Study on the fundamental limits of covert communication over multi-user networks

Abstract

In this dissertation, we study fundamental limits of covert communication over several multi-user network models. Covert communication is a field of secure communication that aims to ensure the reliable communication between legitimate party while keeping the presence of the communication undetectable from an adversary (or warden). There are several practical fields that the covert communication is studied actively such as military communication, satellite communication, and IoT networks. Thus, it is important to examine the covert communication over multi-user networks. In this dissertation, we consider three network scenarios, i.e. interference channels, two-user Gaussian multiple access channel with causal feedback, and block fading channels with a half-duplex relay. We examine the covert capacity (region) in each scenario and corresponding optimal communication strategy. In the case of interference channels, we derive the exact covert capacity region and show that a simple point-to-point based scheme with treating interference as noise is optimal. In addition, we analyze the secret key length required for the reliable and covert communication with the desired rates. The results are extended to the Gaussian case and the case with multiple wardens. For the two-user Gaussian multiple access channel with causal feedback, we show that any causal feedback does not enlarge the covert capacity region and the time-division scheme is optimal for a class of channels. Finally, we propose a DF relaying strategy for the covert communication over block fading channels with a half-duplex relay. A non-convex optimization problem is formulated to maximize the outage throughput of the proposed DF relaying. To solve this problem, we propose a low-complexity algorithm with alternating optimization.