On the Capacity and Diversity-Multiplexing Tradeoff of the Two-Way Relay Channel

TL;DR

This paper analyzes the capacity and diversity-multiplexing tradeoff of a MIMO two-way relay channel, proposing optimal and simplified relay strategies, and establishing capacity scaling laws and optimal tradeoffs.

Contribution

It introduces an iterative algorithm for optimal rate region achievement, proposes a low-CSI amplify-and-forward strategy, and characterizes capacity scaling and tradeoffs in two-way relay channels.

Findings

Iterative algorithm achieves optimal rate region with AF relays.

Dual channel matching requires only local CSI and closely approximates optimal performance.

In large relay regimes, the achievable rate region approaches the upper bound, establishing capacity scaling laws.

Abstract

This paper considers a multiple input multiple output (MIMO) two-way relay channel, where two nodes want to exchange data with each other using multiple relays. An iterative algorithm is proposed to achieve the optimal achievable rate region, when each relay employs an amplify and forward (AF) strategy. The iterative algorithm solves a power minimization problem at every step, subject to minimum signal-to-interference-and-noise ratio constraints, which is non-convex, however, for which the Karush Kuhn Tuker conditions are sufficient for optimality. The optimal AF strategy assumes global channel state information (CSI) at each relay. To simplify the CSI requirements, a simple amplify and forward strategy, called dual channel matching, is also proposed, that requires only local channel state information, and whose achievable rate region is close to that of the optimal AF strategy. In the asymptotic regime of large number of relays, we show that the achievable rate region of the dual channel matching and an upper bound differ by only a constant term and establish the capacity scaling law of the two-way relay channel. Relay strategies achieving optimal diversity-multiplexing tradeoff are also considered with a single relay node. A compress and forward strategy is shown to be optimal for achieving diversity multiplexing tradeoff for the full-duplex case, in general, and for the half-duplex case in some cases.