Theoretical and experimental studies on channel impulse response of short-range non-line-of-sight ultraviolet communications.

A theoretical channel impulse response (CIR) model of short-range non-line-of-sight (NLOS) ultraviolet communications (UVC) in noncoplanar geometry under the single-scatter condition is proposed. Simulation results obtained from the widely accepted Monte-Carlo (MC)-based channel model of NLOS UVC are provided to verify corresponding theoretical results obtained from the proposed theoretical single-scatter CIR model. Additionally, an outdoor experiment with a light-emitting diode (LED) as the light source is first designed to measure the channel step response of NLOS UVC and to further validate the proposed theoretical single-scatter CIR model. By varying the different parameters of the transmitter and the receiver, such as the baseline range, the inclination angle, the azimuth angle, the beam divergence angle, and the field-of-view angle, the results of the proposed theoretical single-scatter CIR model and the MC-based channel model are exhibited and further analyzed in detail. Results indicate that the computational time cost by the proposed theoretical single-scatter CIR model is decreased to less than 0.6% of the MC-based one with comparable accuracy in assessing the temporal characteristics of NLOS UVC channels. Additionally, theoretical results obtained from the proposed theoretical single-scatter CIR model manifest satisfactory agreement with corresponding experimental measurements.

Dynamic Spectrum Sharing Based on Deep Reinforcement Learning in Mobile Communication Systems.

The rapid development of mobile communication services in recent years has resulted in a scarcity of spectrum resources. This paper addresses the problem of multi-dimensional resource allocation in cognitive radio systems. Deep reinforcement learning (DRL) combines deep learning and reinforcement learning to enable agents to solve complex problems. In this study, we propose a training approach based on DRL to design a strategy for secondary users in the communication system to share the spectrum and control their transmission power. The neural networks are constructed using the Deep Q-Network and Deep Recurrent Q-Network structures. The results of the conducted simulation experiments demonstrate that the proposed method can effectively improve the user's reward and reduce collisions. In terms of reward, the proposed method outperforms opportunistic multichannel ALOHA by about 10% and about 30% for the single SU scenario and the multi-SU scenario, respectively. Furthermore, we explore the complexity of the algorithm and the influence of parameters in the DRL algorithm on the training.

Single-collision-induced path loss model of reflection-assisted non-line-of-sight ultraviolet communications.

By considering both scattering and reflection events as collision-induced events (CIEs), an analytical path loss model of reflection-assisted non-line-of-sight (NLOS) ultraviolet communications (UVC) is proposed with single CIE incorporated, which refers to the single-collision-induced (SCI) path loss model. More specifically, the analytical expressions of the received optical energy resulting from single-scatter and single-reflection events in reflection-assisted NLOS UVC are respectively derived. Then, in terms of those two expressions, the expression of the proposed SCI path loss model is obtained. Finally, Monte-Carlo (MC) simulations and experimental results are given to verify the correctness and effectiveness of the proposed SCI path loss model. The results manifest that the proposed SCI path loss model can work well in both coplanar and noncoplanar geometry of the reflection-assisted NLOS UVC.

Single-scatter path loss model of LED-based non-line-of-sight ultraviolet communications: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 4013 (2021)OPLEDP0146-959210.1364/OL.435418.

Single-scatter path loss model of LED-based non-line-of-sight ultraviolet communications.

Light-emitting diodes (LEDs) are popularly used as light sources in non-line-of-sight (NLOS) ultraviolet communications (UVC). However, currently reported single-scatter path loss (PL) models of NLOS UVC links assume that the radiant intensity of the light source is uniformly distributed within the beam divergence angle, which cannot well characterize the light emission pattern of LEDs. In this Letter, we propose a single-scatter PL model for LED-based NLOS UVC systems, and the corresponding analytical expression is derived by modeling the LED emission pattern as a Lambertian distribution. Monte Carlo simulations and experimental results are provided to verify the effectiveness of the proposed PL model. Results show that the proposed model can accurately predict PL results in actual situations. The root-mean-square error of the proposed PL model is only about 1 dB in typical scenarios with experimental results as benchmarks. Also, the proposed model is much easier to implement than the currently reported PL model with the light source model of a uniformly distributed emission pattern.

Flexible NOMA-based NOHO-OFDM scheme for visible light communication with iterative interference cancellation.

In this paper, a flexible non-orthogonal multiple access (NOMA) based optical orthogonal frequency division multiplexing (OFDM) modulation scheme, called non-orthogonal hybrid optical OFDM (NOHO-OFDM), is proposed to increase the achievable data rate of the visible light communication system with high efficiency. In addition, a receiver with iteratively successive interference cancellation (ISIC) is investigated, which can reduce the estimation error. Then, the achievable data rate of the proposed NOHO-OFDM with the ISIC scheme is analyzed. Experiment results show that the bit error rate of the NOHO-OFDM can be significantly reduced by the proposed ISIC scheme, and the NOHO-OFDM is superior than the orthogonal scheme in terms of data rates. Meanwhile, simulation results show that the achievable data rate region of the proposed NOHO-OFDM scheme is larger than that of the orthogonal counterpart.