Diversity-Multiplexing Tradeoff in Space-Time Communications

Multi-antenna communications and space-time coding have been one of the hottest research areas in the communication field in the past few years. There are, however, two distinct points of view on how multiple antennas can be used in a wireless system. The more traditional viewpoint is that multiple antennas provide diversity to combat channel fading: by creating multiple independently faded signal paths between the transmitter and the receiver, more reliable transmission can be achieved. A different and more modern viewpoint is that antennas allow spatial multiplexing : by providing additional degrees of freedom through the use of multiple antennas at both the transmitter and the receiver, much higher data rates can be achieved. These two distinct viewpoints lead to two more or less distinct camps of researchers, using different performance metrics and different tools in their work on space-time codes.

In recent work we propose a new point of view: a multi-antenna system can provide both diversity and spatial multiplexing gains simultaneously, but there is a tradeoff between how much of each type of gains a system can achieve. The viewpoints of maximizing the diversity gain and of maximizing the spatial multiplexing gains are the two extremes in this tradeoff, but it is the entire tradeoff curve which provides the unified picture of the capability of multi-antenna systems. The main analytical result is a simple characterization of the optimal tradeoff curve achievable by any space-time coding scheme. This fundamental limit provides a common benchmark for evaluating the performance of many proposed schemes, be they diversity-based or multiplexing-based, and also provides insights on the design of new schemes. Results were also generalized to the multiple access channel.

L. Zheng and D. Tse, ``Diversity and Multiplexing: A Fundamental Tradeoff in Multiple Antenna Channels'', IEEE Transactions on Information Theory, 2002.

D. Tse, P. Viswanath and L. Zheng "Diversity-Multiplexing Tradeoff in Multiple Access Channels" ,submitted to the Transactions on Information Theory, November 2003.

For a quick overview of the basic ideas, see the following presentation:

" Diversity and Freedom: A Fundamental Tradeoff in Multiple Antenna Channels", MSRI 2002.

This work is supported by the National Science Foundation under grant CCR-0118784, Qualcomm Inc., and a matching grant from the California MICRO program.