We consider DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) low-pass visible light communication (VLC) systems in which water-filling and uniform power loading strategies have almost the same rate performance. However, conventional bit loading based on either water-filling or uniform power loading cannot reach the maximum achievable rate because the number of bits loaded per subcarrier can only be integers. In this paper, we apply pairwise coding (PWC) to solve this problem by performing joint coding on pairs of subcarriers to balance their symbol error rate (SER) performance. We present a closed-form expression for the optimal parameter of PWC and an approximation of its theoretical performance, and propose an enhanced bit loading scheme with PWC to improve the system rate. Numerical results suggest that the enhanced scheme has a significant rate improvement and approaches the maximum achievable rate for moderate to high signal-to-noise ratio (SNR).

Federated learning (FL) can train a global model from clients' local data set, which can make full use of the computing resources of clients and performs more extensive and efficient machine learning on clients with protecting user information requirements. Many existing works have focused on optimizing FL accuracy within the resource constrained in each individual round, however there are few works comprehensively consider the optimization for latency, accuracy and energy consumption over all rounds in wireless federated learning. Inspired by this, in this paper, we investigate FL in wireless networks where client selection and bandwidth allocation are two crucial factors which significantly affect the latency, accuracy and energy consumption of clients. We formulate the optimization problem as a mixed-integer problem, which is to minimize the cost of time and accuracy within the long-term energy constrained over all rounds. To address this optimization problem, we propose a per-round energy drift plus cost (PEDPC) algorithm from an online perspective, and the performance of the PEDPC algorithm is verified in simulation results in terms of latency, accuracy and energy consumption in IID and NON-IID data distributions.

In this paper, we consider the physical-layer security for single-input single-output (SISO) indoor visible light communication (VLC) wiretap systems in the presence of multipath reflections. We derive both the lower and upper bounds on the secrecy capacity in the context of both the symbol- and block-based transmission policies. To enhance the secrecy performance relying on block transmission policy, we propose a low-complexity amplitude scaling (AS) scheme by scaling the amplitudes of different symbols in each block to maximize the achievable secrecy rate. We further provide the upper bound on the optimal secrecy rate achieved by precoding for the sake of validating the effectiveness of the proposed AS scheme. Numerical results suggest that the secrecy performance is severely degraded by inter-symbol interference (ISI) imposed by multipath reflections, which yet can be alleviated by long-block based transmission. Moreover, the proposed AS scheme can enhance the secrecy performance significantly and achieve a secrecy rate very close to the optimal solution.

In this paper, we consider the artificial noise (AN) aided multiple-input single-output (MISO) secure systems in which a multi-antenna transmitter simultaneously transmits the combination of information-bearing signal and artificial noise to a single-antenna legitimate receiver and eavesdropper. At the transmitter front-ends, the signal and/or AN will be amplified by high-power amplifiers (HPAs) which however may work in a nonlinear region and introduce nonlinear distortion. We calculate the received signal-to-noise ratio (SNR) at legitimate receiver and eavesdropper under HPA nonlinearity, and derive the approximated closed-form expression of secrecy rate. Numerical results suggest that the approximated theoretical secrecy performance is very close to Monte Carlo simulations for relatively lower SNRs, and is severely degraded by the nonlinear distortion caused by HPAs as the transmit power increases. Moreover, we further provide a power allocation strategy between information-bearing signal and AN to alleviate the nonlinear distortion.