Secrecy enhancement of multiuser MISO networks using OSTBC and artificial noise

In this paper, we propose a novel physical layer strategy to improve the secrecy performance of multiuser multiple-input single-output networks. In this strategy, orthogonal space-time block code (OSTBC) is employed at an A_A-antenna base station and artificial noise (AN) is employed at an A_J-antenna cooperative relay to enhance the security level of the network. Moreover, two opportunistic scheduling schemes, namely selection combining (SC) and scan-and-wait combining (SWC), are leveraged to select one legitimate user for data transmission. To evaluate the secrecy performance of the proposed OSTBC-SC-AN and OSTBC-SWC-AN schemes, we derive new exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput. Using numerical results, we show that the OSTBC-SWC-AN scheme outperforms the OSTBC-SC-AN scheme when the switching threshold is carefully chosen, while the OSTBC-SC-AN scheme is independent of the switching threshold. We also show that increasing A_A at a low switching threshold or decreasing A_A at a high switching threshold achieves secrecy enhancement for the OSTBC-SWC-AN scheme, while increasing A_A brings better secrecy performance for the OSTBC-SC-AN scheme.