On the tightness of the generalized network sharing bound for the two-unicast-Z network

We study two-unicast-Z networks¹ - two-source two-destination (two-unicast) wireline networks over directed acyclic graphs, where one of the two destinations (say the second destination) is apriori aware of the interfering (first) source's message. For certain classes of two-unicast-Z networks, we show that the rate-tuple (N, 1) is achievable as long as the individual source-destination cuts for the two source-destination pairs are respectively at least as large as N and 1, and the generalized network sharing cut - a bound previously defined by Kamath et. al. - is at least as large as N +1. We show this through a novel achievable scheme which is based on random linear coding at all the edges in the network, except at the GNS-cut set edges, where the linear coding co-efficients are chosen in a structured manner to cancel interference at the receiver first destination.