## 2 Related Work  
Recently, various studies have been published in the area of photon-counting systems. In [17], the design of a MIMO system under the Poisson model is considered, in which a pulse-position modulation (PPM) modulation is used at the transmitter side, a maximum likelihood (ML) detection algorithm under the Poisson model is derived, and optimal as well as sub-optimal decoders are given. In [18], a receiver based on the linear least mean square error (MSE) criterion for FSO communication is presented. The system BER performance is analyzed under on-off keying (OOK) and PPM modulation. In [19], the statistical behavior of underwater fading with different probability density functions (PDFs) is studied. In [20], the authors present a composite quantum iterative multistage measure and MIMO detector at the receiver. In [21], the analytical error probabilities for ML detection in turbulent and non-turbulent cases are derived. In [22], to minimize the probability of detection errors given relay forwarding power budget, a counting and forward relay framework for NLOS communication is presented. In [23], the communication capacity and performance of the Poisson model are investigated. And an upper limit of capacity and a lower limit of error probability are proposed. In [24], the photonic information rate is investigated for a single-photon avalanche diode (SPAD) array, and the effect of dead time on the system is considered. The results show that the photon-counting distribution can be regarded as a Gaussian distribution for sufficiently large arrays. The conventional millimeter-wave hybrid precoding design method mainly decouples the hybrid precoding problem into transmitter-side hybrid precoding design and receiver-side hybrid combiner design, and it regards these two subproblems as matrix decomposition problems, respectively. The hybrid precoding problem is solved by minimizing the Euclidean distance between the product of the analog and digital matrices and the optimal all-digital precoding matrix under unconstrained conditions. In [25], a hybrid precoding method based on the alternating directional multiplier method (ADMM) is proposed. In [26], the symbol-level precoding of a MU-MISO downlink system is investigated. In [27], the objective function of the considered precoding scheme is highly nonconvex and has some complicated constraints, and the authors design a new method based on the ADMM. The convergence conditions of the proposed method are given by considering the order of iterations of the variables. In [28], the design of the symbol-level precoder for MU-MISO downlink communication is investigated, and its decision boundary is studied through the minimum maximum fairness design. To deal with this problem, the dimensionality of the variables is firstly reduced by solving a relaxation problem, after which the ADMM framework is used to efficiently solve the problem. UV has the characteristics of high interference immunity, good confidentiality, and support for non-visual transmission, thus attracting the attention of a large number of researchers. In [29], the authors calculate the optical loss in the FSO communication system operating in the non-visual range, where scattering effects were taken into account, and the obtained results were compared with experimental data at 265 nm (solar-blind UV region). In [30], a 1 \(\times\) 4 communication system was built. The experimental results show that equal-gain combining can provide significant diversity gain, provided that the transmit elevation angle is small or the transceiver distance is short. In [31], wireless UV communication is applied to UAV communication systems to solve the problem of directional awareness. In [32], the spatial diversity technique for the NLOS UV system is investigated, and the BER for different transmitter and receiver configurations is derived.