Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. Pat. No. 7,133,644, which issued on Nov. 7, 2006, which claims the benefit of U.S. Provisional Application No. 60/476,314, filed Jun. 6, 2003, which is incorporated by reference as if fully set forth. 

   FIELD OF THE INVENTION 
   The present invention generally relates to transmitter design in wireless communication systems. More particularly, the present invention relates to digital signal processing (DSP) techniques used to compensate for the impairments introduced in an analog radio transmitter, such as passband distortion, carrier leakage, amplitude imbalance, phase imbalance or the like. 
   BACKGROUND 
   Existing wireless system architectural configurations impose stringent constraints on the system designer with regards to transmitting communication signals. Moreover, such configurations often provide low reliability communication links, high operating costs, and an undesirably low level of integration with other system components. 
   In the radio frequency (RF) section of a conventional low-cost wireless transmitter configured with analog components, a considerable level of distortion occurs when RF signals are processed. Such distortions include carrier leakage, phase imbalance, amplitude imbalance, or the like. Higher cost components with better distortion characteristics that enhance signal quality may be overlooked during the design phase in order to reduce the cost of the end-product. 
   Because the costs of components that process RF analog signals are higher than the components that use DSP, it is desired to provide a digital baseband (DBB) system, including a low cost transmitter with low noise and minimal power requirements, that utilizes DSP techniques as much as is practicable. 
   SUMMARY 
   In order to compensate for performance degradation caused by inferior low-cost analog radio component tolerances of an analog radio, a wireless communication transmitter employs a control process to implement numerous DSP techniques to compensate for deficiencies of such analog components so that modern specifications may be relaxed. By monitoring temperature, bias current or the like, enhanced phase and amplitude compensation, as well as many other RF parameters may be implemented. 
   In a preferred embodiment, the present invention is a digital baseband (DBB) transmitter or a wireless transmit/receive unit (WTRU) which includes an analog radio transmitter, a digital pre-distortion compensation module, a digital direct current (DC) offset compensation module, a digital amplitude imbalance compensation module, a digital phase imbalance compensation module, at least one digital to analog converter (DAC) for interfacing the digital compensation modules with the analog radio transmitter, and at least one controller in communication with the analog radio transmitter and each of the digital compensation modules, wherein the digital compensation modules correct RF parameter deficiencies that occur in the analog radio transmitter. 
   The DBB transmitter may further include a modem for generating in-phase (I) and quadrature (Q) signal components which are input to each of the digital compensation modules, the DAC and the analog radio transmitter. 
   The DBB transmitter may further include a low pass filter (LPF) coupled to each of the I and Q inputs of the digital pre-distortion compensation module. Each LPF may be a root-raised cosine (RRC) filter. 
   The analog radio transmitter may include a power amplifier, a modulator, a power detector, a temperature sensor for monitoring a temperature reading associated with the analog radio transmitter, and a bias current sensor for monitoring a bias current reading associated with the analog radio transmitter. At least one of the digital compensation modules may be activated in response to the bias current sensor or the temperature sensor. 
   The DBB transmitter may further include a memory for storing a plurality of look up tables (LUTs). One of the LUTs may be selected for use by the digital pre-distortion compensation module in response to the temperature reading monitored by the temperature sensor. 
   The power amplifier may be prone to a linearity deficiency. The digital pre-distortion compensation module may be configured to distort the phase and amplitude of the I and Q signal components based on the input power level of the power amplifier as measured by the power detector, and gain and phase characteristics of the power amplifier stored in the selected LUT, such that the power amplifier generates a linear response rather than a distorted response. 
   The modulator may be prone to a carrier leakage deficiency. A minimum detected reading associated with each of the signal inputs may be determined. First and second DC offset compensation values are determined based on the minimum detected readings. The digital DC offset compensation module may be configured to eliminate carrier leakage associated with the modulator by adjusting the respective DC levels of the two signal inputs based on the first and second DC offset compensation values. The modulator may have a local oscillator (LO) frequency at which the minimum detected readings are determined. 
   The modulator may be prone to an amplitude balance deficiency. The digital amplitude imbalance compensation module may be configured to adjust the power level of one of the I and Q signal components, such that the power level of each of the I and Q signal components is the same. 
   The modulator may be prone to a phase balance deficiency. The digital phase imbalance compensation module may be configured to adjust the phase of the I and Q signal components, such that the phase of each of the I and Q signal components are orthogonal to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a block diagram of a transmitter with a DBB compensation processor operating in accordance with the present invention; 
       FIG. 2  shows the individual digital compensation modules that are included in the DBB compensation processor of  FIG. 1 ; 
       FIG. 3  shows an exemplary configuration of the digital compensation modules of  FIG. 2 ; 
       FIG. 4  is a flow chart of an exemplary control process used to compensate for impairments in the transmitter of  FIG. 1 ; 
       FIG. 5  shows an exemplary configuration of the digital pre-distortion compensation module of  FIG. 2 ; 
       FIG. 6  shows an exemplary configuration of the digital DC offset compensation module of  FIG. 2 ; 
       FIG. 7  shows an exemplary configuration of the digital amplitude imbalance compensation module of  FIG. 2 ; and 
       FIG. 8  shows an exemplary configuration of the digital phase imbalance compensation module of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is a DBB transmitter which enables high performance solutions to be shifted from RF to digital baseband by using low performance radio components and compensating in the DBB for the lower radio performance. Thus, the present invention promotes lower cost, lower power consumption and lower hardware complexity. By providing cross optimization between the radio and the DBB, the performance compensation in DBB is tied to the characteristics of the radio that the DBB is integrated with. 
   Preferably, the DBB transmitter 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 DBB transmitter may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components. 
   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. 
     FIG. 1  is a block diagram of a DBB transmitter  100 . The DBB transmitter  100  includes a modem  105  which outputs digital signals including in-phase (I) and quadrature (Q) signal components  110 ,  115 , respectively and passes the digital signals through low pass filters (LPFs)  120 ,  125 , DBB compensation processor  130  and digital to analog conversion (DAC) circuit  135 , which outputs analog signals to analog radio transmitter  150 . LPFs  120 ,  125  may be root-raised cosine (RRC) filters or other suitable filters. The DAC circuit  135  includes DACs  140  and  145 . 
   The DBB transmitter  100  further includes a controller  155  which maintains control over the compensation processor  130  and all of the active components of the analog radio transmitter  150 . Furthermore, controller  155  has access to transmit power control (TPC) signals received by the modem  105  from a base station or other entity, whereby calculations or other functions performed by the controller  155  may depend. 
   The analog radio transmitter  150  includes an antenna  160 , a power amplifier  165 , a modulator  170 , a power detector  175 , a temperature sensor  180  for monitoring the temperature of the analog radio transmitter  150  and a bias current sensor  185  for measuring the bias current of the analog radio transmitter  150 . The components in the analog radio transmitter  150  consist of low cost (i.e., “low-end” quality) components having “relaxed” specifications. For example, the specifications for the power amplifier  165  need not be stringent because of the availability of a pre-distortion compensation module in the DBB compensation processor  130 . 
   Referring to  FIG. 2 , THE DBB compensation processor  130  includes one or more of the following modules used to enhance the performance of the analog radio transmitter  150 :
         1) a digital pre-distortion compensation module  205 ;   2) a digital DC offset compensation module  210 ;   3) a digital amplitude imbalance compensation module  215 ; and   4) a digital phase imbalance compensation module  220 .       

   The digital pre-distortion compensation module  205  is used to correct transmission amplitude characteristics, such as amplitude modulation (AM) to AM and AM to phase modulation (PM) signal characteristics. The amplitude and phase characteristics of the power amplifier  165  in the analog radio transmitter  150  may be determined using a statistical sampling method or it may be based on specifications provided by the manufacturer of the power detector  165 . The digital pre-distortion compensation module  205  estimates the power at the antenna  160  of the analog radio transmitter  150  based on a received signal input and a TPC command received from the modem  105 . Based on known gain and phase characteristics of the power amplifier  165 , the digital pre-distortion compensation module  205  purposely distorts the phase and amplitude of the I and Q signal components, such that the power amplifier  165  generates a linear response, rather than a distorted response. The digital pre-distortion compensation module  205  may refer to a look up table (LUT) or the like to obtain an inverse of such amplifier characteristics. This embodiment of the present invention is advantageous because RF parameter standards such as intermodulation distortion may be met, even though cheap and low quality components (e.g., an amplifier having a low output power rating) are used in the analog radio transmitter  150 . 
   The digital DC offset compensation module  210  is used to correct, (i.e., suppress), carrier leakage associated with the modulator  170  in the analog radio transmitter  150  by adjusting the DC levels of the I and Q signal components based on previously determined (i.e., stored) first and second DC offset compensation values. To determine the DC offset compensation values, the I and Q signal component inputs  110 ,  115  of the DBB transmitter  100  are switched (switch not shown) from the modem  105  to the controller  155 . The controller  155  individually sweeps the DC level of each of the I and Q signal component inputs  110 ,  115  (e.g., from minus to plus sequentially or vice versa) while the power detector  175  is used to determine respective first and second minimum detected readings at the local oscillator (LO) frequency of the modulator  170 . The signal component input that is not currently being swept is temporarily disabled (e.g., the controller  155  turns the unswept signal component input off by setting it to zero). 
   The first and second DC offset compensation values (i.e., compensation factors K 1  and K 2 ) are derived by interpolating the first and second minimum detected readings. The first and second DC offset compensation values are then stored for future reference, whereby the DC levels of the I and Q signal components are adjusted based on the first and second DC offset compensation values, respectively. 
   In an alternate embodiment, the controller  155  may be used in conjunction with a detection algorithm and the power detector  175 . The controller  155  simultaneously sweeps the DC level of each of the I and Q signal component inputs  110 ,  115 . The algorithm determines at least one minimum detected reading by using a coordinate system application, whereby the DC levels of each of the I and Q signal components are applied to an x-axis and y-axis, respectively, while detected readings sensed by the power detector  175  are applied to a z-axis. 
   The digital amplitude imbalance compensation module  215  is used to balance the I and Q signal components, such that the modulator  170  in the analog radio transmitter  150  modulates the I and Q signal components with equal power levels. Assuming that the modulator  170  is cheap and of low quality, the modulator  170  is prone to an amplitude balance deficiency. For example, if the I signal component is 1.0 dB below the Q signal component, the digital amplitude imbalance compensation module  215  will cause the Q signal power level to be reduced by 1.0 dB. Thus, at the output of modulator  170 , the I and Q signal components will be at the same amplitude. Using controller  155 , the I and Q signal components may be turned on and off on an individual basis. For example, if controller  155  turns off the Q signal component, whereby only the I signal component is sent, the controller  155  can determine what power level the power detector  175  in analog radio transmitter  150  is reading. Assuming that the power level is a desired target level, the I signal component is then turned off and the Q signal component is turned back on. The digital amplitude imbalance compensation module  215  adjusts the power level of the Q signal component such that the power detector reads the same power level (i.e., the desired target level) for both the I and Q signal components. 
   The modulator  170  in the analog radio transmitter is also prone to a phase balance deficiency. The digital phase imbalance compensation module  220  is used to balance the phase of the I and Q signal components. The I and Q signal components are activated at the same time and then the power level of both of the I and Q signal components is reduced by 3.0 dB, (i.e., cut in half so that the power level measured by the power detector  175  is equal to the target power level when only one of the signal components is activated with orthogonal I and Q). This procedure is used to establish a reference power level, as measured by the power detector  175 . If the difference between the reference power level and the current power level measured by the power sensor  175  is equal to the desired target power level, the I and Q signal components are orthogonal, whereby the real and imaginary parts have a phase difference of 90 degrees to each other. Based on power level readings performed by the power detector  175  of analog radio transmitter  150 , a phase difference of less than 90 degrees between the I and Q signal components will cause the power detector  175  to read a power level greater than the target power level. A phase imbalance of greater than 90 degrees between the I and Q signal components will cause the power detector  175  to read a power level less than the target power level. The phase is adjusted by the digital phase imbalance compensation module  220  in response to a phase error derived from the difference between the target power level and the power level read by the power detector  175 . 
   The digital compensation modules included in the transmitter DBB compensation processor  130  may be designed according to numerous configurations. However, it is noted that the LPFs  120 ,  125 , must precede the digital pre-distortion compensation module  205 .  FIG. 3  shows a preferred exemplary configuration  300  for the modules of the DBB compensation processor  130 . 
     FIG. 4  is a flow chart depicting the method steps of an exemplary process  400  used to compensate for impairments in the DBB transmitter  100 . In step  405 , an initialization flag is set to one, indicating that the process  400  has begun. In step  410 , a desired communication mode is selected. The communication mode may be TDD, FDD or any other communication mode, such as TDSCDMA, OFDM, CDMA 2000 or the like. 
   In step  415 , if the TDD mode is selected, the TDD mode is initialized. In step  420 , if the FDD mode is selected, the FDD mode is initialized. In step  425 , if another communication mode is selected, it is initialized. In step  430 , the biasing conditions of the analog radio transmitter  150  are monitored by the bias current sensor  185 , indicating, for example, how much current the power amplifier  165  is drawing. 
   In step  435 , the temperature of the analog radio transmitter  150 , or a selected component therein, is monitored by the temperature sensor  180 . In step  440 , the initialization flag is read to determine whether the process  400  has completed at least one cycle (i.e., steps  445 ,  450  and  455  have been implemented). An initialization flag set to one, as implemented in step  405 , indicates that the process  400  has not completed at least one cycle. If the initialization flag is determined in step  440  to be one, in step  445  the pre-distortion compensation parameters are set up for the digital pre-distortion compensation module  205  by selecting one of a plurality of look up tables (LUTs) from an LUT memory  190  based on the temperature monitored by temperature sensor  180  and/or the bias current as measured by the bias current sensor  185 . 
   Amplitude or phase changes associated with the power amplifier  165 , as monitored by the power detector  175  or any other parameter that the programmer and/or designer of the DBB transmitter  100  desires to have monitored may be used to select an LUT from the LUT memory  190 . The LUT memory  190  may reside in the digital pre-distortion compensation module  205 , in the controller  155  or in any other desirable location within DBB transmitter  100 . 
   In step  450 , carrier leakage, (i.e., direct current (DC) levels), on the I and Q signal components is suppressed by the digital DC offset compensation module  210 . In step  455 , amplitude and phase imbalances are compensated by using the digital amplitude imbalance compensation module  215  and the digital phase imbalance compensation module  220 , respectively, as described above. 
   After the process  400  completes one cycle, by completing step  455 , the initialization flag is set to zero in step  460  and the process returns to step  430  whereby if it is determined in step  465  that there was a significant change in the bias current and/or temperature, the steps  445 , 450  and  455  of compensating various parameters of the analog radio transmitter  150  may be repeated. 
   Upon powering up the DBB transmitter  100 , it is envisioned that all of the digital compensation modules in DBB compensation processor  130  would be implemented to optimize the parameters of the analog radio transmitter  150  prior to commencing communications. After the commencement of communications, selective ones of the digital compensation modules  205 ,  210 ,  215 ,  220  may be configured to run on a periodic or continuous basis, or in response to a particular event or user request. For example, if the temperature sensor  180  in the analog radio transmitter  150  detects a certain rise in temperature (e.g., five degrees), the activation of one or more of the digital compensation modules  205 ,  210 ,  215 ,  220  may be desired. 
     FIG. 5  shows an exemplary configuration of the digital pre-distortion compensation module  205  including a power estimation unit  505 , multipliers  510 , 515 ,  520 ,  525 ,  530 , adders  535 ,  540 ,  545 ,  550 , LUT  555  and phase distortion compensation unit  560 . The I and Q signal components are received at power estimation unit  505  which estimates the power using I 2 +Q 2 . The output of the power estimation unit  505  (I 2 +Q 2 ) is multiplied by a transmit power control (TPC) via multiplier  510 , and the resulting product is input into LUT  555 , which provides AM to AM compensation for deficiencies in the analog radio transmitter  150 . The TPC controls the output power of the analog radio transmitter  150 , as designated by the resulting product (I 2 +Q 2 )×TPC. The LUT  555  provides the RF characteristic information associated with the power amplifier  165  and/or other components of the analog radio transmitter  150  such that deficiencies of the amplifier  165 , such as undesired gain compression and/or dynamic range characteristics which cause nonlinearity of the RF output at antenna  160 , may be eliminated. The output of the LUT  555  is multiplied by the I and Q signal components via multipliers  515  and  520 , and the resulting products are added to the I and Q signal components via adders  535  and  540 , respectively. Thus, the amplitude characteristics of the I and Q signal components are altered in accordance with the LUT  555  so as to compensate for distorted amplitude characteristics of the analog radio transmitter  150 . 
   Referring still to  FIG. 5 , the product of (I 2 +Q 2 ) and the TPC is also input into phase distortion compensation unit  560  which provides AM to PM compensation for deficiencies in the analog radio transmitter  150 . The phase distortion compensation unit  560 , operating in conjunction with multipliers  525 ,  530  and adders  545 ,  550 , adjusts the phase differentiation between the I and Q signal components such that they are orthogonal to each other, (i.e., the real and imaginary signal parts have a phase difference of 90 degrees). 
     FIG. 6  shows an exemplary configuration of the digital DC offset compensation module  210  including adders  605  and  610 . Compensation factors K 1  and K 2  are added to the I and Q signal components, respectively, such that carrier leakage is eliminated by canceling out undesired DC offsets. 
     FIG. 7  shows an exemplary configuration of the digital amplitude imbalance compensation module  215  including multiplier  705  and adder  710 . Compensation factor K 1  is multiplied with the Q signal component via multiplier  705 , and the resulting product is then added to the Q component via adder  710 , such that the power level of the Q signal component is adjusted to be the same as the I signal component. Note that the sole purpose of the multiplier  705  is to avoid the unintentional deactivation of the Q signal component should the value of K 1 =0. Alternatively, the configuration of multiplier  705  and adder  710  may be incorporated into the I signal component, or in both of the I and Q signal components. 
     FIG. 8  shows an exemplary configuration of the digital phase imbalance compensation module  220  including adders  805 ,  810  and multipliers  815 ,  820 . In response to a phase error  825  which indicates that the I and Q components are not orthogonal to each other, the phase difference between the I and Q components are adjusted accordingly. 
   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.

Technology Category: h