In this paper, we propose a novel computational method, termed Monte Carlo Integration with Common Volume Sampling (MCI-CVS), which enables direct photon sampling within the common scattering volume. This approach significantly accelerates the convergence of optical wireless scattering channel simulations. Although each sample in MCI-CVS incurs approximately 3.74 times the computational cost of conventional methods, numerical results for ultraviolet scattering channels demonstrate that it reduces the required number of samples by a factor of 1,000 compared with Monte Carlo Integration with Importance Sampling (MCI-IS). As a result, MCI-CVS achieves an overall efficiency gain of approximately 267.3 times faster than MCI-IS and about 1,206 times compared to the baseline Monte-Carlo Simulation (MCS) method. Furthermore, the proposed method effectively addresses inefficient photon path sampling in infrared scattering channels, where low scattering and absorption coefficients degrade sampling efficiency and thus result in slow convergence. By incorporating photon path reuse, MCI-CVS also enables fast generation of two-dimensional scattering fields on a standard personal computer.

This paper demonstrates an automatic beam alignment system for a laser diode (LD)-photomultiplier tube (PMT) based underwater optical wireless communication (UOWC) system. Initially, the imaging characteristics of the underwater laser spot are described. Subsequently, a laser outlet estimation algorithm based on principal component analysis (PCA) is proposed. The direction of the emitted light beam is rapidly determined by reflecting it with a micro-electro-mechanical system (MEMS) mirror to align with the receiver. Experimental results validate the feasibility of the proposed beam alignment method, indicating an acceptable alignment error. Moreover, the proposed system effectively enhances the bit error rate (BER) performance, meeting the BER requirements of the forward error correction (FEC) limit when the receiver moving speed is below 0.45 m/s.

In this paper, we design and demonstrate a beam tracking system on the receiver side for ultraviolet (UV) communication using a single photomultiplier tube (PMT). Specifically, we investigate the properties of the angle-domain channel for UV communication. We characterize the effects of metallic housing and the spatial non-uniformity of the PMT on the angle-domain channel, both of which can significantly impact the channel’s performance. To adjust the orientation of the PMT and maintain satisfactory communication in mobile UV systems, we propose a beam alignment algorithm based on coordinate search and a beam tracking algorithm based on pattern search. A real-time system is developed to test the performance of these algorithms. Experimental results show that the proposed beam alignment algorithm can quickly search for the incident direction, while the beam tracking algorithm effectively tracks a moving transmitter at a rate of 5.5 degrees/sec, ensuring a reliable communication link. The proposed beam alignment and tracking scheme uses only a single PMT, making it lighter and more cost-effective compared to the PMT array scheme, and is thus well-suited for deployment in compact UV communication terminals.

Avalanche photodiodes (APD) are key devices in optical communication, and its nonlinear characteristics have a significant impact on the communication performance of the system. Based on the detection principle of photodetector, we characterize the nonlinearity of avalanche photodiodes, and demonstrate the theoretical prediction results in experiments. For the APD module integrated with trans-impedance amplifer (TIA), the nonlinear AC signal response caused by amplifier saturation is observed and explained. Based on the nonlinear characteristics of the photodetector, we can reasonably set the DC operation point and ensure the signal extraction in nonlinear region.

Typically, a photomultiplier tube (PMT) can be leveraged as the photon detector in the underwater optical wireless communication (UOWC) system to further extend the transmission distance, whose output electric signal can be generally classified into three regimes named discrete-pulse regime, transition regime, and continuous waveform regime according to various received optical power (ROP). In this paper, we experimentally demonstrate a low-power UOWC system that adopted a PMT as the receiver, and a detection method by taking the cumulative sum of the sampled values in a symbol duration is proposed and experimentally evaluated under different ROP and symbol rates. Experimental results show that the proposed detection method outperforms various statistics-based comparison benchmarks. Furthermore, the effectiveness of the proposed method in alleviating strong ambient radiation is experimentally verified.

For an underwater optical wireless communication (UOWC) system, alignment between transmitter and receiver is indispensable, which raises high requirements on the positioning technique. In the practical marine environment, light travels in a bending trajectory rather than in a straight line due to the non-uniformity of the refractive index of seawater, which leads to the position error. In this work, the position error under light beam bending due to refraction index variation is investigated concerning a UOWC system, and a simplified layered model is introduced to approximate the continuous refractive index variation in the practical ocean scenario. Besides, an approximated bending light path function is derived as well. Experimental results showed that a great consistency exists between the predicted positioning error and the measured one, and the potential causes of derivation between these results are further analyzed.

Sampling clock offset (SCO) is a key factor affecting the transmission performance of a vehicular visible light communication (VLC) system. For the traditional on-off keying (OOK) scheme, timing recovery is required for mitigation of the SCO effect in a vehicular light-emitting diode (LED) based VLC system, which suffers nonlinearity. To investigate the impact of the LED nonlinearity on the performance of the timing recovery, a piecewise-parabolic interpolator-based timing recovery method with three different timing error detectors (TEDs) is used to evaluate the transmission performance of 25Mb/s OOK signal in a real-time vehicular VLC system with SCO. The impact of ambient light on the transmission performance is also investigated. It is shown that a SCO of up to ±1000ppm can be effectively compensated with the timing recovery with these TED schemes. A real-time 25Mb/s OOK system is successfully demonstrated over a VLC link up to 60m at a bit error ratio below 3 × 10-3.