Abstract-In this paper, we investigate the secrecy performance for a multiple-input multiple-output (MIMO) wiretap channel in the presence of a multi-antenna eavesdropper. In particular, the legitimate transmitter uses transmit antenna selection (TAS) to transmit on a single antenna with the largest signal-to-noise ratio (SNR) while both the legitimate receiver and the eavesdropper adopt maximal ratio combining (MRC) for reception. We derive exact closed-form expressions for the probabilities of positive secrecy rate and secrecy outage with imperfect feedbacks due to feedback delay and/or error. Furthermore, we derive the asymptotic secrecy outage probability at high signal-to-noise ratio (SNR) for the transmitter-receiver channel, which accurately reveals the secrecy diversity loss due to imperfect feedbacks. Simulation results are provided to verify our analytical results and illustrate the impacts of imperfect feedback on the secrecy performance of such a wiretap system.

Visible light communication (VLC) is an evolving green communication technology for indoor wireless communication networks. VLC utilizes light-emitting diodes (LEDs) for both illumination and communication. To achieve economic and energy-efficient communications, non-orthogonal multiple access (NOMA) improves the system throughput by enhancing spectral efficiency. In indoor VLC networks, the gain ratio power allocation (GRPA) of NOMA allocates greater signal power to the users of poor channels to achieve both fairness and high data rate. However, current GRPA strategies developed for radio frequency channels are not necessarily effective in all VLC channel and illumination cases. This study makes the following contributions. First, a GRPA strategy based on VLC channels is proposed that modifies the power counting and channel proportions enhance the NOMA-VLC throughput. Second, owing to difficulties of directly analyzing NOMA-VLC throughput, an alternative lower bound based on a VLC channel model to facilitate throughput comparisons is proposed. It is proved that the proposed alternative lower bound is asymptotic and compact with respect to NOMA-VLC throughput. Third, for the case of LED heights greater than 1 m, a local extremum solution using the proposed strategy is analytically demonstrated based on the alternative bound of VLC network throughput. Then, for the case of LED heights smaller than 1 m, the necessary but not sufficient condition under which the proposed strategy outperforms previous strategies is provided in VLC scenarios. Finally, the experimental results demonstrate the compactness of the proposed alternate lower bound and the advantages of the proposed strategy. INDEX TERMS Visible light communication, non-orthogonal multiple access, gain ratio power allocation, illumination scenario.

Non-orthogonal multiple access (NOMA) is a promising method for enhancing the throughput in visible light communication (VLC) networks. In NOMA, signal power domain control, called gain ratio power allocation (GRPA), can significantly improve the user sum rate with full-time frequency resource utilization. In indoor NOMA-VLC networks, the scenario in which users are covered by VLC illuminants on the ceiling is typical. First, this paper proposes a novel GRPA strategy for a single VLC cell. Second, due to the difficulty of direct comparisons between our and previously reported GRPA strategies, this paper presents an alternative lower bound for comparability. Third, in two-and threeuser cases, this paper analytically demonstrates that our NOMA-VLC GRPA strategy outperforms the aforementioned strategy, when successive interference cancellation (SIC) is used. Moreover, in the multi-user case, experimental results show that the average user data rate (AUDR) of multi-user cases under our strategy is better than that under the previous strategy. Finally, our paper provides both analytic and numerical results to prove that VLC single-cell system throughput under our strategy is better than that under the previous strategy. The proposed alternative lower bound is also proved to fit the original NOMA-VLC GRPA target asymptotically based on indoor VLC channels. Furthermore, to achieve the same AUDR, our strategy supports worse received conditions than does the previous strategy.

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