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The effects of hardware impairments (HWIs) on the secrecy performance of a reconfigurable intelligent surface (RIS)-assisted millimeter wave system are investigated, where a multi-antenna base station (BS) transmits confidential messages to a single-antenna mobile user (MU) in the presence of a single-antenna passive eavesdropper (Eve). Artificial noise (AN)-aided transmission by the BS is proposed for improving the achievable secrecy rate. As the first step, we construct a system model in the absence of HWIs. For this case, we present optimal solutions for the signal and AN powers, the beamforming design at the BS, and the phase shifts of the RIS elements. Then closed-form expressions are derived for the cumulative distribution functions of the signal-to-noise-ratios experienced at the legitimate and eavesdropping nodes. Finally, a compact solution is presented for the ergodic secrecy rate (ESR) performance. At the next step, we extend our designs and analysis by considering the HWIs of the radio-frequency (RF) modules of the BS, MU and Eve. Our results highlight the detrimental impact of HWIs on the achievable ESR. Finally, we find that the ESR can be further increased by beneficially distributing the tolerable HWIs between the transmission and reception RF chains.

In this paper, the authors deal with secure transmission in two-hop amplify-and-forward network with untrusted relays. To prevent the untrusted relays from intercepting the source message and to achieve positive secrecy rate, the destination-based cooperative jamming technique is used. The authors propose joint secure best relay selection (BRS) and optimal power allocation (OPA) for uplink transmission of a cellular network when the base station is equipped with very large-scale antenna arrays. Then, the common performance metrics such as ergodic secrecy rate (ESR) and secrecy outage probability are evaluated for the optimised network. Moreover, the diversity order and diversity-multiplexing tradeoff of the system are examined. The authors obtain that the diversity order of joint BRS and OPA scenario is equal to number of relays. Finally, some numerical and simulation results are presented to validate the theoretical analysis and to show the advantages offered by the presented systems. They reveal interesting results that, by increasing number of untrusted relay nodes, the system performance improves. For example, for target ESR 2 bits/s/Hz and ten untrusted relay nodes, the optimised network saves about 5 dB for uplink transmission, in comparison with the system applying the BRS and equal power allocation.