多点协作系统中异步干扰特性及其预编码设计

如图4所示为四种情况下用户信噪比(Es/No)与系统平均频谱效率((b/s)/Hz)变化关系。从图3和图4可知，理想的多基站同步协作模式获得的系统均方误差特性和平均频谱效率最好，在异步环境中受制于信号异步干扰影响的多基站协作传输所获得的系统性能明显降低。但若能进一步考虑系统中的信号传输异步特性，并通过一定的预编码矩阵联合优化设计进行预处理，则可明显改善系统性能，有利于减小异步干扰信号的影响。
从以上讨论可看出，即使在完美的定时提前机制下能保证各协作基站到达目标用户期望信号是同步的，但在实际应用中，由于不同的路径传输时延，也很难确保来自其他用户的干扰信号与期望信号同时到达目标用户，因此在协作多基站联合向多用户发射数据时不可避免地会产生干扰信号传输的异步性。相应地，若能在Co-MP系统中充分得知各干扰信号传输的时间异步结构，并联合一定的预编码优化设计准则进行预处理，则可减小异步干扰对系统性能的影响。特别是在高速率数据传输情况下，更应全面而充分地考虑协作系统中各信号传输的时间异步特性。若系统存在定时提前误差，则还应进一步考虑定时抖动对目标用户期望信号的接收影响。此外，在考虑信号传输异步特性基础上如何找到一种低复杂度、高可靠性的预编码优化设计算法，还值得进一步深入研究。
参考文献
[1] DAVID G, MANSOOR S, SHIU Da Shan,et al. From theory to practice: an overview of MIMO space-time coded wireless system [J]. IEEE J. Select. Areas Commun, 2003,21(3): 281-302.
[2] DAI H, MOLISCH A F, POOR H V. Downlink capacity of interference-limited MIMO systems with joint detection[J].IEEE Trans. Wireless Commun., 2004, 3(2): 442-453.
[3] SOMEKH O, ZAIDEL B M, SHAMAI S. Sum rate characterization of joint multiple cell-site processing[J]. IEEE Tran. on Info. Theory, 2007,53(12):4473-4497.
[4] Further Advancements for E-UTRA Physical Layer Aspects(Release 9)[S]. 3GPP Technical Specification, TR 36.814 V0.4.1, Feb. 2009. [Online]. Available: http://www.3gpp.org.
[5] XIAO Shang Hui, ZHANG Zhong Pei, SHI Zhi Pei. Clustered multi-point coordinating transmission systems with the Reduced Overhead [J]. Journal of Communications. 2010, 5(6):493-500.
[6] Boon Loong Ng, JAMIE S, STEPHEN E, HANLY V,et al. Distributed downlink beamforming with cooperative base stations[J]. IEEE Trans. on Information Theory, 2008,54(12): 5491-5499.
[7] SIMEONE O, SOMEKH O, POOR H V, et al. Downlink multicell processing with limited-backhaul capacity[J].
EURASIP J. on Adv. in Signal Process., Article ID:840814, 2009:1-10.
[8] 肖尚辉, 张忠培, 史治平. 多基站协作下行系统中异步空时码构造与分析[J]. 电子科技大学学报, 2010,39(3):335-339.
[9] NG B L, EVANS J S, HANLY S V,et al. Transmit beamforming with cooperative base stations[C]. in Proc. IEEE Intl. Symp. Inf. Th., (Adelaide, Australia), 2005:1431-1435.
[10] SPENCER Q H, SWINDLEHURST A L, HAARDT M. Zero-forcing methods for downlink spatial multiplexing in multiuser MIMO channels[J].IEEE Transactions On Signal Processing, 2004, 52(2): 461-471.
[11] SHAO L, ROY S. Downlink multicell MIMO-OFDM: An architecture for next generation wireless networks [C]. in Proc. IEEE WCNC, Mar.2005.
[12] WINDPASSINGER C, FISCHER R F H,VENCEL T,et al. Precoding in multiantenna and multiuser communications[J]. IEEE Trans.Wireless Commun.,2004,3(4):1305-1316.