Physical Layer Network Coding with Signal Alignment for MIMO Wireless Networks

We propose signal alignment (SA), a new wireless communication technique that enables physical layer network coding (PNC) in multi-input multi-output (MIMO) wireless networks. Through calculated preceding, SA contracts the perceived signal space at a node to match its receive diversity, and hence facilitates the demodulation of linearly combined data packets. PNC coupled with SA (PNC-SA) has the potential of fully exploiting the preceding space at the senders, and can better utilize the spatial diversity of a MIMO network for higher transmission rates, outperforming existing techniques including MIMO or PNC alone, interference alignment (IA) and interference alignment and cancellation (IAC). PNC-SA adopts the seminal idea of 'demodulate a linear combination' from PNC. The design of PNC-SA is also inspired by recent advances in IA, though SA aligns signals not interferences. We study the optimal preceding and power allocation problem of PNC-SA, for SNR maximization at the receiver. The mapping from SNR to BER is then analyzed, revealing that the throughput gain of PNC-SA does not come with a sacrifice in BER. We finally demonstrate general applications of PNC-SA, and show via network level simulations that it can substantially increase the throughput of unicast and multicast sessions, by opening previously unexplored solution spaces in multi-hop MIMO routing.