Underwater Wireless Optical Communication Links Research Articles

Real-Time Video Transmission Over Different Underwater Wireless Optical Channels Using a Directly Modulated 520 nm Laser Diode

We experimentally demonstrate high-quality real-time video streaming over an underwater wireless optical communication (UWOC) link up to 5 m distance using phase-shift keying (PSK) modulation and quadrature amplitude modulation (QAM) schemes. The communication system uses software defined platforms connected to a commercial TO-9 packaged pigtailed 520 nm directly modulated laser diode (LD) with 1.2 GHz bandwidth as the optical transmitter and an avalanche photodiode (APD) module as the receiver. To simulate various underwater channels, we perform laboratory experiments on clear, coastal, harbor I, and harbor II ocean water types. The measured bit error rates of the received video streams are 1.0 × 10-9 for QPSK, 4-QAM, and 8-QAM and 9.9 × 10-9 for 8-PSK. We further evaluate the quality of the received live video images using structural similarity and achieve values of about 0.9 for the first three water types, and about 0.7 for harbor II. To the best of our knowledge, these results present the highest quality video streaming ever achieved in UWOC systems that resemble communication channels in real ocean water environments.

Use of Gaussian beam divergence to compensate for misalignment of underwater wireless optical communication links

The vast majority of underwater wireless optical communication systems use collimated blue/green laser beams to deliver high-speed data over a transmission range of a few metres to tens of metres. However, such systems are extremely susceptible to misalignment of the transmitter and the receiver. The mitigation techniques for misalignment reported in the literature are complex and costly at times. In this study, the authors consider the simple approach of increasing the divergence angle of the transmitted Gaussian beam to mitigate misalignment. Both plane and spherical beams are considered as the limitation cases. Using Monte Carlo simulations, the authors show that an optimum divergence angle for the maximum acceptable lateral offset exists with respect to the receiver sensitivity in clear waters while this is not an efficient method in harbour waters. Results demonstrate that there is a design trade-off between acceptable lateral offset, power loss and channel bandwidth. Furthermore, the authors show how the proposed scheme of beam divergence affects the maximum allowed link span as well as the channel bandwidth for a given distance.

Design of a Hybrid Transceiver Through P-Spice Simulation for Establishing Underwater Optical Wireless Communication Link

In this work, using PSpice Simulator we try to implement an underwater optical wireless communication link and to determine at which wavelength the attenuation is minimum for the given transmission medium characteristics. Because in coastal waters, both inherent and apparent optical properties are influenced by a wide array of physical, biological and chemical processes leading to optical variability. Hence, appropriate channel modeling will be performed. The main challenge for the development of efficient underwater wireless sensor networking solutions posed by the underwater environment is the requirement of different wavelengths of light for different ocean conditions. To overcome this drawback, a hybrid transceiver is designed in such way that the transmitter will transmit a particular wavelength optical signal in order to have least BER depending on the characteristics of the underwater optical channel.

Performance Evaluation of Underwater Wireless Optical Communications Links in the Presence of Different Air Bubble Populations

We experimentally evaluate the performance of underwater wireless optical communication (UWOC) links in the presence of different air bubbles. Air bubbles of different sizes and densities are generated by using an air pipe in conjunction with a submersible water pump of variable flow rate that helps break up large bubbles into smaller bubbles. Received signal intensity measurements show that bubbles significantly degrade the performance of UWOC links. Large bubbles completely obstruct the optical beam and cause a deep fade. However, as the bubble size decreases, the level of deep fade also decreases because the optical beam is less susceptible to complete obstruction, and more light reaches the detector. We also show that beam expansion could help mitigate the performance degradation due to the deep fade caused by air bubbles scatters in the channel.

Performance Studies of Underwater Wireless Optical Communication Systems With Spatial Diversity: MIMO Scheme

In this paper, we analytically study the performance of multiple-input multiple-output underwater wireless optical communication (UWOC) systems with ON–OFF keying modulation. To mitigate turbulence-induced fading, which is amongst the major degrading effects of underwater channels on the propagating optical signal, we use spatial diversity over UWOC links. Furthermore, the effects of absorption and scattering are considered in our analysis. We analytically obtain the exact and an upper bound bit error rate (BER) expressions for both optimal and equal gain combining. In order to more effectively calculate the system BER, we apply Gauss-Hermite quadrature formula as well as approximation to the sum of lognormal random variables. We also apply the photon-counting method to evaluate the system BER in the presence of shot noise. Our numerical results indicate an excellent match between the exact and upper bound BER curves. Also, a good match between the analytical results and numerical simulations confirms the accuracy of our derived expressions. Moreover, our results show that spatial diversity can considerably improve the system performance, especially for channels with higher turbulence, e.g., a $3\times 1$ multiple-input single-output transmission in a 25 m coastal water link with a log-amplitude variance of 0.16 can introduce 8 dB performance improvement at the BER of 10−9.

Performance Analysis of Multi-Hop Underwater Wireless Optical Communication Systems

In this letter, we analytically evaluate the end-to-end bit error rate (BER) of point-to-point multi-hop underwater wireless optical communication (UWOC) systems with respect to all degrading effects of the UWOC channel, namely absorption, scattering, and turbulence-induced fading. To do so, we first derive the BER expression of a single-hop UWOC link as the building block for end-to-end BER evaluation. We also apply Gauss–Hermite quadrature formula to obtain the closed-form solution for the system BER in the case of lognormal underwater fading channel. Numerical results demonstrate that multi-hop transmission, by alleviating channel impairments, can significantly improve the system performance and extend the viable end-to-end communication distance.

Investigation of solar noise impact on the performance of underwater wireless optical communication links.

We investigate the effect of environmental noise, caused by solar radiations under water, on the performance of underwater wireless optical communication (UWOC) systems. Presenting an analytical and generic model for this noise, we examine its impact on the link performance in terms of the bit error rate (BER). This study is conducted for different photo-detector types in the aim of highlighting practical limitations of establishing UWOC links in the presence of subsea solar noise. We show how the solar noise can impact the performance of UWOC links for relatively low operation depths. The results we present provide valuable insight for the design of UWOC links, which are likely to be established at relatively low depths. They can be exploited not only for the purpose of practical UWOC system deployment but also for in-pool experimental set-ups, since they elucidate the effect of ambient light on the measurements.

20-meter underwater wireless optical communication link with 1.5 Gbps data rate.

The video streaming, data transmission, and remote control in underwater call for high speed (Gbps) communication link with a long channel length (~10 meters). We present a compact and low power consumption underwater wireless optical communication (UWOC) system utilizing a 450-nm laser diode (LD) and a Si avalanche photodetector. With the LD operating at a driving current of 80 mA with an optical power of 51.3 mW, we demonstrated a high-speed UWOC link offering a data rate up to 2 Gbps over a 12-meter-long, and 1.5 Gbps over a record 20-meter-long underwater channel. The measured bit-error rate (BER) are 2.8 × 10-5, and 3.0 × 10-3, respectively, which pass well the forward error correction (FEC) criterion.

Impulse response modeling for general underwater wireless optical MIMO links

In underwater wireless optical communications (UWOC), channel modeling plays a key role in investigating the propagation properties of light beams through UWOC links as well as evaluating overall system performance. We consider UWOC multiple-input and multiple-output (MIMO) systems with multiple light sources and detectors, and focus on the impulse response that is capable of characterizing the temporal behavior of UWOC links. We propose a weighted Gamma function polynomial (WGFP) to model the impulse response of general UWOC MIMO links with arbitrary numbers of light sources and detectors. Numerical results of Monte Carlo simulations have validated the proposed WGFP model in a turbid water environment.

General Stochastic Channel Model and Performance Evaluation for Underwater Wireless Optical Links

In underwater wireless optical communications (UWOC), absorption and scattering characterize the link properties since photons may suffer these two processes with energy loss and direction change, respectively, when interacting with water molecules or suspended particles. In this work, we consider the effects of absorption and scattering on the probability distribution, i.e., normalized intensity distribution, of photons in space and time domains. Our prior work proposed a stochastic channel model to represent the spatial–temporal probability distribution of propagated photons only for nonscattering and single scattering components of UWOC links. However, multiple scattering will dominate the scattering behavior of the underwater environment with long communication distance and/or more turbid water type. In this work, we take into account all three types of components including nonscattering, single and multiple scattering, and present a more general stochastic channel model which fits well with Monte Carlo simulations in turbid water environment such as coastal and harbor water. Based on the proposed channel model, we also evaluate the performance of path loss, scattering richness, and attenuation of UWOC links. Numerical results suggest that multiple scattering can compensate the path loss overestimated by traditional approaches. Furthermore, scattering richness and attenuation tend to increase but have opposite effects to raise and reduce the received probabilities of higher order scattered photons, respectively, as link range increases.

Impulse Response Modeling for Underwater Wireless Optical Communication Links

In underwater wireless optical communication (UWOC) links, multiple scattering may cause temporal spread of beam pulse characterized by the impulse response, which therefore results in inter-symbol interference (ISI) and degrades system error performance. The impulse response of UWOC links has been investigated both theoretically and experimentally by researchers but has not been derived in simple closed-form to the best of our knowledge. In this paper, we analyze the optical characteristics of seawater and present a closed-form expression of double Gamma functions to model the channel impulse response. The double Gamma functions model fits well with Monte Carlo simulation results in turbid seawater such as coastal and harbor water. The bit-error-rate (BER) and channel bandwidth are further evaluated based on this model for various link ranges. Numerical results suggest that the temporal pulse spread strongly degrades the BER performance for high data rate UWOC systems with on-off keying (OOK) modulation and limits the channel bandwidth in turbid underwater environments. The zero-forcing (ZF) equalization designed based on our channel model has been adopted to overcome ISI and improve the system performance. It is plausible and convenient to utilize this impulse response model for performance analysis and system design of UWOC systems.

Characterization of the Beam-Spread Function for Underwater Wireless Optical Communications Links

Optical links are currently being considered for high-bandwidth underwater communications at short ranges (<100 m). To predict the performance of these links, a firm understanding of how the inherent optical properties of water affect the encoded optical signal is needed. Of particular interest is the impact of scattering due to particulates. Typically, the link loss is computed using the beam attenuation coefficient, which describes the attenuation of nonscattered light due to absorption and scattering. This approach is insufficient, as it neglects the contribution of scattered light to the total received signal. Given the dynamic nature of underwater platforms, as well as the dynamic nature of the environment itself, knowledge of the angular dependence of forward-scattered light is imperative for determining pointing and tracking requirements as well as overall signal to noise. In this work, the theory necessary to describe spatial spreading of an optical beam in the presence of scattering agents underwater is reviewed. This theory is then applied to a performance prediction model that is validated via laboratory experiments. Finally, the model is used to study the impact of spatial spreading on an underwater optical link.