Multi-User Beam Training and Transmission Design for Covert Millimeter-Wave Communication

Millimeter-wave (mmWave) communication has emerged as a promising means for supporting high-rate covert communication. However, the use of antenna arrays with beamforming at mmWave requires precise beam alignment between legitimate parties, and this procedure may entail large beam training overhead and create additional signal leakage to eavesdroppers. In this work, we consider a multi-user mmWave communication system and address the problem of designing proper covert beam training and data transmission between legitimate parties Alice and Bobs, while keeping the underlying communication undetectable from warden Willie. We first propose a novel Covert Multi-user Beam Training Strategy (CMBTS) that adopts multi-finger beam codebook to reduce the probability of communication being detected and to enable simultaneous training for multiple users. With the proposed CMBTS, a joint optimization framework for covert beam training and data transmission with a friendly jammer is developed to maximize the effective covert throughput while ensuring the covertness constraint at warden is met. We further propose an algorithm that combines successive convex approximation and inexact block coordinate descent methods to solve the problem efficiently. Numerical results validate the effectiveness of the CMBTS proposed and confirm its superior performance as compared to several beam training baselines (including exhaustive and hierarchical search) tailored to the covert communication setup considered. Among them, CMBTS achieves the best successful alignment probability and the largest effective covert throughput yet with the least training overhead.