Single Photon Detection Technology in Underwater Wireless Optical Communication:Modulation Modes and Error Correction Coding Analysis

This study explores the application of single photon detection(SPD)technology in underwater wireless optical communication(UWOC)and analyzes the influence of different modulation modes and error correction coding types on communication performance.The study investigates the impact of on-off keying(OOK)and 2-pulse-position modulation(2-PPM)on the bit error rate(BER)in single-channel intensity and polarization multiplexing.Furthermore,it compares the error correction performance of low-density parity check(LDPC)and Reed-Solomon(RS)codes across different error correction coding types.The effects of unscattered photon ratio and depolarization ratio on BER are also verified.Finally,a UWOC system based on SPD is constructed,achieving 14.58 Mbps with polarization OOK multiplexing modulation and 4.37 Mbps with polarization 2-PPM multiplexing modulation using LDPC code error correction.

Green indoor optical wireless communication systems:Pathway towards pervasive deployment

The Optical Wireless Communication(OWC)offers the high capacity of optical fiber communication with the flexibility of wireless communication.Since it works in the optical region of the ElectroMagnetic(EM)spectrum,it guarantees safety and security which are critical in radio and microwave frequency communication.The principal objective of this paper is to analyze the indoor OWC systems on these guaranteed features,and safety and security are jointly denoted by the term green.The high obstacle impermeability of optical signals and their directivity strengthen the security of indoor OWC data transmission.The confidentiality and authenticity of optical wireless data can also be preserved with the Quantum Key Distribution(QKD).This paper provides a technological overview and a review of literature about the OWC system that helps to identify the challenges in the path of a ubiquitous deployment of green wireless communication systems.Significant advancements in the sources and detectors are discussed together with the coding,modulation and multiplexing techniques for making highly robust OWC links.The ubiquitous deployment of green OWC necessitates the development of optical transmitters and receivers,performance enhancement techniques,incorporation of uplink and energy harvesting abilities,and safety and security enhancement techniques.Hence,a special emphasis is placed on these aspects and their challenges towards the green implementation.Furthermore,the paper explores some significant indoor applications based on the OWC that have great impacts on the Next Generation Networks(NGN)and the Internet of Things(IoT).

Towards 5th Generation Wireless Communication Systems

This paper introduces the general landscape of next-generation wireless communication systems （5G）, including the impetus and requirements of 5G and the candidate technologies that might help 5G achieve its goals. The following areas, which the author considers particularly relevant, are discussed： deteetion of and access to free spectrum over bands of a heterogeneous nature, ex- treme densification of networks （massive base station deployments）, extreme increase in the number of antennas in base station ar- rays and their interaction with a novel waveform, integration of both wireless and optical sides of telecom networks, and study of wireless networks from the perspective of complex systems science. The author discusses recent research conducted by his team in each of these research areas.

Petahertz communication:Harmonizing optical spectra for wireless communications

With the rapid deployment of Next-Generation Networks(NGN),the research community has initiated discussions on an entirely new suite of optical enabling techniques.To pave the way for the development of future wireless networks,this article aims to unify the existing infrared,visible light,and ultraviolet subbands while also exploring the potential of the Petahertz(PHz)band to support extremely bandwidth-thirsfy telepresence style applications.Our focus is on the emerging Petahertz Communication(PetaCom)framework,scenario-dependent propagation channels,modulation schemes,system performance,multiple access techniques,and networking.We conclude with a range of PetaCom challenges and open research issues.

Optical Wireless Communications

The optical spectrum can serve as a good spectrum resource for wideband wireless communications. The advantages of optical wireless communications （OWC） mainly lie in two aspects： the potential large transmission bandwidth due to the high-frequency carrier, and thecommunication security due to no radio-frequency radiation. Thus OWC can be applied in the seenarios where the radio silence is required or the radio frequency radiation may cause explosions, for example in the battle field or some special ar- eas in the storehouses.

Short-Range Optical Wireless Communications for Indoor and Interconnects Applications

Optical wireless communications have been widely studied during the past decade in short-range applications, such as indoor highspeed wireless networks and interconnects in data centers and high-performance computing. In this paper, recent developments in high-speed short-range optical wireless communications are reviewed, including visible light communications （VLCs）, infrared indoor communication systems, and reconfigurable optical interconnects. The general architecture of indoor high-speed optical wireless communications is described, and the advantages and limitations of both visible and infrared based solutions are discussed. The concept of reconfigurable optical interconnects is presented, and key results are summarized. In addition, the challenges and potential future directions of short-range optical wireless communications are discussed.

Subcarrier Intensity Modulated Optical Wireless Communications:A Survey from Communication Theory Perspective

Subcarrier intensity modulation with direet detection is a modulatiou/detection technique tbr optical wireless communication systems, where a pre-modulated and properly biased radio frequency signal is modulated on the intensity of the optical carrier. The most important benefits of subcarrier intensity modulation are as follows： 1） it does not provide irreducible error floor like the conventional on-off keying intensity modulation with a fixed detection threshold; 2） it provides improved spectral efficiency and supports higher order modulation schemes; and 3） it has much less implementation complexity compared to coherent optical wireless communications with heterodyne or homodyne detection. In this paper, we present an up-to-date review of subcarrier intensity modulated optical wireless communication systems. We survey the error rate and outage performance of subcarrier intensity modulations in the atmospheric turbulence channels considering different modulation and coding schemes. We also explore different contemporary atmospheric turbulence fading mitigation solutions that can be employed for subcarrier intensity modulation. These solutions include diversity combining, adaptive transmission, relay assisted transmission, multiple-subcarrier intensity modulations, and optical orthogonal frequency division multiplexing. Moreover, we review the performance of subcarrier intensity modulations due to the pointing error and synchronization error.

Call for Papers ZTE Communications Special Issue on Optical Wireless Communications

Optical wireless communication, which serves as an important alternative to the radio-frequency communication, shows great potential to a lot of applications, such as indoor communication, secure communication, and battlefield communication. It has attracted increasing research interests from both academia and industrial fields. Moreover, optical wireless communication has been applied to the indoor communication and positioning system. The upcoming special issue of ZTE Communication will focus on the cutting-edge research and application on the communication system and related signal processing in optical wireless communication. The expected publication date will be in Mar. 2016. It includes （but not limited to） the following research directions：

Timing Synchronization Method for Asymmetrically Clipped DC Biased Optical OFDM System

In a typical intensity-modulated direct-detection optical system, the information is carried on the intensity of the optical signal, and therefore can only be positive. Existing synchronization methods for RF-based OFDM system cannot be used directly in asymmetrically clipped DC biased optical of DM (ADO-OFDM) system. In order to solve the above problem, according to the characteristics of ADO-OFDM signal, the modified training symbol is presented. The simulation results show that the modified synchronization method gives good performance.

Optical engineering of infrared PbS CQD photovoltaic cells for wireless optical power transfer systems

Infrared photovoltaic cells(IRPCs)have attracted considerable attention for potential applications in wireless optical power transfer(WOPT)systems.As an efcient fber-integrated WOPT system typically uses a 1550 nm laser beam,it is essential to tune the peak conversion efciency of IRPCs to this wavelength.However,IRPCs based on lead sulfde(PbS)colloidal quantum dots(CQDs)with an excitonic peak of 1550 nm exhibit low short circuit current(Jsc)due to insufcient absorption under monochromatic light illumination.Here,we propose comprehensive optical engineering to optimize the device structure of IRPCs based on PbS CQDs,for 1550 nm WOPT systems.The absorption by the device is enhanced by improving the transmittance of tin-doped indium oxide(ITO)in the infrared region and by utilizing the optical resonance efect in the device.Therefore,the optimized device exhibited a high short circuit current density of 37.65 mA/cm^(2)under 1 sun(AM 1.5G)solar illumination and 11.91 mA/cm^(2)under 1550 nm illumination 17.3 mW/cm^(2).Furthermore,the champion device achieved a record high power conversion efciency(PCE)of 7.17%under 1 sun illumination and 10.29%under 1550 nm illumination.The PbS CQDs IRPCs under 1550 nm illumination can even light up a liquid crystal display(LCD),demonstrating application prospects in the future.

Optical SDMA for applying compressive sensing in WSN

In order to apply compressive sensing in wireless sensor network, inside the nodes cluster classified by the spatial correlation, we propose that a cluster head adopts free space optical communication with space division multiple access, and a sensor node uses a modulating retro-reflector for communication. Thus while a random sampling matrix is used to guide the establishment of links between head cluster and sensor nodes, the random linear projection is accomplished. To establish multiple links at the same time, an optical space division multiple access antenna is designed. It works in fixed beams switching mode and consists of optic lens with a large field of view（FOV）, fiber array on the focal plane which is used to realize virtual channels segmentation, direction of arrival sensor, optical matrix switch and controller. Based on the angles of nodes＇ laser beams, by dynamically changing the route, optical matrix switch actualizes the multi-beam full duplex tracking receiving and transmission. Due to the structure of fiber array, there will be several fade zones both in the focal plane and in lens＇ FOV. In order to lower the impact of fade zones and harmonize multibeam, a fiber array adjustment is designed. By theoretical, simulated and experimental study, the antenna＇s qualitative feasibility is validated.

50 m/187.5 Mbit/s real-time underwater wireless optical communication based on optical superimposition

In this paper,an optical pulse amplitude modulation with 4 levels(PAM-4)using a fiber combiner is proposed to enhance the data rate of a field-programmable gate-array-based long-distance real-time underwater wireless optical communication system.Two on-off keying signals with different amplitudes are used to modulate two pigtailed laser diodes,respectively,and the generated optical signals are superimposed into optical PAM-4 signals by a fiber combiner.The optical PAM-4scheme can effectively alleviate the nonlinearity,although it reduces the peak-to-peak value of the emitting optical power by 25%.A real-time data rate of 187.5 Mbit/s is achieved by using the optical PAM-4 with a transmission distance of 50 m.The data rate is increased by about 25%compared with the conventional electrical PAM-4 in the same condition.

A comprehensive review of optical wireless power transfer technology

Ground-breaking optical wireless power transfer(OWPT)techniques have gained significant attention from both academia and industry in recent decades.Powering remote systems through laser diodes(LDs)to either operate devices or recharge batteries offers several benefits.Remote LDs can remove the burden of carrying extra batteries and can reduce mission time by removing battery swap-time and charging.Apart from its appealing benefits,laser power transfer(LPT)is still a challenging task due to its low transfer efficiency.In this paper,we discuss the necessity and feasibility of OWPT and discuss several projects,working principle,system design,and components.In addition,we show that OWPT is an essential element to supply power to Internet-of-Things(IoT)terminals.We also highlight the impacts of dynamic OWPT.We outline several OWPT techniques including optical beamforming,distributed laser charging(DLC),adaptive-DLC(ADLC),simultaneous lightwave information and power transfer(SLIPT),Thing-to-Thing(T2T)OWPT,and high intensity laser power beaming(HILPB).We also deal with laser selection,hazard analysis,and received photovoltaic(PV)cell selection for OWPT systems.Finally,we discuss a range of open challenges and counter measures.We believe that this review will be helpful in integrating research and eliminating technical uncertainties,thereby promoting progress and innovation in the development of OWPT technologies.

Machine-vision-based acquisition,pointing,and tracking system for underwater wireless optical communications

Due to the proliferation of underwater vehicles and sensors,underwater wireless optical communication(UWOC)is a key enabler for ocean exploration with a strong reliance on short-range bandwidth-intensive communications.A stable optical link is of primary importance for UWOC.A compact,low-power,and low-cost acquisition,pointing,and tracking(APT)system is proposed and experimentally demonstrated to realign the optical link within 0.04 s,even when the UWOC transmitter and receiver are in relative motion.The system successfully achieves rapid auto-alignment through a 4 m tap water channel with a relatively large number of bubbles.Furthermore,the required minimum illumination value is measured to be as low as7.1 lx,implying that the proposed APT scheme is robust to dim underwater environments.Meanwhile,mobility experiments are performed to verify the performance of the APT system.The proposed system can rapidly and automatically align moving targets in complex and unstable underwater environments,which can potentially boost the practical applications of UWOC.

Real-time UWOC miniaturized system based on FPGA and LED arrays and its application in MIMO

In order to alleviate the impact of turbulence on the performance of underwater wireless optical communication(UWOC)in real time,and achieve high-speed real-time transmission and low cost and miniaturization of equipment,a 2×2 real-time multiple-input and multiple-output(MIMO)high-speed miniaturized UWOC system based on a field-programmable gate array(FPGA)and a high-power light-emitting diode(LED)array is designed in this Letter.In terms of multiplexing gain,the imaging MIMO spatial multiplexing and high-order modulation for the first time are combined and the real-time high-speed transmission of PAM-4 signal based on the LED array light source in 12 m underwater channel at 100 Mbps rate is implemented,which effectively improves the throughput of the UWOC system with a high-power commercial LED light source.In light of diversity gain,the system employs the diversity of repeated coding scheme to receive two identical non-return-to-zero on-off keying(NRZ-OOK)signals,which can compensate the fading or flickering sublinks in real time under the bubble-like simulated turbulence condition,and has high robustness.To our knowledge,this is the first instance of a high rate and long-distance implementation of a turbulence-resistant real-time MIMO miniaturized UWOC system based on FPGA and high-power LED arrays.With spatial diversity or spatial multiplexing capabilities,its low cost,integrity,and high robustness provide the system with important practical prospects.

SIMO detection schemes for underwater optical wireless communication under turbulence

In underwater optical wireless communication(UOWC),a channel is characterized by abundant scattering/absorption effects and optical turbulence.Most previous studies on UOWC have been limited to scattering/absorption effects.However,experiments in the literature indicate that underwater optical turbulence(UOT)can cause severe degradation of UOWC performance.In this paper,we characterize an UOWC channel with both scattering/absorption and UOT taken into consideration,and a spatial diversity receiver scheme,say a singleinput–multiple-output(SIMO) scheme,based on a light-emitting-diode(LED) source and multiple detectors is proposed to mitigate deep fading.The Monte Carlo based statistical simulation method is introduced to evaluate the bit-error-rate performance of the system.It is shown that spatial diversity can effectively reduce channel fading and remarkably extend communication range.

Ultraviolet communication technique and its application

With recent developments of deep ultraviolet(DUV)light-emitting diodes and solar-blind detectors,UV communication(UVC)shows great potential in replacing traditional wireless communication in more and more scenarios.Based on the atmospheric scattering of UV radiation,UVC has gained considerable attention due to its non-line-of-sight ability,omnidirectional communication links and low background noise.These advantages make UVC an ideal option for covert secure communication,especially for military communication.In this review,we present the history and working principle of UVC with a special focus on its light sources and detectors.Comprehensive comparison and application of its light sources and detectors are provided to the best of our knowledge.We further discuss the future application and outlook of UVC.Hopefully,this review will offer valuable insights into the future development of UVC.

Modulation Techniques for Li-Fi

Modulation techniques for light fidelity （Li-Fi） are reviewed in this paper. Li-Fi is the fully networked solution for nmltiple users that combines communication and illumination simultaneously. Light emitting diodes （LEDs） are used in Li-Fi as visible light transmitters, therefore, only intensity modulated direct detected modulation techniques can be achieved. Single carrier modulation techniques are straightforward to be used in Li-Fi, however, computationally complex equalization processes are required in fre- quency selective Li-Fi channels. On the other hand, multiearrier modulation techniques offer a viable solution for Li-Fi in terms of power, spectral and computational efficiency. In particular, orthogonal frequency division multiplexing （OFDM） based modulation techniques offer a practical solution for Li-Fi, especially when direct current （DC） wander, and adaptive bit and power loading techniques are considered. Li-Fi modulation techniques need to also satisfy illumination requirements. Flickering avoidance and dimming control are considered in the variant modulation techniques presented. This paper surveys the suitable modulation techniques for Li-Fi including those which explore time, frequency and colour domains.

Application of phase‑conjugate beams in beam correction and underwater optical wireless communication subject to surface wave turbulence

Water surface wave turbulence is one of the factors afecting the performances of underwater optical wireless communication(UOWC)systems.In our research,a phase-conjugate beam was used to correct the beam distortion and enhance the communication performances when a system is subject to surface wave turbulence.The phase-conjugate beam was generated by a phase-conjugate mirror(PCM),and a turbulence generator was used to generate surface wave turbulence in the experiment.We calculated the beam centroid distribution and the results showed that the phase-conjugate beam had a better propagation performance than the distorted beam at the diferent water depths.The root mean square(RMS)of the beam centroid for the phase-conjugate beam was 11 times less than that for the distorted beam,which meant that the phase-conjugate beam could efectively correct the beam drift.We further investigated the scintillation index and the signal-to-noise ratio(SNR);the results showed that the phase-conjugate beam was able to reduce the scintillation and an obvious improvement in SNR could be obtained.This research has the potential to be applied in UWC.

Experimental demonstration of underwater optical wireless power transfer using a laser diode

We experimentally demonstrate an underwater optical wireless power transfer （OWPT） using a laser diode （LD） as a power transmitter. We investigate the characteristics of a solar cell and a photodiode （PD） as a power receiver. We optimize the LD, the PD, and the solar cell to achieve the maximum transfer efficiency. The maxi- mum transfer efficiency of the back-to-back OWPT is measured as 4.3% with the PD receiver. Subsequently, we demonstrate the OWPT in tap and sea water. Our result shows an attenuation of 3 dB/m in sea water.