Diversity-Multiplexing Tradeoff for the MIMO Static Half-Duplex Relay

TL;DR

This paper analyzes the fundamental limits of MIMO static half-duplex relay channels, deriving the optimal diversity-multiplexing tradeoff and revealing how relay antenna count affects performance compared to full-duplex systems.

Contribution

It provides a general DMT upper bound for MIMO static half-duplex relays, explicit formulas for symmetric cases, and introduces new asymptotic analysis of spherical integrals.

Findings

DMT matches full-duplex when relay has one antenna.

DMT is below full-duplex when relay has multiple antennas.

Derived explicit DMT expressions for symmetric antenna configurations.

Abstract

In this work, we investigate the diversity-multiplexing tradeoff (DMT) of the multiple-antenna (MIMO) static half-duplex relay channel. A general expression is derived for the DMT upper bound, which can be achieved by a compress-and-forward protocol at the relay, under certain assumptions. The DMT expression is given as the solution of a minimization problem in general, and an explicit expression is found when the relay channel is symmetric in terms of number of antennas, i.e. the source and the destination have n antennas each, and the relay has m antennas. It is observed that the static half-duplex DMT matches the full-duplex DMT when the relay has a single antenna, and is strictly below the full-duplex DMT when the relay has multiple antennas. Besides, the derivation of the upper bound involves a new asymptotic study of spherical integrals (that is, integrals with respect to the Haar measure on the unitary group U(n)), which is a topic of mathematical interest in itself.