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.

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.

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.

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.

Characterization of Blue-Green Light Non-Line-of-Sight Transmission in Seawater

The blue-green light in the 450 nm to 550 nm band is usually used in underwater wireless optical communication (UWOC). The blue-green light transmission in seawater is scattered by the seawater effect and can achieve communication in non-line-of-sight (NLOS) transmission mode. Compared to line-of-sight (LOS) transmission, NLOS transmission does not require alignment and can be adapted to various underwater environments. The scattering coefficients of seawater at different depths are different, which makes the scattering of light in different depths of seawater different. In this paper, the received optical power and bit error rate (BER) of the photodetector (PD) were calculated when the scattering coefficients of blue-green light in seawater vary from large to small with increasing depth for NLOS transmission. The results show that blue-green light in different depths of seawater in the same way NLOS communication at the same distance, the received optical power and BER at the receiver are different, and the received optical power of green light is greater than that of blue light. Increasing the forward scattering coverage of the laser will suppress the received optical power of the PD, so when performing NLOS communication, appropriate trade-offs should be made between the forward scattering coverage of the laser and the received optical power.

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.

Underwater Bessel-like beams with enlarged depth of focus based on fiber microaxicon

Underwater optical wireless communication,which is useful for oceanography,environmental monitoring,and underwater surveillance,suffers the limit of the absorption attenuation and Mie–Rayleigh scattering of the lights.Here,Bessel-like beams generated by a fiber microaxicon is utilized for underwater wireless propagation.Underwater,the cone angle for generating Bessel-like beams starts from 46°,which is smaller than that in air for Bessel-like beams.When the cone angle of the fiber microaxicons is about 140°,the depth of focus underwater,which is four times as long as the depth of focus in air,has enlarged about 28μm,36.12μm,and 50.7μm for 470 nm,520 nm,and 632 nm visible lights.The transmission distance of the Bessel beams for visible lights has been simulated by using Henyey–Greenstein–Rayleigh phase function methods and spectral absorption by bio-optical model due to Monte Carlo methods.The results show that the propagation distance could reach 4000 m,which overcome the limit of the Mie–Rayleigh scattering and absorption attenuation underwater.

Security enhancement for OFDM-UWOC system using three-layer chaotic encryption and chaotic DFT precoding

Security is one of the key issues in communications,but it has not attracted much attention in the field of underwater wireless optical communication(UWOC).This Letter proposes a UWOC encryption scheme with orthogonal frequency division multiplexing(OFDM)modulation,based on the three-layer chaotic encryption and chaotic discrete Fourier transform(DFT)precoding.The three-layer chaotic encryption processes are bit stream diffusion,in-phase/quadrature encryption,and time-frequency scrambling.With multi-fold data encryption,the scheme can create a keyspace of 9.7×10^(179),effectively resisting brute force attacks and chosen-plaintext attacks.A 3 Gbit/s encrypted OFDM signal is successfully transmitted over a 7 m water channel.