Ergodic capacity analysis of dual-hop ZF/MRT relaying systems with co-channel interference

In this paper, we analyze the ergodic capacity of dual-hop amplify-and-forward systems where the relay node is equipped with multiple antennas and subject to co-channel interference (CCI) and the additive white Gaussian noise. Specifically, we consider a heuristic precoding scheme, where the relay node first applies the zero-forcing principle to eliminate the interference, and then steers the signal towards the destination with the maximum ratio transmission scheme. Two separate power constraints are investigated, namely, the short-term/long-term power constraint. New exact closed-form ergodic capacity expressions are derived for both cases. It is demonstrated that the short-term power constraint case only achieves a slightly higher capacity, indicating that the long-term power constraint maybe preferred in practice as it requires less control and incurs smaller overhead. In addition, the coupled effect of the number of antennas N and number of interferers M is examined. Our findings show that a slight change in N or M does not cause a significant variation of the ergodic capacity when the difference between N andM is large. On the other hand, when the difference between N and M is small, a slight change in N or M results in a substantial variation of the ergodic capacity.