Diversity reception technology is introduced into ultraviolet communication area in this article with theory analysis and practical experiment. The idea of diversity reception was known as a critical effective method in wireless communication area that improves the Gain significantly especially for the multi-scattering channel. A theoretical modeling and simulation method are proposed to depict the principle and feasibility of diversity reception adopted in UV communication. Besides, an experimental test-bed using ultraviolet LED and dual receiver of photomultiplier tube is setup to characterize the effects of diversity receiving in non-line-of-sight (NLOS) ultraviolet communication system. The experiment results are compared with the theoretical ones to verify the accuracy of theoretical modeling and the effect of diversity reception. Equal gain combining (EGC) method was adopted as the diversity mechanism in this paper. The research results of theory and experiment provide insight into the channel characteristics and achievable capabilities of ultraviolet communication system with diversity receiving method.

Visible light communication (VLC) is considered an important complementary technology for extremely high sixth-generation (6G) data transmission and has become part of a hybrid 6G indoor network architecture with an ultradense deployment of VLC access points (APs) that presents severe challenges to user mobility. An adaptive handover mechanism, which includes a seamless handover protocol and a selection algorithm optimized with a deep reinforcement learning (DRL) method, is proposed to overcome these challenges. Experimental simulation results reveal that the average downlink data rate with the proposed algorithm is up to 48% better than those with traditional RL algorithms and that this algorithm also outperforms the deep Q-network (DQN), Sarsa and Q-learning algorithms by 8%, 13% and 13%, respectively.

One primary challenge in wireless ultraviolet communications (UVCs) is the inter-symbol-interference (ISI), which may block the detection of current informative signal, especially when channel-related characteristics are unknown. In this paper, we propose a UV channel-related Bayesian scheme that can simultaneously estimate the channel characteristics and detect informative signals, which therefore can address the ISI disturbance. By investigating the UV single-scattering photon model, the dynamic behaviors of the channel state information (CSI), which involve the uncertain signal and the unknown channel parameters are formulated. Hence, a sequential Bayesian process is suggested to estimate the UV CSI. Numerical analysis shows that the proposed scheme can obtain a promising estimation performance (i.e., the relative errors are less than 4%), and gain an extra 4dB detection performance compared with imperfect maximum-likelihood sequence detection (MLSD) scheme.

A non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is established by using a Monte Carlo simulation method based on photon tracing. This method considers the multiple-scattering effects of UV signal propagation in the atmosphere, and simulates the condition of dual receivers for diversity reception. The channel characteristics of the UV communication are obtained by simulating the photon arrival probabilities. The model is employed to study the characteristics of NLOS UV scattering channels for a variety of scattering conditions, including the separation distance between transmitter and receiver, transmit/receive elevation angle, beam divergence, and field of view. The model has advantages in reliable prediction of UV communication for the dual-receiver condition, as validated by outdoor experiments at fixed elevation angles.

A digital pre-distortion waveform shaping scheme combined with a blue filter is proposed to optimize both the rise and fall times of a light-emitting diode (LED) and the optical receiver current of the signal of the real-time visible light communication (VLC) system. The proposed scheme is implemented on a field-programmable gate array (FPGA) and a digital-to-analog converter-based testbed, which is flexible and reconfigurable by programming the FPGA to match different LED characteristics and varied data rates. A 262-Mbps non-return-to-zero on-off keying modulation-based real-time VLC-link with a bit error rate of less than 1.0$\times$10$^{-6}$ is achieved over a transmission distance of 5.0 m, which uses a single white phosphorous LED with a limited power of 0.1-W.