This paper proposes XOR-BiDO, a data delivery scheme (DDS) suited for multihop underwater acoustic networks characterized by regular bidirectional traffic streams. DDSs for multihop underwater networks usually assume a single-sink configuration, where most of the transmissions flow in a single direction. Throughput performance decreases and fairness suffers when such DDSs are subjected to heavy bidirectional traffic streams due to increase of queueing delays at relay nodes. This paper proposes the integration of an opportunistic XOR network coding layer into BiDO, a scheme previously designed for single-sink underwater networks. This coding layer actively and opportunistically mixes data frames whenever a coding opportunity arises. By effectively transmitting more data in fewer transmissions at each relay node, the result is a scheme that (1) maximizes usage of the underwater acoustic channel's scarce bandwidth, (2) provides better fairness for bidirectional traffic and (3) improves energy-efficiency of submerged devices, while retaining the opportunistic-routing and network-wide overhearing features of BiDO. Using simulations, we demonstrate the network performance improvement of XOR-BiDO in terms of packet latency, fairness and energy-efficiency for bidirectional network traffic. While maintaining comparable reliability over a 10-hop network, XOR-BiDO achieves a 12% improvement in latency and consumes 24% less energy per delivered packet against BiDO.
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