Low-Complexity Power Control and Energy Harvesting Algorithms for Wiretap Channels Employing Finite-Alphabet Input Schemes

We discuss the design rules of an energy-efficient wireless wiretap channel in this paper. On the transmitter side, we investigate the optimal power control policy for secrecy rate maximization over wiretap channels with the bit-interleaved coded modulation scheme. On the receiver side, an energy harvesting algorithm is used to collect wireless signal energy without degrading the secrecy rate performance. These objectives are challenging as the closed-form solution to the mutual information of finite-alphabet input schemes remains unknown. In this paper, we derive a closed-form relationship between the secrecy rate and the signal-to-noise ratio (SNR) by transforming the SNR from the linear domain to the logarithm domain. The optimal power control policy (PCP) can be easily obtained by exhaustive search in a small interval, whereas the traditional search is performed over the entire SNR range. We also propose a sub-optimal PCP algorithm that significantly reduces the search complexity with only a minor performance loss. The optimal power splitting ratio is studied to save energy while achieving the target secrecy rate. We show that if the transmission power is too high, the receiver is unable to simultaneously harvest energy from the wireless signals and achieve the target secrecy rate.