High-Performance MIMO Transceiver Design for Single and Multiuser Wireless Communications

未来一代无线系统的愿景将是提供宽带接入、无缝全球漫游、互联网/数据/语音和高度互动的多媒体服务，如随时随地的多方视频会议和虚拟远程呈现。提供这种无处不在的高容量高质量传输将需要在空中达到几Mbps。虽然目前的2.5G和不断发展的3G技术分别承诺高达348kbps和2Mbps的突发速率，但在繁忙时段，每个用户的平均吞吐量预计不会超过171kbps。为了纠正这一点，已经提出了各种先进技术，其中多单元发射机和接收机天线配置，即多输入多输出（MIMO）天线系统，最近成为提高容量的重大突破，远远超出了目前使用的系统。MIMO技术已经被提议作为3GPP（第三代合作伙伴计划）第6版的一种选项，并且一种名为BLAST（贝尔实验室分层时空）架构的著名MIMO方案的标准化正在进行中。从物理上讲，从不同天线发射或接收的无线电信号（如果它们相隔很远）是非常不同的，可以从它们所谓的空间特征中区分出来。该原理允许多个信号流在空间域中复用，从而导致MIMO天线可以实现巨大的容量。然而，技术上的挑战是，最佳的MIMO性能需要最大似然（ML）解码，不幸的是，这是非确定性多项式时间困难（NP-hard）。即使多路复用信号的数量适中（例如，5个），紧凑的移动设备和强大的基站也不可行。更糟糕的是，在真实环境中，由于蜂窝系统的频率重用，共信道干扰（与同一无线电信道共享的其他信号）的存在将进一步导致ML解码更加难以实现。该项目的目标是设计用于高性能解码的低复杂度MIMO接收器，并研究在上行（从许多移动站到基站）和下行（从基站到许多移动站）的多用户环境中使用这种接收器，并采用适当的发射器设计。长期以来，人们一直在研究导致复杂度降低的次优解码。虽然相对实用，但它要么只适用于特定的调制，要么需要牺牲信道中已经存在的大量分集。此外，大多数次优检测器仅针对单个MIMO链路设计，并且要求接收天线的数量至少等于复用信号流的数量。然而，目前尚不清楚它们如何在蜂窝网络中工作，在蜂窝网络中，相同的频率通道被更积极地重复使用，天线阵列基本上超负荷。这种情况将在本项目中处理。这项研究很重要，因为最终频谱高效的蜂窝网络应该允许每个无线电频道由尽可能多的用户共享，这将导致天线阵列过载。在这方面，我们的目标是开发一种非线性信号处理方案，该方案可以放置比接收天线数量更多的零，超过线性处理所能实现的限制。根据单用户或多用户、上行或下行信道的不同系统约束，将设计出在复杂性和性能之间进行最佳权衡的各种MIMO接收器，从而使MIMO技术在实际中可行。开发的探测器应该可扩展到具有大量用户/天线的系统，因此，MIMO技术，如BLAST和空时编码（在3G/4G+系统中）可以实际部署，以在干扰有限的蜂窝环境中产生有希望的性能。

The vision of future generation of wireless systems will be the provision of broadband access, seamless global roaming, Internet/data/voice and highly interactive multimedia services such as multiparty videoconference and virtual telepresence anywhere anytime. Providing such ubiquitous high-capacity high-quality transmission will require several Mbps to be achieved over air. Though current 2.5G and evolving 3G technologies promise burst rates up to 348kbps and 2Mbps, respectively, the average throughput per user is not expected to be more than 171kbps during busy hours. To rectify this, various advanced techniques have been proposed, among which multi-element transmitter and receiver antenna configurations known as multiple-input multiple-output (MIMO) antenna systems have recently emerged as a significant breakthrough for enhancing the capacity far beyond systems in use today. MIMO techniques have already been proposed as an option for 3GPP (Third Generation Partnership Project) Release 6, and the standardisation of a well-known MIMO scheme called BLAST (Bell Labs Layered Space-Time) architecture is underway.Physically, radio signals, transmitted from or received at, different antennas (if they are separated far apart) are very different that they can be distinguished from their so-called spatial signatures. This principle allows multiple signal streams to be multiplexed in spatial domain, leading to tremendous capacity achievable by MIMO antenna. The technical challenge, however, is that the optimum MIMO performance requires maximum-likelihood (ML) decoding, which is unfortunately non-deterministic polynomial-time hard (NP-hard). Neither a compact mobile device nor a powerful base station is feasible even if the number of multiplexed signals is moderate (e.g., 5). Worse of all, in real environments, the presence of co-channel interference (other signals that share with the same radio channel) as a result of frequency reuse for cellular systems, will further cause ML decoding even more difficult to implement.The objective of this project is to design low-complexity MIMO receivers for high-performance decoding and study the use of such receivers in multiuser environments in both up (from many mobile stations to a base station) and downlinks (from a base station to many mobile stations) with appropriate transmitter designs.Suboptimal decoding leading to reduced complexity has long been investigated for MIMO detection. Though relatively practical, it either works only for specific modulation or needs to scarify much diversity that already exists in the channel. Besides, most suboptimal detectors are designed for single MIMO link only and require the number of receive antennas to be at least equal to the number of multiplexed signal streams. It is, however, not clear how they would work in a cellular network where the same frequency channel is more aggressively reused and the antenna array is largely overloaded. Such scenario will be dealt with in this project. The study is important, for ultimately spectral-efficient cellular networks ought to allow every radio channel to be shared by as many users as possible, which gives rise to overloaded antenna array. In this respect, we aim to develop a non-linear signal processing scheme that can place more nulls than the number of receive antennas, exceeding the limit of what linear processing can achieve.Subject to different system constraints for single, or multiuser, uplink or downlink channels, various MIMO receivers with the best tradeoff between complexity and performance will be devised that makes MIMO technologies practically viable. The developed detectors should be scalable for a system with a large number of users/antennas, and as a consequence, MIMO technologies such as BLAST and space-time coding (in 3G/4G+ systems) can be practically deployed to yield promising performance in interference-limited cellular environments.