Research On Underwater Wireless Optical Communication System And Its Single-Photon MIMO Detection Theory

The ocean is the cradle of life.The total area of the ocean is about 360 billion square kilometers,accounting for about 71% of the earth’s surface.The average depth of the ocean is about 3,795 meters.The ocean contains more than 1.3 billion cubic kilometers of water,accounting for about 97% of the total water on the earth.The ocean is also the granary of the future,which can provide human beings with abundant marine biological resources such as fish and shellfish,it also rich in marine mineral resources such as oil,inspiring people to explore unremittingly.However,the exploration of the ocean is less than 5%.With the development of economy,countries around the world pay more and more attention to the value of the ocean.Ocean informatization is an important part of ocean development,underwater information transmission or underwater communication is an indispensable key technology,which has attracted wide attention from scientific research,industry,commerce,and military all over the world.Obviously,because underwater wired communication needs to lay expensive cables,it is not suitable for mobile submarine terminals.At present,the underwater acoustic wave communication with sound waves as the information carrier is the earliest and very mature technology,which can realize the information transmission of tens of kilometers underwater.However,the underwater acoustic wave communication system has high energy consumption,low speed,small communication capacity,large delay and certain harm to marine life,so it does not have the requirements of high-speed and large-capacity underwater communication.In addition,due to the skin effect of underwater radio frequency communication,the transmission distance is extremely short,and the transmission quality is not high.In recent years,underwater wireless optical communication(UWOC)with light waves as the information carrier has been widely developed,because the blue-green light in the visible band in the electromagnetic spectrum has a relatively small low loss coefficient underwater,and the light wave It has the advantages of large capacity,fast transmission rate,high bandwidth,good confidentiality,and low cost.Therefore,the underwater visible light communication or underwater wireless optical communication using blue-green light as the information carrier has aroused the scientific community,industry and the world.The attention of academia has developed rapidly in just a few decades.Underwater wireless optical communication is wireless communication with light waves as the information carrier.Compared with the traditional electrical communication system,the main difference is that the system includes electrical/optical conversion and optical/electrical conversion parts,and the channel of the system is a relatively special marine/underwater medium environment.Currently,commercial UWOC systems on a global scale have not yet appeared on a large scale,and designing a UWOC commercial/prototype system that can be used in actual seas is one of the ultimate goals.This paper describes the typical structure of the UWOC system and introduces the underwater full-duplex video wireless optical communication prototype system designed and developed by our team in detail.This prototype UWOC system takes field programmable gate array(FPGA)as the core in hardware,and realizes 1 Mbps video transmission in an indoor 10 m water tank.Duplex communication mode and transmission protocol,and have larger transmit optical power,but also need to solve the self-interference problem involved in full-duplex communication.The ocean/underwater medium channel has a great influence on the transmission of light waves.Therefore,the basic characteristics of the ocean light channel are comprehensively described in this paper.The seawater medium will absorb and scatter light waves.The typical value is 0.4 d B/m,which is much larger than 0.1-4 d B/km for acoustic waves.Coupled with unfavorable factors such as the harsh and complex biochemical environment of sea water and ocean turbulence,the transmitted optical wave power attenuates very quickly.The optical link distance of the UWOC system is usually It is only about 100 meters,far below the order of magnitude of tens of kilometers for underwater acoustic communication.Therefore,in order to further enhance the application potential of UWOC to marine informatization,how to realize underwater long-distance optical communication is an important research direction.After the transmitted light wave is attenuated by underwater longdistance transmission,the optical power reaching the photodetector at the receiving end is very weak.According to the wave-particle duality of light,the light wave shows obvious particle nature at this time,that is,the amount of single photon is reached.level,beyond the sensitivity limit of conventional avalanche photodiodes,and it is difficult to recover useful signals from being drowned in thermal and avalanche noise.Using a single-photon detector with high sensitivity and low noise in the UWOC system is a solution.The single-photon detector can respond to the single-photon signal and determine the information symbol by counting the incident photons in the symbol period.It is expected to improve the Communication problems under underwater long-distance weak light links.This paper discusses common single-photon detectors such as photomultiplier tubes(PMTs),single-photon avalanche diodes(SPADs),and silicon photomultipliers(Si PMs),analyzes the basic characteristics of the single-photon response of these devices,and studies how to use them.The method of photon counting is used to judge information and achieve the purpose of information transmission.In practical applications,the underwater channel fading caused by ocean turbulence and the alignment of the transmitter and receiver of the UWOC system cannot be avoided.For any communication system,the link structure of the communication system has a great influence on the performance of the system.Multiple-input multiple-output(MIMO)structures and technologies have been widely used in the field of mobile communications,and the application of MIMO technology to the UWOC field is also an important research trend.The multiplexing technology of MIMO can effectively improve the communication capacity of the system,and the diversity technology of MIMO can effectively suppress the fading effect of the underwater channel,and in addition,it can increase the detection area of the receiving end,thereby reducing the alignment requirements of the sending and receiving ends.In this paper,the detection problem of MIMO under underwater single-photon channel is specially proposed,and its link structure and analysis model are preliminarily established,and the influence of different turbulence models on system performance is analyzed.There is little research work and further research is needed.The combination of single-photon detectors and MIMO technology is an important innovation point of this paper,that is,the single-photon detector arrays such as PMT/SPAD/Si PM are applied to the UWOC system,and the single-photon detector array can detect multiple sub-channels simultaneously.The single-photon pulse sequence after long-distance transmission is finally restored to the original electrical signal after signal processing such as signal combining,photon counting,and coincidence judgment.This team is the first to propose this single-photon detector-based MIMOUWOC system.This system has many advantages,such as high sensitivity,high energy utilization,low noise,anti-channel fading,and reduced alignment requirements at both ends of the transceiver.This structure proposal provides a potential solution for realizing underwater long-distance optical communication.This paper introduces the 6×2 and 6×3 MIMO-UWOC systems based on PMT array and Si PM array designed by the team.The hardware structure of the system,photon detection theory,system experimental setup,and characterization of key performance parameters of the system are detailed respectively.discussion.However,the system with this new structure needs further research on many core issues such as background photon suppression,photon count distribution,and photon signal decision.