Title: Cooperative Protocols for the Relay Channel at Low SNR
Authors: A. Salman Avestimehr and David N. C. Tse (Poster I, 3)

ABSTRACT

Here, the cooperative scenario is modeled by a slow Rayleigh fading relay channel where the relay operates on a half-duplex mode and transmits and receives on different frequency bands (so called frequency-division (FD) relay channel). There are two regimes of interest that one can look at for this channel: high SNR and low SNR.

In this presentation we focus on the low SNR regime and we analyze different cooperative protocols and we present a scheme, called Bursty Amplify and Forward, which achieves the epsilon-outage capacity of the relay channel for small outage probabilities epsilon, and we give a simple characterization of the outage capacity.

In order to find the outage capacity we will first use the max-flow min-cut bound to find an upper bound on the epsilon-outage capacity of the frequency-division (FD) relay channel. Then we investigate the outage performance of two classes of cooperative protocols: Amplify-Forward (AF) and Decode-Forward (DF). We show that with AF protocol we get the same outage rate as when there is no cooperation and only the direct link is used. On the other hand with DF protocol we can get full diversity gain. But still there is a gap between the outage rate of DF protocol and the upper bound on the outage rate of the relay channel. Then we investigate the performance of Bursty Amplify-Forward (BAF) protocol, where the source only transmits with a low duty cycle but transmitting at high power when transmitting. We show that, somewhat surprisingly, this simple protocol closes the gap and achieves the optimal outage capacity of the relay channel, in the limit of low SNR and low probability of outage.