Uncoordinated cooperative jamming for secret communications

We consider a Gaussian wiretap channel model with a single-antenna source, destination and eavesdropper. The communication is assisted by multiple multiantenna helpers that transmit noise to confound the eavesdropper. First, we consider a nulling scheme, in which each helper independently transmits noise, designed to maximize the system secrecy rate while creating no interference to the destination. In this scheme, each helper requires only local relay-destination channel state information (CSI). When global CSI is available at the relays, the nulling scheme is not optimal. The optimal jamming noise structure is also provided under global CSI. Interestingly, it is shown both analytically and via simulations that, despite its reduced CSI requirements, the nulling scheme may have secrecy rate performance that is very close to the optimal one. The probability of outage of the nulling scheme is provided in closed form based on the statistics of the eavesdropper CSI.