Delay in cooperative communications: Higher multiplexing gain in Gaussian interference channels with full-duplex transmitters

Delay, guaranteeing causality, is inevitable in cooperative communication systems. Traditionally, delay granularity has been limited to one symbol; however, channel delay is in fact governed by channel memory and can be shorter. For example, the delay requirement in orthogonal frequency-division multiplexing (OFDM), captured in the cyclic prefix, is typically much shorter than the OFDM symbol itself. This perspective is used to study the two-user Gaussian interference channel with full-duplex transmitters. By superimposing the signal from the other node on its own signal, each transmitter cancels the interference at its receiver. Among other results, it is shown that under a mild condition, the maximum multiplexing gain of this channel is in fact two, rather than the limit of one, previously proved under the traditional constraint of causal delay [1]. Further, the optimal power allocation among orthogonal sub-carriers, which maximizes the achievable sum-rate, is shown to be a generalization of the well-known water filling.