Underwater Optical Wireless Communication

The communication of underwater wireless information is of great interest to the armed, engineering and science as it plays an important role in protective statement, pollution control, oil control and conservation, offshore considerations and oceanographic research. To smooth all these activities, the number of vehicles or unmanned devices that are organized underwater increases which requires a large bandwidth and a large capacity for the transmission of information under water. UOWC has many potential uses ranging from deep oceans to coastal waters.

This article gives a comprehensive description of recent progress in the UOWC. The characterization of channels, modulation schemes, coding techniques and various UOWC-specific noise sources is discussed. Not only does this document provide a thorough exploration of underwater optical communication, it also aims to develop new ideas that support the growth of future underwater communication.

 INTRODUCTION

Optical Fiber Chemical Sensors

In recent years, the interest in optical wireless communication in terrestrial, space and submarine connections has increased as it is able to provide high data rates with low power and bulk supplies.

For example, the first Chinese used campfire towers to repossess information about soldiers around 1000 BC. The first Greek and Roman armies used polished shields to reflect the sunlight and to 800 BC. To show. In 1880, Alexander Graham Bell developed a wireless telephone system that used sunlight as a transmission medium. This system is considered the world’s first opto-electronic communication system [1], [2]. In the 1960s, the invention of the laser as an optical source changed the future of wireless optical communication (OWC) [3].

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From that moment on, a number of terrestrial OWC applications appeared. Due to the strong impact of seawater attenuation on visible light and limited knowledge of water optics, the early development of UOWC was far behind terrestrial optical communication (Free Space). In recent years, however, there is increasing research activity at UOWC. There is an urgent need for a comprehensive survey that can provide researchers with a basic understanding of the UOWC and knowledge of innovative UOWC research.

In the current years, with increasing global climate change and scarcity of resources, interest in studying the ocean contemplation system has increased. The technology of underwater wireless communication (UWC) enables the realization of oceanic exploration systems and therefore attracts much attention. UWC refers to the communication of data in an unsupervised marine environment through the use of radio operators i.e. radio frequency (RF) waves, sound waves and optical waves. In view of the limited RF bandwidth and acoustic techniques and the growing need for high speed underwater data transmission, underwater optical communication (UOWC) has become an attractive and viable alternative.

Radio Frequency (RF) electromagnetic waves (EM) are a good option for underwater wireless communication when used to transmit high-speed data over short distances. The speed of the EM waves depends mainly on the permeability, the permittivity, the conductivity and the volume charge density. It has been observed that the attenuation of RF waves increases with increasing frequency and is greatly attenuated by the seawater. On the other hand, optical waves have a large bandwidth, but are affected by other propagation effects due to temperature changes, scattering, scattering, and beam steering. Underwater wireless communication is limited to short distances because of the high water absorption in the optical frequency band and the high cleaning of the backside of the suspended particles. However, there is an optical window with relatively little attenuation of the blue-green wavelengths of the EM spectrum under water. For this reason the UOWC has noted a growing interest in the development of blue-green sources and detectors. The blue-green wavelengths can demonstrate high bandwidth communication at medium distances (up to 100 meters).

Evaluation of Related UOWC Inspections

With the increase in UOWC research activities, several short articles were available to explore the topic. Khalighi et al. [11] I have reviewed some recent work on UOWC in channel models, modulation and coding schemes, and preliminary work. The authors also presented the performance study of a typical UOWC system under various simplified expectations. Johnson et al. [9] conducted a survey on UOWC channel models. Some typical UOWC modeling approaches have been briefly discussed, such as the Beer-Lambert law, the radiation transfer function and the Monte Carlo method. Johnson et al. [4] introduced UOWC and focused mainly on aquatic optical properties. Arnon [10] evaluated the performance of the connection in a typical UOWC system and introduced a number of image problems associated with UOWC systems. For the development of acoustic communication under water many fonts are available, but for UOWC only a few papers are available. The editions available for UOWC do not provide a holistic coverage of the topic. Throughout the period, the interest of the UOWC remains limited to military applications [8]. The huge market preference of UOWC has not been achieved so far. At the beginning of the decade of 2010, only a few limited UOWC products were launched.

EXPERIMENTAL SETUPS AND PROTOTYPES OF UOWC

This section first introduces some digital modulation techniques implemented in UOWC systems. The advantages and limitations of each modulation scheme are presented. Next we will discuss the UOWC channel coding techniques. Finally we will summarize this section and classify the related literature on modulation and coding schemes for UOWC.

Modulation and coding techniques:

The modulation techniques of the UOWC channel have received much attention in recent years as they can significantly affect the performance of the system. Since UOWC can be considered for implementing FSO communication in an underwater environment, conventional intensity modulation (IM) techniques used in FSO communication systems can also be applied to UOWC systems. On-Off Encoding Modulation (OOK) is the most popular IM scheme in FSO communication systems. This modulation scheme can also be implemented in UOWC systems. OOK modulation is a binary modulation scheme. During an OOK transmission, an optical pulse occupation part or the total duration of a bit represents a single data bit ‘1’. On the other hand, the absence of an optical pulse gives a single data bit ‘0’. It is gives two pulse formats in the OOK modulation scheme: return to zero format (RZ) and non-return to zero format (NRZ). In the RZ format, a pulse having a duration that takes only part of the duration of the bit is defined as ‘1’; however, the pulse occupies the entire duration of the bit in the NRZ scheme.

It has been shown that the RZ-OOK achieves higher energy efficiency than the NRZ-OOK, but at the cost of consuming more channel bandwidth. Due to the strong absorption and scattering effects in the underwater environment, the transmitted OOK signal suffers several channel fluctuations.

In PPM, all transmitted M bits are modulated as a single pulse in one of the time intervals of 2 M and the pulse position represents the transmitted information (Fig .2). The main disadvantage of PPM modulation is the strict time management requirement. Any time fluctuation or asynchronization significantly affects the BER performance of the system. In recent years, several researchers have investigated the performance of the PPM scheme on UOWC channel models. He and Yan [7] investigated the performance of the 4-PPM scheme for the numerical RTE channel model. They found that the corresponding BER for the PPM scheme is nearly equal to OOK modulation and has much higher energy and spectral efficiency. More complex PPMs such as 8-PPM or 16-PPM can be used to improve bandwidth efficiency. Sui et al. [11] proposed a modified PPM scheme for UOWC. You have to modify PPM can maintain a similar energy efficiency and noise reduction performance as conventional PPM.

UOWC system design:

In PPM, all transmitted M bits are modulated as a single pulse in one of the time intervals of 2 M and the pulse position represents the transmitted information (Fig .2). The main disadvantage of PPM modulation is the strict time synchronization requirement. Any time fluctuation or asynchronization significantly affects the BER (Bit-Error Rate) performance of the system. In recent years, several researchers have investigated the performance of the PPM scheme on UOWC channel models. He and Yan [7] investigated the performance of the 4-PPM scheme for the numerical RTE channel model. They found that the corresponding BER for the PPM scheme is nearly equal to OOK modulation and has much higher energy and spectral efficiency. More complex PPMs such as 8-PPM or 16-PPM can be used to improve bandwidth efficiency. Sui et al. [11] proposed a modified PPM scheme for UOWC. You have to modify PPM can maintain a similar energy efficiency and noise reduction performance as conventional PPM.

 TRANSMITTER:

The market for optical modules is already well developed and is widely used in FSO optical fibers and communication systems. Therefore, UOWC has the advantage of technological maturity of the wavelength of interest. Depending on the requirements and considering the power and mass limitation of submarine systems, the choice of LED or laser in the blue-green part of the spectrum may vary. In general, blue-green LEDs are preferred for buoy systems operating in shallow water. For systems operating in oceanic waters of crystalline waters, laser-based systems are preferred. The power output of lasers or LEDs in the cyan spectrum ranges from 10 mW to 10 W. Both LEDs and lasers have their own advantages and disadvantages when making the decision of the source in the UOWC system. Lasers have fast switching time and high optical performance, but LEDs are cheap, simple, less temperature dependent and more reliable. The LED-based system is less susceptible to underwater effects than the laser because of its large viewing angles.

RECEIVER:

The receiver in UOWC must have a wide FOV, high gain, and high SNR. The most common blue-green photographic sensors are: PMT, semiconductor photoelectric sensors, and biologically inspired quantum photographic sensors. PMT is a type of vacuum tube that is very sensitive to light. It is characterized by high gain, low noise level, high frequency response and large detection range. However, they are a poor choice for UOWC because of their size, higher energy consumption and fragility. In addition, PMTs can be damaged if exposed to excessive light. In [5] PMT is used together with a variable gain amplifier in the receiver to establish a communication link in a laboratory configuration for 500 kbps on return to zero format and 1 Mbps on non-return to zero format for a distance of 3.66 m. Omnidirectional optical signal transmission and reception is presented using PMT together with a hemispherical diffuser in [1] for a range of 100 m at a data rate of 1 Mbps.

 MODULATORS:

Choosing a modulation technique is a very important decision to design a communication system. The modulation can be done directly or via an external modulator. Direct modulation is the easiest way to detect where the current-controlling light source is directly modulated. The direct modulation of the lasers by the pump source is very simple, but suffers from the phenomenon called chirp, which limits the data rate and linkage area in the UOWC system. In optically pumped solid-state lasers, non-linearites in the system lead to relaxation oscillations that prevent direct modulation of the laser. The above techniques (OOK, NRZ-OOK, RZ-OOK and PPM) are used depending on the requirements of the channel.

LOS/NLOS UOWC SYSTEMS:

Some commercial UOWC products were developed in the early 2000s, large-scale commercial applications of UOWC systems have not been made so far. Most UOWC systems are experimental demonstrations and prototypes in laboratory environments. In the remainder of this section, we will provide a complete summary of recent advances in experimental research at the UOWC. The purpose of this summary is not to present the entire UOWC experimental literature in detail, but to provide a general description of the most recent work on UOWC experiments relating to various applications and approaches.

Depending on the link configuration, experimental configurations and UOWC prototypes can be divided into two categories: experimental LOS (Line-of-Sight) configurations and experimental NLOS (Non-Line-of-Sight) configurations. The LOS UOWC link configuration is similar to FSO communication configurations [4]. On the transmitter side, the information bits are generated by a personal computer (PC) and modulated in optical carriers. In several UOWC experiments, the modulated optical signal is further amplified by an optical amplifier and then transmitted through the lens, which is precisely aligned to focus the light. The water tank or water pipe is used to model the submarine transmission route.

To mimic the different refraction conditions and turbidity of the underwater environment, Maalox is added to the water to attenuate as a blasting medium. Light [7]. At the receiver side, the optical signal passes through an optical filter and a focusing lens. On the other hand, NLOS UOWC’s experimental links focus mainly on the application of scope and images under water. Alley et al. [5] proposed an NLOS imaging system using a blue 488 nm laser as the illuminator. The experimental results show that this NLOS configuration significantly improves the SNR of the images compared to the conventional LOS imaging system. A similar experimental approach using a pulse-modulated laser in the NLOS configuration for detection, removal, imaging and communication under water was presented in [2].

RETROREFLECTORS IN UOWC:

The retroreflector is an optical device that can reflect any incident light back to its source (11). With this advantageous characterization, a UOWC modulator retroreflector system has been introduced. When the retroreflector link is modulated, an active transceiver projects a beam of light into the retroreflector. During the reflection process, the modulator modulates the light beam and adds information. This information is then captured and demodulated by the active transceiver. The main benefit of modulating the UOWC system with a retroreflector is that most of the power, weight of the device, volume, and targets are shifted to the active end of the connection, so the passive end can benefit from the small dimensions, the relatively low power and the target requirements. [7] A retroreflector-based connection configuration achieves a shorter connection distance than the LOS-based connection configuration due to the additional attenuation caused by the folded transmission path. On the other hand, the BER performance curve of a reflective connection configuration due to reflection in the random sea surface is generally not a monotonic function of linkage removal. A typical UWSN may contain many submarine sensor nodes.

SMART TRANSCEIVERS OF UOWC:

A smart, compact and adaptable UOWC transceiver should be proposed that can reduce relocation requirements with minimized volume and energy costs. Simpson et al. [7] proposed a new UOWC frontend and introduced the concept of intelligent transmitters and receivers. The intelligent quasi-omnidirectional transmitter can estimate the water status from the backscattered light detected by the adjacent intelligent receiver. Depending on the specific water conditions, the transmitter may perform various actions, such as changing the wavelength of the transmitted light, to improve the connection performance. The transmitter may also electronically change the direction of the beam according to the angle of arrival of the detected signal. On the receiver side, a segmented lens array architecture has been implemented to increase the overall field of view. By using the arrival angle estimation information, the intelligent receiver can also adjust and align the FOV toward the desired signals to improve the SNR of the received signal. In addition, the CDMA technique has also been implemented at the ends of the transmitter and the receiver to improve the performance of the system.

Advantages of UOWC

UOWC systems are used for fast underwater communication between multiple fixed or mobile nodes. They have great potential for applications in the UWSN. There are three UWC options for implementing the UWSN: acoustic, RF and optical [12]. To highlight the advantages and unique features of UOWC, we will compare the UOWC with the RF and acoustic methods. The acoustic method is the most commonly used technology in UWC. It has a long history of applications that can be dated to the end of the 19th century. After a major expansion of military applications in the two world wars, the underwater acoustic communication system has become a proven popular technology that has been applied to almost every aspect of UWSNs [2]. In view of the extreme amplitude of the ocean and the strong damping effects of seawater on other transmission sources such as optical waves and high frequency waves, the most attractive advantage of underwater acoustic communication is that a long connection can be achieved by several tens of kilometers [4].

Although the acoustic method is the most widely used method to achieve UWC, it also has certain technical limitations. Since the typical frequencies of underwater acoustics are between tens hertz and hundreds of kilohertz, the data transmission rate of the acoustic link is relatively low (typically of the order of kbps) [10]. Secondly, due to the slow propagation velocity of the sound wave in the water (about 1500 m / s for pure water at 20 degrees Celsius), the acoustic connection experiences a significant delay in communication (usually in seconds). Therefore, applications that require a large amount of real-time data exchange cannot be supported. Third, acoustic transceivers are often bulky, expensive and consume a lot of energy. They are not economical for large-scale implementations of UWSN [5]. In addition, acoustic technology can also affect marine life, which uses sound waves for communication and navigation [10]. Underwater radio communication can be considered as an extension of terrestrial radio communication. Underwater radio communication has two major advantages.

First, the RF wave can make a relatively smooth transition through the air / water interface as compared to the acoustic wave and the optical wave. This advantage can be exploited to achieve boundary communication combining the terrestrial RF communication system and the underwater RF communication system. Second, the HF process is more tolerant of turbulence and water turbidity than optical and acoustic methods [2]. The deadly limitation that prevents the development of the HF method underwater is the short connection range. Since seawater, which contains a lot of salt, is a conductive transmission medium, RF waves can propagate only a few meters at low frequencies (30-300 Hz) [11]. In addition, submerged RF systems require a huge transmit antenna and expensive transceivers that consume energy. Compared to the acoustic approach and the RF approach, UOWC has the highest data rate, lowest link delay, and lowest implementation cost. UOWC can achieve a data rate in the order of Gbps at moderate distances of several tens of meters [9]. This high speed advantage ensures the realization of many real-time applications, such as underwater video transmission.

Challenges of UOWC

Although UOWC has many advantages over acoustics and RF techniques, achieving UOWC remains a challenging task. The main challenges of UOWC are listed below.

The optical signal suffers from strong absorption and dispersion. Although the wavelength of the transmission light in the blue and green spectrum has been carefully selected [2] to minimize the attenuation effect of transmission due to the unavoidable interactions of photons with water molecules and other particles. In water absorption and dispersion still strongly attenuate and cause the transmitted light signal to pass through Bleaching several ways. Due to the influence of absorption and dispersion, the UOWC has a low BER performance at a distance of several hundred meters in a murky water environment. In an underwater environment, problems such as chlorophyll can absorb blue and red lights. These materials and other dissolved colored organic materials (CDOM) can increase the turbidity of the water and therefore reduce the removal of light propagation. In addition, the CDOM concentration also changes with the variation of the ocean depth, which changes the corresponding light attenuation coefficients [7]. These undesirable effects increase the complexity of UOWC systems.

Underwater optical connections are temporarily interrupted because the optical transceivers are misaligned. In several UOWC systems, blue / green or LED lasers were used as light sources due to their narrow divergence characteristics. However, an exact alignment condition is required [11]. Since the submarine environment is turbulent at relatively shallow depths, link misalignment is frequently performed, particularly in UOWC applications with deep depths based on vertical buoys [3]. Random movements of the sea surface cause serious problems with connection loss [1].

The implementation of UOWC systems requires reliable underwater equipment. The underwater environment is complex. The flow, pressure, temperature and salinity of seawater have a major impact on the performance and life of UOWC equipment [6]. Considering the fact that UOWC devices cannot use the sub-solar energy and prolonged underwater running time, the reliability of the device’s batteries and the power efficiency of the devices are essential [4].

CONCLUSION

Improving the underwater communication system is required due to the greater number of unmanned vehicles in space and under water. Traditional underwater communication is based on acoustic signals, and despite significant advances in the field, it is difficult to provide sufficient bandwidth with acoustic communication with low latency. RF signals for UOWC can only be used in ELF due to the high absorption of electromagnetic signals at radio frequencies. The use of optical fibers or coaxial cables limits the range and maneuverability of underwater operation. Underwater optical communication offers great potential for increasing traditional acoustic communication due to its high data rates, low latency, lower power consumption, and smaller package size. In addition, this technology can greatly benefit from the advances in wireless optical terrestrial communication. However, the distance and range of the underwater optical beam will be affected by water type, demolition, dispersion and other propagation losses. UOWC uses the blue-green wavelength of the visible spectrum because it provides a low attenuation window and enables high bandwidth (on the order of MHz) communication at moderate distances (10-100m). In addition, a typical UOWC with a point-to-point connection requires strict tracking and tracing systems, especially for mobile platforms. The question of the Holy Grail is still how reliable optical underwater communication can be achieved over a long distance (preferably of the order of Km). Achieving this ultimate goal requires more research at the UOWC, and this area of ​​research is still at an early stage. Next, we will suggest possible future research directions for UOWC research.

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