Diversity in Communications:
From Source Coding to Wireless Networks

Abstract

In transmission over networks, random packet losses degrade performance. Diversity here would be to create conduits with independent packet loss patterns. For example, this can be done by transmission over multiple routes. In order to utilize these conduits, multiple description source coding generates multiple codeword streams to describe a source. The design goal is to have a graceful degradation in performance (in terms of distortion) when only subsets of the transmitted streams are received. We present the design of multiple description lattice quantizers that cover the entire spectrum of the distortion profile, ranging from symmetric or balanced to successively refinable.

As the size of wireless devices shrink and they become pervasive, an attractive architecture is that of a peer-to-peer distributed (ad-hoc) network. Here the source of randomness is the link condition between users. Diversity arises by viewing the different users as conduits for relaying information from source to destination, i.e., multiuser diversity. Recently it has been shown that fixed ad-hoc networks of large scale have per-user session rates that decrease with network size. We have found that even with users restricted to one-dimensional mobility, the (long-term) per-user session rate is of O(1), i.e., large-scale networks can be built without sacrificing the per-user throughput. .The multiuser diversity is utilized since nodes encounter several different nodes in their mobility path and use them as relays.

In wireless transmission, multipath fading is a source of randomness. Here diversity arises by utilizing independent realizations of fading in several domains; time (mobility), frequency (delay spread) and space (multiple antennas). Recent results have shown that multiple antenna spatial diversity (space-time) communication can not only provide robustness, but also dramatically improve reliable data rates. We show that even with (computationally) simple structures the achievable data rate grows linearly with the number of antennas, when the number of antennas become large. .We also summarize our related results on this topic.

Therefore, diversity plays not only a role in robustness, it can also result in remarkable gains in achievable performance over several disparate applications. We end the talk with a brief glimpse of other problems we have been working on, which are steps in the overall research program.

Parts of this talk are joint work with Neil Sloane, Vinay Vaishampayan, Matthias Grossglauser and David Tse.

Biography

A list of Telecommunications and Signal Processing Seminars is available at from the ECE department Web pages under "Seminars". The Web address for the Telecommunications and Signal Processing Seminars is http://signal.ece.utexas.edu/seminars