Patent Publication Number: US-8982995-B1

Title: Communication device and method of multipath compensation for digital predistortion linearization

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the present disclosure relate to signal processing, and more particularly to systems, and devices and a method of multipath compensation for digital pre-distortion linearization. 
     2. Description of Related Art 
     In wireless communication, a multi carrier signal is transmitted using a power amplifier (PA). Radio transmitter amplifiers in most telecommunications systems are required to be “linear”, which means accurately reproducing the input signal. An amplifier that compresses input signals or has a non-linear input/output relationship causes the output signal to splatter onto adjacent radio frequencies. This causes interference to other radio channels. 
     Predistortion is a technique used to improve the linearity of radio transmitter amplifiers. The multi carrier PA (MCPA) transmission should transmit the signal at a very high efficiency while maintaining acceptable signal quality and a high adjacent channel power ratio (ACPR) to meet the spectral emissions mask (SEM) requirements. 
     The transmission system would employ a digital pre-distortion (DPD) linearizer to suppress the intermodulation distortion to achieve high ACPR. To achieve this goal, the DPD linearizer compares observed transmission signals with observed feedback signals and derives the optimum solution that is applied to the DPD function to reverse the effects of the PA non-linearity, and improve the ACPR. 
     In addition to PA distortion, however, multipath distortions also exist along a transmission path. These linear distortions may degrade the DPD linearization performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one embodiment of a multi carrier power amplifier (MCPA) transmission system; 
         FIG. 2  shows a block diagram of one embodiment of a multi carrier power amplifier (MCPA) transmission system according to the present disclosure; and 
         FIG. 3  shows a block diagram of one embodiment of a DPD algorithm and the Multipath compensation processing unit. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the accompanying drawings in which like references indicate similar elements. Various embodiments illustrate different features of the disclosure. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references can mean “at least one.” Description of components in the embodiments is given for the purpose of illustrating rather than limiting. 
       FIG. 1  shows a block diagram of a MCPA transmission system  10  with a digital pre-distortion (DPD) processor  13 , with feedback for closed loop adaptation. The feedback of the DPD processor  13  comprises feedback signals captured and transmitted through a feedback path from a radio frequency (RF) switch  18  which selects the captured feedback signal between the output of the PA  15  and the output of the transmitter filter  16 . The feedback path comprises the RF switch  18  and an analog feedback unit  19 . A multi-carrier combiner  11  transmits a multi-carrier signal to a crest factor reduction (CFR) processor  12 . The multi-carrier signal denoted as a function x(t) of time variable t. The multi-carrier combiner  11  is an processor that combines multiple carriers on a common spectrum, where each carrier has an gain of γ 1  at a frequency f i  to produce the composite signal y(t) expressed as follows: 
     
       
         
           
             
               
                 
                   
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     where γ i  and f i  are the carrier gain and frequency of the individual carrier, respectively with a subscript of variable i, j is a unit imaginary number, and N γ  is a total number of the multiple carriers. Signal of the i th  carrier is denoted as x i (t). 
     The multi-carrier signal is processed by the crest factor reduction processor  12  to reduce the peak to average ratio (PAR) so that the waveform can be transmitted at levels with a root mean square (rms) value closer to the saturated PA output power. The signal after the CFR processor  12  is then processed with a DPD engine  13  before passing the signal to the analog/RF transmitting path  14  followed by the power amplifier  15  and the transmission filter  16 . 
     To suppress the intermodulation distortion (IMD), the DPD engine  13  performs a DPD algorithm  1100  to process the captured transmission signals with the captured feedback signals to extract the inverse PA characteristic, such as inverse PA characteristic of PA  15 , and then applies the inverse PA characteristic as an inverse transfer function in the DPD processor to reverse the non-linearity of PA, such as non-linearity of PA  15 . 
     The DPD engine  13  applies the linearization processing to correct distortion, and can improve the ACPR. However, this is based on a premise that the observation from the feedback path, such as a path in  FIG. 1  is the same to the observation at the antenna output. In practice, the transmission signal is corrupted and captured feedback signal is not the same as the antenna output. Over transmission lines with many radio frequency/intermediate frequency (RF/IF) modules, the analog signal is corrupted by many multipaths or reflected signals. Even though the multipath or reflected signals have the same characteristics with the desired signal, they have different amplitudes, time delays and are at different phases. When the desired feedback signal is added to many multipaths or reflected signals having different delay times and phases, they can add constructively at some frequencies and destructively at other frequencies causing amplitude and delay variations. These amplitude and delay variations over the transmission and feedback bandwidth cause the distortion in the captured feedback signals for the observation feedback path and will degrade the DPD linearization performance. 
     Amplitude variations and the group delays may be caused by multipath signals. A multipath signal having different amplitude and phase is added to the desired it would cause amplitude variations and different delays over the frequency. This substantially limits the DPD linearization performance, especially when the delay between the multipath signal and the desired signal is large. 
     For the case of multiple reflections, the phase differences cause different peaks and valleys, and the amplitude variations and group delay variations are increased. Over temperature the spectral response profile can change, so it is necessary to adaptively correct this response. 
     Many other components have similar characteristics too. For example, the transmission cable to the power amplifier input, and the cable from the PA output to the feedback receiver can include a significant reflected signal with large delay. These can cause substantial distortions to the feedback path. 
     This disclosure shows a method to compensate for the effects of the multipath and reflection signals and other distortions to substantially improve the DPD linearization performance. An electronic wireless communication device in accordance with the present disclosure may comprise an MCPA transmission system. The disclosure focuses on a proposed method of multipath compensation for digital pre-distortion linearization. Embodiments of systems, devices, and a method of multipath compensation for digital pre-distortion linearization is given in the following. The disclosed systems and communication devices may be implemented as a stand-alone device, or integrated into various network gateway devices or network terminal devices. The various network gateway devices comprise base stations, bridges, routers, switches, or hot spots or access points for wireless networking. The network terminal devices comprise set-top boxes, cell phones, tablet personal computers, laptop computers, multimedia player, digital cameras, personal digital assistants (PDAs), navigation devices, or mobile internet devices (MID). 
       FIG. 2  shows a typical wireless transmission communication system. A CFR processing block  12  first receives and processes the multi-carrier signal to generate post CFR signals. The CFR processor  12  performs crest factor reduction (CFR) to reduce the peak to average ratio (PAR) or so called crest factor of the multi-carrier signal in the digital domain 
     The output of the CFR processing block  12  is sent to a transmission filter compensation  100  that is operable to correct for the distortion of the transmission filter  700 , and then up-converted to a sampling rate R TX  using a Up-Sampler  200 . The TFC  100  processes the post CFR signals to generate post TFC signals. The up-sampler up-samples the post TFC signals to a higher sampling rate that is wide enough to process an intermodulation bandwidth of the post TFC signals to generate up-sampled signals. 
     A digital pre-distortion (DPD) engine  300  generates the predistorted signal by applying digital pre-distortion to the up-sampled signals, which is then transmitted through a transmitting multipath compensation filter  350  followed with the DAC  400 , an analog transmission path  500 , and a power amplifier (PA)  600 . This DPD engine  300  applies digital pre-distortion to the up-sampled signals between the up-sampler  200  and the PA  600  to compensate for non-linearity of the PA  600  and other non-linear analog components of the communication device  10 A, and preferably facilitates the predistorted signals to have an inverse non-linearity to non-linearity of the PA  600 . Since the predistorted signal has inversed non-linearity with the PA non-linearity, the net effect of the DPD engine  300  and PA  600  produces a linear PA where the intermodulation is suppressed. The out-of-band residue intermodulation products are then removed with a transmission filter  700  that can either be a duplexer filter or a simplex filter. 
     The DAC  400  converts the predistorted signals to generate analog predistorted signals, and transmits the analog predistorted signals to the PA. 
     With reference to  FIG. 3 , a DPD linearizer algorithm unit  1100  comprises a DPD coefficients estimator  1160  and other components. To maximize the linearization performance, it is desirable to have the analog transmission path  500 , the PA  600 , and an analog feedback receiving path to have no linear distortion, leaving the DPD coefficients estimator  1160  in the DPD linearizer  1100  to evaluate the optimum filter coefficients. The DPD linearizer algorithm unit  1100  generates and outputs a DPD solution to the DPD engine  300  to enable the DPD engine  300  perform a DPD algorithm according to the DPD solution. 
     The DPD linearizer algorithm unit  1100  generates the DPD solution based on the predistorted signals sampled from the DPD engine  300  and feedback signals sampled from amplified signals output by the PA  600 . The DPD linearizer algorithm unit  1100  receives feedback signals through the analog feedback receiving path which comprises a radio frequency (RF) switch  800 , an analog feedback receiver  900 , and an analog to digital converter (ADC)  1000 . The DPD linearizer algorithm unit  1100  generates the DPD solution to suppress multi path distortion of the feedback signals sampled from amplified signals. 
     Block  100 —Transmission Filter Compensation 
     The transmission filter compensation (TFC)  100  is operable to reverse the linear distortion of a transmission filter  700 , such as a duplexer filter to assure the correct gain flatness of signals processed by the communication device  10 A and consistent group delay from the source signal to the transmission filter  700 . This linear distortion includes the amplitude ripple and group delay distortions to the transmission signal. 
     The TFC  100  is a programmable complex finite impulse response (FIR) with P taps. The output of the filter is expressed as y n , where: 
     
       
         
           
             
               
                 
                   
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     The filter coefficients v i  may be obtained from a transmission compensation algorithm unit, is the number of coefficients. The x n-i  represents an input signal of the transmission compensation filter  100 . The TFC  100  processes the post CFR signals to generate post TFC signals. The TFC  100  generates the post TFC signals to inverse and suppress distortion induced by a transmission path which comprises the up-sampler  200 , the DPD engine  300 , the DAC  400 , and the PA  600 . 
     Block  200 —Up-Sampler Filter 
     A up-sampler filter  200  increases the sampling rate of the signal output by the transmission filter compensation  100  to a rate R TX  that is large enough to cover the intermodulation bandwidth required for correction of the significant intermodulation products of the signal. For example, if the signal bandwidth of the output of the TFC  100  is 100 MHz, having a sampling rate of 125 Ms/s, and the significant intermodulation distortion (IMD) is the 5 th  order, then the 100 MHz signal may need to be resampled to support a fifth order IMD, or 500 MHz bandwidth. This requires an up-sampling order of 4. 
     Block  300 —DPD Engine 
     A DPD engine  300  can be a polynomial-based linearizer, but can also be other DPD techniques, such as look up table technique, or other methods which perform non-linear signal processing to suppress the IMD caused by the non-linearity in the PA. The disclosure discussed here does not include the details of such a non-linear DPD engine, but focuses more on the methods to suppress the linear distortion to enhance or improve the performance of the DPD engine and the amount of non-linear correction which can be achieved. 
     Theoretically, the DPD engine  300  implements a process to convert the desired incoming signal by applying the distortion that is defined from the DPD linearizer algorithm unit  1100  in order to generate the predistorted signal, which has the inverse nonlinearity to compensate nonlinearity of the PA and other analog components.
 
 z=DPD ( y )= PA   −1 ( y )  (3)
 
     where y is the DPD input of the DPD engine  300  and z is the predistorted signal output by the DPD engine  300 . If DPD is truly the inverse of the PA then the output of the PA would be the same as that of the input signal. From an illustration point of view, the PA output, p, is expressed as:
 
 p=PA ( z )= PA ( DPD ( y ))= PA ( PA   −1 ( y ))= y   (4)
 
     And since DPD input y has no out-of-band intermodulation, it is expected that the output of the PA  600  has suppressed intermodulation. 
     However, if the transmission and feedback signals are corrupted with multipaths and reflections, the DPD engine  300  is no longer the truly inverse of the PA  600 , and thus the PA  600  has some residue IMD. Block  1100  would attempt to find the necessary correction factors in time, amplitude and phase, and apply these corrections to the feedback path to improve to the best possible linearization. 
     Block  350 —Transmission Multipath (MP) Compensation Filter 
     The transmission multipath (MP) compensation filter  350  uses a N Q  taps FIR fitter to reverse the effects of the multipaths or reflections occurred in the transmitting path and the power amplifier. The filter is expressed as: 
               o   n     =       ∑     i   =   1       N   Q       ⁢           ⁢       Q   i     ⁢     j     n   -   i                 
where j and o are the input and output of the transmission multipath (MP) compensation filter  350 , Q n  is the coefficients and N Q  is the number of taps of the filter  350 . During the system calibration, or in DPD operation, the filter coefficients Q n  are adjusted to achieve the best signal to noise ratio between the source signal and the data from the feedback signals.
 
     Block  400 —Digital to Analog Converter (DAC) 
     A DAC  400  converts the digital signal from a former stage, a transmission multipath compensation filter  350 , into analog signal for RF transmission to a next stage, an analog transmission path  500 . The DAC  400  is required to have high dynamic range so that the DAC quantization noise is not a substantial contribution to the system noise in the transmitting path. 
     Block  500 —Analog Transmission 
     The analog transmitting path  500  up-converts the DAC signal into an RF signal that is sent to the PA  600  for power amplification. 
     It is preferred to maintain the analog transmitting path  500  with low distortion, such as low amplitude gain variations and also low group delay variations over the entire predistortion bandwidth. 
     Block  600 —Power Amplifier 
     The RF power amplifier  600  converts a low power RF signal into the high power transmission signal for transmission. The power amplifier  600  may be a multi stage power amplifier, and should be optimized to have high efficiency by operate at near saturation zone of the power amplifier. 
     Block  700 —Transmission Filter 
     A Transmission Filter  700  rejects the residue out-of-band noise of the high power transmission signal to prevent from interfering with an up-link receiver path, and other frequency bands as required by the Federal Communications Commission (FCC). 
     The Transmission Filter  700  can be a part of a duplexer for frequency division duplexing (FDD) systems or a simplex filter for time division duplexing (TDD) systems. 
     Since it&#39;s difficult to maintain the transmission signal with minimum distortion, the transmission compensation filter  100  can correct the signal distortion during signal transmission. Because of the sharp amplitude transition at the band edges, the transmission filter  700  can cause undesirable distortion such as sharp amplitude roll-off and rapid group delay changes that degrades the error vector magnitude (EVM). 
     Block  800 —RF Switch 
     The RF switch  800  is operable to switch feedback signals from the output of the PA  600  or the output of the transmission filter  700  to the receiver path. The RF switch operates in two modes: 
     (1) DPD Linearization Mode for linearization of the PA: In the DPD linearization mode of the RF switch  800 , the RF switch  800  is set to capture the signal from the output of the PA  600  to perform the DPD linearization processing 
     (2) Transmission Filter Compensation Mode for equalization of the transmission filter variations and group delays: In the transmission filter compensation mode of the RF switch  800 , the RF switch is set to capture the output signal from the output of the transmission filter  700  or input of antenna  17  to perform the transmission filter compensation. 
     Most of the time, the RF switch is set to the DPD linearization mode to provide the fast update of the DPD coefficient W, and occasionally, the RF switch is set to the Transmission filter compensation mode when needed. 
     Block  900 —Analog Feedback Receiver 
     An analog feedback receiver  900  down-converts the RF signal from the PA  600  into an intermediate frequency (IF) or baseband signal that can be digitized using an analog to digital converter (ADC)  1000 . It is preferred to maintain the analog feedback receiver path with low distortion, such as low amplitude gain variations and low group delay variations over the entire bandwidth of the feedback path. 
     Block  1000 —Analog to Digital Converter (ADC) 
     An ADC  1000  converts the analog signal into a digital signal for digital processing to linearize the transmitting path including the PA  600  and equalize the transmission filter  700 . The ADC is required to have sufficient dynamic range so that ADC quantization noise is not a limiting factor in the quality of the equalization processing. 
     Block  1100 —DPD Linearizer Algorithm 
       FIG. 3  shows the block diagram of the DPD linearizer algorithm unit  1100 . The signal processing is performed on the samples T i  of captured transmission signal T and feedback sample F i  of captured feedback signal F to extract the best possible solution polynomial coefficient set W for the DPD engine  300 . 
     Block  1110 —Transmission Capture random access memory (RAM) 
     The transmission output signal output from the DPD engine  300  is captured for the DPD linearization algorithm  1100  before output to the DAC  400 . This is the predistorted signal, and thus has a wide signal bandwidth. A total of N TX  baseband IQ samples, captured at sampling rate R TX  are stored in the transmission capture RAM  1110 . 
     Block  1120 —Feedback Capture RAM 
     The output of the ADC  1000  is the digital feedback signal, to be converted to baseband IQ signal if the feedback is at IF. A total of N FB  baseband IQ samples, at a sampling rate R FB , of the feedback ADC are stored in a feedback capture RAM  1120  for subsequent linearization processing or transmission compensation processing. 
     Block  1130 —Resample and Delay 
     A resampler  1130  comprises an interpolation filter and a delay pointer connected to a decimation filter which can be implemented using a polyphase structure. The interpolation filter resamples the feedback signals received by the feedback capture RAM  1120  to a higher sampling rate, K·R TX , where R TX  is the sampling rate of the digital predistortion engine  300 , K is an integer that is reasonably large, such as 4, 6, or 8. The feedback signal is then processed by the delay pointer to adjust the correlation position. 
     The decimation filter receives and decimates the feedback signals from the delay pointer to the rate of R TX . The interpolation filter receives correlation position n and set the pointer in the delay pointer to adjust the signal with a resolution of 
     
       
         
           
             
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     The interpolated samples enable the resampler to adjust the pointer for the best time delay that has highest SNR or ACPR in order to achieve the best DPD performance. 
     Block  1140 —Correlation 
     The correlation unit  1140  performs a correlation process to search for the best timing offset t 1  between the interpolated feedback signal and the captured transmission signals from a transmission capture RAM  1110 . The correlation process can be accomplished by searching for the location of peak signal value SNR n . 
                     SNR   ⁡     (   n   )       =                ∑     i   =   1     L     ⁢           ⁢       U   i     ⁢     T     i   -   n     ′              2                  ∑     i   =   1     L     ⁢           ⁢       U   i     ⁢     U     i   -   n     ′              ·            ∑     i   =   1     L     ⁢       T   i     ⁢     T     i   -   n                  -              ∑     i   =   1     L     ⁢           ⁢       U   i     ⁢     T     i   -   n     ′              2                 (   5   )               
where
         L is the integration length.   U i  is the output samples of the DPD engine  1170 , i is a integer variable belonging to {1:L}.   T i  is the output of the transmission capture RAM  1110 , i is a integer variable belonging to {1:L}.   U i ′ and T i ′ are the complex conjugates of the samples U i  and T i , respectively.
 
In searching for the maximum value of SNR, the correlation position n can be extracted and applied to the block  1130  to adjust the delay. The resampler  1130  aligns the captured transmission signal and the captured feedback signal according to the determined best timing offset and the correlation position n.
       

     Block  1150 —Feedback Multipath Compensation Filter 
     A feedback multipath (MP) compensation filter  1150  suppresses different multipaths or reflections that distort the feedback signal, so that the DPD algorithm processing can determine the optimum solution for the DPD engine  300 . The multipath compensation filter  1150  performs finite impulse response (FIR) digital filtering to suppress multi path distortion of the feedback signals sampled from amplified signals. The filter is expressed as 
     
       
         
           
             
               
                 
                   
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     where N P  is the number of taps, P i &#39;s are the filter coefficients, and G and H are the input and output of the feedback MP Compensation filter  1150 . G n-i  and H n  are respectively samples of G and H. 
     The feedback compensation optimization process (block  1190 ) searches for the optimum filter coefficients P i , and apply the optimum filter coefficients P i  at the feedback MP filter  1150 . 
     Block  1160 —DPD Coefficients Estimator 
     The DPD coefficients estimator  1160  computes the DPD polynomial coefficients W with lowest rms error using normal equation and provides the DPD polynomial coefficients W to DPD engine  1170 . The DPD coefficients estimator  1160  generates the DPD polynomial coefficients W as a first digital predistortion (DPD) solution. This processing can be tailored for different linearization designs, and is out of the scope of this disclosure. 
     Block  1170 —DPD Engine 
     The DPD engine  1170  applies the polynomial coefficients W extracted from the DPD coefficients estimator (Block  1160 ) to produce the predistorted signal U that is sent to a DPD performance estimator  1180 . The DPD engine  1170  can be tailored for different linearization designs, and is out of the scope of this disclosure. 
     Block  1180 —DPD Performance Estimator 
     The DPD engine  1170  generates predistorted signal U according to a DPD solution W. A DPD performance estimator  1180  compares the predistorted signal U to the captured transmission signals T to determine the SNR or the ACPR of a DPD solution currently utilized by DPD engine  1170 . 
     The SNR of the DPD solution is computed as in Equation 5. The ACPR of the DPD solution is computed as 
                   ACPR   =     10   *       log   10     ⁡     (       P   FUN       P   IMD       )                 (   7   )                 P   FUN     =       ∑     i   =   1     L     ⁢           ⁢     F   ⁡     (              T   i          2     ,     f   FUN     ,   B     )                 (   8   )                 P   IMD     =       ∑     i   =   1     L     ⁢           ⁢     F   ⁡     (                T   i     -     Z   i            2     ,     f   IMD     ,   B     )                 (   9   )               
where f FUN  is the frequency of a fundamental carrier, f IMD  is the frequency of the observed sideband, B is the carrier bandwidth, and F(x,f,B) is the bandpass filter of signal x with an IF centering at frequency f and bandwidth B. This process can be implemented with digital signal processing using captured transmission and feedback signals.
 
     Block  1190 —Feedback Multipath (MP) Compensation Search 
     The feedback MP compensation search engine  1190  searches for the optimum delay point n as a second time offset and searches for the compensation filter coefficients P for utilization by the MP compensation filter  1150  as indicated in Equation (6). The search is performed to find the delay n and the filter coefficient P that would give the best SNR or best ACPR as computed in a DPD performance estimator  1180 . The feedback MP compensation search engine  1190  has two search options. 
     Option 1—Brute Force Search Method for n and P 
     A brute-force method is to search for the delay n, the amplitude a i , and the phase θ i  of P i  (i=1 to N p ). A set illustrated in the following description is enclosed by a curly bracket “{ }.” The search is operated in conjunction with block  1130 , block  1150 , block  1160 , block  1170  and block  1180  with the purpose to find the maximum SNR or ACPR, as shown in the following pseudo codes:
         1—Block  1190  sets n=0, {a i  }=0,{θ i }=0   2—Block  1130  activates delay n   3—Block  1150  activates {a i } and {θ i }   4—Block  1160  computes W, but only apply the weight to Block  1170  for performance estimation   5—Block  1170  computes U,   6—Block  1180  computes SNR and/or ACPR   7—Block  1190  adjusts n, {a i } and {θ i }   8—Repeat steps 2-7 until the highest SNR and/or ACPR are found. The value of n, {a i } and {θ i } are the optimum values for Block  1150  that provides the weight W for the best SNR and/or ACPR.       

     Option 2—Direct Computation of P i    
     This method directly computes the correction filter P i =a i e jθ     i     
     
       
         
           
             
               
                 
                   
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     The coefficient P can be solved by a normal equation, expressed as
 
( F′F )· P =( F′T )  (11)
 
where
 
     
       
         
           
             
               
                 
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                   P   =     [           P   1               P   2             …             P   Np           ]             (   13   )               T   =     [           t   1               t   2               t   3               t   4               t   5             …             t   NN           ]             (   14   )               
where F={f n } is the captured feedback samples, T={t n } is the captured transmission samples, F′ is the conjugate of F, and NN is the number of samples that is used to process to estimate the coefficient set P.
 
     Solving the above normal equation, the filter coefficients P i  can be extracted.
 
 P   i   =a   i   e   jθ     i     (15)
 
     The filter coefficients P i  is then applied for Block  1150 . 
     Block  1195 —DPD Solution Verification Unit 
     The DPD performance estimator  1180  receives predistorted signals U from DPD engine  1170  which is based on a DPD algorithm solution W and provides the SNR or ACPR results of the predistorted signals. The DPD solution verification unit  1195  reviews the computed SNR and/or the ACPR of the DPD algorithm solution. A SNR threshold SNR T  and an ACPR threshold ACPR T  are pre-specified. If a SNR value SNR 1  and a ACPR value ACPR 1 , for example, are associated with a specific i th  DPD algorithm solution W i , the DPD solution verification unit  1195  performs a determination as shown in the following pseudo codes:
         If SNR 1 &gt;SNR T  or if ACPR 1 &gt;ACPR T : Good DPD Solution   Otherwise: :Bad DPD Solution       

     If the i th  DPD solution is good, then the DPD solution verification unit  1195  passes the i th  DPD solution W, to the DPD engines  300  and  1170  to process, since the i th  DPD solution W, can gives good SNR and ACPR to the DPD/PA linearization loop. If not, the DPD solution verification unit  1195  ignores this i th  DPD solution and the DPD engines  300  continues to use the previous DPD solution until an updated DPD solution is found which meets the verification criteria. 
     The method of multipath compensation for DPD improves the digital pre-distortion (DPD) linearization performance by suppressing the linear distortion caused by the reflected signals and distortion in the transmitting and feedback paths. This allows the DPD engine to dedicate most of its linearization resources for suppression of non-linearity in the PA and the remaining distortions. The purpose of reflection suppression is to suppress effects of the distortion on the transmitting and receiving paths such that the DPD processor can achieve more effective linearization. 
     The component blocks in the communication device  10 A may be integrated or at least partly integrated as an integrated circuit (IC) chip. 
     The foregoing disclosure of various embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the disclosure is to be defined only by the claims appended hereto and their equivalents.