Performance Analysis of SIMO-UWOC System

In order to meet the reliability requirements of communication for underwater resource exploration, this study develops an underwater wireless optical communication (UWOC) system utilizing a blue semiconductor laser as the light source. At the receiver, a fully digital automatic gain control (AGC) module, implemented on a field-programmable gate array (FPGA), is designed to mitigate signal fluctuations induced by underwater turbulence. Digital filtering techniques, including median filtering (MF) and bilateral edge detection filtering (BEDF), are also employed to improve signal demodulation reliability. An improved Reed–Solomon (RS) coding scheme is further adopted to significantly reduce the bit error rate (BER). The design of a highly reliable UWOC system was realized based on the above techniques. The system’s performance was evaluated across a range of signal-to-noise ratios (SNRs) and bubble intensities. The results show that the digital AGC module can provide a gain range from −3.2 dB to 16 dB, adapting to varying signal strengths, which greatly bolsters the system’s resilience against underwater turbulence. Filtering techniques and RS coding further enhance the system’s immunity to interference and reduce the system BER. Communication experiments were conducted over various distances under three distinct water quality conditions. The results demonstrate that, within the detection range of the avalanche photodiode (APD), the system consistently maintained a BER below 3.8 × 10−3 across all water types, thereby confirming its high reliability. In clear seawater, the system demonstrated reliable information transmission over a 10 m distance at a data rate of 10 Mbps, achieving a BER of 2 × 10−8. Theoretical calculations indicate that the maximum transmission distance in clear seawater can reach 111.35 m.

We provide an experimental study of the channel characteristics in an underwater wireless optical communication (UWOC) system given by a 35-m transmission distance in various water types. A UWOC system using a low-power 488-nm laser diode is established to comprehensively evaluate the influence of the link distance, water turbidity, receiver parameters, and link misalignment on the communication link power. The results show that the link misalignment-induced power loss is significant and the effect of receiver parameters on power is limited. However, a higher turbidity or longer signal transmission distance can help to reduce the link misalignment effect and manifest the receiver parameters effect on the received power. In particular, in turbid waters with an attenuation coefficient of 0.471 m − 1, the change of the signal reception power curve is small over a certain degree of the link misalignment in the 35-m physical distance, and the reasonable configuration of receiver parameters effectively improves the signal reception quality in the UWOC system. These results can provide theoretical guidance for optimizing the UWOC system.

Single LED-based 46-m underwater wireless optical communication enabled by a multi-pixel photon counter with digital output

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is shown to be an efficient modulation scheme for underwater optical wireless communication (UOWC) systems due to its high power efficiency and excellent flexibility. Differing from conventional OFDM, the modulation parameters of OFDM-IM consist of not only M-ary constellations but also subcarrier indices. In this Letter, we propose and demonstrate an intelligent automatic modulation recognition (AMR) scheme for OFDM-IM in UOWC systems by using Swin Transformer (Swin-T). Moreover, transfer learning with a hybrid training approach is further proposed to substantially enhance the AMR performance. Experimental results successfully demonstrate the feasibility and superiority of the proposed intelligent AMR scheme based on Swin-T with transfer learning and hybrid training in UOWC systems. More specifically, a recognition accuracy of 96% can be obtained by using Swin-T with transfer learning and hybrid training for a relatively small received signal-to-noise ratio (SNR) of 3.6 dB.

In underwater wireless optical communication (UWOC) systems, using single-photon avalanche diodes (SPAD) as a detector can improve the detection sensitivity and thus improve the transmission distance. However, the signal detection for SPAD-based systems is greatly challenged by the complex optical channel characteristics and SPAD nonlinear distortion. In order to mitigate the nonlinear distortion caused by SPAD detectors, signal-dependent noise (SDN), and the effect of underwater channel attenuation on the transmitted signal in UWOC systems, this paper proposes a signal detection scheme using deep neural networks with embedded SDN variance (DNN-SDN). This scheme performs joint equalization and demodulation of the received signal, avoiding information loss and error propagation between these two processes, thereby achieving more accurate signal recovery. In this work, we first characterize the statistical noise model for the SPAD array and establish a nonlinear relationship between the number of detected photons and the transmitted signal power. The SDN variance is integrated as an input feature during network training. Performance is quantified via a joint loss function with parameters optimized through a systematic ablation study. Finally, the performance of the proposed signal detection scheme is also compared with other schemes in different waters. Simulation results demonstrate that the scheme can effectively mitigate the effects of SPAD-induced nonlinear distortion and SDN on the signal, and the proposed method extends the transmission distance by 20%-40%.

In this paper, a low-complexity two-level chaotic encryption scheme is introduced and experimentally demonstrated to improve the physical layer security of a 450-nm laser underwater optical wireless communication (UOWC) system using discrete Fourier transform spread discrete multi-tone (DFT-S DMT) modulation. In the first encryption stage, the original bit stream is encrypted with a chaotic sequence based on a one-dimensional Logistic map. In the second encryption stage, the real and imaginary components of the DFT-S symbols are further encrypted with a pair of separate chaotic sequences, which are generated from a two-dimensional Logistic iterative chaotic map with infinite collapse (2D-LICM). The experimental results indicate that the encryption operation has no negative effect on the performance of the proposed UOWC system. For chaotic encryption, the DFT-S DMT gives a better performance than the DMT scheme under different water turbidities. 55-m/4.5-Gbps and 50-m/5-Gbps underwater transmissions are successfully demonstrated by the chaotic encrypted DFT-S DMT scheme. To the best of our knowledge, this is the first time to verify the feasibility of chaotic encryption in a high-speed UOWC system.

A 25-Gbps underwater wireless optical communication (UWOC) system with a two-stage injection-locked 680-nm red-light vertical-cavity surface-emitting laser (VCSEL) transmitter to enhance the frequency response and a laser beam expander to expand the collimated beam diameter over a 5-m highly turbid harbor water link is proposed and practically demonstrated. In highly turbid harbor water link, the overall attenuation coefficient at 680 nm is smaller than that at 520 and 450 nm, thereby a 680-nm red-light VCSEL transmitter is adopted in this proposed 5 m/25 Gbps UWOC system rather than a 520-nm green-light laser diode (LD) transmitter or a 450-nm blue-light LD transmitter. A satisfactory bit error rate performance (3 × 10–9) and a clear eye diagram are acquired in real time. This proposed UWOC system with a two-stage injection-locked 680-nm VCSEL transmitter and a laser beam expander brings important improvements in the scenario characterized by high turbidity.

Underwater wireless optical communication (UWOC) has emerged as a key technology for next-generation underwater wireless data transmission. Semantic communication is regarded as a promising communication paradigm to improve the efficiency and robustness of UWOC systems. However, existing underwater wireless optical semantic communication (UWOSC) system lacks the capability to adapt the code rate to different channel states, leading to intelligence inadequacy and coding inefficiency. In this paper, we introduce environment semantics into the UWOSC system for the first time, and propose an environment semantics aided underwater wireless optical semantic communication (ESA-UWOSC) framework. Based on this framework, we develop a system leveraging computer vision and deep learning for adaptive rate transmission. The ESA-UWOSC system is realized by a channel state perception mechanism (CSPM) for extracting environment semantics from the channel environment image to obtain the channel gain, a link network (LinkNet) for enabling variable code rate transmission of source semantic information, and a prediction network (PreNet) for predicting the optimal code length that satisfies the transmission task requirement. The combined operation of these three modules enables the ESA-UWOSC system to actively perceive channel state and perform adaptive rate transmission accordingly. In a series of experiments conducted on an emulated UWOC experimental platform, the results show that the ESA-UWOSC system outperforms the baseline scheme, achieving superior performance with lower bandwidth consumption, especially in harsh channel environments. Moreover, the results also demonstrate that the ESA-UWOSC system can achieve adaptive rate transmission based on the perceived channel state to meet transmission task requirement.

Improving the spectrum efficiency (SE) is an effective method to further enhance the data rate of bandwidth-limited underwater wireless optical communication (UWOC) systems. Non-orthogonal frequency-division multiplexing (NOFDM) with a compression factor of 0.5 can save half of the bandwidth without introducing any inter-carrier-interference (ICI) only if the total number of subcarriers is large enough, and we termed it as half-spectrum OFDM (HS-OFDM). To the best of our knowledge, this is the first reported work on a closed-form HS-OFDM signal in the discrete domain from the perspective of a correlation matrix. Due to the special mathematical property, no extra complex decoding algorithm is required at the HS-OFDM receiver, making it as simple as the conventional OFDM receiver. Compared with traditional OFDM, HS-OFDM can realize the same data rate, but with a larger signal-to-noise ratio (SNR) margin. To fully use the SNR resource of the communication system, we further propose a digital power division multiplexed HS-OFDM (DPDM-HS-OFDM) scheme to quadruple the SE of conventional OFDM for the bandwidth-starved UWOCs. The experimental results show that HS-OFDM can improve the receiver sensitivity by around 4dB as opposed to conventional 4QAM-OFDM with the same data rate and SE. With the help of the DPDM-HS-OFDM scheme, the data rate of multi-user UWOC can reach up to 4.5Gbps under the hard-decision forward error correction (HD-FEC) limit of a bit error rate (BER) of 3.8×10-3. Although there is some performance degradation in comparison with single-user HS-OFDM, the BER performance of multi-user DPDM-HS-OFDM is still superior to that of conventional single-user 4QAM-OFDM. Both single-user HS-OFDM and multi-user DPDM-HS-OFDM successfully achieve 2Gbps/75m data transmission, indicating that the DPDM-HS-OFDM scheme is of great importance in bandwidth-limited UWOC systems and has guiding significance to underwater wireless optical multiple access.

Blue/green light-emitting diodes (LEDs) have been studied extensively in medium/short-distance underwater optical wireless communication (UOWC), while available commercial bandwidth-limited LEDs cannot support the increased capacity. In this work, a 175-µm blue mini-LED with a high 3 dB modulation bandwidth of 580 MHz was designed and employed to implement a high-speed UOWC system to improve the bandwidth while maintaining high emitting optical power. As a result, we achieve the data rate of 3.8 Gb/s and the rate-distance product of 7.6 Gb/s·m of the PAM-4 signal with the 3.7e-3 bit error rate (BER) over the 2-m underwater channel by using a single-pixel 175-μm GaN-based mini-LED and an avalanche photodiode (APD). To the best of our knowledge, it is the highest rate-distance product among the UOWC systems based on a single-pixel LED and a single APD. In addition, we design multiple sets of comparison experiments to provide a comprehensive investigation regarding channel loss, transmission capacity, and computation complexity between the UOWC system with and without water.

Underwater wireless optical communication (UWOC) has been considered a promising technology for high-speed underwater transmission. Some Gb/s level UWOC systems applying visible light have been demonstrated with a transmission distance of several meters or more. Many of the previous works focus on the advanced technologies to push the systems’ capacity–distance performance. However, practical environmental factors issue such as flow turbulence and temperature variation are seldom studied through specific statistical/theoretical models. In this paper, a UWOC system using a 450 nm blue light laser source was set up using a 1.5-m water tank with mirrors located on both sides for single or multiple reflections corresponding to different transmission distances. The blue laser was modulated by a 1.25 Gbps NRZ-OOK format with PRBS of 7, 24 or 31, respectively, for system performance comparison. The bit error rate (BER) values were measured in 1.5, 3.0 and 6 m, respectively, for system evaluation. At room temperature, the BER value was down to 10 × 10−8 for a 1.25 Gbps data rate in a 6 m transmission. Then, the UWOC transmission system experiment was carried out under several environmental parameters such as temperature, turbulence, artificial seawater by adding salt to simulate practical application in river or sea. When a submerged motor with an output of 1200 L/h was used as a water flow turbulence source, the impact to BER and transmission quality was negligible. For the temperature change issue, the experiment shows that around the original temperature of 25 °C had the best BER as compared to other temperature ranges from 10 to 50 °C. For artificial seawater issues by adding salt to simulate the real seawater environment. The transmission distance was only 3-m instead of 6 m, mainly due to particle scattering and water disturbance. With the motor pump on, the power penalty was 1 dB at 10 × 10−8 BER when compared to the motor pump off.

Underwater wireless optical communication (UWOC) systems have been identified as a suitable replacement technology with high data rates over relatively medium transmission ranges for underwater communication. In the research, we propose a new optical hybrid modulation format for carrying the letters over underwater wireless optical communication (UWOC) system. This hybrid modulation format combines pulse position modulation (PPM) with digital pulse interval modulation (DPIM) to increase the transporting speed. The proposed UWOC system is experimentally investigated in the laboratory over a clean and turbulent water channel. The message was electrically modulated in the transmitter with PPM-DPIM format and then applied to a laser diode. The laser diode emitted light at a wavelength of 650 nm with a power of 60 mW. At the receiver, the photodiode, with its amplifier circuits, sends the received signal to the microprocessor to restore the origin message. The results showed that the received power and signal-to-noise ratio (SNR) decreased with elongating the transmission distance. In addition, the attenuation is raised with increasing distance. The bubbles source is also situated at distances of 25 cm, 50 cm, and 75 cm from the transmitter in turbulent waters. The results showed that locating the bubble source decreases the system's performance. The best results were obtained when the bubble source was 75 cm away from the transmitter. Finally, the hybrid modulation formats (PPM-DPIM) can increase the speed of letter transmission, reduce the bit error rate (BER), and minimize the effect of scattering and attenuation in water channels.

Experimental demonstration of underwater wireless optical OFDM communication system with a single SPAD receiver

BER variation of an optical wireless communication system in underwater turbulent medium with any temperature and salinity concentration

In this letter, we analytically evaluate the end-to-end bit error rate (BER) of point-to-point multi-hop underwater wireless optical communication (UWOC) systems with respect to all degrading effects of the UWOC channel, namely absorption, scattering, and turbulence-induced fading. To do so, we first derive the BER expression of a single-hop UWOC link as the building block for end-to-end BER evaluation. We also apply Gauss-Hermite quadrature formula to obtain the closed-form solution for the system BER in the case of lognormal underwater fading channel. Numerical results demonstrate that multi-hop transmission, by alleviating channel impairments, can significantly improve the system performance and extend the viable end-to-end communication distance.