## 1 Introduction  
The introduction of a single photon avalanche diode (SPAD) in the optical wireless communication (OWC) system can effectively expand communication distance [1, 2], and reliable transmission can be established in the case of the limited signal power [3]. However, the lower bound of the SPAD receiver bit error rate (BER) is determined by the background light. The SPAD detector cannot distinguish the background photon and signal photon only in one individual avalanche pulse, which requires a longer symbol time to improve the accuracy of the signal transmission by recording a mass of pulse counts, resulting in a low data rate of the SPAD receiver [4]. In addition, the influence of dead time on SPAD communication performance cannot be ignored. Dead time causes the counting loss and counting block. In a certain symbol period, the dead time after a photon arrival affects the detection of the next photon, resulting in the counting loss. In adjacent symbol periods, dead time may lead to inter slot interference (ISI), resulting in count blocking [5, 6]. The symbol time of SPAD is generally longer than the dead time, which makes it difficult to realize high-speed applications. How to use a high-sensitivity SPAD to realize high-speed communication under noisy ambient light conditions has attracted increasingly widespread attention. In order to realize high-speed communication under the background light, array-receiving pattern can be used for SPAD to expand the dynamic range [7]. Ref. [8] uses a SPAD array with 60 pixels to realize high-speed transmission in 800 lx background light flux intensity, with the maximum data rate at 200 MB/s. In Ref. [9], the author concluded that for SPAD arrays with the same total effective detection size, the dynamic range is positively correlated with the number of pixels. In order to achieve a high performance detection under the intensive background light condition, the SPAD's array scale has to be increased. This approach may lead to a decrease in SPAD detection efficiency. Ref. [10] proposes that if the photon detection efficiency of SPAD array is higher than 14%, the receiver based on the SPAD array has better sensitivity than the APD receiver in background light. Although large-scale SPAD arrays can achieve faster data rates and larger dynamic range, the effect of decreased detection efficiency on the performance of the receiver cannot be ignored [11]. Although the receiver photon detection efficiency may be affected, the improvement of the dynamic range can provide a stronger ability to resist shot noise. Applying photon detection techniques with large dynamic ranges to the communication field to enhance the anti-background lighting capacity of the receiver is a potential solution, such as the time-gated single photon detection technology [12]. The time-gated SPAD detection scheme can be used to improve the dynamic range of time-correlated single photon counting (TCSPC) [13]. Laser radar and biomedical devices also reduce the photon incidence of specific time through the time-gated scheme to improve the detection accuracy [14]. At present, external time-gated techniques have been widely studied in the photon detection field [15; 16], but the use of time-gated schemes in optical wireless communication systems is still in the initial research stage [17; 18]. In communication applications, time-handed technology can effectively reduce the effects of background photons. However, time-gated techniques reduce the impact of background photons by decreasing exposure time, but limited exposure times also reduce the incidence of signal photons [19]. Therefore, we build the photon count model and BER model of the time-gated mode based on the photon counting characteristics of the SPAD. We use the minimum BER as the optimization target, give the optimal gate-ON time according to the signal photons, background photons and blocking time to optimize the communication performance of the SPAD receiver.