Technological progress in the field of wireless communications over the past few years has only been matched by the increasing demand for sophisticated services at lower costs. A significant breakthrough was achieved in the design of efficient wireless communication systems with the advent of the diversity concept. A communication system is said to have diversity if there are multiple paths, in time, frequency or space, between the transmitter and receiver. Diversity enables the system to mitigate the effects of multipath fading on signals transmitted through the wireless medium. Spatial diversity exploits the availability of multiple spatial paths between the transmitter and receiver by placing antenna arrays at either end. In addition to improving the reliability of communication by creating redundant copies of the transmitted information at the receiver, wireless transceivers with multiple antennas exploit the spatial degrees of freedom to multiplex multiple streams of data and achieve significant gains in spectral efficiencies. Spatial diversity is crucial to reliable communication over slow-fading wireless channels, where it is hard to achieve low error probabilities due to the lack of time diversity.
In this thesis, we design spatial diversity techniques for slow-fading wireless channels. There are two parts to this thesis: In Part I we propose spatial diversity techniques for point-to-point single-user wireless systems, while in Part II we propose multiuser cooperative diversity techniques for multiuser wireless communication systems.
In the first part, we propose a set of new wireless communication techniques for multiple-input, multiple-output (MIMO) channels over Rayleigh slow-fading wireless channels. In the last decade, several techniques have been proposed to achieve the multiplexing and diversity benefits and low error rates over MIMO systems. However, achieving these objectives at an affordable computational complexity remains a challenging problem. We introduce MIMO transceivers that achieve high data rates and low error rates using a class of MIMO systems known as layered space-time (ST) architectures, which use low complexity, suboptimal decoders such as successive cancellation (SC) decoders.
We propose a set of improved layered space-time architectures and show that it is possible to achieve near-optimal error performance over MIMO channels while requiring just SC decoding at the receiver. We show that these architectures achieve high rate and diversity gains while maintaining simple decoders with affordable computational complexity. We also show that some of the proposed layered space-time architectures could find applications in multiple-access communications as low-complexity solutions for achieving near-optimum performance.
In the second part of this thesis, we propose novel techniques for cooperative communication between terminals in multiuser wireless communication systems. Cooperative communication over wireless networks is a concept where neighboring terminals share their antennas and signal processing resources to create a “virtual transmit array”. In addition to transmitting their own information to the destination, users in a cooperative communication system listen to transmission from other users and relay this information to the destination, thus creating multiple paths between transmitter and receiver. Cooperation amongst users creates a new form of diversity, known as cooperative diversity, which helps improve the reliability of all the users in a network collectively, compared to each user communicating independently with the destination.
Current cooperation protocols show that it is possible to improve the diversity gains significantly over multiple access channels, improving the outage performance at high SNR. The concept of cooperation necessitates that a user spend time, bandwidth, energy and signal processing resources to help the communication of other users in the network. This raises a fundamental question on the tradeoff between spending resources for one’s own communication and helping other users. Several state-of-the-art cooperation schemes, while effectively harnessing the diversity benefits of cooperation, incur a high penalty in transmission rate, thus affecting the outage performance. Specifically, the transmission rates of these protocols do not scale well as the number of users in the network increases.
We start with a simple three node multiple-access system where two users are communicating with a common destination. We propose new high-rate cooperation strategies which achieve the full diversity gain offered by the cooperative channel for this simple system. We propose a new framework to address the tradeoff between cooperation and independent transmission over a multiple access channel and determine the conditions under which each idea is better than the other. Finally, we propose a high rate cooperation protocol which achieves the maximum diversity over a multiple access system with an arbitrary number of users and achieves high rates which scale favorably as the number of users increases.