Structured interference-mitigation in two-hop networks

We consider two-hop S-R-D Gaussian networks with a source (S), a relay (R) and a destination (D), some of which experience additive interference. This additive interference, which renders the channels state-dependent, is either a) experienced at the destination D and known non-causally at the source S, or b) experienced at the relay R and known at the destination D. In both cases, one would hope to exploit this knowledge of the channel state at some of the nodes to obtain “clean” or interference-free channels, just as Costa's dirty-paper coding does for one-hop channels with state non-causally known to the transmitter. We demonstrate a scheme which achieves to within ½ bit of a “clean” channel. This novel scheme is based on nested-lattice code and a Decode-and-Forward (DF) relay. Intuitively, this strategy uses the structure provided by nested lattice codes to cancel the “integer” (or lattice quantized) part of the interference and treats the “residual” (or quantization noise) as noise.