  In this work, we analyze the performance of full-duplex relay selection
(FDRS) in spectrum-sharing networks. Contrary to half-duplex relaying,
full-duplex relaying (FDR) enables simultaneous listening/forwarding at the
secondary relay(s), thereby allowing for a higher spectral efficiency. However,
since the source and relay simultaneously transmit in FDR, their superimposed
signal at the primary receiver should now satisfy the existing interference
constraint, which can considerably limit the secondary network throughput. In
this regard, relay selection can offer an adequate solution to boost the
secondary throughput while satisfying the imposed interference limit. We first
analyze the performance of opportunistic FDRS with residual self-interference
(RSI) by deriving the exact cumulative distribution function of its end-to-end
signal-to-interference-plus-noise ratio under Nakagami-m fading. We also
evaluate the offered diversity gain of relay selection for different
full-duplex cooperation schemes in the presence/absence of a direct
source-destination link. When the adopted RSI link gain model is sublinear in
the relay power, which agrees with recent research findings, we show that
remarkable diversity gain can be recovered even in the presence of an
interfering direct link. Second, we evaluate the end-to-end performance of FDRS
with interference constraints due to the presence of a primary receiver.
Finally, the presented exact theoretical findings are verified by numerical
simulations.
