Problems of Underwater Optical Links Modeling

Guest Editorial

Communication underwater is severely limited when compared to communications in air because water is essentially opaque to electromagnetic radiation except in the visible band. Even in the visible band, light penetrates only a few hundred meters in the clearest waters and much less in waters made turbid by suspended sediment or high concentrations of marine life. Consequently, acoustic techniques have been developed for underwater communication systems and now represent a relatively mature and robust technology. Acoustic systems are capable of long range communication, but offer limited data rates and significant latency (due to the speed of sound in water). We are developing an optical communication system that complements and integrates with existing acoustic systems resulting in an underwater communications capability offering high data rates and low latency when within optical range combined with long range and robustness of acoustics when outside of optical range. Amongst a wide array of applications, this combination of capabilities will make it possible to operate self-powered ROVs from support vessels or platforms without requiring a physical connection to the ROV. Such a capability will help simplify operations and potentially reduce costs through the use of less capable surface vessels. New deployment strategies may offer game-changing opportunities within all areas of undersea activities. For example, rapid event response will be enhanced and repair and maintenance of the emerging ocean observatory infrastructure will become more cost effective. Such through-water communications will likewise enable exchange of large data files from fixed sensors using AUVs (or ROVs) as data mules, shuttling real-time video from untethered vehicles for inspection, identification, and other related operations. Interconnectivity for dense arrays of underwater sensors without the need for expensive and difficult to install undersea cables is also possible. An unmanned battery operated vehicle, dedicated to a subsea node, that can be wirelessly operated though a combination of acoustic and optical communications, will be an important asset for both scientific exploration and commercial applications. We have demonstrated robust multi-point, low power omnidirectional optical communications over ranges of 100 meters at data rates up to 10 mega bits per second using a few tens of Watts of battery power with small, inexpensive transmitters and receivers. During the next few years, we will be exploring applications of this new technology directly in support of ongoing science and engineering programs. High-speed underwater optical communication is an enabling technology that has many potential applications in a range of environments from the deep sea to coastal waters. This development effort will enhance infrastructure for scientific research and commercial use by providing technology to efficiently communicate between surface vessels, underwater vehicles and seafloor infrastructure.

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

There is a speedy development withinside the place of broadband wi-fi communications as the global call for in elevating for large bandwidth and extra mobility. The speedy increase withinside the previous few a long time withinside the WAN's and LAN's is particularly because of growing improvement in excessive potential and occasional loss optical fiber. But, the main downside in optical fiber cable communique is that optical fiber receives broken in a long-haul installation. Thus, underwater communication has paved the way for overcoming this disadvantagefor programs of underwater statement and sea monitoring, the importance of underwater wi-fi optical communication has expanded. Attenuation and fading of light propagation, as well as impairment in the performance of underwater wireless optical communications, are caused by elements such as underwater absorption, scattering, and turbulence (UWOC). UWOC is a short-range wireless communication system that allows for greater data throughput than classic acoustic communication systems while using less power and posing fewer computing problems. The main challenge for underwater wireless communication, on the other hand, is from the fundamental properties of ocean or sea water. Understanding the feasibility and minimising the BER, as well as boosting the reliability of high data rate underwater communication lines, is the main topic of this paper. This study's purpose is to conduct underwater optical communication research while also producing new ideas to aid in the development of future underwater communication systems.

This paper has presented our interesting in wireless underwater communications for different ocean water types. Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this paper is to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. As well as we have developed the underwater optical wireless communication systems to have shorter ranges, that can provide higher bandwidth (up to several hundred Mbit/s) communications by the assistant of exciting high brightness blue LED sources, and laser diodes suggest that high speed optical links can be viable for short range application. The received signal power, signal to noise ratio, bit error rate, transmitted signal bandwidth, and transmission bit rates are the major interesting parameters for different ocean water mediums as a criteria of the best signal transmission characteristics of short wireless optical communications over wide range of the affecting parameters.

Underwater acoustic channels are affecting by many factors like time varying multipath propagation, Doppler spread, and salinity, which can greatly limit the quality of data rate and transmission distance. Underwater optical communication represents a crucial technology that supporting higher data rate, secure links and low latency. In his paper the use of 64- Quadrature Amplitude Modulation (64-QAM) in underwater optical communication systems is investigated. The system is designed to offer higher data rate than those of acoustic communication system. The proposed design achieve data rate of 10 Gbps over a challenging transmission distance of 50 km. In our study, a simulation technique is used to evaluate the performance of the proposed design in the underwater conditions, focusing on factors such as Bit Error Rate (BER), constellation diagram and Quality factor (Q-factor). The achieved BER is of about 10-10 with a minimum signal to noise ratio (SNR) of about 30dB. This paper provides valuable results for the design of next-generation underwater communication systems for applications such as pollution monitoring, oil control and oceanography research.

Underwater vehicles are utilized in various fields, such as exploration, submarine research, and industrial applications. However, underwater communication is often a formidable challenge because conventional communication technologies are ineffective for underwater vehicles. Therefore, laser communication systems for underwater vehicles are receiving more and more attention. This paper presents a novel approach by investigating comprehensive design criteria and analysis of laser communication systems for underwater vehicles. Firstly, the basic principles and working mechanisms of underwater laser communication systems are explained. Then, the main factors affecting system performance and design criteria are discussed in detail. These criteria include communication distance, data rate, power consumption, optical properties of the underwater environment, and system stability. Different laser modulation techniques and communication protocols are also evaluated. The paper also focuses on simulation, channel diversity, and test methods that can be used to assess the performance of laser communication systems in different underwater environments.

Underwater acoustic communications are required for controlling and networking a variety of underwater or sea surface objects. Due to the unique characteristics of the underwater acoustic channel, e.g., complicated multipath propagation, selective frequency fading, extreme Doppler effect and heavy ambient noise, FH-MFSK techniques play a vital role in robust underwater acoustic communication, especially in shallow water. However, based on detecting the frequency of the receiving signals, the demodulation scheme for FH-MFSK may be sensitive to the frequency shifting caused by underwater Doppler shifts. This paper focuses on how to design the frequency demodulation scheme and to improve the performance of underwater FH-MFSK acoustic communication system. By using an original Double-Line Interpolation algorithm with Hanning window, we propose a demodulation scheme to enhance the tolerance of the Doppler shift and improve the detection rate. Simulations and demodulation performance were tested. Experimental results show a promising future for FH-MFSK demodulation in underwater acoustic communication system.

Due to the power and transmission rate limitations, it is challenging to achieve underwater acoustic (UWA) communications over long distances from the unmanned underwater vehicle (UUV) to the shore station. As an alternative solution, the distributed acoustic sensing (DAS) underwater communication uses submarine optical cable to acquire and demodulate acoustic communication signals. There is little research on the channel and noise of DAS underwater communication. This paper analyzes the channel characteristics and the background noise of a DAS underwater communication system with armored optical cable based on the experimental measurements. Channel analysis shows that the envelope amplitudes of impulse responses follow the Burr distribution. The amplitudes get increased concentration at lower values as the sound source goes closer to the position right above the optical cable. Moreover, we observe that the received signals are influenced by the adjacent segments, leading to multiple arrivals within each acoustic bounce. Noise analysis shows that for frequencies lower than 1 kHz, unlike the Wenz noise spectrum primarily caused by shipping and wind, the −18 dB/octave decrease in power spectral density (PSD) of DAS noise is mainly caused by the equipment and cable themselves. The noise also exhibits full-band fluctuations. In underwater communications, full-band noise poses significant challenges for signal design. As far as we know, this article is the first report on the field experiment using a DAS underwater communication system equipped with armored optical cable, providing a reference for future underwater applications.

Ocean turbulence is an important factor affecting the development of underwater wireless optical communication (UWOC). To improve the error characteristics of the underwater optical communication system, we propose a differential detection-based multi-receiver-multi-transmitter (MIMO) underwater laser communication transmission method. Additionally, we derive the expressions for calculating the average BER of the MIMO underwater wireless optical communication system with differential detection and non-differential detection in the case that the two transmitted beams are completely uncorrelated. The error characteristics of the MIMO system are simulated and analyzed from the perspective of ocean turbulence intensity and link distance. The simulation results show that the differential detection method has a lower average BER compared to the non-differential detection method in the case of moderate-to-strong ocean turbulence. In addition, the differential detection methods do not have the error floor effect, and non-differential detection methods have the error floor effect. The more the turbulence intensity affects the average BER of the MIMO UWOC system with the increase of the communication link distance, the more obvious is the effect of the turbulence intensity on the average BER of the MIMO UWOC system. Accordingly, the simulation analysis shows that the differential detection method is more suitable for the construction of communication links under long-distance and medium-strong turbulence.

With the deepening exploration of ocean, underwater wireless communication technology has become the key to restrict its development. Aiming at the requirement of high speed and long distance underwater wireless communication, an underwater bidirectional link communication system for marine commercial was designed. The laser LD and LED dual emission light sources were adopted at the transmitting end, the system design scheme of dual emission light sources was proposed, and the feasibility of the scheme by designing the corresponding drive circuits was verified. At the receiving end, 5 mm large-area APD and high-sensitivity PMT dual receiving detector were used to receive optical signals, which were suitable for long-distance and high-speed communication. The information processing part of the system was completed by FPGA, and information interaction was carried out with PC through network communication. Finally, the whole system was designed and commercialized. The underwater simulation experiment shows that the communication rate of the system can reach 60 Mbps when the communication distance is 5 m and the bit error rate is 10−7. The communication rate of the system can reach 10 Mbps when the communication distance is 60 m and the bit error rate is 10−7.

Most traditional underwater communication systems rely upon low data-rate acoustic communications, which cannot propagate behind objects and is confined to very narrow-band channels. We propose an alternative method supporting robust broadband radio frequency (RF) communications in low signal-to-noise ratio (SNR) undersea environments. This paper presents an integrated treatment involving broadband RF antennas and digital radio systems, potentially enabling 1 Mbps underwater communications at a medium range of 100 to 1000 meters. We describe the broadband underwater antenna design results coupled with the application of spectrally efficient multi-carrier modulation and iterative message-passing low-density parity check codes. We also present the channel modeling and underwater link-level simulation results at 5 MHz, 10 MHz, and 15 MHz bandwidths.

Most traditional underwater communication systems rely upon low data-rate acoustic communications, which cannot propagate behind objects and is confined to very narrow-band channels. We propose an alternative method supporting robust broadband radio frequency (RF) communications in low signal-to-noise ratio (SNR) undersea environments. This paper presents an integrated treatment involving broadband RF antennas and digital radio systems, potentially enabling 1 Mbps underwater communications at a medium range of 100 to 1000 meters. We describe the broadband underwater antenna design results coupled with the application of spectrally efficient multi-carrier modulation and iterative message-passing low-density parity check codes. We also present the channel modeling and underwater link-level simulation results at 5 MHz, 10 MHz, and 15 MHz bandwidths.

Unlike Wi-Fi which uses radio frequencies, Li-Fi technology uses light as a data transmission medium. Li-Fi belongs to the higher wireless optical communication category and operates in the visible light band, which has higher data transfer rates than Wi-Fi. This paper discusses data transmission using Li-Fi technology for an underwater wireless optical communication system. Based on the experimental results, data transmission carried out without going through the water tends to be higher through water media but is still capable of alternative underwater communication systems.

In the past decades, researchers/scientists have paid attention to the physical layer of underwater communications (UWCs) due to a variety of scientific, military, and civil tasks completed beneath water. This includes numerous activities critical for communication, such as survey and monitoring of oceans, rescue, and response to disasters under the sea. Till the end of the last decade, many review articles addressing the history and survey of UWC have been published which were mostly focused on underwater sensor networks (UWSN), routing protocols, and underwater optical communication (UWOC). This paper provides an overview of underwater acoustic (UWA) physical layer techniques including cyclic prefix orthogonal frequency division multiplexing (CP-OFDM), zero padding orthogonal frequency division multiplexing (ZP-OFDM), time-domain synchronization orthogonal frequency division multiplexing (TDS-OFDM), multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM), generalized frequency division multiplexing (GFDM), unfiltered orthogonal frequency division multiplexing (UF-OFDM), continuous phase modulation orthogonal frequency division multiplexing (CPM-OFDM), filter bank multicarrier (FBMC) modulation, MIMO, spatial modulation technologies (SMTs), and orthogonal frequency division multiplexing index modulation (OFDM-IM). Additionally, this paper provides a comprehensive review of UWA channel modeling problems and challenges, such as transmission loss, propagation delay, signal-to-noise ratio (SNR) and distance, multipath effect, ambient noise effect, delay spread, Doppler effect modeling, Doppler shift estimation. Further, modern technologies of the physical layer of UWC have been discussed. This study also discusses the different modulation technology in terms of spectral efficiency, computational complexity, date rate, bit error rate (BER), and energy efficiency along with their merits and demerits.