Research On Physical-layer Security Of Cooperative Relay System

In cooperative relay system, the communication distance and transmission rate can beimproved through the collaboration between relay nodes, which makes the system have broadapplication prospect. With more nodes and more complex network structure, it is difficult for thedeployment of high-level security mechanisms. As a result, the system faces severe challenges insecurity. Using physical-layer security technology is the feasible approach to solve this problem.In the domain of relay physical-layer security, there exist the following problems to solve.1)The relay system model cannot associate the relay channel and system security well. There arerelatively few researches on the system model of different scenarios.2) The rich channelcharacteristics constructed by multiple relay nodes haven’t been made full use of. Researches onmulti-relay cooperation are not enough.3) The use of channel characteristic difference betweenrelay and legitimate receiver is insufficient. The work on untrusted relay node still needs to beimproved. To solve these problems, this paper concentrates in the physical-layer security incooperative relay system. We study multi-node secure transmission mechanism with untrustedrelay in two scenarios where source node has multiple antennas or only a single antenna. Themain work is as follows:1. When the sender has multiple antennas, we propose a physical layer security transmissionmechanism of relay system based on joint channel characteristics and artificial noise. First of all,we combine the two channels before and after the relay into one, and get the joint channelcharacteristics. Then in the null space of the channel characteristics, the artificial noise is addedto make the internal wire-tappers and external eavesdroppers get none of the information.Secrecy capacity and optimal signal transmission power ratio are calculated. Simulation resultsshow that secrecy capacity first increase with the signal transmission power ratio and thendecrease. The trend of the secrecy capacity, changed by increasing relay number, relates to thedistribution of eavesdroppers.2. In the same scenario as above, we propose another physical-layer security mechanismbased on the randomized diversity of relays. By use of complex channel characteristicsconstructed by multi-relay nodes, we can control the received signal of relays through adjustingthe antenna weights. Legitimate receiver merges all the signals from the relays, gets the originalinformation, and obtains the diversity gain. Every relay’s received signal contains randomweights, so that the relay cannot understand the information. The optimal antenna transmissionweights are derived. Simulation results show that the bit-error ratios of eavesdroppers are around50%. Contrast to last mechanism, this mechanism is a little worse in performance but easier touse.3. When the sender has only a single antenna, a physical-layer security mechanism based oninterference cancellation in time-division by external nodes is proposed. The transmission isdivided into two phases: first, signals with artificial noise from the sender and interference nodesare sent to the relay; second, the original information is restored by receiver with the help ofinterference nodes. Since the signals received by the relay and the eavesdropper are always accompanied with severe interference, the system’s security is well guaranteed. Simulationresults show that: when the eavesdropper and interference nodes are in different locations, thesystem always has a large secure-transmission rate; when interference nodes are in the vicinity ofthe sender, the system has the best security performance.