  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 0.5 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.
