In underwater optical wireless communication (UOWC) systems, using single photon avalanche photondiode (SPAD) as the detector can improve the transmission distance. However, the signal detection for SPAD-based systems is greatly challenged by the complex optical channel characteristics and SPAD nonlinear distortion. To address this issue, a novel deep learning aided signal detection scheme is proposed in this paper. By exploiting the physical mechanism and prior expert knowledge of the signal processing, a two-connected multilayer perception (MLP) network is integrated into the receiver. The first subnetwork is regarded as a channel compensation block while the second one works as a demodulator. With sophisticated numerical optical channel model and SPAD non-Poisson model, large amounts of training data are utilized to train the proposed model offline. Afterwards, the online data are recovered with the trained network. Simulation results verify that significant bit error ratio (BER) improvement can be achieved with the proposed scheme.

Integrated sensing and communication (ISAC) is a key technology in the new generation of wireless communication systems. The ability to ensure robust sensing performance for different targets becomes essential for ISAC systems. However, existing works have not been successful in guaranteeing this ability. In this letter, we propose a novel robust joint beamforming scheme to address this issue. Our approach formulates the robust joint beamforming problem as a min-max optimizer, aiming to minimize the maximum Cramér-Rao bound (CRB) of multi-target direction of arrival (DOA) estimation while ensuring the signal-to-interference-plus-noise ratio (SINR) of multi-user communication. The intractable min-max matrix diagonal element problem can be elegantly transformed into an eigenvalue optimization problem, which can be solved using a semidefinite relaxation (SDR) technique. Numerical results demonstrate that our proposed scheme can achieve robust multi-target DOA estimation for different targets, regardless of SINR thresholds and the number of communication users.

Underwater visible light communication (UVLC) is a promising technique to provide high-speed data transmission in the water. Since it is quite difficult to supply power to the underwater devices in the complex and severe marine environment, energy consumption would be a main consideration in the practical deployment of UVLC system. In this paper, we investigate an asymmetric clipped optical orthogonal frequency division multiplexing and non-orthogonal multiple access (ACO-OFDM-NOMA) scheme for the uplink UVLC system to realize energy saving and support massive device connectivity. A mixed integer nonlinear programming problem of jointly optimizing the user-subcarrier pairing and power allocation is formulated to maximize the system utility. To tackle this problem, we propose a low-complexity iterative algorithm to obtain an optimal solution. Simulation results verify that the proposed algorithm converges quickly, and the uplink ACO-OFDM-NOMA system using the proposed algorithm has better utility compared to the uplink ACO-OFDM-NOMA system using the greedy algorithm and the traditional uplink ACO-OFDM access (ACO-OFDMA) system.

In this paper, a promising phase-modulated underwater wireless optical communication (UWOC) system with silicon photomultiplier (SiPM) based receiver is proposed for the first time and its feasibility has been experimentally demonstrated in a laboratory environment. The phase modulation enables the additional degree of freedom available for encoding of information in UWOC, given that previous state-of-the-art systems mainly employ intensity modulation (IM) schemes with direct detection (DD). In the proposed UWOC system, the information is encoded in the phase of light wave carrier based on differential phase-shift keying (DPSK). A highly sensitive receiver is built from SiPM, which is able to compensate the transmission loss and dramatically enhances the performance of the DPSK UWOC system. We experimentally show the feasibility of the proposed system at a data rate of 200 Mbps. For comparison, the commonly used avalanche photodiode (APD) based receiver is also tested. Comparing with the APD, the use of SiPM reduces the BERs by about two orders of magnitudes. The minimum required optical power for achieving a BER below the FEC threshold is about −40.2 dBm, which is about 11.6 dB lower than the case of APD.

The silicon photomultiplier (SiPM) based orthogonal frequency division multiplexing (OFDM) system is a promising solution to realize both the longer distance and higher data rate in visible light communication (VLC). However, it suffers from nonlinear distortions induced by SiPM. The Volterra nonlinear equalization (VNLE) is commonly used at the receiver to compensate for the nonlinear distortions, but it has a huge implementation complexity. As an alternative, the memory polynomial based equalization (MPE) offers significantly reduced complexity but at the cost of degrading BER performance. In this paper, we propose and demonstrate a promising simplified VNLE equalizer for SiPM based OFDM-VLC systems according to SiPM's characteristics. Results show that the proposed equalizer outperforms the MPE in terms of BER and significantly reduces the computational complexity of the conventional VNLE. With the proposed equalization scheme, a data rate of 1.35Gbps has been achieved. To the best of the author's knowledge, this is the highest data rate achieved using SiPM in VLC with optical OFDM.

The orthogonal frequency division multiplexing (OFDM), which has high spectral efficiency, is an attractive solution for silicon photomultiplier (SiPM)-based optical wireless communication (OWC) systems to boost data rates. However, the currently available SiPMs are not optimized for implementing the OFDM receiver. Incorporating different types of SiPM at the OFDM receiver results in different data rates at the same condition. Therefore, the receiver designer requires a method for predicting the performance of SiPMs and then selects the best one to build the optimum receiver. In this paper, we first investigate the origin of SiPM’s power-dependent frequency response. The investigation outcome is then used to create a method for predicting the subcarrier SNR. Combining the estimated subcarrier SNR with the bit-loading scheme, we finally propose a general approach for estimating the fundamental OFDM data rate an SiPM chip can support at a given received power. Results are then presented that can be used by the future receiver designer as a guideline to find the best type of SiPM to build the optimum OFDM receiver.