Distributed Adaptive Beamforming and Nullforming for Covert Wireless Communications

In this paper, we focus on the problem of distributed coherent beamforming in wireless networks where multiple distributed transmitters adjust the phases of their signals to form a directional and targeted communication link to a client receiver. The quality-of-service (QoS) and security are key aspects of robust and covert wireless networks. Although the security can be enhanced by exploiting information about the locations of adversaries, such information may not be available in practical networks since the adversaries are often passive. Therefore, we propose transmission strategies which divide transmitters into two groups where one group forms a beam towards the client and the other group broadcasts interference in order to obfuscate adversaries. As the interference may degrade the QoS of the client, the latter group steers a null to the client to alleviate the interference at the client. The proposed scheme can be performed in a fully distributed manner with only two bits of feedback information from the client and without any knowledge about the locations of potential adversaries.