Physical layer network coding: A cautionary story with interference and spatial reservation

Physical layer network coding (PLNC) has the potential to improve throughput of multi-hop networks. However, most of the works are focused on the simple, three-node model with two-way relaying, not taking into account the fact that there can be other neighboring nodes that can cause/receive interference. The way to deal with this problem in distributed wireless networks is usage of MAC-layer mechanisms that make a spatial reservation of the shared wireless medium, similar to the well-known RTS/CTS in IEEE 802.11 wireless networks. In this paper, we investigate two-way relaying in presence of interfering nodes and usage of spatial reservation mechanisms. Specifically, we introduce a reserved area in order to protect the nodes involved in two-way relaying from the interference caused by neighboring nodes. We analytically derive the end-to-end rate achieved by PLNC considering the impact of interference and reserved area. A relevant performance measure is data rate per unit area, in order to reflect the fact that any spatial reservation blocks another data exchange in the reserved area. The numerical results carry a cautionary message that the gains brought by PLNC over one-way relaying may be vanishing when the two-way relaying is considered in a broader context of a larger wireless network.