Abstract:
A method that includes interference control at transmitters and interference mitigation at receivers in a wireless communication system is disclosed. Embodiments of the present invention exploit the interference sensitivity at neighboring terminals by taking into account the reciprocity of propagation radio channels in Time Division Duplexing systems (TDD). It can be applied to design the resource allocation for downlink (DL) and uplink (UL) transmissions in a wireless communication system. The methods include the self-configuration of the transmit power, the transmit precoder and the receive filter, at each transmitter and receiver in a multi-cell network. Systems are also provided and configured for implementing the methods of the invention.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/862,566, filed Aug. 6, 2013, the contents of such application being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the field of wireless communications. More specifically, the invention relates to methods and systems for decentralized managing of interference in multi-antenna systems, that includes transmitter design for controlling the generated interference and receiver design for mitigating the interference, through the self-configuration of the transmit power, the transmit precoding, and the receive filter, in a multi-cell network based on Time Division Duplexing (TDD). The proposed methods rely on propagation channel reciprocity for decentralized interference management. The invention is applicable to, but not limited to, a communication resource access, particularly for an enhanced downlink or uplink packet-based data transmission employing Orthogonal Frequency Division Multiple Access (OFDMA)-based system used in the 3rd Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE-Advanced) standard. 
       BACKGROUND OF THE INVENTION 
       [0003]    With the objective of improving the system efficiency and providing a homogenous coverage, the wireless communication networks envision the deployment of multiple-input multiple-output (MIMO) systems, full frequency reuse, and a denser deployment of transmitters. In such scenario, efficient interference management techniques are expected to play a crucial role in improving the transmission data rates. 
         [0004]    The MIMO interfering channel is a generic model for cellular communication systems, in which a plurality of transmitters, each equipped with multiple antennas, wish to send independent messages to their intended receiver while generating interference to all remaining receivers. Unfortunately, the optimal transmit/receive strategy with linear spatial filters that maximize the weighted sum of data rates of the system is not known because of the non-convexity of the problem. 
         [0005]    Already existing centralized interference management approaches allow finding locally optimal solutions, but they require of a central unit to collect the Channel State Information (CSI) of all receivers, and further the complexity of the solution scales with the number of transmitters times number of receivers. For those reasons, most current research focuses on decentralized techniques. Recent decentralized procedures consider the exchange of control-plane messages among transmitters to convey the degradation of the data rate due to the created interference (also called interference prices). This knowledge allows to control the interference at the transmitters by selecting the transmit power and the linear transmit precoder. On the other hand, the maximization of the weighted sum of data rates can be obtained by iteratively minimizing the weighted sum of Mean Square Errors (MSEs) [Christensen-08][Shi-11], where the error weighting matrices are chosen accordingly to the inherent relation between the data rate and the Minimum MSE (MMSE). 
         [0006]    Nevertheless, all these decentralized procedures require the interfering channels be estimated and reported by the receivers to interfering transmitters. Hence, channel estimation errors on the interfering links and the overhead associated to the reporting of channel gains have a detrimental effect on the overall potential gains. 
         [0007]    Other solutions proposed in the field are detailed in the patent documents below. They are mainly focused on wireless cellular communication systems, where transmitters are Base Stations (BSs) and receivers are User Equipments (UEs). The downlink (DL) transmission is understood as the transmission from BSs to UEs, and the uplink (UL) transmission as the transmission from UEs to BSs.
       U.S. Pat. No. 6,718,184, incorporated by reference herein, describes a linear precoding technique for an antenna array at the BS that is able to balance the coverage and the generated interference towards other cells by studying the quotient between the DL received power at the desired in-cell UE and the DL power at the other-cells UE. To that end, they estimate the covariance matrix of the desired signal and the covariance matrix of the interfered UEs using UL transmissions. The procedure is not able to guarantee a certain SINR or link quality to the UEs.   EP-A-2045930, incorporated by reference herein, discloses a method for decentralized yet iterative power control in a network of nodes with the target of guaranteeing a certain C/I. Convergence of the method is proved. The method exploits Time Division Duplex (TDD) access mode and hence channel reciprocity, but it is not applicable to a multi-antenna transmission system.   U.S. Pat. No. 8,023,955, incorporated by reference herein, enables decentralized schedulers by exploiting the reciprocity of radio channels in TDD or Frequency Division Duplex (FDD) systems by preemptively controlling inter-cell interference levels in the downlink transmission. It applies for the interference control in the UL transmission.   WO-A-2010/148371, incorporated by reference herein, provides inter-cell interference coordination in femtocell networks where the UE determines the information from interfering cells and reports this information to them either directly or indirectly through a backhaul link. The information reported by the UE is used for beamforming design so as to mitigate the interference in the direction of said UE. Communication among cells is required.   EP-A-2045930, incorporated by reference herein, describes methods for inter-cell interference coordination using resource partitioning in which UEs are allocated to orthogonal time or frequency resources. WO-A-2010/148371 discloses methods and apparatus for beamforming for femtocells, such as in LTE wireless networks, to provide inter-cell coordination and interference mitigation based on a macrocell UE obtaining information regarding an interfering Home evolved Node B (HeNB) and the HeNB may adjust an output based on the information.   U.S. Pat. No. 8,023,855 divulges a computer program for performing a method of inter-cell control by allocating uplink transmission resources in a mobile station of a wireless communication system where a figure of merit for transmission to the BS is calculated for adjusting an uplink transmission resource parameter at the UE.   WO-A-2011/088465, incorporated by reference herein, reveals a method for interference mitigation in a wireless communication system where interference information including an amount of uplink interference experienced at a BS or serving cell is determined (comparing the interference value to a target value) and scheduling signal transmission is performed within the serving cell based at least in part upon the interference information.   EP-B-2045930, incorporated by reference herein, discloses a method for communicating between a transceiver apparatus and a communication partner in repetitive radio frames wherein the step of transmitting during the transmission time slot of the second radio frame comprises transmitting the second transmit signal with the second transmission power being dependent on the interference power and a control component.   U.S. Pat. No. 8,401,480, incorporated by reference herein, refers to a method for transmitting feedback information at a UE in a wireless communication system that performs a Coordinated Multi-Point (CoMP) operation, comprising measuring noise and interference variances corresponding to a signal strength or an interference level using reference signals received from one or more neighbor BSs.   U.S. Pat. No. 8,260,206, incorporated by reference herein, reveals a method for uplink and downlink inter-cell interference coordination (ICIC) by a HeNB where a data exchange with a UE is performed, a measurement report is received and transmit power is reduced with a first slew rate and increased with a second slew rate.   WO-A-2011/055943, incorporated by reference herein, discloses a method for UL transmission control in a wireless communication system including the steps of receiving from a first UE information on a frequency band having an UL interference occurring therein with respect to a second BS and allocating an UL resource for the first UE based upon the frequency band.       
 
       SUMMARY OF THE INVENTION 
       [0019]    This disclosure generally relates to a method and systems for a decentralized managing of interference in a wireless communication system, including interference control at transmitters and interference mitigation at receivers, which takes into account the reciprocity of propagation radio channels in Time Division Duplexing systems (TDD). In a possible embodiment that optimizes the DL performance metrics of a wireless cellular communication system, the transmitters and receivers are Base Stations (BSs) and User Equipments (UEs), respectively. 
         [0020]    In another embodiment, the UL performance metrics is optimized, where the transmitters and receivers are User Equipments (UEs) and Base Stations (BSs), using the same methods described below. 
         [0021]    The first aspect of the invention concerns to a method, where each BS has knowledge of an estimated version of the channel towards its associated UEs, but not of the neighboring ones associated to other BSs, comprising:
       a) sensing an uplink transmission at each BS from the UEs associated to other BSs;   b) processing at said BSs said uplink transmission for determining interference information;   c) adjusting each BS a transmit filter for a downlink transmission to associated UEs under an optimization transmission criterion and to mitigate interference towards UEs associated to other BSs.   d) said UEs on receipt of said downlink transmission adjusting its receive filter under an optimization reception criterion, and   e) adjusting each UE a transmit filter using said receive filter for a further uplink transmission to be sensed by all BSs. Said adjusting of the uplink transmit filter at said UE is implemented in the digital signal processing (DSP) block at the UE.       
 
         [0027]    Said adjustment of the downlink transmit filter at said BS is preferably implemented in the digital signal processing (DSP) block at the BS controller; 
         [0028]    As transmit filter for downlink or uplink transmission is understood both the control of the transmit power and the selection of the transmit precoder. As receive filter for downlink transmission is understood the linear processing applied at UE for data demodulation. 
         [0029]    Said adjusting of the transmit filter at BS in step c), the receive filter at UE in step d), and the transmit filter at UE in step e) is done in a decentralized way, where each transmitter (BS or UE) establishes the parameters of its transmission (transmit power and transmit precoding) based on local information available, and each receiver (UE) establishes the parameters of its reception (receive filter) based on local information available. For each BS, the local information for transmission includes knowledge of the channel towards its associated UEs and knowledge of the interference information that is acquired in step b). For each UE, the local information for reception includes knowledge of the equivalent channel (that is, the combined effect of the propagation channel and the transmit filter) with its associated BS and knowledge of the covariance matrix of the received interference-plus-noise. For each UE, the local information for transmission includes knowledge of the receive filter used for downlink reception, and possibly other system-established parameters. 
         [0030]    For some embodiments, the method comprises further iterations of steps a) to e) to achieve a given level of interference mitigation. 
         [0031]    For some embodiments of the method, at least one among the plurality of BSs and active UEs are equipped with one, two, three, four or more antennas, where said antennas can be collocated or distributed. 
         [0032]    For some embodiments of the method, steps a) to e) are performed for at least one carrier frequency in a multicarrier system. 
         [0033]    For some embodiments of the method, said processing of step b) comprises computing an interference cost matrix which is estimated as a function of the covariance matrix of the received interference-plus-noise signal at each BS from said uplink transmission from UEs associated to other BSs. 
         [0034]    Said interference cost matrix reflects the interference created by BS towards UEs associated to other BSs. 
         [0035]    For some embodiments of the method, said optimization of step c) of the downlink transmit filter of said BS to its associated UEs is done by taking into account: i) an estimated version of the channel towards its associated UEs, and ii) the interference cost matrix. 
         [0036]    For some embodiments of the method, said optimization of step c) of the downlink transmit filter of said BS to its attached UE is done by taking into account: i) an estimated version of the channel towards its associated UEs, ii) the interference cost matrix that reflects the interference created by BS towards UEs associated to other BSs, and iii) the covariance matrix of the interference-plus-noise at the associated UEs. 
         [0037]    Said adjusting of the downlink transmit filter as a function of the interference cost matrix allows to control the levels of transmitted power, and implicitly the levels of experienced interference by UEs associated to other BSs, and either maximize the total weighted sum of data rates of the system or minimize the total weighted sum of mean square errors of the system. 
         [0038]    Said adjusting of the downlink transmit filter may be computed according to a robustness criterion that takes into account the error in channel estimation between each UE and its serving BS. 
         [0039]    Said adjusting of the downlink transmit filter at said BS is implemented in the digital signal processing (DSP) block at the BS controller. 
         [0040]    For one embodiment of the method, said downlink from each BS to corresponding served UE and said uplink from served UE to its BS do not involve data transmission, so that only spatially filtered symbols are transmitted. 
         [0041]    For one embodiment of the method, said downlink from each BS to corresponding UE served includes data transmission, and said uplink from served UE to its BS does not involve data transmission. 
         [0042]    For one embodiment of the method, said downlink from each BS to corresponding UE served does not include data transmission and said uplink from served UE to its BS involves data transmission. 
         [0043]    For one embodiment of the method, both said downlink and said uplink involve data transmission. 
         [0044]    For some embodiments of the method, each UE at step d) estimates the equivalent channel (that is the combined effect of the propagation channel and the transmit filter) towards its BS and the covariance matrix of the interference-plus-noise, and implements a downlink receive filter. 
         [0045]    For some embodiments of the method, said UE based on said downlink receive filter, updates at step e) a corresponding uplink transmit filter. 
         [0046]    Said adjusting of the uplink transmit filter at said UE is implemented in the digital signal processing (DSP) block at the UE. 
         [0047]    For some embodiments of the method, each UE at step e) uses sounding reference signals defined in LTE-A release 11 properly configured for uplink transmission. 
         [0048]    The method and systems of the present invention are applicable for any femtocell, picocell, microcell or macrocell base station with an IP-based backhaul link operating ADSL, ADSL2, ADSL2+, VDSL2, FTTx technologies or any other backhaul specification to the Core Network or to the Internet serving mobile or fixed wireless equipment terminals equipped with one, two, three, four or more antennas and configured to operate IS-95, CDMA, GSM TDMA, GPRS, EDGE, UMTS, WCDMA, OFDM, OFDMA, TD-SCDMA, HSDPA, LTE, LTE-A, WiMaX, 3GPP, and/or 3GPP2 signals. 
         [0049]    The present invention circumvents the limitations of the prior art proposals by providing three main advantages:
       i) Provides a method that avoids the estimation of the interfering channels (propagation channels from BSs to UEs attached to other BSs in the same frequency/time resource). This way complexity, overhead and the impact of errors associated to the estimation of the interfering channels are significantly reduced.   ii) Provides a method that avoids the exchange of control-plane messages among transmitters over the backhaul so as to manage interference. This way the impact of non-ideal backhaul links is significantly reduced.   iii) Provides a method that can be implemented in a totally decentralized manner and which solution is fully scalable.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The previous and other advantages and features will best be understood by reference to the following detailed description of several illustrative and not limitative embodiments when read in conjunction with the accompanying drawing, wherein: 
           [0054]      FIG. 1   a  illustrates the intended downlink signals from BSs towards its associated UEs in a TDD wireless communication system. 
           [0055]      FIG. 1   b  illustrates the interfering downlink signals from BSs towards UEs attached to neighbor BSs in a TDD wireless communication system. 
           [0056]      FIG. 1   c  illustrates the received uplink signals when all UEs transmit simultaneously in a TDD wireless communication system. 
           [0057]      FIG. 2  illustrates a method for DL transmit and receive filters design according to the first embodiment of the invention. 
           [0058]      FIG. 3  illustrates a method for UL transmit and receive filters design according to the first embodiment of the invention. 
           [0059]      FIG. 4  illustrates a method for computing a system parameter according to an embodiment of the invention. 
           [0060]      FIG. 5   a  illustrates a method related to the second embodiment of the invention in a TDD wireless communication system. 
           [0061]      FIG. 5   b  illustrates another method regarding the third embodiment of the invention in a TDD wireless communication system. 
           [0062]      FIG. 5   c  illustrates a further method related to the fourth embodiment of the invention in a TDD wireless communication system. 
           [0063]      FIG. 6  is a block diagram of a system describing the second embodiment of the invention. 
           [0064]      FIG. 7  is another block diagram of a system regarding the third embodiment of the invention. 
           [0065]      FIG. 8  is a further block diagram related to a system of the fourth embodiment of the invention. 
           [0066]      FIG. 9  shows a list of parameters for a simulation scenario. 
           [0067]      FIGS. 10   a  and  10   b  are used layout configurations for a simulation scenario. 
           [0068]      FIGS. 11   a  and  11   b  show the sum throughput achieved on each resource block (RB) or subband for the layout configurations depicted in  FIG. 10 . 
           [0069]      FIG. 12  shows a comparison of assumptions, requirements and implications of different methods for UL non-data transmission using sounding reference signals (SRS) in LTE-Advanced release 11 with the objective of broadcasting the interference cost. 
           [0070]      FIG. 13   a  illustrates a method for UL non-data transmission using SRS in LTE-Advanced. 
           [0071]      FIG. 13   b  illustrates another method for UL non-data transmission using SRS in LTE-Advanced release 11. 
           [0072]      FIG. 14   a  illustrates a further method for UL non-data transmission using SRS and component subband specific UL power control in LTE-Advanced release 11. 
           [0073]      FIG. 14   b  illustrates another method for UL non-data transmission using a new type of SRS and component subband specific UL power control in LTE-Advanced release 11. 
           [0074]      FIG. 15  illustrates a method related to the second embodiment of the invention in a TDD LTE-Advanced release 11 wireless communication system using SRS for UL non-data transmission. 
           [0075]      FIG. 16  details in a Table the list of abbreviations and symbols used in this description. 
           [0076]      FIG. 17  shows the definition of linear transmit filter (including transmit power and transmit precoding) and linear receive filter in a multi-antenna wireless communication system between one transmitter and one receiver. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0077]    The notation used in this disclosure is detailed in the following. Scalars are denoted by italic letters. Boldface lower-case and upper-case letters denote vectors and matrices, respectively. The function log e ( ) refers to the base-2 logarithm. For a given matrix A, its transpose matrix is denoted as A T , its conjugate matrix as A*, its transpose conjugate matrix as A H  and the matrix inverse as A −1 . det(A) operator refers to the determinant of A, Tr(A) to the trace of A, and E{A} to the expectation operator of each component in A. Matrix I refers to the identity matrix. 
         [0078]    In all the equations involving transmit or receive filters, the per-antenna power of the signal is normalized relative to the transmitter (BS or UE) maximum transmitted power. 
         [0079]    Embodiments of the present invention are found in the following methods and systems for decentralized self-configuration of the power, transmit precoding and receive filter (or decoding), in a multi-cell deployment. In these embodiments, BSs and UEs can be equipped with one, two, three, four or more antennas. The antennas at each BS can be either spatially distributed or collocated. 
         [0080]    In reference to  FIG. 1   a  and  FIG. 1   b , they illustrate the intended DL signals and the interfering DL signals, respectively, in a communication between several BSs and several UEs present in the network. BS  105  is serving UE  101 , BS  106  is serving UE  102 , BS  107  serves UE  103  and BS  108  serves UE  104 . In  FIG. 1   a , links  115 ,  126 ,  137  and  148  represent the path gain between each BS and its serving UE. The interfering path gains are illustrated in  FIG. 1   b.    
         [0081]    In reference to  FIG. 1   c , it illustrates the received UL signals in a communication between several UEs and several BSs, where the received UL signal at each BS comes from a plurality of UEs. Links  151 ,  161 ,  171 ,  181 ,  152 ,  162 ,  172 ,  182 ,  153 ,  163 ,  173 ,  183 ,  154 ,  164 ,  174  and  184  represent the path gain between each UE and each BS. 
         [0082]      FIG. 17  illustrates the role of a linear transmit filter (T) and a linear receive filter (R) in a wireless communication between one transmitter and one receiver. It can be applied either for DL or for UL transmission. The stream of transmitted symbols b  171  is spatially filtered into the transmitted signal x  174  through the linear transmit filter T that includes transmit power P 1/2    172  and transmit precoding V  173  as follows: x=Tb=VP 1/2 b. The spatially filtered transmitted signal x has a length equal to the number of antennas at transmitter. After the wireless propagation through the MIMO channel  175 , the received signal is denoted by y  176 , which is spatially filtered using the linear receive filter R  177  in order to recover the stream of transmitted symbols as follows: {circumflex over (b)}=R H y  178 . 
         [0083]    With the objective of maximizing the total weighted sum of data rates of the system, constrained by the maximum transmitted power at the BS i , the DL transmit filters of BSs (or equivalently the transmit beam-formers, when one layer is transmitted per UE) are designed as the solution to the following optimization problem: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    where N is the total number of BS-UE pairs considering that each BS serves a single user in a given time/frequency resource, μ i  is the user priority coefficient of UE i , r i  is the data rate of UE i , T i  is the DL transmit filter of the signal transmitted to UE i , P i   max  is the maximum available transmit power at BS i  and Tr(.) denotes the trace operator. The proper update of the user priority coefficients (μ i ) can shape the system performance, from the greedy to the proportional fair operation mode [Kelly-98]. The data rate of UE i  is understood as the Shannon Capacity limit for the transmission towards UE i : 
         [0000]        r   i =log 2 det( I+H   i,i   T   i   T   i   H   H   i,i   H   N   i   −1 ),  (0)
 
         [0000]    where H i,i  denotes the complex channel matrix between BS i  and its own UE i , which contains the channel gains between each UE antenna element and each BS antenna element, det(.) stands for the determinant operator, I refers to the identity matrix and N i  corresponds to the covariance matrix of the received noise-plus-interference at UE i : 
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         [0000]    where H j,i  denotes the complex channel matrix between BS j  and UE i , T j  is the DL transmit filter matrix of the signal transmitted to UE j  attached to neighbor BS j  and Σ i  refers to the covariance matrix of the received noise at UE i . In case the noise could be modeled as additive white Gaussian noise: Σ i =σ i   2 I, being σ i   2  the received noise power. Finally, superscripts H and −1 indicate the Hermitian transpose and the inverse operation, respectively. 
         [0084]    Due to the interference existing on the scenario, the previous problem is not convex and the optimal solution cannot be guaranteed. Nevertheless, it can be shown [Christensen-08] [Shi-11] that one solution can be obtained by solving the following problem that considers minimization of the total weighted sum of mean square errors (MSEs) of the system, constrained by the maximum transmitted power at the BS i : 
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         [0000]    where W i  is the error weighting matrix associated to UE i  and E i  corresponds to the MSE-matrix, which contains in its diagonal the MSE for the symbols transmitted towards UE i : 
         [0000]        E   i   =I+R   i   H   H   i,i   T   i   T   i   H   H   i,i   H   R   i   −R   i   H   H   i,i   T   i   −T   i   H   H   i,i   H   R   i   +R   i   H   N   i   R   i ,  (0)
 
         [0000]    where R i  corresponds to the DL receive filter at UE i , defined in continuation in (0). 
         [0085]    Problem in (0) is convex on DL transmit filters for a fixed DL receive filters, and the other way round. Further, optimal expressions for DL transmit filters can be derived analytically for a fixed set of DL receive filters, and the other way round. So, a local optimum of the problem in (0) can be found by alternate optimization between DL transmit filters and DL receive filters. 
         [0086]    In case 
         [0000]        W   i =μ i   I,   (0)
 
         [0000]    then, the sum of mean square error of the system is minimized. In contrast, see [Christensen-08][Shi-11], the maximization of the total weighted sum of data rates of the system (problem presented in (0)) is obtained by using: 
         [0000]        W   i =μ i   Ē   i   −1 ,  (0)
 
         [0000]    being Ē i  the MSE-matrix obtained every time the transmitters and receivers are updated (in the previous iteration of the alternate optimization between DL transmit filters and DL receive filters). 
         [0087]    The problem in (0), which is solved in a centralized mode in the prior art, is solved in this invention in a decentralized way; each BS i  optimizes the DL transmit filter T i  towards its associated UE i  according to: 
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         [0000]    where γ i  is an interference cost matrix that reflects the interference created by BS i  towards users in neighboring cells and is equal to: 
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                             j 
                             = 
                             1 
                           
                           N 
                         
                         
                           j 
                           ≠ 
                           i 
                         
                       
                        
                       
                         
                           H 
                           
                             i 
                             , 
                             j 
                           
                           H 
                         
                          
                         
                           R 
                           j 
                         
                          
                         
                           W 
                           j 
                         
                          
                         
                           R 
                           j 
                           H 
                         
                          
                         
                           H 
                           
                             i 
                             , 
                             j 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where H i,i  denotes the complex channel matrix between BS i  and UE j  attached to BS j , and R j is the DL receive filter of the signal transmitted to UE j  by its serving BS, presented in Error! Reference source not found.continuation in (0). 
         [0088]    The decentralized problem in (0) is convex on the DL transmit filter T i  and the DL receive filter R i , separately, and the optimal expressions can be analytically derived. 
         [0089]    Transmit filter design at each BS: for a given interference cost matrix γ i , problem in (0) can be solved at each BS i  by alternate optimization between the DL transmit filter and the DL receive filter, which expressions are given next: 
         [0000]        T   i =( H   i,i   H   R   i   W   i   R   i   H   H   i,i +γ i +λ i   I ) −1   H   i,i   H   R   i   W   i  
 
         [0000]        R   i =( H   i,i   T   i   T   i   H   H   i,i   H   +N   i ) −1   H   i,i   T   i,   (0)
 
         [0000]    where λ i  is a scalar parameter that allows to meet the transmit power constraint in (0). 
         [0090]    Such alternate optimization between transmit and receive filters at each BS requires knowledge of the interference cost matrix γ i , an estimated version of the MIMO propagation channel and the interference-plus-noise covariance matrix at the UE N i . So, each UE should report N i  to the serving BS. In case such report is not possible, alternative approximations can be used: i) if the UE can report the interference-plus-noise received power (denoted by P N     i   ), then N i =P N     i   I could be used in (0), ii) if no kind of report from the UE is possible the transmit filters at each BS could be computed following the first equation in (0) based on the knowledge of. Further alternatives could be proposed. 
         [0091]    Such optimization at each BS is implemented in the digital signal processing (DSP) block at the BS controller. 
         [0092]    The DL transmit filter T i  includes both the transmit power (represented by a diagonal real-valued matrix P i   1/2 ) and the transmit precoding (represented by a spatial complex-valued matrix V i ). So, after the optimization and for practical implementation, T i  can be decomposed as: 
         [0000]        T   i   =V   i   P   i   1/2 .  (0)
 
         [0093]    Receive filter design at each UE: once designed the DL transmit filters for each BS i , T i , the DL receive filter at each UE i  is obtained following an MMSE criterion as: 
         [0000]        R   i =( H   i,i   T   i   T   i   H   H   i,i   H   +N   i ) −1   H   i,i   T   i .  (0)
 
         [0000]    which can be implemented at each UE based on the estimation of the equivalent channel H i,i T i  that includes propagation and transmit filter, and the estimation of the covariance matrix of the received interference-plus-noise signal N i . In 3GPP LTE-A [3GPP-TR36829], DL receive filter in (0) corresponds to the linear MMSE interference rejection combiner (LMMSE-IRC). 
         [0094]    The decentralized problem in (0) can easily be extended to the following cases:
       1) Each BS serves multiple UEs simultaneously on the same time and frequency resource (multi-user MIMO). The decentralized problem to be solved at BS i  is:       
 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             minimize 
                             
                               
                                 { 
                                 
                                   T 
                                   
                                     n 
                                     i 
                                   
                                 
                                 } 
                               
                               , 
                               
                                 
                                   { 
                                   
                                     R 
                                     
                                       n 
                                       i 
                                     
                                   
                                   } 
                                 
                                 
                                   ∀ 
                                   
                                       
                                   
                                    
                                   n 
                                 
                               
                             
                           
                           
                             ∀ 
                             i 
                           
                         
                          
                         
                             
                         
                          
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               1 
                             
                             
                               N 
                               i 
                             
                           
                            
                           
                             Tr 
                              
                             
                               ( 
                               
                                 
                                   W 
                                   
                                     n 
                                     i 
                                   
                                 
                                  
                                 
                                   E 
                                   
                                     n 
                                     i 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                       + 
                       
                         Tr 
                          
                         
                           ( 
                           
                             
                               ϒ 
                               i 
                             
                              
                             
                               T 
                               
                                 n 
                                 i 
                               
                             
                              
                             
                               T 
                               
                                 n 
                                 i 
                               
                               H 
                             
                           
                           ) 
                         
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       
                         s 
                         . 
                         t 
                         . 
                         
                             
                         
                          
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               1 
                             
                             
                               N 
                               i 
                             
                           
                            
                           
                             Tr 
                              
                             
                               ( 
                               
                                 
                                   T 
                                   
                                     n 
                                     i 
                                   
                                 
                                  
                                 
                                   T 
                                   
                                     n 
                                     i 
                                   
                                   H 
                                 
                               
                               ) 
                             
                           
                         
                       
                       ≤ 
                       
                         P 
                         i 
                         max 
                       
                     
                   
                    
                   
                       
                   
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 where sub-index n i  refers to user n served by BS i , N i  is the total number of users attached to BS i . The MSE-matrix E n     i    the received noise-plus-interference covariance matrix N n     i    at user n served by BS i  are given by 
               
             
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       E 
                       
                         n 
                         i 
                       
                     
                     = 
                     
                       I 
                       + 
                       
                         
                           R 
                           
                             n 
                             i 
                           
                           H 
                         
                          
                         
                           H 
                           
                             i 
                             , 
                             
                               n 
                               i 
                             
                           
                         
                          
                         
                           T 
                           
                             n 
                             i 
                           
                         
                          
                         
                           T 
                           
                             n 
                             i 
                           
                           H 
                         
                          
                         
                           H 
                           
                             i 
                             , 
                             
                               n 
                               i 
                             
                           
                           H 
                         
                          
                         
                           R 
                           
                             n 
                             i 
                           
                         
                       
                       - 
                       
                         
                           R 
                           
                             n 
                             i 
                           
                           H 
                         
                          
                         
                           H 
                           
                             i 
                             , 
                             
                               n 
                               i 
                             
                           
                         
                          
                         
                           T 
                           
                             n 
                             i 
                           
                         
                       
                       - 
                       
                         
                           T 
                           
                             n 
                             i 
                           
                           H 
                         
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                           H 
                           
                             i 
                             , 
                             n 
                           
                           H 
                         
                          
                         
                           R 
                           
                             n 
                             i 
                           
                         
                       
                       + 
                       
                         
                           R 
                           
                             n 
                             i 
                           
                           H 
                         
                          
                         
                           N 
                           
                             n 
                             i 
                           
                         
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                           R 
                           
                             n 
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                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   
                     
                       N 
                       
                         n 
                         i 
                       
                     
                     = 
                     
                       
                         
                           
                             ∑ 
                             
                               
                                 n 
                                 j 
                               
                               = 
                               1 
                             
                             
                               N 
                               n 
                             
                           
                           
                             
                               n 
                               j 
                             
                             ≠ 
                             
                               n 
                               i 
                             
                           
                         
                          
                         
                           
                             H 
                             
                               i 
                               , 
                               
                                 n 
                                 i 
                               
                             
                           
                            
                           
                             T 
                             
                               n 
                               j 
                             
                           
                            
                           
                             T 
                             
                               n 
                               j 
                             
                             H 
                           
                            
                           
                             H 
                             
                               i 
                               , 
                               
                                 n 
                                 i 
                               
                             
                             H 
                           
                         
                       
                       + 
                       
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             N 
                           
                           
                             m 
                             ≠ 
                             n 
                           
                         
                          
                         
                           
                             ∑ 
                             
                               
                                 m 
                                 j 
                               
                               = 
                               1 
                             
                             
                               N 
                               m 
                             
                           
                            
                           
                             
                               H 
                               
                                 j 
                                 , 
                                 
                                   n 
                                   i 
                                 
                               
                             
                              
                             
                               T 
                               
                                 m 
                                 j 
                               
                             
                              
                             
                               T 
                               
                                 m 
                                 j 
                               
                               H 
                             
                              
                             
                               H 
                               
                                 j 
                                 , 
                                 
                                   n 
                                   i 
                                 
                               
                               H 
                             
                           
                         
                       
                       + 
                       
                         
                           Σ 
                           
                             n 
                             i 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
       
         
           
             2) Per-antenna or per group of antennas power constraints are included to the optimization problem. The decentralized problem to be solved is: 
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           minimize 
                           
                             ∀ 
                             i 
                           
                         
                         
                           
                             T 
                             i 
                           
                           , 
                           
                             R 
                             i 
                           
                         
                       
                        
                       
                           
                       
                        
                       
                         Tr 
                          
                         
                           ( 
                           
                             
                               W 
                               i 
                             
                              
                             
                               E 
                               i 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       Tr 
                        
                       
                         ( 
                         
                           
                             ϒ 
                             i 
                           
                            
                           
                             T 
                             i 
                           
                            
                           
                             T 
                             i 
                             H 
                           
                         
                         ) 
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       
                         s 
                         . 
                         t 
                         . 
                         
                             
                         
                          
                         
                           Tr 
                            
                           
                             ( 
                             
                               
                                 B 
                                 i 
                               
                                
                               
                                 T 
                                 i 
                               
                                
                               
                                 T 
                                 i 
                                 H 
                               
                             
                             ) 
                           
                         
                       
                       ≤ 
                       
                         
                           P 
                           i 
                           max 
                         
                          
                         
                             
                         
                          
                         l 
                       
                     
                     = 
                     1 
                   
                   , 
                   … 
                    
                   
                       
                   
                   , 
                   L 
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 where B i  denotes a matrix with all zeros except for the diagonal elements that refer to the antennas indexes included in the lth group of antennas power constraint or individual power constraint, in which it has a one. If per-antenna power constraints are considered, L coincides with the number of transmit antennas. 
               
             
             3) A plurality of carrier frequencies, as in a multicarrier system, is available. The decentralized problem to be solved is: 
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               minimize 
                               
                                 ∀ 
                                 i 
                               
                             
                             
                               
                                 T 
                                 i 
                                 k 
                               
                               , 
                               
                                 R 
                                 i 
                                 k 
                               
                             
                           
                           
                             ∀ 
                             
                                 
                             
                              
                             k 
                           
                         
                          
                         
                             
                         
                          
                         
                           Tr 
                            
                           
                             ( 
                             
                               
                                 W 
                                 i 
                                 k 
                               
                                
                               
                                 E 
                                 i 
                                 k 
                               
                             
                             ) 
                           
                         
                       
                       + 
                       
                         Tr 
                          
                         
                           ( 
                           
                             
                               ϒ 
                               i 
                               k 
                             
                              
                             
                               T 
                               i 
                               k 
                             
                              
                             
                               T 
                               i 
                               k 
                             
                           
                           ) 
                         
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       
                         s 
                         . 
                         t 
                         . 
                         
                             
                         
                          
                         
                           Tr 
                            
                           
                             ( 
                             
                               
                                 T 
                                 i 
                                 k 
                               
                                
                               
                                 T 
                                 i 
                                 
                                   k 
                                    
                                   
                                       
                                   
                                    
                                   H 
                                 
                               
                             
                             ) 
                           
                         
                       
                       ≤ 
                       
                         P 
                         i 
                         
                           k 
                            
                           
                               
                           
                            
                           max 
                         
                       
                     
                   
                    
                   
                       
                   
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 where super-index k refers to the carrier frequency and P i   k max  is the available transmit power for carrier frequency k at BS i . 
               
             
             4) Maximum transmission rate constraints are considered, as it happens for the maximum modulation coding schemes (MCS) allowed at the LTE. The decentralized problem to be solved is: 
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           minimize 
                           
                             ∀ 
                             i 
                           
                         
                         
                           
                             T 
                             i 
                           
                           , 
                           
                             R 
                             i 
                           
                         
                       
                        
                       
                           
                       
                        
                       
                         Tr 
                          
                         
                           ( 
                           
                             
                               W 
                               i 
                             
                              
                             
                               E 
                               i 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       Tr 
                        
                       
                         ( 
                         
                           
                             ϒ 
                             i 
                           
                            
                           
                             T 
                             i 
                           
                            
                           
                             T 
                             i 
                             H 
                           
                         
                         ) 
                       
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       s 
                       . 
                       t 
                       . 
                       
                           
                       
                        
                       
                         Tr 
                          
                         
                           ( 
                           
                             
                               T 
                               i 
                             
                              
                             
                               T 
                               i 
                               H 
                             
                           
                           ) 
                         
                       
                     
                     ≤ 
                     
                       
                         P 
                         i 
                         max 
                       
                        
                       
                           
                       
                        
                       
                         
 
                       
                       - 
                       
                         
                           log 
                           2 
                         
                          
                         
                             
                         
                          
                         
                           det 
                            
                           
                             ( 
                             
                               E 
                               i 
                             
                             ) 
                           
                         
                       
                     
                     ≤ 
                     
                       r 
                       i 
                       max 
                     
                   
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 where r i   max  is the transmission rate at UE, supported by the maximum modulation and coding scheme. 
               
             
           
         
       
     
         [0103]    Previous cases can be properly combined to encompass all features. 
       EMBODIMENTS 
       [0104]    The objective of Embodiment 1 is to describe the communication process and the configuration of the system parameters required to acquire the interfering cost, (0), at each BS present in the wireless communication system. Embodiments 2, 3 and 4 provide a detailed description of the systems for implementing the method of the invention. More specifically, they describe different iterative procedures so as to iterate between DL and UL transmissions through which the self-configuration of the transmit filter (including transmit power and transmit precoding) and the receive filter is achieved while preemptively managing the interference created in the network. 
       Embodiment 1 
       [0105]    The knowledge of the interference cost matrix γ i , (0), at BS i  allows managing the interference created by BS i  towards unintended UEs attached to other BSs. 
         [0106]    In this regard,  FIG. 2  illustrates how the DL transmit and receive filters are designed in a deployment with 2 BSs and 1 UE attached to each BS. First, each BS i  designs its DL transmit filter T i  (along with R i ) based on the equation (0), using the knowledge of the channel matrix towards its associated UE H i,i  ( 201 ,  208 ), the weighting matrix W i  ( 203 ,  210 ) and an estimation of the interference cost matrix {circumflex over (γ)} i  ( 202 ,  209 ). The design is done by solving the decentralized optimization problem in (0) (or one of the extended versions in (0), (0), (0) or (0)) with γ i ={circumflex over (γ)} i  ( 204 ,  211 ). Then, the DL receive filter R i  is computed at each UE i  ( 205 ,  212 ) by means of equation Error! Reference source not found.(0) based on the knowledge of the equivalent complex channel matrix towards its associated BS H i,i T i  ( 205 ,  212 ), and the covariance matrix of the received interference-plus-noise N i  ( 207 ,  214 ). At each UE i , the knowledge of H i,i  and T i  is obtained jointly through the estimation of a single variable: H i,i T i . 
         [0107]    Each BS can have an estimation of the interference cost matrix γ i  by exploiting the received signal in the uplink if the following two conditions are satisfied: 
         [0108]    1) propagation channel reciprocity can be assumed, as in a TDD system with slow varying channel for duplexing UL and DL transmissions, and 
         [0109]    2) UEs transmit simultaneously in the UL with a UL transmit filter (defined in next equation (0)) that is designed for each UE, based on its DL receive filter R i , its weighting matrix W i  and its maximum transmitted power. 
         [0110]    If the first condition is satisfied, the covariance matrix of the received interference-plus-noise signal at BS i  in the UL transmission Ψ i  is: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Ψ 
                       i 
                     
                     = 
                     
                       
                         
                           
                             ∑ 
                             
                               j 
                               = 
                               1 
                             
                             N 
                           
                           
                             j 
                             ≠ 
                             1 
                           
                         
                          
                         
                           
                             H 
                             
                               i 
                               , 
                               j 
                             
                             T 
                           
                            
                           
                             
                               T 
                               ← 
                             
                             j 
                           
                            
                           
                             
                               T 
                               ← 
                             
                             j 
                             H 
                           
                            
                           
                             H 
                             
                               i 
                               , 
                               j 
                             
                             * 
                           
                         
                       
                       + 
                       
                         
                           Σ 
                           ← 
                         
                         i 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   0 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where  T   j  denotes the UL transmit filter of the signal transmitted by UE j  and            i  refers to the covariance matrix of the received noise at BS i . Superscripts T and * indicate the transpose and the complex conjugate operation, respectively. 
         [0111]    Furthermore, if the UL transmit filters are designed according to: 
         [0000]                  i   =√{square root over (F)}R   i   *W   i   1/2* ,  (0)
 
         [0000]    where F&lt;1 is a common cell-wide scaling factor (the same for all UEs in the system) that allows to restrict the transmit power used at UEs, and the superscript refers to the Cholesky decomposition W i =W i   1/2 (W i   1/2 ) H , then the interference cost matrix γ i  can be estimated from the received signal at BS i  as a function of the complex conjugate of the covariance of the received interference-plus-noise signal in the UL transmission (Ψ i ): 
         [0000]      {circumflex over (γ)} i   =F   −1 Ψ i *,  (0)
 
         [0000]    where {circumflex over (γ)} i  denotes the estimation of the interference cost matrix {circumflex over (γ)} i . 
         [0112]      FIG. 3  illustrates how each BS i  obtains knowledge of the interference cost matrix γ i  thanks to the proper design of UL transmit filters at the UEs, for a simple deployment with 2 BSs and 1 UE attached to each BS. UL transmit filters are designed according to (0) ( 303 ,  308 ), based on the knowledge of the DL receive filter R i  ( 301 ,  306 ) and the weighting matrix W i  ( 302 ,  307 ). Then, each BS i  estimates its interference cost matrix γ i  based on the covariance matrix of the received interference-plus-noise signal in the UL. 
         [0113]    Notice that, in the UL transmission there is no need for BSs to receive data content from the signals transmitted by UEs. Hence, spatially filtered symbols can be transmitted. 
         [0114]    Due to the fact that UL transmit filters design of UE i  depends on the weighting matrix W i ,  FIG. 4  illustrates how the weighting matrix W i  is calculated at UE i  following equation (0) when the objective is the maximization of the system weighted sum of data rates with constraints on the total transmitted power at BSs. The proper design of W i  depends on the user priority coefficient of UE i μ i  ( 403 ) and on its MSE-previous matrix Ē i  that can be calculated using (0), which is obtained at UE i  based on the knowledge of the equivalent complex channel matrix towards its associated BS H i,i T i  ( 401 ), and the interference-plus-noise covariance matrix N i  ( 402 ). 
       Embodiment 2 
       [0115]      FIG. 6  illustrates the block diagram corresponding to Embodiment 2 as applied to a wireless cellular communication system. First, each base station BS i  acquires or updates the knowledge of the complex channel matrix towards its own UE i  H i,i  ( 601 ), which contains the channel gains between each UE antenna element and each BS antenna element. The channel state acquisition can be done either at the BS by exploiting channel reciprocity or at the UE and then reported to the BS. Once this information is available at BS i , the DL transmit and receive filters are calculated in a decentralized manner as it is described in Embodiment 1 and  FIG. 2 , and DL data transmission is carried out ( 602 ). Based on the knowledge of the DL receive filter at each UE, the UL transmit filter is designed as it is described in Embodiment 1 and  FIG. 3  ( 603 ) and an UL non-data transmission is carried out ( 604 ). The UL non-data transmission by non-associated UE j  allows BS i  to estimate the interference cost matrix based on the covariance matrix of the received UL interference-plus-noise signal and, based on this estimation, the DL transmit filters are updated in a decentralized manner ( 605 ) as it is described in Embodiment 1 and  FIG. 2 . Then, previous steps are repeated from  602  to  605 . 
         [0116]      FIG. 5   a  illustrates these steps as applied to a TDD wireless communication system.  501 ,  503 ,  505  and  507  correspond to intervals devoted for DL data transmissions;  502 ,  504  and  506  refer to intervals allocated for UL non-data transmissions. Each time that an UL non-data transmission (short interval duration) is carried out, the DL transmit and receive filters are updated and system performance (in terms of weighted sum of data rates) is increased. 
       Embodiment 3 
       [0117]      FIG. 7  illustrates the block diagram corresponding to Embodiment 3 as applied to the DL of a wireless cellular communication system. First, each base station BS i  acquires or updates the knowledge of the complex channel matrix towards its own UE i  H i,i  ( 701 ), which contains the channel gains between each UE antenna element and each BS antenna element. The channel state acquisition can be done either at the BS by exploiting channel reciprocity or at the UE and then reported to the BS. Once this information is available at BS i , the DL transmit filters are calculated in a decentralized manner as it is described in Embodiment 1 and  FIG. 2 , and DL non-data transmission is carried out ( 702 ). Based on the knowledge of the received interference-plus-noise at each UE, the UL transmit filter is designed as it is described in Embodiment 1 and  FIG. 3  ( 703 ) and an UL non-data transmission is carried out ( 704 ). The UL non-data transmission allows BS i  to estimate the interference cost matrix based on the covariance matrix of the received UL interference-plus-noise signal and, based on this estimation, the DL transmit filters are updated in a decentralized manner as it is described in Embodiment 1 and  FIG. 2  ( 705 ). The DL and UL non-data transmissions are repeated until the optimal performance is achieved or the maximum allowed number of iterations is reached. Then, DL data transmission is carried out ( 707 ), where transmit and receive filters are computed in a decentralized manner as it is described in Embodiment 1 and  FIG. 2  as long as the channel coefficients do not change. 
         [0118]      FIG. 5   b  illustrates these steps as applied to a TDD wireless communication system.  508 ,  510  and  512  correspond to intervals devoted for DL non-data transmissions;  509 ,  511  and  513  refer to intervals allocated for UL non-data transmissions; and  514  corresponds to the interval devoted for DL data transmission. Each time that an UL non-data transmission is carried out, the DL transmit filters are updated and they are used for DL non-data transmission in order to update the DL receive filters. This alternate DL and UL non-data transmission can be repeated one or several times until, at the end, DL data transmission is carried out. 
       Embodiment 4 
       [0119]      FIG. 8  illustrates the block diagram corresponding to Embodiment 4 as applied to a wireless cellular communication system. First, each base station BS i  acquires or updates an estimation of the complex channel matrix towards its own user i H i,i  ( 801 ), which contains the channel gains between each UE antenna element and each BS antenna element. The acquisition of the channel state estimation can be done either at the BS by exploiting channel reciprocity or at the UE and then reported to the BS. Once this information is available at BS i , the downlink transmit and receive filters are calculated in a decentralized manner as it is described in Embodiment 1 and  FIG. 2 , and DL data transmission is carried out ( 802 ). Based on the knowledge of the DL receive filter at each UE, the UL transmit filter and the UL receive filter are designed as it is described in Embodiment 1 and  FIG. 3  ( 803 ) and UL data transmission is carried out ( 804 ). The UL data transmission in addition to exchange data with BS it allows BS i  to estimate the interference cost matrix based on the covariance matrix of the received UL interference-plus-noise signal. 
         [0120]    During UL data transmission reference signals should be considered in order that each BS could estimate the actual transmit filter used by its associated UE and, hence, receive the data. 
         [0121]    The DL transmit filters are updated in a decentralized manner as it is detailed in Embodiment 1 and  FIG. 2  ( 805 ). Then, previous steps are repeated from  802  to  805 . 
         [0122]      FIG. 5   c  illustrates these steps as applied to a TDD wireless communication system.  515 ,  517  and  519  correspond to intervals devoted for DL data transmissions;  516  and  508  refer to intervals allocated for UL data transmissions. Each time that an UL data transmission is carried out, the DL transmit and receive filters are updated and system performance is increased. 
       Simulation Results 
       [0123]    The simulated scenario follows the Small Cell Scenario #2a in [3GPP-SCE]. A list of parameters is provided in  FIG. 9 . 
         [0124]    The simulations targeted the Scenario #2a with one cluster per macrocell area and 4 or 10 small cells per cluster. In [3GPP-SCE], the cluster defines the area where small cells are deployed.  FIG. 10  shows the cluster, macrocell, small cell and user location for each layout configuration. 
         [0125]    The following interference management techniques are evaluated:
       Decentralized Coordinated Beamforming with precoding design based on Channel       
 
         [0127]    Reciprocity for interference coordination (Decent. CB with CR in figures): interference management technique detailed in Embodiment 1.
       Decentralized Beamforming without Interference Management (Decent. B without IM in figures): No management of interference is performed, each BS designs its transmit filters to combat the received noise plus interference at its associated UE.   Enhanced Inter-Cell Interference Coordination with Time Domain Muting and a muting ratio equal to 5/10 (eICIC TDM 5/10 in figures): interference is managed thanks to almost blank subframes, in which each BS is muted 5 subframes and transmits the other 5 subframes.       
 
         [0130]    The performances of said techniques are evaluated for users served by small cells in the 3.5 GHz band. 
         [0131]    The performance indicator is Sum Throughput (ST) per macrocell area measured in bits/s/Hz and defined as: 
         [0000]      ST=total amount of data for all users in 3.5 GHz/total amount of observation time/total amount of bandwidth/number of macrocells 
         [0132]      FIG. 11  shows the ST achieved on each resource block (RB) or subband for the layout configurations depicted in  FIG. 10 . Significant ST gains are observed thanks to the proposed interference management technique for precoding design based on channel reciprocity. 
       Application to the 3Gpp Lte-Advanced Release 11 
       [0133]    Embodiments 2 and 3 of the present invention use a UL non-data transmission to get the desired information for DL precoding design. Said step can be done in LTE-Advanced standard by using the already defined UL Sounding Reference Signals (SRS) properly configured. 
       SRS Configuration 
       [0134]    The SRS configuration involves:
       Periodicity of the SRS: 2, 5, 10, 20, 40, 80, 160 or 320 ms;   SRS mode: frequency-hopping mode or wide-band mode. In the frequency-hopping mode the SRS are transmitted on a specific subband, while in the wide-band mode the SRS are transmitted in all the UL bandwidth; and   Hopping scheduling (in case of frequency-hopping mode), which indicates the subband to be used by the UE at each time instant.       
 
         [0138]    For the embodiments of the present invention, the proper configuration of the SRS involves frequency-hopping mode because the sounding has to be different for each subband the user is scheduled to as the DL interference to be managed varies among subbands. The periodicity of the SRS and the hopping scheduling depend on the specific method to be adopted. However, as SRS are allocated to the last OFDM symbols of the Uplink Pilot Time Slot (UpPTS) in the synchronization (SYNC) subframe in a TDD system [3GPP-SCE], the minimum periodicity of the SRS is 5 ms which corresponds to the DL-to-UL switch-point periodicity. 
         [0139]    The limitations in the LTE-Advanced release 11 are:
       Component carrier specific UL power control is available, but there is not component subband specific UL power control. So, for a given time instant, the user can only do the UL sounding adjusting the UL transmit filter in a specific subband. In case that component subband specific UL power control was available, a user could be assigned to multiple subbands and do the UL sounding for these subbands simultaneously in time.   The baseline for SRS operation is non-precoded and antenna-specific, i.e. SRS are transmitted using only one antenna. This fact implies that, as the UE has more than 1 DL receive antenna, multiple SRS transmissions multiplexed in time are needed (the same as the number of DL receive antennas) to get the desired UL signal at the cost of increasing the received noise. If a new type of SRS was defined, only 1 SRS transmission would be required independently of the number of DL receive antennas at the UE.   The current SRS have a minimum length equivalent to 4 RBs. If a new type of SRS was defined, more sounding granularity could be obtained to adapt the design according to channel variations.       
 
         [0143]    In the following, different methods are described based on the modifications that the LTE-Advanced standard could admit or not. For each method, assumptions, SRS configuration, requirements and implications are detailed in  FIG. 12 . Small Cell Scenario #2a in [3GPP-SCE] and a low mobility scenario are used, such that the channel coherence time (T c ) is 25 ms at 3.5 GHz band and user speed of 3 Km/h. Then, the maximum SRS periodicity is 20 ms. The total bandwidth is described by the number of resource blocks (RBs): B=16, and S refers to the number of subbands in which the B RBs are divided. 
         [0144]    Methods A and B in the sequel do not require any modification in LTE-Advanced release 11. 
         [0145]    In Method A, the sounding is done in all subbands simultaneously in time, so that each UE can only be scheduled to a single subband as it is not able to apply component subband power control. 
         [0146]    In Method B, the sounding for each subband is done in different time instants, hence implying that a specific UE could be scheduled to multiple subbands but the number of subbands is limited by the channel coherence time. 
         [0147]    Method C uses component subband specific UL power control. Such procedure becomes independent of the number of subbands and allows more flexibility in the user scheduling process and SRS periodicity than methods A and B. 
         [0148]    Method D uses component subband specific UL power control and the definition of a new type of SRS including precoding and a minimum length equivalent to 1 RB. Method D is non-dependent of the number of subbands, the number of receive antennas at user nor inter-subband channel variations, hence allowing more flexibility in the user scheduling process, SRS periodicity and sounding granularity than previous methods. 
         [0149]      FIG. 13   a  and  FIG. 13   b  illustrate Method A and Method B, respectively, in a time/frequency grid for B=16 RBs and S=4 subbands. Assuming that a specific BS has 2 users to be scheduled, a possible scheduling of UEs in the available subbands is shown. 
         [0150]      FIG. 14   a  displays Method C in a time/frequency grid for B=16 RBs and S=4 subbands.  FIG. 14   b  shows Method D in a time/frequency grid for B=16 RBs and S=16 subbands. Assuming that a specific BS has two users (user  1  and user  2 ) to be scheduled, a possible scheduling of users in the available subbands is shown. 
       Practical Implementation 
       [0151]    Embodiment 2 of the present invention is the most suitable procedure for implementation in 3GPP LTE-Advanced, as the use of UL non-data transmission to broadcast the interference cost allows not degrading UL data transmission and the presented procedure is able to dynamically include new users appearing and disappearing in the system. In this regard,  FIG. 15  provides the detailed steps as applied to a TDD LTE-Advanced wireless communication system. The transmission is carried out in five slots alternating DL and UL transmissions, each including control-plane and data-plane information transmission. 
       1 St  Slot (DL): 
       [0000]    
       
         
           
             Control-plane transmission ( 1501 ): DL control-plane information is broadcasted, from which the user updates the receive filter R 0 ; 
             Data-plane transmission ( 1502 ): DL data transmission is carried out by means of transmit filter T 0  at a specific MCS 0 ; 
           
         
       
     
       2 nd  Slot (UL): 
       [0000]    
       
         
           
             Sounding reference signals transmission ( 1503 ): BSs acquire knowledge of the interference cost matrix to properly update transmit filters T 1  in next DL slot ( 1506 ,  1507 ); 
             Control-plane transmission ( 1504 ): users communicate to the serving BS the most suitable MCS 1  to be applied in the next DL slots ( 1507 ,  1511 ); 
             Data-plane transmission ( 1505 ): UL data transmission is carried out; 
           
         
       
     
       3 rd  Slot (DL): 
       [0000]    
       
         
           
             Control-plane transmission ( 1506 ): update of the DL receive filter R 1 ; 
             Data-plane transmission ( 1507 ): DL data transmission is carried out with DL transmit filter T 1  designed at  1503 , and MCS 1  reported at  1504 ; 
           
         
       
     
       4 th  Slot (UL): 
       [0000]    
       
         
           
             Control-plane transmission ( 1508 ): MCS 2  to be applied in the next DL slot ( 1510 ,  1511 ) is reported; 
             Data-plane transmission ( 1509 ): UL data transmission is carried out 
           
         
       
     
       5 th  Slot (DL): 
       [0000]    
       
         
           
             Control-plane transmission ( 1510 ): update of the DL receive filter R 2 ; 
             Data-plane transmission ( 1511 ): DL data transmission is carried out with DL transmit filter T 1  designed at  1503 , and MCS 2  reported at  1508 ; 
           
         
       
     
         [0163]    While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention. 
       REFERENCES 
       [0000]    
       
         [3GPP-SCE] 3GPP R1-130856, Evaluation Assumptions for SCE. 
         [3GPP-TR36829] 3GPP Technical Report 36.829, “Enhanced performance requirement for LTE User Equipment (UE)”, v11.1.0 Release 11, January 2013. 
         [Christensen-08] S. Christensen, R. Agarwal, E. Carvalho, J. M. Cioffi, “Weighted Sum-Rate Maximization using Weighted MMSE for MIMO-BC Beamforming Design,”  IEEE Trans. on Wireless Commun ., vol. 7, no. 12, pp. 4792-4799, December 2008. 
         [Shi-11] Q. Shi, M. Razaviyayn, Z. Luo, C. He, “An Iteratively Weighted MMSE Approach to Distributed Sum-Utility Maximization for a MIMO Interfering Broadcast Channel”, IEEE Trans. on Signal Processing, vol. 59, no. 9, pp. 4331-4340, September 2011. 
         [Kelly-98] F. P. Kelly, A. K. Maulloo, D. K. H. Tan, “Rate control in communication networks: shadow prices, proportional fairness and stability”,  Journal of the Operational Research Society , vol. 49, April 1998.