# Angular Circle Array Multiple Input Multiple Output Underwater Optical Wireless Communications +
## 1 Introduction  
Underwater wireless communication (UWC) refers to the transmission of data through wireless carriers in an underwater environment, primarily including underwater acoustic communication, underwater radio frequency (RF) communication, and underwater wireless optical communication (UWOC). Although underwater acoustic communication is commonly used for wireless links [1], it has limitations such as low data transmission rates, high latency, and high energy consumption [2]. Additionally, RF communication suffers from high attenuation, which increases dramatically with frequency [3]. Considering the need for high-speed transmission, UWOC technology is a feasible alternative. UWOC not only uses an ultra-wide range of unregulated spectrum to overcome spectrum crisis but also offers advantages like high confidentiality, small footprint, and low latency [4]. Early UWOC modeling studies focused on single-input single-output (SISO) communication systems in case of line-of-sight (LOS) links. [PERSON] et al. [5] demonstrated 1 Gbps underwater laser communication over a 2 m path in a laboratory environment. Since then, UWOC has become an emerging topic, and researchers have explored higher data rates and longer transmission distances. [PERSON] et al. [6] successfully achieved a data rate of 500 Mbps through a 100 m tap-water channel by using a green laser diode (LD) and non-return-to-zero on-off keying (NRZ-OOK) modulation. [PERSON] et al. [7] demonstrated a20.09 Gbps transmission over a 1.2 m underwater link with PS bit-loading DMT modulation. Additionally, [PERSON] et al. [8] pushed the transmission distance and data rate to 100.6 m and 3 Gbps, respectively, using a wideband photomultiplier tube. Recently, our group proposed a MIMO scheme, which increased the channel capacity [9]. In fact, UWOC terminals are mostly carried on mobile platforms with low load capacity, such as autonomous underwater vehicles (AUVs) and frogmen. The swaying of these platforms with the currents is inevitable, making the establishment and maintenance of the UWOC link challenging. Additionally, ocean optical applications, such as undersea monitors and tsunami warning systems, require extensive coverage areas for the UWOC systems [10]. To address these issues, one approach is to increase the beam divergence angle and receiver field of view (FOV). However, this leads to more severe absorption and scattering introduced by underwater channels, increasing link loss and time dispersion, thus shortening the communication distance and reducing the channel capacity of the communication systems, especially in turbid water. Traditional wireless communication often uses higher-order modulation to address limited channel capacity, but this method of trading power for bandwidth is not practical in underwater environments with high attenuation. Multiplexing techniques play a vital role in increasing the channel capacity by utilizing spatial freedom resources to combine multiple low-speed data streams into one high-speed stream. Therefore, several researchers have further investigated UWOC using MIMO techniques. [PERSON] et al. [11] studied the performance of a UWOC system using on-off keying modulation at a data rate of 500 Mbps over a link range of 30 m. The system uses a transmit/receive diversity scheme, and the closed-form analytical bit error rate (BER) expressions of single-input single-output (SISO), single-input multiple-output (SIMO), MISO, and MIMO links for un-coded and RS-coded cases have been computed using the hyperbolic tangent distribution and validated with Monte Carlo simulation results. Their results show that the MIMO technique effectively reduces the BER of UWOC systems. [PERSON] et al. [12] obtained the outage probability expression for a UWOC MIMO communication system in analytical form using the Meijer-G function. In addition to diversity techniques, researchers have also investigated multiplexing techniques for MIMO communication systems. [PERSON] et al. [13] developed a 2 \(\times\) 2 UWOC MIMO channel model.