Opportunistic Power Control for Physical Layer Security

We introduce opportunistic power control (OPC) as a physical-layer security technique to enable wireless communications with secrecy in fading channels, enabling secure communication in the presence of an eavesdropper. OPC is a scheme where the transmitter only transmits when the channel gain is above a certain threshold, and a larger modulation alphabet than necessary is employed to maintain a target spectral efficiency. When the transmitter does transmit, it transmits with less power due to the higher channel gain. This effect reduces the signal power received by the eavesdropper, inducing decoding errors for the eavesdropper and enhancing the secrecy. Hence, the secrecy improves as the threshold increases. However, since the transmitter does not always transmit, buffers are necessary at the transmitter and receiver to smooth out gaps in the transmission, introducing delay. A higher threshold results in a longer delay before that threshold is met. Due to the direct relationship between the threshold and the delay, there exists a tradeoff between secrecy and delay. We obtain a closed-form expression for the average achievable secrecy rate with OPC under Rayleigh fading and optimize over the target SNR to obtain the maximum average achievable secrecy rate. We then present numerical results to quantify the incurred delay. We find that with OPC, the secrecy improves with increasing delay, reaching the aforementioned maximum, beyond which, there is no benefit in secrecy nor in delay. We also find that communication with a positive average achievable secrecy rate is possible even when the eavesdropper channel is better than the main channel on average, and that for the maximum average achievable secrecy rate, there is an inverse relationship between the target spectral efficiency and the secrecy, and a direct relationship between the target spectral efficiency and the delay.