Abstract:
A method and apparatus that reduces the amount of constellation rotation due to power amplifier (PA) insertion phase variation during activation (i.e., turn on) of a transmitter. This is accomplished by applying an instantaneous phase rotation during the transmitter turn on at digital baseband to counteract and minimize unwanted phase variations.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. provisional application No. 60/562,664, filed Apr. 15, 2004, 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 due to activating a transmitter.  
       BACKGROUND  
       [0003]     In a conventional time division duplex (TDD) system, a transmitter within the system is temporarily deactivated during receive time slots in order to reduce power consumption. However, once a turn on command is initiated, the bias current of a power amplifier (PA) in the transmitter requires a significant amount of time to move from its shut down value to its desired quiescent value. This type of time varying bias results in a proportional change in the PA insertion phase, which in turn will result in a rotation of the signal constellation.  
         [0004]     An insertion phase variation will occur even if the input power of the transmitter is held constant. As a result, performance degradation of phase sensitive receiver algorithms at the beginning of time slots will occur unless the insertion phase variation is reduced and kept within some acceptable level when the transmitter is activated (i.e., turned on). A method and system for compensating for phase variations caused by activating the transmitter is desired.  
       SUMMARY  
       [0005]     The present invention provides a method and a system that reduces the amount of constellation rotation due to insertion phase variation.  
         [0006]     The insertion phase variation is adjusted when a transmitter or a power amplifier therein is turned on (i.e., activated). This is accomplished by incrementally applying an instantaneous phase rotation during transmitter turn on at digital baseband to counteract and minimize unwanted phase variations. The present invention is applicable to TDD, frequency division duplex (FDD), orthogonal frequency division multiplexing (OFDM), code division multiple access (CDMA) and time division synchronous CDMA (TDSCDMA) systems.  
         [0007]     The transmitter includes at least one amplifier which causes the insertion phase deviation when transitioning from a deactivated state to an activated state, and means for establishing a period for the transmitter to transition to a quiescent state value associated with the activated state. The transmitter further includes a means for setting a target insertion phase that corresponds to the quiescent state value, and means for incrementally adjusting the insertion phase of the transmitter until the quiescent state value is equal to the target insertion phase when the amplifier is activated.  
         [0008]     The means for incrementally adjusting the insertion phase of the transmitter includes an accumulator and a function unit (e.g., a look up table (LUT)) in communication with the accumulator. The function unit outputs at least one insertion phase rotation control function in response to power gain command values accumulated by the accumulator. 
     
    
     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 radio transmitter operating in accordance with the present invention;  
         [0011]      FIG. 2  is a timing diagram showing time slots separated by a guard period during which the transmitter of  FIG. 4  may be activated; and  
         [0012]      FIG. 3  is a flow chart of a process including steps implemented to continuously counteract the effects of phase offsets introduced into the transmitter of  FIG. 1  due to the powering up of an amplifier therein. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The present invention provides a method and system that performs phase insertion adjustments in a transmitter when it transitions from a deactivated state to an activated state.  
         [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 transmitter  100  operating in accordance with the present invention. The transmitter includes a pair of digital to analog converters (DACs)  105 ,  110 , a modulator  115 , a radio frequency (RF) variable gain amplifier (VGA)  120  and a PA (power amplifier)  125 . The RF VGA  120  and PA  125  are selectively enabled and disabled by control line  130 . Furthermore, the transmitter  100  includes multipliers  135 ,  140 ,  145 ,  150  and adders  155  and  160 . The transmitter  100  generates signals based on a real (Re) I signal component  165  and an imaginary (jIm) Q signal component  170 . The phase of the signal components Re and jIm by x degrees (e jx ) are rotated as described by Equation 1 below:
 ( Re+jIm )× e   jx =( Re+jIm )×(Cos ( x )+ j  Sin ( x ))  Equation 1 
         [0017]     The transmitter  100  deactivates the RF VGA  120 , the PA  125  and other power consuming components of the transmitter  100  during the occurrence of receive time slots in a TDD type system, thus essentially deactivating the transmitter  100 . Based on the condition of control line  130 , and the output  174  of an accumulator  173 , a function unit (e.g., an LUT)  175  is used to provide a phase offset (x) to compensate for phase variations caused by deactivating or activating the transmitter  100 . Alternatively, other devices and/or techniques may be used in lieu of the accumulator  173 .  
         [0018]      FIG. 2  is a timing diagram  200  showing a receive time slot  205  and a transmit time slot  210  separated by a guard period  215  during which transmitter (i.e., amplifier) switching  220  occurs.  
         [0019]      FIG. 3  is a flow chart of a process  300  including steps implemented to continuously counteract the effects of phase offsets introduced into the transmitter  100  due to the powering up of at least one amplifier therein. In step  305 , a period is established for adjusting the output power level of the transmitter  100 , and at least one power amplifier therein, for the transmitter  100  to transition from a deactivated state (“OFF”) value to a desired quiescent (“ON”) state value when the transmitter  100  is activated. The insertion phase of the transmitter  100  will overshoot the “ON” value and eventually settle. In step  310 , a reference target insertion phase is set that corresponds to the quiescent state value of the transmitter  100 . The function unit  175  detects a change on the control line  130  and instantaneously adjusts the insertion phase of the transmitter  100  by inputting incremented and/or decremented power gain control values into the accumulator  173  via an accumulator input  176 . The same power gain control values are provided to the RF VGA  120 . The transmitter  100 , and thus the amplifiers  120 ,  125 , therein, are activated (i.e., turned on) during the guard period  215 . Because it takes a substantial amount of time for an applied AGC incremented value to settle in the transmit chain, the transmitter  100  is required to provide sufficient time periods for implementing ramp up and ramp down of the transmitter  100 .  
         [0020]     In step  315 , an incremented power gain command value provided on input  176  is input into accumulator  173 . In step  320 , the accumulator  173  outputs an accumulated value  174  to the function unit  175 . In step  325 , the function unit  175  outputs one or more rotation functions of “x”, (e.g., sin (x), cos (x)), to instantaneously adjust the phase of the transmitter  100 . In step  330 , a determination is made as to whether the accumulated value output  174  is equal to the target insertion phase. If, in step  330 , the accumulated value output  174  is determined not to be equal to the target insertion phase, the process  300  repeats steps  315 ,  320 ,  325  and  330  until the accumulated value  174  is determined to be equal to the target insertion phase.  
         [0021]     In a preferred embodiment of the present invention, the function unit  175  at digital baseband instantaneously adjusts the insertion phase of the transmitter  100  by providing a value of “x” such that it is sufficiently close to the desired quiescent value at the time of transmitter switching. The value of “x” is then gradually reduced to zero such that the overall insertion phase deviation is maintained from the quiescent value to within some acceptable amount. In order to provide both continuous and discrete phase adjustments, the accumulator  173  may be used to indicate to the function unit  175  what the current power is by summing up several incremented and/or decremented power gain control values  176 .  
         [0022]     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.