  A small-cell network with multiple transmitters and unreliable wireless
backhaul is considered for secrecy enhancement. The small-cell network is
operating under a spectrum sharing agreement with a primary network in a
cognitive radio system. A constraint on the desired outage probability at the
primary receiver is assumed as a part of the spectrum sharing agreement. The
reliability of the wireless backhaul links are modeled by a set of independent
and identically distributed Bernoulli random variables. A sub-optimal and an
optimal small-cell transmitter selection (TS) scheme is proposed to improve the
performance of the system, depending on the availability of channel state
information. Selection schemes are designed for the scenario where knowledge is
available regarding which backhaul links are active. The corresponding secrecy
outage probabilities along with their asymptotic expressions are derived. It is
shown that the secrecy performance is significantly improved compared to the
case where knowledge of the active backhaul links is unavailable.
