Jing Xu

Underwater wireless transmission of high-speed QAM-OFDM signals using a compact red-light laser.

We first study the transmission property of red light in water in terms of extinction coefficient and channel bandwidth via Monte Carlo simulation, with an interesting finding that red light outperforms blue-green light in highly turbid water. We further propose and experimentally demonstrate a broadband underwater wireless optical communication system based on a simple and cost-effective TO56 red-light laser diode. We demonstrate a 1.324-Gb/s transmission at a bit error rate (BER) of 2.02 × 10-3 over a 6-m underwater channel, by using 128-QAM OFDM signals and a low-cost 150-MHz positive-intrinsic-negative photodetector, with a record spectral efficiency higher than 7.32 bits/Hz. By using an avalanche photodetector and 32-QAM OFDM signals, we have achieved a record bit rate of 4.883 Gb/s at a BER of 3.20 × 10-3 over a 6-m underwater channel.

Underwater Laser Communication Using an OFDM-Modulated 520-nm Laser Diode

In this letter, we propose and experimentally demonstrate an underwater wireless optical communication system based on a simple and cost-effective TO56 transversal multi-mode green-light laser diode (LD) and a low-cost 150-MHz positive-intrinsic-negative detector. We use quadrature amplitude modulation (QAM)-orthogonal frequency-division multiplexing modulation, with fine frequency granularity and high spectral efficiency, to tackle the challenging issue imposed by the ragged and uneven frequency response of the green LD within a limited bandwidth. We have demonstrated that a bit rate of 1.118 Gb/s (net bit rate: 0.927 Gb/s) can be achieved at a bit-error-rate of 2.98 × 10-3, with the assistance of bit loading (using both 16-QAM and 256-QAM signals). A spectral efficiency as high as 6.18 bit/s/Hz is also obtained in the proposed underwater wireless optical communication system.

26 m/5.5 Gbps air-water optical wireless communication based on an OFDM-modulated 520-nm laser diode

We experimentally demonstrate a high-speed air-water optical wireless communication system with both downlink and uplink transmission employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) and a single-mode pigtailed green-light laser diode (LD). This work is an important step towards the future study on optical wireless communications between underwater platforms and airborne terminals. Over a 5-m air channel and a 21-m water channel, we achieve a 5.3-Gbps transmission without power loading (PL) and a 5.5-Gbps transmission with PL in the downlink. The corresponding bit error rates (BERs) are 2.64×10-3 and 2.47×10-3, respectively, which are below the forward error correction (FEC) criterion. A data rate of 5.5 Gbps with PL at a BER of 2.92×10-3 is also achieved in the uplink.

10-m 9.51-Gb/s RGB laser diodes-based WDM underwater wireless optical communication

The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10-3, 2.0 × 10-3 and 2.3 × 10-3, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10-3.

Demonstration of data-carrying orbital angular momentum-based underwater wireless optical multicasting link

One-to-many data information transfer, also known as multicasting, is desired in underwater wireless communications when distributing signal between multiple users. In this paper, by exploiting the space domain (spatial phase structure) of lightwaves, we propose and demonstrate an orbital angular momentum (OAM)-based underwater wireless optical multicasting link. 2-meter underwater transmission of 4-fold green light (520 nm) OAM modes multicasting (OAM-6, OAM-3, OAM+3, OAM+6), each channel carrying 1.5-Gbaud 8-ary quadrature amplitude modulation (8-QAM) with orthogonal frequency-division multiplexing (OFDM) signal, is demonstrated in the experiment. The OAM spectrum after underwater propagation suffers some degradation with the crosstalk between multicasting OAM channels and other unwanted channels measured to be less than −6 dB. Bit-error rate (BER) performance is characterized with ~2 dB penalty. Higher-order modulation signals (16-QAM-OFDM, 32-QAM-OFDM) are also considered in free-space OAM multicasting link for comparison.

Experimental investigation of multi-band OCT precoding for OFDM-based visible light communications

In this paper, we propose and experimentally demonstrate a channel-independent multi-band orthogonal circulant matrix transform (MB-OCT) precoding, to efficiently combat the severe frequency-selective fading of visible light communications (VLC). The proposed MB-OCT precoding exhibits an attractive ladder-like signal-to-noise-ratio (SNR) profile, thus can significantly reduce system BER by applying different quadrature amplitude modulation (QAM) level to different sub-bands. The impacts of sub-band number, signal bandwidth, and length of cyclic prefix (CP) on bit error rate (BER) of the VLC system are investigated. We experimentally compare BER performance of the proposed MB-OCT precoding with that of the conventional MB discrete Fourier transform (MB-DFT) precoding and the adaptive-loaded discrete multitone (DMT). The results show that the MB-OCT precoding outperforms the MB-DFT precoding and the single-band case for different data rates. Furthermore, it exhibits reduced implementation complexity and comparable BER performance with the adaptive-loaded DMT. For ~700-Mb/s VLC system with 2-m transmission distance, the BER is reduced from 1.53 × 10-2 to 1.17 × 10-4 by using the proposed MB-OCT precoding.

Security weaknesses of underwater wireless optical communication

In this work, for the first time, we uncover that the level of security we have traditionally taken for granted on underwater wireless optical communication (UWOC) may not always be there. We first numerically investigate the security weaknesses of UWOC via Monte Carlo simulation. With the link distance increasing or the water becoming more turbid, the simulation results indicate that the possibility of information leakage increases, which may pose a great threat to the security of UWOC. By using a high-sensitivity multi-pixel photon counter (MPPC) placed aside the water tank, a 5-MHz square wave signal is successfully tapped at 1-m, 3-m, and 5-m underwater transmission distances, which preliminarily verifies the probability of information leakage. We further experimentally demonstrate an UWOC system with potential eavesdropping employing a 2.5-Gb/s orthogonal frequency division multiplexing (OFDM) signal. After transmitting through a 15-m underwater channel, the OFDM signal is eavesdropped by a mirror at 7.8 m. Both the normal receiver at 15 m and the eavesdropping receiver at 7.8 m can achieve a bit error rate (BER) below the forward error correction (FEC) limit of 3.8 × 10-3, which validates that UWOC indeed suffers potential safety hazard.

OFDM and self-coherent detection based satellite-to-ground communication system

In this paper, we propose a scheme using Orthogonal Frequency Division Multiplexing (OFDM) and self-coherent detection to enhance satellite-to-ground data links, with improved communication data rate and detection performance. We analyzed this free space optical communication systems theoretically, with preliminary experimental results being demonstrated. In the experiment, quadrature phase shift keying and 16-quadrature amplitude modulation OFDM signals were transmitted and detected by coherent receiver respectively.

Experimental evalution of underwater wireless optical transmission link employing 4-fold orbtial angular momentum (OAM) multicasting

We demonstrate an underwater wireless optical transmission link by employing 4-fold orbital angular momentum (OAM) multicasting. The OAM spectrum degradation and crosstalk between multicasted OAM channels and their neighboring ones after 2-m underwater propagation are studied.

An image processing method for floating benzene slick detection on water surface

In the last 20 years, transport of hazardous chemicals by ship has increased along with the high risk of spill events, which could cause acute and long-term environmental effects. Among hazardous chemicals, benzene and its similar chemicals are carcinogenic. In this paper, the aim of this work is to propose an accurate, quantitative, and non-polluted method to detect floating benzene on a water surface. Benzene of 14 to 18 ml was dropped on the water, and their images were obtained at spectral band of 600nm. Otsu thresholding segmentation was adopted to divide the image into the background and the target briefly, after which morphological operations were used to eliminate the noise components of the background. Then Roberts edge operator was used to detect the edge of the target, and least-squares fitting of ellipses was carried out to link the incomplete edge in order to recover the full region of floating benzene in the image. As a result, the recovered region coincided closely with the area of floating benzene in the origin images. The spreading area of the floating benzene was calculated from the recovered region, and was correlated to the actual amount of benzene. High correlationship was observed, which indicated that the proposed image detection method has potential for floating benzene slick on water surface under mild windy condition.