  Energy harvesting (EH) and network coding (NC) have emerged as two promising
technologies for future wireless networks. In this paper, we combine them
together in a single system and then present a time switching-based network
coding relaying (TSNCR) protocol for the two-way relay system, where an energy
constrained relay harvests energy from the transmitted radio frequency (RF)
signals from two sources, and then helps the two-way relay information exchange
between the two sources with the consumption of the harvested energy. To
evaluate the system performance, we derive an explicit expression of the outage
probability for the proposed TSNCR protocol. In order to explore the system
performance limit, we formulate an optimization problem to minimize the system
outage probability. Since the problem is non-convex and cannot be directly
solved, we design a genetic algorithm (GA)-based optimization algorithm for it.
Numerical results validate our theoretical analysis and show that in such an EH
two-way relay system, if NC is applied, the system outage probability can be
greatly decreased. Moreover, it is shown that the relay position greatly
affects the system performance of TSNCR, where relatively worse outage
performance is achieved when the relay is placed in the middle of the two
sources. This is the first time to observe such a phenomena in EH two-way relay
systems.
