant power level at the input to the ADC.  
         [0004]     When the AGC is adjusted, a phase offset is introduced into the communication signal which degrades the performance of the phase-sensitive communication system. A method and system is desired for canceling the phase offset of the communication signal caused by adjusting the AGC.  
       SUMMARY  
       [0005]     The present invention is incorporated into a communication system which includes an AGC circuit, a receiver, an analog to digital converter (ADC) and an insertion phase variation compensation module. The AGC circuit receives and amplifies communication signals. The gain of the AGC circuit is continuously adjusted. The AGC circuit outputs an amplified communication signal to the receiver which, in turn, outputs an analog complex signal to the ADC. The ADC outputs a digital complex signal to the insertion phase variation compensation module which counteracts the effects of phase offsets introduced into the communication signal due to the continuous gain adjustments associated with the AGC circuit. The analog and digital complex signals include in-phase (I) and quadrature (Q) signal components.  
         [0006]     The gain of the AGC circuit is continuously adjusted in response to a gain control signal. Estimates of the phase offsets are provided to the insertion phase variation compensation module as a function of the gain control signal.  
         [0007]     The insertion phase variation compensation module may receive the digital I and Q signal components from the ADC and output altered I and Q signal components having different phase characteristics than the digital I and Q signal components. The communication system may further include a modem which receives the altered I and Q signal components. The modem may include a processor which generates the gain control signal. The processor may calculate how much power is input to the ADC.  
         [0008]     The communication system may further include a look up table (LUT) in communication with the processor and the insertion phase variation compensation module. The LUT may receive the gain control signal from the processor and provide estimates of the phase offsets to the insertion phase variation compensation module as a function of the gain control signal. The provided estimates may include a Sin function and a Cos function of a phase offset, x. The insertion phase variation compensation module may have a real, Re, input associated with a digital I signal component and an imaginary, Im, input associated with a Q signal component and, based on the estimates provided by the LUT, the insertion phase variation compensation module may output an I signal component having a phase that is adjusted in accordance with the function (Cos(x)×Re)−(Sin(x)×Im) and a Q signal component having a phase that is adjusted in accordance with the function (Sin(x)×Re)+(Cos(x)×Im). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     A more detailed understanding of the invention may be had from the following description of a preferred example, given by way of example and to be understood in conjunction with the accompanying drawing wherein:  
         [0010]      FIG. 1  is a block diagram of a communication system including an insertion phase variation compensation module that cancels out phase offsets introduced into a communication signal by an AGC circuit in accordance with the present invention;  
         [0011]      FIG. 2  is an exemplary configuration of the insertion phase variation compensation module of  FIG. 1 ; and  
         [0012]      FIG. 3  is a flow chart of a process including steps implemented to continuously counteract the effects of phase offsets introduced into a communication signal by the AGC circuit of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The present invention provides a method and system that cancels out the phase difference introduced into an RF or IF communication signal, (i.e., data stream), by performing AGC adjustments.  
         [0014]     Preferably, the method and system disclosed herein is incorporated into a wireless transmit/receive unit (WTRU). Hereafter, a WTRU includes but is not limited to a user equipment, mobile station, fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless environment. The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
         [0015]     The present invention is applicable to communication systems using time division duplex (TDD), frequency division duplex (FDD), code division multiple access (CDMA), CDMA 2000, time division synchronous CDMA (TDSCDMA), orthogonal frequency division multiplexing (OFDM) or the like.  
         [0016]      FIG. 1  is a block diagram of a communication system  100  operating in accordance with the present invention. Communication system  100  includes an AGC circuit  105 , a receiver  110 , an analog to digital converter (ADC)  115 , an insertion phase variation compensation module  120  and a modem  125 . The AGC circuit  105  and the ADC  115  may be incorporated into receiver  110 . The AGC circuit  105  may include a single gain stage or multiple gain stages. Furthermore, the insertion phase variation compensation module  120  may be incorporated into the modem  125 .  
         [0017]     The modem  125  includes a processor  130  which calculates how much power is input to the ADC  115 . The modem  125  receives complex I and Q signal components  135 ,  140 , from the insertion phase variation compensation module  120 , and, via processor  130 , outputs a gain control signal  145  to the AGC circuit  105 . The gain control signal  145  includes a gain factor used by the AGC circuit  105  to set the amplitude of an RF and/or IF communication signal  150 . The gain control signal  145  is also output from the processor  130  to a look up table (LUT)  155  which uses the gain control signal  145  to provide the insertion phase variation compensation module  120  with an estimate of the phase offset that is introduced into the communication signal  150 . Alternatively, a predefined polynomial or any other method may be used in lieu of the LUT  155  to provide the estimate of the phase offset.  
         [0018]     Each time the gain level of the gain stage(s) of the AGC circuit  105  is changed, an associated phase offset, i.e., phase rotation, may be introduced into the communication signal  150 . Thus, an estimate of the phase offset (x) as a function of the gain provided by the AGC circuit  105  may be determined on a continuous basis by accessing the LUT  155 , a predefined polynomial, or any other method that can map a full range of AGC values associated with the AGC circuit  105  to a phase offset estimate.  
         [0019]      FIG. 2  shows an exemplary configuration of the insertion phase variation compensation module  120  which rotates the phase characteristics of the I and Q signal components of a digital complex signal output from the ADC  115  based on the gain control signal  145 , so as to counteract the effects of phase offsets introduced into a communication signal  150  by the AGC circuit  105 . Thus, the modem  125  is not affected by the phase offsets and the performance of the communication system  100  is not degraded. Different gain levels will introduce different gain offsets into the communication signal  150 .  
         [0020]     As shown in  FIG. 2 , the insertion phase variation compensation module  120  includes multipliers  205 ,  210 ,  215 ,  220  and adders  225  and  230 . The insertion phase variation compensation module  120  receives a real (Re) I signal component  250  and an imaginary (Im) Q signal component  260  from the ADC  115  and rotates the phase of the signal components Re and jIm by x degrees (e jx ) as described by Equation 1 below: 
 
( Re+jIm )× e   jx =( Re+jIm )×(Cos( x )+ j Sin( x ))   Equation 1 
 
         [0021]     The outcome of the real output, Re, is described by Equation 2 below: 
 
 {circumflex over (R)}e =(Cos( x )× Re )+( j   2 ×Sin( x )× Im )=(Cos( x )× Re )−(Sin( x )× Im )   Equation 2 
 
 Note that if x is close to zero, then Cos(x)=1.0 and Sin(x)=x, as described by Equation 3 below: 
 
 {circumflex over (R)}e=Re−Im×x    Equation 3 
 
         [0022]     The output of the imaginary output, Î m, is described by Equation 4 below: 
 
 Î m =(Sin( x )× Re )+(Cos( x )× Im )   Equation 4 
 
 Note that if x is close to zero, then Cos(x)=1.0 and Sin(x)=x, as described by Equation 5 below: 
 
 Î m=Im+Re×x    Equation 5 
 
         [0023]     Thus, as depicted by Equation 2, the real signal component  250  is multiplied by a Cos(x) function  280  specified by the LUT  155  via the multiplier  215  and the imaginary signal component  260  is multiplied by a Sin (x) function  270  also specified by the LUT  155  via the multiplier  210 , whereby the output of the multiplier  210  is subtracted from the output of the multiplier  215  by the adder  225 . Furthermore, as depicted by Equation 4, the real signal component  250  is multiplied by a Sin(x) function  270  specified by the LUT  155  via the multiplier  205  and the imaginary signal component  260  is multiplied by a Cos(x) function  280  also specified by the LUT  155  via the multiplier  220 , whereby the output of the multiplier  220  is added to the output of the multiplier  205  by the adder  230 .  
         [0024]      FIG. 3  is a flow chart of a process  300  including steps implemented to continuously counteract the effects of phase offsets introduced into a communication signal  150  received by the AGC circuit  105 . In step  305 , the gain control signal  145  is provided to the AGC circuit  105 . In step  310 , the AGC circuit  105  adjusts the gain of a communication signal  150  in response to the gain control signal  145 , the adjustment causing a phase offset to be introduced into the communication signal  150 . In step  315 , an estimate of the phase offset is provided to the insertion phase variation compensation module  120  as a function of the gain control signal  145 . In step  320 , the insertion phase variation compensation module  120  adjusts the phase of the communication signal  150  based on the provided estimate. The process  300  repeats on a continuous basis.  
         [0025]     While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention described hereinabove.