Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefits of U.S. provisional patent application No. 61/213,176 filed on May 14, 2009, which is herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This present disclosure relates to the field of wireless communications, and more specifically, to the distortions and impairment&#39;s corrections of Multiple Input Multiple Output (MIMO) systems with linear and nonlinear components and unwanted interactions and correlations between multiple input signals. 
       BACKGROUND 
       [0003]    MIMO refers to a system with multiple inputs and multiple outputs. The definition of MIMO system is extended to wireless communication topologies in which multiple modulated signals, separated in frequency or space domain, are simultaneously transmitted through a single/multiple branch radiofrequency (RF) front-end. 
         [0004]    MIMO systems, with modulated signals separated in space domain, refer to wireless topologies with multiple branches of RF front-ends, with all branches simultaneously involved in signal transmission. These types of MIMO systems are considered as Multi-branch MIMO systems. 
         [0005]    MIMO systems, with modulated signals separated in frequency domain, refer to systems where multiple signals modulated in different carrier frequencies are concurrently transmitted through a single branch RF front-end. These types of MIMO systems are considered as Multi-frequency MIMO systems. Examples of multi-frequency MIMO systems are concurrent dual-band and multi-carrier transmitters. The system in frequency domain comprises two independent baseband signals as the multiple inputs and two up-converted and amplified signals at two carrier frequencies as the multiple outputs. In fact, this type of MIMO system uses a single branch RF front-end to transmit multiple signals. 
         [0006]    RF MIMO systems are composed of linear and nonlinear components and/or sub-blocks which may results in signal quality degradation. For example, the power amplifier (PA) is one of the main building blocks of the RF front-end that has a significant nonlinear behavior. This nonlinear relation between the input signal and the amplified output signal of the transmitter results in significant distortions on the output signal. These distortions significantly degrade the output signal&#39;s quality and result in poor data communications. In this regard, different techniques to compensate for these distortions were proposed in order to improve the linearity of the RF radio front-end. 
         [0007]    Also, there are unwanted and unavoidable interactions and correlations between the different signals in a MIMO system. These interactions are combined with the linear and nonlinear distortions in each branch of the MIMO system to generate more complex distortion effects, which considerably degrade the performance of the MIMO system. The effect of these complex distortions cannot be eliminated or reduced with conventional signal processing algorithms applied to Single Input Single Output (SISO) systems. 
         [0008]    Therefore, there is a need for a signal processing technique for MIMO systems that compensates for any distortion, interactions, and crosstalk in the system in order to improve the signal quality of the transmission link. 
       SUMMARY 
       [0009]    MIMO systems require special processing architectures, which compensate for the complex distortions in order to transmit and/or receive good quality signals. Processing architectures that are conventionally used with SISO system do not consider the interactions between the different input signals of the MIMO systems, This requires a more complex processing architecture that considers the effect of interaction between the multiple input signals. 
         [0010]    Therefore, according to the present invention, there is provided a method for multiple-input multiple-output impairment pre-compensation comprising: receiving a multiple-input signal; generating a pre-distorted multiple-input signal from the received multiple-input signal; generating a multiple-output signal by feeding the pre-distorted multiple-input signal into a multiple-input and multiple-output transmitter; estimating impairments generated by the multiple-input and multiple-output transmitter; and adjusting the pre-distorted multiple-input signal to compensate for the estimated impairments. 
         [0011]    According to the present invention, there is also provided a pre-compensator for use with a multiple-input and multiple-output transmitter, comprising: a multiple-input for receiving a multiple-input signal; a matrix of pre-processing cells for generating a pre-distorted multiple-input signal from the received multiple-input signal; and a multiple-output for feeding the pre-distorted multiple-input signal to the multiple-input and multiple-output transmitter; wherein the pre-processing cells are configured so as to estimate impairments generated by the multiple-input and multiple-output transmitter and adjust the pre-distorted multiple-input signal to compensate for the estimated impairments. 
         [0012]    The present invention further relates to a compensator for use with a multiple-input and multiple-output transmitter, comprising: a multiple-input for receiving a multiple-input signal; a matrix of processing cells for generating a distorted multiple-input signal from the received multiple-input signal; and a multiple-output for feeding the pre-distorted multiple-input signal; wherein the pre-processing cells are configured so as to estimate impairments generated by the multiple-input and multiple-output transmitter and adjust the pre-distorted multiple-input signal to compensate for the estimated impairments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Embodiments of the invention will be described by way of example only with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  is a block diagram of a Multiple Input Multiple Output (MIMO) system with a pre-compensator; 
           [0015]      FIG. 2  is a block diagram of an example of pre-distortion linearization technique in the form of a cascade of a signal processing block and a transmitter; 
           [0016]      FIG. 3  is a graph of measured output spectrums of a Single Input Single Output (SISO) transmitter with and without digital pre-distortion linerization technique; 
           [0017]      FIG. 4  is a block diagram of a dual branch MIMO transmitter; 
           [0018]      FIG. 5  is a block diagram of a MIMO system with a digital pre-compensator having four cells and a dual branch MIMO transmitter; 
           [0019]      FIG. 6  is a graph of measured output spectrum of a dual branch MIMO transmitter with and without the pre-compensation signal processing technique and for the linear MIMO transmitter; 
           [0020]      FIG. 7  is a block diagram of a MIMO system with a number N of RF paths cascaded with a digital pre-compensator; 
           [0021]      FIG. 8  is a block diagram of a dual-band transmitter with dual inputs and single branch nonlinear transmitter; 
           [0022]      FIG. 9  is a graph of the output signal from a nonlinear transmitter which shows intra-band and inter-band distortions; 
           [0023]      FIG. 10  is a block diagram of a system comprising a multi-cell processing pre-compensator cascaded with a dual-band transmitter; 
           [0024]      FIG. 11  is a block diagram of a system comprising a multi-cell processing pre-compensator cascaded with a multi-carrier transmitter; and 
           [0025]      FIG. 12  is a diagram of a processing cells matrix for MIMO systems. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Linear and nonlinear distortions are the main sources of performance degradation in RF front-ends. These distortions affect the signal quality and lead to an unacceptable data communication. In situations where both linear and nonlinear distortions are present simultaneously, the conventional signal processing algorithms are not able to eliminate and compensate for these distortions. To overcome this drawback, there is provided a signal processing to simultaneously compensate for both linear and nonlinear distortions and impairments. 
         [0027]    Referring to  FIG. 1 , there is shown an example of a system  100 , having multiple inputs  110  and multiple outputs  140 , comprising a Multiple Input Multiple Output (MIMO) RF front-end  130  having degraded performance due to the nonlinear behavior of the integrated RF PAs and the coupling effects between the multiple RF paths. In this case, the MIMO RF front-end  130  suffers from a joint effect of linear and nonlinear distortions. A MIMO pre-compensator processing block  120  is cascaded to the MIMO RF front-end  130  to compensate for all linear and nonlinear distortions of the MIMO system  100 . 
         [0028]    Referring now to  FIG. 2 , there is shown an example of pre-distortion linearization  200  for a Single Input Single Output (SISO) transmitter, which may be used to illustrate the basic concept behind signal pre-processing methods. Pre-distortion linearization  200  includes a signal processing block  220 , which pre-processes the input baseband signal  210  to generate a pre-distorted baseband signal  230 . Then the pre-distorted signal  230  is supplied to the nonlinear transmitter  240  to produce an output signal  250 . Both the signal processing block  220  and the transmitter  240  have nonlinear behavior; however, the cascade of both block  220  and transmitter  240  has a linear response. Therefore, the output signal  250  is a linear amplified version of the input baseband signal  210 . If ƒ(x) is a function that models the nonlinear behavior of the transmitter  240  extracted using the baseband input signal (z)  230 , and the equivalent complex envelope of sampled RF signal at the output of the transmitter (y)  250 , the pre-distortion function of the signal processing block  220 , g(x), has to satisfy the following set of relations: 
         [0000]        y =ƒ( z ) and  z=g ( x )
 
         [0000]      ƒ( g ( x ))= G   0   x    Equation 1
 
         [0000]    where
       G 0  is the linear or small-signal gain of the transmitter  240 .       
 
         [0030]      FIG. 3  shows the output spectrum of the nonlinear transmitter  240  presented in  FIG. 2  with and without using the signal processing block  220  (with linearization and without linearization, respectively, in  FIG. 3 ). The use of the signal processing block  220  significantly reduces the out-of-band distortion of the signal and improves quality of the signal. 
         [0031]    In transmitters for multi-branch MIMO systems, the transmitter&#39;s linear and nonlinear distortions on each branch may be coupled because of the interference and crosstalk between the multiple front-ends of the transmitter. Indeed, crosstalk or coupling is more likely to happen between the paths in the case of multiple RF paths with the same operating frequency and power. This crosstalk phenomenon is expected to be more significant in integrated circuit (IC) design, where the size of the prototype is a critical design parameter. 
         [0032]    Referring to  FIG. 4 , there is shown a dual branch MIMO transmitter  420  as an example of a multi-branch MIMO system  400 . The dual branch MIMO transmitter  420  comprises low pass filters  430 A and  430 B, up-converters  435 A and  435 B and a local oscillator (L.O.)  440 , and nonlinear transmitters  445 A and  445 B. The transmitters  445 A and  445 B exhibit nonlinear and/or linear distortion behaviors. The distortion behaviors may include but not limited to nonlinear power response of the active devices such as the power amplifier, frequency response, memory effect, branch imbalance, DC and carrier offset, and/or image interference. 
         [0033]    The crosstalk or coupling in dual branch MIMO transmitter may be classified as linear crosstalk,  455 , and/or nonlinear crosstalk,  450 . The crosstalk is considered linear when the effect of the crosstalk at the output of the transmitter  460  can be modeled as a linear function of the interference  460 B and desired signal  460 A. In other words, the input signals  410  affected by linear crosstalk  455  do not pass through nonlinear components such as  445 A and  445 B. Conversely, the nonlinear crosstalk  450  affects the input signals  410  before it passes through nonlinear components such as  445 A and  445 B. The nonlinear crosstalk produces undesired signal  460 C at the output of the dual branch MIMO transmitter  400 . The sources of nonlinear crosstalk  450  may be interferences in the chipsets between the different paths of the MIMO transceiver and leakage of RF signals through the common local oscillator  440  path. 
         [0034]    Referring now to  FIG. 5 , there is shown a MIMO system  500  comprising a digital pre-compensator  520  with dual inputs and dual outputs cascaded in front of a dual branch MIMO transmitter  540  similar to the one illustrated in  FIG. 4  (with components  550 A,  550 B,  555 ,  560 A and  560 B of  FIG. 5  corresponding to components  435 A,  435 B,  440 ,  445 A and  445 B of  FIG. 4 ). The digital pre-compensator  520  uses a matrix of four processing cells  515  in order to compensate for the dual branch nonlinearities and any crosstalk and interference (impairments) between the two RF paths. The digital pre-compensator  520  comprises means, for example the processing cells, using the input signals  530  and output signals  570  of the dual branch MIMO transmitter  540  to estimate any nonlinearities and interferences (impairments) and identify a proper processing function for each of the four processing cells  515 . After identification, the input signals  510  are supplied to the four processing cells  515  to generate and adjust the pre-distorted signals  530 . Then the pre-distorted signals  530  are supplied to the dual branch MIMO transmitter  540 . The cascade of the digital pre-compensator  520  and the dual branch MIMO transmitter  540  exhibit linear behavior. The output signals  570  are the linear amplified version of the input signals  510  without the effect of the transmitter linear and nonlinear distortions and crosstalk on the quality of the signals. Therefore, the digital pre-compensator  520  compensate for all the linear and nonlinear distortions and crosstalk (impairments) in the different branches of the MIMO transmitter  540 . 
         [0035]    Referring to  FIG. 6 , there is shown the measured output spectrum of the dual branch MIMO transmitter  540  for three cases: case-1) in the presence of −20 dB crosstalk and without using the digital pre-compensator  520 , case-2) in the presence of −20 dB crosstalk and using the digital pre-compensator  520 , and case-3) for a perfect MIMO transmitter without any crosstalk and nonlinearities. The output spectrum of case-2 with −20 dB crosstalk and digital pre-compensator  520  is almost following the one in case-3; this demonstrates that the digital pre-compensator  520  can compensate for the effect of both transmitter nonlinearities and crosstalk (impairments). 
         [0036]    Referring to  FIG. 7 , there is shown an example of a system  700  comprising a digital pre-compensator with multiple inputs and multiple outputs  720 , having a RF front-end  740  with a number N of outputs  750 . The digital pre-compensator  720  can be modeled as a N×N matrix  725  where each cell of the matrix represents a processing block. For example, D i,j  represents the processing block between the i th  input signal and the j th  output of the digital compensator  720 . The matrix representation of the digital compensator block based on the input signals  x   710  and output signals  Y   730  can be expressed as follows: 
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         [0000]    is an N×K matrix, and β k (x(n)) is defined as: 
         [0000]      β k ( x ( n ))=| x ( n )| k−1   x ( n )   Equation 3
 
         [0000]    and,  x =[x(1) x(2) . . . x(N)] T  is an N×1 vector representing N samples of the input signal, and K and Q are the maximum polynomial order and memory depth. 
         [0037]    Referring to  FIG. 8 , there is shown an example of a multi-frequency MIMO system  800  in the form of a dual-band transmitter  820  having inputs  810  and output  860 . The dual-band transmitter  820  consists of low pass filters  830 A and  830 B, up-converters  835 A and  835 B, local oscillators (L.O.)  840 A and  840 B, and nonlinear transmitter  850 . The input signals are up-converted to carrier frequencies ω 1  and ω 2  from local oscillators  840 A and  840 B using up-converters  835 A and  835 B. The up-converted signals from the up-converters  835 A and  835 B are combined by means of a power combiner  845  and are supplied, after combination, to the dual-band transmitter  850 . The dual-band transmitter  850  exhibits nonlinear and/or linear distortions (impairments) behaviors. The distortion behaviors may include but not limited to nonlinear power response of the active devices such as the power amplifier, frequency response and memory effect. 
         [0038]    Referring to  FIG. 9 , there is shown the output signal of the dual-band transmitter  820  presented in  FIG. 8 . Due to nonlinear behavior of the dual-band transmitter  820 , the output signal  900  of the transmitter  820  consists of desired signals  910 A and  910 B at carrier frequencies ω 1  and ω 2 , intra-band distortions  920 , and inter-band distortions  930 A and  930 B. 
         [0039]    Referring now to  FIG. 10 , there is shown a system  1000  comprising a digital multi-cell processing pre-compensator  1020  with dual inputs  1010  and dual pre-distorted outputs  1015  cascaded in front of a dual-band transmitter  1030  similar to the one illustrated in  FIG. 8  (with components  1035 A,  1035 B,  1040 A,  1040 B,  1045 A,  1045 B,  1050  and  1060  of  FIG. 10  corresponding to components  830 A,  830 B,  840 A,  840 B,  835 A,  835 B,  845  and  850  of  FIG. 8 ). The digital multi-cell pre-compensator  1020  uses a matrix of two processing cells,  1025 A and  1025 B, in order to compensate for the dual-band transmitter&#39;s nonlinearities and any intra-band distortions (impairments) between the two RF signals. The digital multi-cell pre-compensator  1020  with dual inputs  1010  and dual outputs  1015  comprises means, for example the processing cells  1025 A and  1025 B, using the input signals  1010  and output signal  1070  of the dual-band transmitter  1030  to estimate any nonlinearities and distortions (impairments) and identify a proper processing function for each of the two processing cells PC1  1025 A and PC2  1025 B. After identification, the input signals  1010  are supplied to the two processing cells  1025 A and  1025 B to generate and adjust the pre-distorted signals  1015 . Then the pre-distorted signals  1015  are supplied to the dual-band transmitter  1030 . The cascade of the digital compensator  1020  and the dual-band transmitter  1030  exhibits linear behavior. The output signal  1070  is the linear amplified version of the input signals  1010  without the effect of the transmitter&#39;s nonlinearities and intra-band distortions (impairments) on the quality of the output signal. Therefore, the digital multi-cell processing pre-compensator block  1020  compensate for all the linear and nonlinear distortions (impairments) of the dual-band transmitter  1030 . 
         [0040]    Referring to  FIG. 11 , there is shown a system  1100  comprising a digital multi-cell processing pre-compensator  1120  with dual inputs  1110  and pre-distorted output  1150  cascaded in front of a multi-carrier transmitter  1160 . The digital multi-cell pre-compensator  1120  uses a matrix of four processing cells,  1125 A,  1125 B,  1130 A and  1130 B, in order to compensate for the multi-carrier transmitter&#39;s  1160  nonlinearities and any intra-band and inter-band distortions (impairments) between the two RF signals. The digital multi-cell pre-compensator  1120  comprises means, for example the processing cells  1125 A,  1125 B,  1130 A and  1130 B, using the input signals  1110  and the output signal  1170  of the multi-carrier transmitter  1160  to estimate any nonlinearities and distortions (impairments) and identify a proper processing functions for each of the four processing cells PC1  1125 A, PC2  1125 B, PC3  1130 A, and PC4  1130 B. The processing cells PC1  1125 A and PC2  1125 B compensate for the intra-band distortions and transmitter&#39;s nonlinearities around carrier frequencies ω 1  and ω 2 . The processing cells PC3  1130 A and PC4  1130 B compensate for the inter-band distortions at frequency bands centered at 2ω 1 -ω 2  and 2ω 2 -ω 1 . The pre-distorted output signals of the processing blocks are then up-converted to designated carrier frequencies using the up-converters  1135 A,  1135 B,  1135 C, and  1135 D. Finally, the up-converted signals are combined in power combiner  1145  and feed the input of the nonlinear multi-carrier transmitter  1160 . The cascade of the digital multi-cell pre-compensator  1120  and the dual-band transmitter  1160  exhibit linear behavior. The output signal  1170  is the linear amplified version of the input signals  1110  without the effect of the transmitter&#39;s nonlinearities, inter-band, and intra-band distortions (impairments) on the quality of the output signal. Therefore, the digital multi-cell pre-compensator  1120  compensates for all the linear and nonlinear distortions of the multi-carrier transmitter  1160 . 
         [0041]    Referring to  FIG. 12 , there is shown a system comprising a digital pre-compensator  1220  with multiple inputs  1210  and multiple outputs  1250  used for forward behavior modeling and simulation of the linear/nonlinear behavior of multi-branches and multi-frequencies MIMO systems. The digital pre-compensator  1200  is modeled as a N×N matrix with N 2  cells  1230 , with N inputs  1210  and N outputs  1250 , where each cell of the matrix represents a processing block. For example, D(i,j) represents the processing block where the input of the processing cell is the i th  input  1210  of the MIMO system and the output of the processing cell is the input of the function f j , which its output is the j th  output  1240  of the digital pre-compensator  1220 . The functions f i    1240  can be modeled as linear or nonlinear functions with/without considering the memory of the system. 
         [0042]    Depending on the architecture of the MIMO system, the digital compensator with multiple inputs and multiple outputs  1220  can be added before or after the MIMO system as pre-compensator or post-compensator. 
         [0043]    Therefore, as taught by the above disclosure: 
         [0044]    The pre-distorted multiple-input signal may be adjusted to introduce linear and nonlinear distortions on each signal path of the multiple-input signal to compensate for estimated impairments; and 
         [0045]    The pre-distorted multiple-input signal may be adjusted to introduce interference between each signal path of the multiple-input signal to compensate for estimated impairments. 
         [0046]    Each of the above described pre-processing cells may include nonlinear processing blocks compensating for multiple-input multiple-output nonlinear distortions and an effect of interferences between signal paths of the multiple-input signal and signal paths of the multiple-output signal. The nonlinear processing blocks process the multiple-input signal and the multiple-output signal to determine a desired multiple-output signal that pre-compensates for the nonlinear distortions; and estimating a nonlinear function for each nonlinear processing block. 
         [0047]    Each of the above described pre-processing cells may include linear processing blocks compensating for multiple-input multiple-output linear distortions and an effect of interferences between signal paths of the multiple-input signal and signal paths of the multiple-output signal. The linear processing blocks process the multiple-input signal and the multiple-output signal to determine a desired multiple-output signal that pre-compensates for the linear distortions, and estimate a linear function for each linear processing block. 
         [0048]    Each of the above described pre-processing cells of the matrix may comprise nonlinear processing blocks compensating for multiple-input multiple-output nonlinear distortions and an effect of interferences between signal paths of the multiple-input signal and signal paths of the multiple-output signal, and linear processing blocks compensating for the multiple-input multiple-output linear distortions and the effect of interferences between the signal paths of the multiple-input signal and the signal paths of the multiple-output signal. The non-linear and linear processing blocks process the multiple-input signal and the multiple-output signal to determine a desired multiple-output signal that pre-compensates for the non-linear and linear distortions, estimate a non-linear function for each non-linear processing block, and estimate a linear function for each linear processing block. 
         [0049]    Each of the above described pre-processing cells of the matrix may model a behavior of multi-input multi-output system and may include a nonlinear processing block to compensate for the multiple-input multiple-output system linear distortions and an effect of interferences between signal paths of the multiple-input signal and signal paths of the multiple-output signal, and a linear processing block to compensate for the multiple-input multiple-output system linear distortions and the effect of interferences between the signal paths of the multiple-input signal and the signal paths of the multiple-output signal. Each of the non-linear and linear processing blocks process the multiple-input signal and the multiple-output signal to determine a desired multiple-output signal that pre-compensates for the non-linear and linear distortions, estimate a non-linear model for each non-linear processing block, and estimate a linear model for each linear processing block. 
         [0050]    Those of ordinary skill in the art will realize that the description of the system and methods for digital compensation are illustrative only and are not intended to be in any way limiting. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. Furthermore, the disclosed systems can be customized to offer valuable solutions to existing needs and problems of the power efficiency versus linearity tradeoff encountered by designers of wireless transmitters in different applications, such as satellite communication applications and base and mobile stations applications in wireless communication networks. 
         [0051]    In the interest of clarity, not all of the routine features of the implementations of signal pre-compensation processing mechanism are shown and described. It will, of course, be appreciated that in the development of any such actual implementation of the network access mechanism, numerous implementation-specific decisions must be made in order to achieve the developer&#39;s specific goals, such as compliance with application-, system-, network- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the field of telecommunication networks having the benefit of this disclosure. 
         [0052]    In accordance with this disclosure, the components, process steps, and/or data structures described herein may be implemented using various types of operating systems, computing platforms, network devices, computer programs, and/or general purpose machines. In addition, those of ordinary skill in the art will recognize that devices of a less general purpose nature, such as hardwired devices, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), or the like, may also be used. Where a method comprising a series of process steps is implemented by a computer or a machine and those process steps can be stored as a series of instructions readable by the machine, they may be stored on a tangible medium. 
         [0053]    Systems and modules described herein may comprise software, firmware, hardware, or any combination(s) of software, firmware, or hardware suitable for the purposes described herein. Software and other modules may reside on servers, workstations, personal computers, computerized tablets, PDAs, and other devices suitable for the purposes described herein. Software and other modules may be accessible via local memory, via a network, via a browser or other application in an ASP context, or via other means suitable for the purposes described herein. Data structures described herein may comprise computer files, variables, programming arrays, programming structures, or any electronic information storage schemes or methods, or any combinations thereof, suitable for the purposes described herein. 
         [0054]    Although the present invention has been described hereinabove by way of non-restrictive illustrative embodiments thereof, these embodiments can be modified at will within the scope of the appended claims without departing from the spirit and nature of the present invention.

Technology Category: h