Physical layer security with uncoordinated helpers implementing cooperative jamming

1A wireless communication network is considered, consisting of a source, a destination and an eavesdropper, each equipped with a single antenna. The communication is assisted by multiple multi-antenna helpers, which independently transmit jamming noise to confound the eavesdropper. The optimal structure of the jamming noise that maximizes the secrecy rate is derived. A suboptimal solution that results in nulling at the legitimate receiver is also considered. Unlike the optimal solution, which requires global channel information, the suboptimal solution requires local channel information only, i.e., each helper needs to know only its own channel to the destination. Although the nulling solution results in lower secrecy rate as compared to the optimal solution, simulations show that the difference is rather small, with the inexpensive and easy to implement nulling scheme performing near optimal.