# Unified Performance Analysis of Direct Air-to-Underwater RF/UWOC Systems
## 1 Introduction  
Underwater wireless optical communication [1; 2; 3; 4] has become a suitable and efficient transmission solution with high security, high data rate, low energy consumption, low cost, and short delay. In recent years, hybrid RF/UWOC systems have been widely used due to their ability to meet various requirements for high-speed underwater data transmission [5; 6; 7]. Collaborative RF/UWOC communication systems generally use amplification forwarding (AF) or decoding forwarding (DF) to establish connections between underwater vehicles and ground base stations within the ocean. Specifically, the source node sends signals to relay buoys with communication, sensing, and processing capabilities through FSO/RF links. Subsequently, the relay converts the electrical signal into an optical signal and sends the signal to the terminal through the UWOC link. Asymmetric Cloned Short-Term Memory (LSTM)-Depth Determined Policy Gradient (DDPG) algorithm is used to solve traversal and rate optimization problems in satellite-air-ground integrated network systems [8]. A dual-hop system module based on single hop and a space-air-ground integrated network (SAGIN) was proposed for uplink satellite communication with hybrid FSO/RF links [9]. The power-adaptive strategy based on truncated channel reversal for hybrid FSO/RF systems using an adaptive combination is adopted [10]. A wireless backhaul hybrid transmission [11] system used an opportunistic selection combination (OSC) and a maximum-ratio combination (MRC) with decoding and forward (DF) protocols. An adoption \(\alpha-\mu\) communication of distributed and AF protocols is considered [12]. The fading RF link is mixed with a pointing error, while the FSO link adopts the performance of a dual-hop hybrid RF/FSO system with Nakagami-\(m\) fading [13]. This paper analyzesthe channel capacity of the Space-to-Ground (SAG)-FSO transmission/Hybrid Single-Hop (SH)-FSO/Radio Frequency (RF) transmission (SAG-FSO/SH-FSO/RF) communication system [14]. It proves that direct detection intensity modulation has more advantages than heterodyne detection technology. The pointing error caused by the beam drift significantly impacts the performance of communication systems, and there are many types of pointing errors in free space. Considering the influence of the pointing error in free space on strong atmospheric turbulence under nonzero line of sight [15], [PERSON] et al. used the pointing error of the generalized nonzero positive distance model to analyze the precise performance of \(\mathcal{F}\) distribution in atmospheric turbulence through assisting AF relay [16]. In addition, we also analyzed the effects of atmospheric turbulence and pointing errors on optical communication systems under foggy conditions. The system outage probability performance was analyzed under fog and pointing error conditions [17]. An RF/UWOC system with a fixed-gain amplification relay has been studied, and new closed-form expressions for CDF and PDF with end-to-end signal-to-noise ratios have been derived [18]. The UWOC chain uses spatial diversity, and the precise and upper bound error rate (BER) expressions for optimal and equal-gain combinations are obtained [19]. In atmospheric turbulence, different distribution functions represent different levels of turbulence intensity. Several distribution functions were unified, and the statistical results of atmospheric turbulence channels with pointing errors under random fog conditions were derived and expressed in the form of multidimensional Fox-H functions [20]. Recently, the mixture exponential generalized gamma (EGG) distribution model [21] was proposed to describe UWOC links. The author applied the moment generating function (MGF)-based method [22] to analyze the performance of Alamouti codes based on mixed exponential generalized gamma distribution. The performance of multi-input multi-output (MIMO) systems was analyzed using the EGG channel model combined with the PDF-based [23] method.