  In this paper, we analyze the ergodic capacity of a dual-hop
amplify-and-forward relaying system where the relay is equipped with multiple
antennas and subject to co-channel interference (CCI) and the additive white
Gaussian noise. Specifically, we consider three heuristic precoding schemes,
where the relay first applies the 1) maximal-ratio combining (MRC) 2)
zero-forcing (ZF) 3) minimum mean-squared error (MMSE) principle to combine the
signal from the source, and then steers the transformed signal towards the
destination with the maximum ratio transmission (MRT) technique. For the
MRC/MRT and MMSE/MRT schemes, we present new tight analytical upper and lower
bounds for the ergodic capacity, while for the ZF/MRT scheme, we derive a new
exact analytical ergodic capacity expression. Moreover, we make a comparison
among all the three schemes, and our results reveal that, in terms of the
ergodic capacity performance, the MMSE/MRT scheme always has the best
performance and the ZF/MRT scheme is slightly inferior, while the MRC/MRT
scheme is always the worst one. Finally, the asymptotic behavior of ergodic
capacity for the three proposed schemes are characterized in large $N$
scenario, where $N$ is the number of relay antennas. Our results reveal that,
in the large $N$ regime, both the ZF/MRT and MMSE/MRT schemes have perfect
interference cancelation capability, which is not possible with the MRC/MRT
scheme.
