  This paper investigates the secrecy performance of sub-optimal transmitter
selection (SS) and optimal selection (OS) schemes in a system comprising
multiple transmitters, multiple eavesdroppers, and a single destination in
frequency-selective fading channels with single carrier cyclic prefix
modulation. Considering unreliable backhaul links between the access point and
the transmitters, we analyze secrecy performance in two scenarios: when
backhaul activity knowledge is available (KA) and when backhaul activity
knowledge is unavailable (KU). Closed-form expressions for the secrecy outage
probability (SOP) and ergodic secrecy rate (ESR) are derived. We also provide
the corresponding asymptotic expressions when the backhaul links are unreliable
as well as when the backhaul links are active in both KA and KU scenarios. We
show that in the backhaul KA scenario, the asymptotic SOP and the slope of the
ESR are governed by the number of transmitters and the backhaul reliability
factor. In contrast, only the backhaul reliability factor influences these two
parameters in the backhaul KU scenario. These parameters are independent of the
number of eavesdroppers in both backhaul KA and KU scenarios. We also observe
that the secrecy diversity order of both selection schemes is identical when
all backhaul links are active.
