System and method for improving the robustness of spatial division multiple access via nulling

[0001] This application is a continuation of U.S. patent application Ser. No. 11/695,575 filed Apr. 2, 2007, entitled “System and Method for Improving the Robustness of Spatial Division Multiple Access Via Nulling,” now issued as U.S. Pat. No. 7,450,673 on Nov. 11, 2008, which in turn claims priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application Ser. No. 60/836,716, filed Aug. 10, 2006, and entitled “SDMA with Robust Co-Channel Interference Nulling through Wide Beam-Width Multi-User Beamforming,” the entire contents of each of which are hereby incorporated by reference.

[0002] A communication channel in a wireless communication network can be shared by different wireless stations in the network. One example of channel sharing is that wireless stations, such as customer premises equipment (CPEs), transmit signals on the same frequency at different times or on different frequencies at the same time.

[0003] A wireless communication network that employs spatial division multiple access (SDMA) utilizes spatial diversity to increase the capacity of a network. In such a system, the CPEs sharing the same communication channel transmit signals on the same frequency at the same time.

[0004] In order to prevent the signals transmitted by the CPEs on the same frequency at the same time from interfering with one another, a base transceiver station (BTS) needs to isolate the signals in such a way that they will not reach unintended wireless stations. In other words, these CPEs must be able to reliably detect and retrieve the signals that are sent to them.

[0005] There are two common methods to provide isolation among the CPEs sharing the same communication channel in a wireless communication network that employs SDMA. They are polarization isolation and spatial isolation. Polarization isolation is a more technically challenging method, and yet, it only provides a limited degree of isolation among the CPEs. In an environment with severe multi-path, polarization isolation only provides a difference of 5 to 10 dB in gain between the signals and interference.

[0006] An antenna array system on a BTS in a wireless communication network provides a practical solution for spatial isolation. The BTS selects a set of CPEs to participate in SDMA such that the degree of isolation among them is greater than a predetermined threshold. Spatial isolation among CPEs is achieved by using beamforming and interference nulling for antenna arrays.

[0007] For example, in a system employing SDMA, the BTS determines the spatial signatures of CPEs A and B, which are identified as candidates for sharing a communication channel, and generates a different beamforming weighting vector for CPEs A and B by using their spatial signatures jointly.

[0008] When the BTS transmits a signal to CPE A, the beamforming weighting vector of CPE A is applied to the antenna array. The antenna beam pattern created with the beamforming weighting vector has a nulling angle positioned toward the direction of arrival (DOA) of the antennae beam pattern of CPE B, i.e., CPE A will not receive signals intended for CPE B. The same mechanism is also applied to CPE B. The method described above is called SDMA via nulling.

[0009] One issue related to an SDMA via nulling method is that the effectiveness of antenna nulling is very sensitive to the accuracy of the beamforming weighting vector generated from the spatial signatures. If the beamforming weighting vector is not accurate enough, employing an SDMA operation might not lead to an improvement in system capacity. It might even make the channel unusable for the CPEs sharing the same channel, which subsequently reduces the overall capacity of the wireless communication network.

[0010] For example, in order to support 16 QAM modulation in a wireless network employing SDMA, each CPE must have an SINR greater than 20 dB. Assume that CPEs A and B both have an SINR greater than 20 dB and both support 16 QAM modulation before sharing a communication channel. If the wireless communication network employing SDMA via nulling cannot provide an SINR greater than 20 dB for both CPEs A and B, employing SDMA will bring down the communication channel for both of them.

[0011] SDMA via nulling eliminates co-channel interference (CCI) by applying beamforming weighting vectors of the CPEs that are almost orthogonal to each other. The effectiveness of the elimination of the CCI by employing SDMA via nulling depends on the accuracy of the spatial signatures of a CPE.

[0012] However, in reality, the spatial signatures calculated from receiving signals are never ideal; therefore, it is not uncommon for a CCI leakage to occur in the wireless communication network employing SDMA via nulling. A CCI leakage produces a fixed noise level and puts a hard limit on the bit error rate (BER) of the wireless communication network. As such, what is desired is a system and method for providing a robust SDMA via nulling.

[0013] The present invention discloses a base transceiver station (BTS) equipped with a plurality of antennas for improving the robustness of spatial division multiple access via nulling. The BTS comprises of a first matrix module receiving a plurality of signals from one or more customer premises equipments (CPEs) through the plurality of antennas and producing correspondingly a first plurality of covariance matrices representing the plurality of signals, a second matrix module receiving the first plurality of covariance matrices and generating correspondingly a set of derivative spatial signature matrices representing the CPEs respectively, a third matrix module receiving the derivative spatial signature matrices and producing correspondingly a second plurality of covariance matrices representing interferences of the CPEs, and an eigenvector module generating a plurality of beamforming vectors for the CPEs from the plurality of derivative spatial signature matrices and the second plurality of covariance matrices.

[0014] The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

[0020] The following detailed description of the invention refers to the accompanying drawings. The description includes exemplary embodiments, not excluding other embodiments, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

[0021] The present invention discloses a system and method that improves the robustness of spatial division multiple access (SDMA) via nulling. The method disclosed in the present invention uses novel sets of the spatial signatures of customer premises equipments to generate beamforming weighting vectors for the CPEs to share a communication channel.

[0022] Rather than using the spatial signatures calculated from the receiving signals of a CPE to generate a beamforming weighting vector, the method disclosed in the present invention calculates derivative spatial signature matrices of a CPE and subsequently produces a covariance matrix of interference. A beamforming weighting vector is generated by using the derivative spatial signature matrices and the covariance matrix of interference of the CPEs sharing the same communication channel.

[0023] By applying a beamforming weighting vector generated by the aforementioned method to an antenna array on a base transceiver station, the antenna beam pattern of a CPE has a wider nulling angle positioned toward the direction of co-channel interference. The wider nulling angle makes an SDMA via nulling method more robust, because a small error in a covariance matrix of interference has less effect on the efficiency of the method.

[0024] FIGS. 1A and 1B illustrate a system that generates beamforming weighting vectors for the CPEs sharing a communication channel in a wireless communication network employing SDAM via nulling. FIG. 1A is a block diagram illustrating the first part of the system that calculates derivative spatial signature matrices for each CPE. FIG. 1B is a block diagram illustrating the second part of the system that generates beamforming weighting vectors for the CPEs sharing the communication channel.

[0025] FIG. 1A shows five modules: a receiver module 110, a covariance matrix module 120, a spatial signature module 130, a derivative spatial signature matrix module 140, and a memory module 150. Assume that there are L CPEs sharing a communication channel.

[0026] The m antennas on a BTS receives a signal transmitted from CPE k at a receiving period i, and the BTS forms a vector of receiving signals

[0027] $Y_i^k=\left[\begin{array}{c}{y}_{i1}^k\\ y_{i2}^k\\ ⋮\\ y_{\mathrm{im}}^k\end{array}\right]112,$
where kε{1, . . . , L) and y_ij^k is the receiving signals received by antenna j at a receiving period i, where jε{1, . . . , M). The vector 112 is stored in the memory module 150. The receiver module 110 receives signals continuously and all the receiving vectors 112 are stored in the memory module 150.

[0028] The covariance module 120 takes a set of N^k receiving vectors 112 of CPE k from the memory module 150 and produces a covariance matrix of receiving signals 122 according to the following equation:

[0029] ${\mathrm{COV}}^k=\frac{1}{N^k}\sum _{i=1}^{N^k}\left[\begin{array}{c}{y}_{i1}^k\\ y_{i2}^k\\ ⋮\\ y_{\mathrm{im}}^k\end{array}\right]\left[\begin{array}{cccc}{y}_{i1}^{k^{*}}& y_{i2}^{k^{*}}& \dots & y_{\mathrm{im}}^{k^{*}}\end{array}\right],$
where (y_im^k)* is the conjugate-transpose of y_im^k. The covariance matrix of receiving signals COV^k 122 is stored in the memory module 150. The covariance matrix module produces a covariance matrix of receiving signals continuously and all the covariance matrices 122 are stored in the memory module 150.

[0030] The spatial signature module 130 calculates a spatial signature 132 of CPE k by using the covariance matrix of receiving signals 122. The spatial signatures 132 are stored in the memory module 150. The spatial signature module calculates spatial signatures continuously and all spatial signatures are stored in the memory module 150.

[0031] The derivative spatial signature matrix module 140 calculates a set of s^k derivative spatial signature matrices 142 of CPE k from a set of spatial signatures 132 calculated by the spatial signature module 130. The set of derivative spatial signature matrices 142, denoted as {R₁^k, . . . , R_sk^k}, is stored in the memory module 150.

[0032] The BTS uses the system described in FIG. 1A to calculate a set of derivative spatial signature matrices of every CPE while the system described in FIG. 1B uses the derivative spatial signature matrices of all L CPEs to generate the beamforming weighting vectors of all L CPEs sharing a communication channel in a wireless communication network employing SDMA via nulling.

[0033] FIG. 1B shows a beamforming weighting vector module 160, which is composed of two modules: an interference covariance module 162 and an eigenvector module 166. The beamforming weighting vector module 160 generates the beamforming weighting vector of CPE k by using the derivative spatial signature matrices of a set of L CPEs.

[0034] The interference covariance module 162 produces a covariance matrix of interference 164 of CPE k by using the derivative spatial signature matrices of all L CPEs, excluding CPE k, according to the following equation:

[0035] $\sum _{j=1,j\ne k}^L\left(\frac{1}{s^j}\sum _{i=1}^{s^j}{R}_i^j{R}_i^{j^H}\right),$
where R_i^jH is the conjugate-transpose of R_i^j. Lines 154 and 156 depict two of the derivative spatial signature matrices of CPEs, excluding CPE k, while a line 152 depicts the derivative spatial signature matrix of CPE k. These derivative spatial signature matrices are retrieved from the memory module 150.

[0036] Based on the covariance matrix of interference 164 and the derivative spatial signature matrices 152, the eigenvector module 166 generates a beamforming weighting vector W^k 168 from the following eigenvalue equation:

[0037] ${\left(\sum _{j=1,j\ne k}^L\left(\frac{1}{s^j}\sum _{i=1}^{s^j}{R}_i^j{R}_i^{j^H}\right)+{\sigma }_n^{2}I\right)}^{-1}\left(\sum _{i=1}^{s^k}{R}_i^k{R}_i^{k^H}\right)).$
W^k is the eigenvector corresponding to the largest eigenvalue of the equation.

[0038] The method to obtain beamforming weighting vectors in a wireless communication network employing the SDMA via nulling is applicable to other wireless communication networks that support multiple access, such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), orthogonal frequency division multiplex multiple access (OFDM-MA) and any combinations of the above. In addition, frequency division duplex (FDD) and time division duplex (TDD) also allow multiple access in a wireless communication network.

[0039] FIGS. 2A and 2B depict a system module 200 that calculates derivative spatial signature matrices of a CPE, an interference covariance module 230, and an eigenvector module 240. The system module 200 includes a receiver module 210 of a BTS in an OFDMA-based WiMax system, a covariance matrix module 220, and a memory module 250.

[0040] FIGS. 2A and 2B show two applications of the system and method disclosed in the present invention in an OFDMA-based WiMax system with TDD employing SDMA via nulling. Assume CPEs A and B share one communication channel in a wireless communication network employing SDMA via nulling. In an OFDM system, a receiving signal is denoted as a unit of symbols.

[0041] In FIG. 2A, the m antennas on the BTS receive OFDM symbols transmitted from CPE A at a receiving period i, and the BTS forms a vector of receiving signals 212, denoted as

[0042] $Y_i^A=\left[\begin{array}{c}{y}_{i1}^A\\ y_{i2}^A\\ ⋮\\ y_{\mathrm{im}}^A\end{array}\right],$
where the receiving symbols received by an antenna k are shown as y_ik^A, where kε{1, . . . , m). The vector 212 is stored in the memory module 250. The receiver module 210 receives OFDM symbols from CPE A continuously and all receiving vectors 212 are stored in the memory module 250.

[0043] The same operation is also applied to CPE B. A vector of receiving OFDM symbols 214 at time j, is denoted as

[0044] $Y_j^B=\left[\begin{array}{c}{y}_{j1}^B\\ y_{\mathrm{j2}}^B\\ ⋮\\ y_{\mathrm{jm}}^B\end{array}\right],$
where the receiving symbols received by antenna j are shown as y_jk^B, where kε{1, . . . , m). The receiver module 210 receives OFDM symbols from CPE B continuously and all receiving vectors 214 are stored in the memory module 250.

[0045] The covariance matrix module 220 takes a set of N^A receiving vectors 212 of CPE A from the memory module 250 and produces a covariance matrix of receiving signals of CPE A according to the following equation:

[0046] ${\mathrm{COV}}^A=\frac{1}{N^A}\sum _{i=1}^{N^A}\left[\begin{array}{c}{y}_{i1}^A\\ y_{i2}^A\\ ⋮\\ y_{\mathrm{im}}^A\end{array}\right]\left[\begin{array}{cccc}{y}_{i1}^{A^{*}}& y_{i2}^{A^{*}}& \dots & y_{\mathrm{im}}^{A^{*}}\end{array}\right],$
where (y_im^A)* is the conjugate-transpose of (y_im^A). The covariance matrix of receiving signals COV^A 222 is stored in the memory module 250. The covariance matrix module 220 produces a covariance matrix of receiving signals of CPE A continuously, and all the covariance matrices of receiving signals 222 are stored in the memory module 250.

[0047] The same operation is also applied to CPE B. A covariance matrix of receiving signals of CPE B is produced according to the following equation:

[0048] ${\mathrm{COV}}^B=\frac{1}{N^B}\sum _{i=1}^{N^B}\left[\begin{array}{c}{y}_{i1}^B\\ y_{i2}^B\\ ⋮\\ y_{\mathrm{im}}^B\end{array}\right]\left[\begin{array}{cccc}{y}_{i1}^{B^{*}}& y_{i2}^{B^{*}}& \dots & y_{\mathrm{im}}^{B^{*}}\end{array}\right],$
where (y_im^B)* is the conjugate-transpose of (y_im^B). COV^B 224 is stored in the memory module 250. The covariance matrix module 220 produces a covariance matrix of receiving signals of CPE B continuously, and all the covariance matrices of receiving signals 224 are stored in the memory module 250. The memory module 250 has a set of m+1 covariance matrices of receiving signals 252 of CPE A, denoted as {COV₁^A, COV₂^A, . . . , COV_m^A, COV^A}, and a set of m+1 covariance matrices of receiving signals 254 of CPE B, denoted as {COV₁^B, COV₂^B, . . . , COV_m^B, COV^B}.

[0049] Using the covariance matrices of receiving signals 254 of CPE B, the interference covariance module 230 produces a covariance matrix of interference 232 of CPE A according to the following equation:

[0050] $\sum _{i=1}^m{\mathrm{Cov}}_i^B+{\mathrm{Cov}}^B.$
Similarly, the interference covariance module 230 uses the covariance matrices of receiving signals 252 of CPE A to produce a covariance matrix of interference 234 of CPE B according to the following equation:

[0051] $\sum _{i=1}^m{\mathrm{Cov}}_i^A+{\mathrm{Cov}}^A.$

[0052] Using the covariance matrix of interference 232 and the last covariance matrix of receiving signals COV^A in 252 of CPE A, the eigenvector module 240 generates a beamforming weighting vector 242 of CPE A, denoted as W^A, using the following eigenvalue matrix:

[0053] ${\left[\frac{1}{m+1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^B+{\mathrm{Cov}}^B\right)+{\sigma }_n^{2}I\right]}^{-1}\left({\mathrm{Cov}}^A\right).$
W^A is the eigenvector corresponding to the largest eigenvalue of the matrix.

[0054] Similarly, using the covariance matrix of interference 234 and the last covariance matrix of receiving signals COV^B in 254 of CPE B, the eigenvector module 240 generates a beamforming weighting vector 244 of CPE B, denoted as W^B, using the following eigenvalue matrix:

[0055] ${\left[\frac{1}{m+1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^A+{\mathrm{Cov}}^A\right)+{\sigma }_n^{2}I\right]}^{-1}\left({\mathrm{Cov}}^B\right).$
W^B is the eigenvector corresponding to the largest eigenvalue of the matrix.

[0056] FIGS. 2A and 2B use the same system module 200. The difference between FIG. 2A and FIG. 2B is that in FIG. 2B the eigenvector module 240 generates the beamforming weighting vectors of CPE A and B by using the covariance matrices of interference 232 and 234.

[0057] A beamforming weighing vector 246 of CPE A, denoted as W^A, is generated using the following eigenvalue matrix:

[0058] ${\left[\frac{1}{m+1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^B+{\mathrm{Cov}}^B\right)+{\sigma }_n^{2}I\right]}^{-1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^A+{\mathrm{Cov}}^A\right).$
W^A is the eigenvector corresponding to the largest eigenvalue of the matrix. Similarly, a beamforming weighting vector 248 of CPE B, denoted as W^B, is generated using the following eigenvalue matrix:

[0059] ${\left[\frac{1}{m+1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^B+{\mathrm{Cov}}^B\right)+{\sigma }_n^{2}I\right]}^{-1}\left(\sum _{i=1}^m{\mathrm{Cov}}_i^B+{\mathrm{Cov}}^B\right).$
W^B is the eigenvector corresponding to the largest eigenvalue of the matrix.

[0060] FIG. 3 is another application of the system and method disclosed in the present invention in an OFDMA-based WiMax system with TDD employing SDMA via nulling. FIG. 3 depicts a system 300, which is the same as the system 200 in FIG. 2A, a transformation matrix module 310, a derivative spatial signature matrix module 320, an interference covariance matrix module 330, and an eigenvector module 340.

[0061] The memory module 250 in the system 300 has a set of m+1 covariance matrices of receiving signals {COV₁^A, COV₂^A, . . . , COV_m^A, COV^A} for CPE A and a set of m+1 covariance matrices of receiving signals {COV₁^B, COV₂^B, . . . , COV_m^B, COV^B} for CPE B.

[0062] Using the m+1 covariance matrices of receiving signals of CPE A, the transformation matrix module 310 produces m transformation matrices 312 for CPE A, denoted as T^A, based on the following equations: T_i^A=COV_i+1^A(COV_i^A)⁻¹, where iε{1, . . . , m−1), and T_m^A=COV^A(COV_m^A)⁻¹. If (COV_m^A)⁻¹ does not exist, the m-th transformation matrix T_m^A is produced based on the following equation: T_m^A=COV_m+1^A(COV_m^AH COV_m^A)⁻¹COV_m^AH. The transformation matrices 312 are stored in the memory module 250.

[0063] Similarly, the transformation matrix module 310 uses the M+1 covariance matrix of receiving signals of CPE B to produce m transformation matrices 314 for CPE B, denoted as T^B, based on the following equations: T_i^B=COV_i+1^B(COV_i^B)⁻¹, where iε{1, . . . , m−1), and T_m^B=COV^B(COV_m^B)⁻¹. If (COV_m^B)⁻¹ does not exist, the m-th transformation matrix T_m^B is produced based on the following equation: T_m^B=COV_m+1^B(COV_m^BHCOV_m^B)⁻¹COV_m^BH. The transformation matrices 314 are stored in the memory module 250.

[0064] The derivative spatial signature matrix module 320 calculates a set of n derivative spatial signature matrices 322 from the set of transformation matrices 312 of CPE A according to the following equation: R_i^A=T_i^ACov^A, where iε{1, . . . , n) and n≦m. The last matrix in the set of the covariance matrices of receiving signals is COV^A. The set of derivative spatial signature matrices 322 is stored in the memory module 250.

[0065] The derivative spatial signature matrix module 320 calculates a set of n derivative spatial signature matrices 324 from the set of transformation matrices 314 of CPE B according to the following equation: R_i^B=T_i^BCov^B, where iε{1, . . . , n) and n≦m. The last matrix in the set of the covariance matrices of receiving signals is COV^B. The set of derivative spatial signature matrices 324 is stored in the memory module 250. The number of derivative spatial signature matrices for each CPE is predetermined according to the requirements of the wireless communication network.

[0066] Using the derivative spatial signature matrices 324 of CPE B, the interference covariance matrix module 330 produces a covariance matrix of interference 332 of CPE A according to the following equation:

[0067] $\sum _{i=1}^n{R}_i^B.$
Similarly, the interference covariance matrix module 330 uses the derivative spatial signature matrices 322 of CPE A to produce a covariance matrix of interference 334 of CPE B according to the following equation:

[0068] $\sum _{i=1}^n{R}_i^A.$

[0069] The eigenvector module 340 generates the beamforming weighting vectors of CPEs A and B by using the covariance matrices of interference 332 and 334. A beamforming weighting vector 342 of CPE A, denoted as W^A, is generated using the following eigenvalue matrix:

[0070] ${\left[\frac{1}{m}\left(\sum _{i=1}^m{R}_i^B\right)+{\sigma }_n^{2}I\right]}^{-1}\left(\sum _{i=1}^m{R}_i^A\right).$
W^A is the eigenvector corresponding to the largest eigenvalue of the matrix. In the same fashion, a beamforming weighting vector 344, of CPE B, denoted as W^B, is generated using the following eigenvalue matrix:

[0071] ${\left[\frac{1}{m}\left(\sum _{i=1}^m{R}_i^A\right)+{\sigma }_n^{2}I\right]}^{-1}\left(\sum _{i=1}^m{R}_i^B\right).$
W^B is the eigenvector corresponding to the largest eigenvalue of the matrix.

[0072] The method disclosed in the present invention can reduce the noise caused by a CCI leakage by a significant level and is superior to existing methods. The method disclosed in the present invention increases the robustness of SDMA via nulling by creating an antenna beam pattern that has a wider nulling angle positioned toward the DOA of CCI.

[0073] The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.

[0074] Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.