Two Birds and One Stone: Gaussian Interference Channel with a Shared Out-of-Band Relay

Details Two Birds and One Stone: Gaussian Interference Channel with a Shared Out-of-Band Relay Two Birds and One Stone: Gaussian Interference Channel with a Shared Out-of-Band Relay

2011-04-03

arxiv.org/abs/1104.0430v1

Computer Science

Information Theory

Submitted to IEEE Transactions on Information Theory

Scientific paper

The two-user Gaussian interference channel with a shared out-of-band relay is considered. The relay observes a linear combination of the source signals and broadcasts a common message to the two destinations, through a perfect link of fixed rate R_0 bits per channel use. The out-of-band nature of the relay is reflected by the fact that the common relay message does not interfere with the received signal at the two destinations. We are interested in designing "oblivious" relay strategies that incrementally improve upon the communication strategy in place, i.e., the source encoding strategy (apart from rate allocation) is independent of the relay presence. An ideal relay strategy would have a two-for-one gain, i.e., for every common bit relayed, the rate of each user is increased by one bit. We show that this ideal upper bound cannot be always achieved. Still, for a range of weak interference regimes and for R_0 below a certain threshold depending on signal-to-noise and interference-to-noise ratios (SNR and INR), we show an oblivious relay strategy that incrementally improves the sum rate by 2R_0-Delta bits per channel use, where Delta=1.95 is a constant for all channel coefficients and R_0. For a symmetric interference channel, the approximate two-for-one gain is achieved in the regime of log INR / log SNR < 2/3. A similar general result is also found for an asymmetric interference channel. It is further shown that an oblivious relay strategy achieves the entire capacity region of the interference-relay channel model at hand to within the constant gap Delta = 1.95 bits, for R_0 below a threshold that depend on the ratio log INR / log SNR. Instrumental to this result is the design of a suitable quantize-and-forward (QF) strategy at the relay, that can benefit both destinations.

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