Network coding for three unicast sessions: Interference alignment approaches

We propose interference alignment techniques, originally developed for wireless interference channels, for the problem of network coding across unicast sessions. We describe two general approaches (namely, coding at the edge or in the middle of the network) and one specific example of each approach (namely, symbol extension method and ergodic alignment, respectively). We discuss the conditions for feasibility of alignment and their relation to network structure. We also compare alignment to alternative approaches. For three unicast sessions with min-cut one, we show that whenever alignment is possible, alternative approaches can also achieve half the min-cut. However, for more than three sessions and/or for min-cut per session greater than one, we show examples where alignment is necessary.