Covert mmWave Communication When the Warden Locates in the Beam Direction

In this letter, the beamforming scheme based on random frequency diverse array (RFDA) is newly investigated to enhance the performance of covert mmWave communications. First, the closed-form expressions of covertness constraint are derived for the RFDA and the conventional phase array (PA) beamforming schemes, respectively. Then, the optimal transmit power and blocklength maximizing the average effective covert throughput (AECT) are derived for both beamforming schemes. Considering the pessimistic scenarios that Willie locates in its desired direction as Bob (but different distances), our examination shows the enhancement of maximum AECT by adopting the RFDA beamforming scheme. Besides, in this case, the maximum AECT for the RFDA beamforming scheme can be improved while that for the PA beamforming scheme remains unchanged as the number of antennas enlarges. Furthermore, it is revealed that the superior performance of the RFDA beamforming scheme is influenced by Willie's location, and the range of Willie in which the RFDA beamforming scheme outperforms the PA beamforming scheme is obtained.