Deaf cooperation for secrecy with a multi-antenna helper

In this paper, we investigate the roles of cooperative jamming (CJ) and noise forwarding (NF) in improving the achievable secrecy rates of a Gaussian wiretap channel (GWT) when the helper node is equipped with multiple antennas. We decompose the channel from the helper to the eavesdropper into two orthogonal components: one is aligned in the direction of the channel between the helper and the legitimate receiver (direct component) and the other is in the orthogonal direction to the channel between the helper and the legitimate receiver (orthogonal component). We then propose a strategy in which the helper uses the orthogonal component to transmit pure Gaussian noise as in the CJ strategy while he uses the direct component for either CJ or NF depending on the given channel conditions. We explicitly derive the optimal power control policy for this strategy and give the achievable secrecy rates when the direct component is used to perform CJ or NF. We hence derive the channel conditions where CJ is better than NF over the direct component and vice-versa.