Convex Optimization Based Robust Spatial Multiplexing Techniques for Downlink Multiuser Wireless Systems

由于视频流、音乐和交互式服务等丰富的多媒体应用，对无线接入高速数据传输的需求正在稳步增长。期望是使无线访问能够提供与有线对等物提供的相同的数据速率和服务质量（QoS）。这需要提供超过100mb /s的无线接入，最好是1Gb/s的无线接入。无线系统的数据速率可以通过利用发射器和接收器上的多个天线提供的空间分集来提高。基站可以使用下行链路空间复用技术在同一频带内同时为多个用户服务。对于基于频分双工（FDD）的系统，在发送端使用多天线需要从接收器反馈信道状态信息（CSI）。由于信道估计误差、估计的量化（由于比特反馈的有限预算）以及发射机和接收机之间的相对运动，这种信道状态信息在发射机上使用时总是不准确的。例如，当信道由于反馈延迟而变化较快时，到发射机使用信道状态信息时，真正的前向信道将发生变化。真实信道与发射机估计信道之间的误差会严重降低发射机的分集性能。因此，在设计发射机分集技术以提高容量或覆盖范围时，考虑信道状态信息误差是非常重要的。基于无线多用户系统发射机分集技术的丰富理论和实验结果，我们建议开发先进的信号处理技术来缓解基站信道状态信息不完善这一非常重要和实际的问题，这是在高度敌对的下行信道环境中使用这些分集技术的主要限制。该研究将集中于开发鲁棒波束形成/发射分集技术，该技术可以使用二阶锥规划和半定规划等凸优化技术，对信道状态信息误差具有弹性。该技术的性能（误码率（BER）、编码误码率（CBER）和吞吐量）将与传统的非鲁棒技术进行比较，这些非鲁棒技术使用模拟和由Malcolm Macleod教授提供的QinetiQ提供的实际现场数据获得各种信道衰落曲线。该项目具有与世界知名的强大波束形成和阵列处理技术专家Alex Gershman教授的欧洲合作的明显优势，这个机会之窗不应该失去，因为我们相信这为英国在国际水平的强大设计和阵列信号处理领域的地位提供了一个很好的工具。

The demand for high speed data transmission over wireless access is growing steadily due to rich multimedia applications such as video streaming, music and interactive services. The expectation is to enable wireless access to provide the same data rate and quality of services (QoS) as that provided by wire-line counterparts. This requires provision of beyond 100 Mb/s and preferably 1Gb/s wireless access. The data rates of wireless systems can be increased by exploiting spatial diversity provided by multiple antennas at the transmitter and receivers. A basestation could serve multiple users simultaneously in the same frequency band using downlink spatial multiplexing techniques. For a frequency division duplex (FDD) based system, the use of multiple antennas at the transmitter however requires feedback of channel state information (CSI) from the receiver. Such channel state information when used at the transmitter will always be inaccurate due to channel estimation error, quantization of the estimates (due to finite budget for bit feedback) and relative motion between the transmitter and receiver. For example, when the channel is changing moderately fast, due to feedback delay, by the time the transmitter uses the channel state information, the true forward channel would have changed. The error which is the difference between the true channel and the estimates available at the transmitter could seriously degrade the transmitter diversity performance. Therefore, it is very important to consider the channel state information error when designing transmitter diversity techniques for the enhancement of capacity or coverage.Motivated by the rich theoretical and experimental results on the benefits of transmitter diversity techniques for wireless multiuser systems, we propose to develop advanced signal processing techniques to mitigate this very important and practical problem of imperfect channel state information at the basestation, a major limitation for using these diversity techniques in a highly hostile downlink channel environment. The research will be focused on developing robust beamformers/transmit diversity techniques that are resilient to channel state information error, using convex optimization techniques such as second order cone programming and semidefinite programming. The performance (in terms of bit error rate (BER), coded BER (CBER) and throughput) will be compared against conventional non-robust techniques for various channel fading profiles obtained using simulations and real field data provided through QinetiQ, facilitated by Professor Malcolm Macleod. This project has a distinct advantage of European collaboration with Prof. Alex Gershman who is a world renowned expert on robust beamforming and array processing techniques, and this window of opportunity should not be lost as we believe this provides an excellent vehicle for UK's standing in the field of robust design and array signal processing at an international level.