Fundamental properties of on-off transmission scheme for wiretap channels

This work reveals some fundamental properties of an on-off transmission (OOT) scheme, in which a transmitter sends signals occasionally as per the capacity of the main channel in order to achieve physical layer security. To this end, we first identify the widely used hybrid secrecy outage probability as a function of the transmission probability and the conditional secrecy outage probability of the OOT scheme. This indicates, for the first time, that the hybrid secrecy outage probability can be achieved by the OOT scheme. We then derive a lower bound on the conditional secrecy outage probability of the OOT scheme in case of transmission, which is solely determined by the average signal-to-noise ratios (SNRs) of the main channel and eavesdropper's channel. Finally, we re-investigate the OOT scheme within an absolutely completely passive eavesdropping scenario, in which even the average SNR of the eavesdropper's channel is not required. Specifically, we derive an easy-evaluated expression for the average conditional secrecy outage probability of the OOT scheme by adopting an annulus threat model.