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.

A wireless relay channel is considered, consisting of a multi-antenna source, a single-antenna destination, a single-antenna eavesdropper and a set of multi-antenna relays (helpers) that act as jammers to the eavesdropper. Each helper knows only its own link to the receiver and independently transmits jamming noise, lying in the null space of its own link to the destination, thus causes no interference to the destination. The source, knowing the main channel explicitly and having statistical information on the eavesdropper channel, designs the input covariance matrix so that the secrecy rate is maximized subject to an outage probability constraint and a sum power constraint. We show that the optimal input covariance matrix has rank one. Assuming that the eavesdropper channels follow a zero-mean Gaussian distribution with known covariance, the outage probability and outage secrecy rate are derived in closed form. Simulation results in support of the analysis are provided.

Cooperative jamming (CJ) schemes support secure wireless communication in the presence of one or more eavesdroppers. Larger numbers of cooperative relays provide better secrecy rate, while increasing the communication and synchronization needs associated with cooperative beamforming. For low density networks (small number of relays) the secrecy rate changes rapidly with an increase in the number of relays, while for higher density networks increasing the number of relays has significantly smaller effect on the secrecy rate. This research considers a resource-aware approach: instead of using all available relays, choose the smallest set of active relays that meet a predetermined performance goal. The problem is formulated as a knapsack problem that may be solved through an exhaustive search. As this search method has exponential complexity, three heuristic algorithms are proposed, offering significant complexity reduction. The first one relies on the individual relay secrecy rate with a greedy algorithm. The second one is based on the beamforming weights norms, and the third one successively selects relay nodes that minimize the performance gap between the temporal secrecy rate and the given secrecy rate goal. Simulation results demonstrate that relatively high secrecy rate may be achieved with a small number of active relays. The first method performs better for low secrecy rate threshold levels, while the second does so at high threshold. The third method combines the advantages of the previous two and offers performance very close to the optimum at the expense of higher complexity than the first two.