In recent years, wireless optical communication (WOC) has received increasing attention. WOC is applicable to diverse applications, including those pertaining to terrestrial and underwater communications. Terrestrial WOC is similarly known as free-space optical (FSO) communication. Compared with the existing radio frequency (RF) communication system, FSO systems offer the advantages of large bandwidth, high data rate, reusable equipment and wavelength, high security, and anti-electromagnetic interference. WOC applied in an underwater environment is known as underwater wireless optical communication (UWOC). Compared with underwater acoustic and underwater RF communications, UWOC offers a higher transmission rate, lower link delay, and lower implementation cost. Whether on land or underwater, turbulence fading significantly affects the performance of WOC systems. Transmit-laser selection (TLS) has been proposed as an effective technique for mitigating fading. For the typical TLS under ideal assumptions, the laser source with the highest instantaneous received signal-to-noise ratio (SNR), i.e., the best laser source, is selected for signal transmission. However, a suboptimal or worse source may be selected owing to channel-estimation and/or feedback errors. Additionally, to accommodate deep-ocean explorations, land platforms may be required to transmit information to underwater platforms through reliable wireless links, which are typically vertical links. Therefore, in this study, we propose a mixed communication system (FSO-VUWOC) comprising FSO communication and vertical UWOC (VUWOC) with generalized transmit-laser selection (GTLS) to realize land and ocean integrated communication. The GTLS considers the selection of non-optimal light sources due to various errors in the actual scene.

First, considering the atmospheric turbulence channel and the multilayer cascade vertical underwater turbulence channel based on a gamma-gamma distribution, as well as applying the Meijer G function and Gauss hypergeometric function, we derive the closed-form outage-probability expression for the FSO-VUWOC system with GTLS. Subsequently, by performing an asymptotic analysis of the outage probability under a high SNR, we obtain the diversity order of the system. Finally, the derivation results above are verified via numerical simulation.

The outage-probability and diversity-order performances of the system were simulated and verified, and the performances of the FSO-VUWOC system with GTLS (where the number of light sources was set to five) and without GTLS (where the number of laser sources was one) were compared. First, we analyzed the outage probability of the FSO-VUWOC system under different VUWOC and FSO link distances (Figs. 3 and 4). As the link distance increases, the outage probability increases rapidly. Moreover, the outage-probability performance of the GTLS system under different VUWOC and FSO link distances is significantly better than that of the system without GTLS. Second, we investigated the effect of the selected laser-source index on the outage-probability performance (Fig. 5). Compared with the system without GTLS, the system with GTLS yields better outage-probability performance. For the system with GTLS, its outage-probability performance deteriorates as the selected laser-source index increases. For example, the system without GTLS requires an average SNR of 55.5 dB to achieve a target outage probability of 10^-3. Meanwhile, the system with GTLS requires an average SNR of 22.5 dB to achieve a target outage probability of 10^-3 under the ideal condition where the best laser source is selected. If the second-, third-, and fourth-best laser sources are selected owing to channel-estimation and/or feedback errors, then the required average SNR increases to 28.5 dB, 35.0 dB, and 44.0 dB, respectively. Third, we investigated the effect of the number of laser sources on the outage probability (Fig. 6). The outage probability decreases significantly as the number of laser sources increases. For example, when the number of laser sources is three and the second-best laser source is selected, to achieve a target outage probability of 10^-3, the system with GTLS requires an average SNR of 40.0 dB. If the number of laser sources increases to four and five, then the required average SNR decreases to 33.5 dB and 28.5 dB, respectively. Therefore, increasing the number of laser sources within the scope of hardware cost effectively improves the performance of the FSO-VUWOC system. Finally, the effects of the VUWOC and FSO link distances, the laser-source index selected, and the number of laser sources on the diversity order were analyzed (Figs. 7, 8, and 9). The results show that the diversity order of the FSO-VUWOC system with GTLS depends on the number of laser sources in the two links, the laser-source index selected, and the minimum turbulence parameter.

In this study, a mixed communication system (FSO-VUWOC) comprising FSO communication and VUWOC with GTLS is proposed to realize integrated land and ocean communication. Under a multilayer cascaded gamma-gamma ocean turbulence channel and a gamma-gamma atmospheric turbulence channel, we derived the exact closed-form outage-probability expression based on the Meijer G function and the Gauss hypergeometric function. Subsequently, we analyzed the diversity-order expression under a high SNR based on the outage-probability results. Finally, the simulation results are presented to verify the accuracy of our derivation, and the effects of the link distances of the VUWOC and FSO communication, the laser-source index selected, and the number of laser sources on the outage probability and diversity order were analyzed. The analysis and simulation results show that the FSO-VUWOC system with GTLS outperforms the FSO-VUWOC system without GTLS in terms of outage probability. Additionally, increasing the link distance and utilizing non-optimal laser sources deteriorates the outage probability and diversity order. By contrast, increasing the number of laser sources significantly improves the system performance.