Underwater Communication System Research Articles

Investigation of 3 dB Optical Intensity Spot Radius of Laser Beam under Scattering Underwater Channel.

An important step in the design of receiver aperture and optimal spacing of the diversity scheme for an underwater laser communication system is to accurately characterize the two-dimensional (2D) spatial distribution of laser beam intensity. In this paper, the 2D optical intensity distribution and 3 dB optical intensity spot radius (OISR) are investigated due to the dominating optical intensity of laser beam being within the 3 dB OISR. By utilizing the Henyey–Greenstein function to compute the scattering angles of photons, the effects of the scattering underwater optical channel and optical system parameters on 3 dB OISR are examined based on the Monte Carlo simulation method. We have shown for the first time that in the channel with a high density of scattering particles, the divergence angle of the laser source plays a negligible role in 3 dB OISR. This is an interesting phenomenon and important for optical communication as this clearly shows that the geometric loss is no longer important for the design of receiver aperture and optimal spacing of the diversity scheme for the underwater laser communication system in the highly scattering channel.

Underwater Wireless Optical Communication Based on DPSK Modulation and Silicon Photomultiplier

In this paper, a promising phase-modulated underwater wireless optical communication (UWOC) system with silicon photomultiplier (SiPM) based receiver is proposed for the first time and its feasibility has been experimentally demonstrated in a laboratory environment. The phase modulation enables the additional degree of freedom available for encoding of information in UWOC, given that previous state-of-the-art systems mainly employ intensity modulation (IM) schemes with direct detection (DD). In the proposed UWOC system, the information is encoded in the phase of light wave carrier based on differential phase-shift keying (DPSK). A highly sensitive receiver is built from SiPM, which is able to compensate the transmission loss and dramatically enhances the performance of the DPSK UWOC system. We experimentally show the feasibility of the proposed system at a data rate of 200 Mbps. For comparison, the commonly used avalanche photodiode (APD) based receiver is also tested. Comparing with the APD, the use of SiPM reduces the BERs by about two orders of magnitudes. The minimum required optical power for achieving a BER below the FEC threshold is about −40.2 dBm, which is about 11.6 dB lower than the case of APD.

Deep Learning Aided Signal Detection for SPAD-Based Underwater Optical Wireless Communications

In underwater optical wireless communication (UOWC) systems, using single photon avalanche photondiode (SPAD) as the detector can improve the transmission distance. However, the signal detection for SPAD-based systems is greatly challenged by the complex optical channel characteristics and SPAD nonlinear distortion. To address this issue, a novel deep learning aided signal detection scheme is proposed in this paper. By exploiting the physical mechanism and prior expert knowledge of the signal processing, a two-connected multilayer perception (MLP) network is integrated into the receiver. The first subnetwork is regarded as a channel compensation block while the second one works as a demodulator. With sophisticated numerical optical channel model and SPAD non-Poisson model, large amounts of training data are utilized to train the proposed model offline. Afterwards, the online data are recovered with the trained network. Simulation results verify that significant bit error ratio (BER) improvement can be achieved with the proposed scheme.

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

ОБ ИСПОЛЬЗОВАНИИ БИСТАТИЧЕСКИХ ПОДВОДНЫХ ОПТИЧЕСКИХ СИСТЕМ СВЯЗИ

Atmospheric and underwater optical communication lines have much in common due to variable conditions for the transmission of laser radiation along the path of the communication line, both in time and in space. The passage of a laser beam through a cloudy medium is accompanied by a decrease in the radiation intensity and an increase in the intensity of the scattered flux, which forms illumination, the intensity of which decreases with distance from the laser beam. As a rule, at the scales of distances at which the radiation of the optical range decays by several orders of magnitude, water in natural systems is homogeneous. The analysis shows that water in natural reservoirs is a light-attenuating medium both due to attenuation on dispersed particles and due to scattering on nanobubbles. The article analyzes the capabilities of atmospheric and underwater optical communication systems (POS). The results of modeling bistatic PIC in a reservoir containing particles with a diameter of 0.8 ?m and a concentration of 2 х 107 cm-3 are presented. It was shown that the parameters of the bistatic PIC pulses are significantly affected by nanobubbles in water. In natural reservoirs, the use of systems with direct line of sight is preferable. It has been shown that bistatic PICs, in which the scattering region is located on the side of the receiver unit relative to the transmitter - receiver line, have the greatest energy potential and minimal intersymbol interference. In natural reservoirs, it is possible to use such bistatic PICs for communication with a frequency of less than 20 MHz at distances of not more than 20 m. As a scattering region in bistatic PIC, you can use the bottom of the reservoir, the surface of the water or the region of water with a higher turbidity than the rest of the reservoir.

Time Jitter, Turbulence and Chromatic Dispersion in Underwater Optical Wireless Links

The performance of an underwater optical wireless communication link is investigated by taking into account—for the first time and to the best of our knowledge—the simultaneous influence of the chromatic dispersion, the time jitter and the turbulence effects, by assuming chirped longitudinal Gaussian pulse propagation as information carriers. The estimation procedure is presented and a novel probability density function is extracted in order to describe the irradiance fluctuations at the receiver side. Furthermore, the availability of the link is investigated by means of its probability of fade and various numerical results are presented using typical parameters for the underwater optical wireless communication systems.

Analysis of underwater wireless optical communication system performance.

Because the underwater channel environment is complicated, it is difficult to do an actual experiment in the ocean to analyze the performance of underwater wireless optical communication (UWOC) systems. In this study, the spot-expansion characteristics and time-domain broadening characteristics of underwater wireless optical signals are simulated and analyzed by a Monte Carlo statistical method. Thus, what we believe is an improved underwater channel transmission-attenuation model and time-domain broadening model based on UWOC are proposed, so the transmission distance characteristics of the UWOC system are obtained by combining the system parameters, and the transmission-rate characteristics can be analyzed by using the Shannon-Hartley theorem. The results show that the transmission distance is linear with the receiver sensitivity, and the transmission rate decays exponentially with the transmission distance and is limited by the receiver sensitivity in the UWOC system.

Performance analysis for multi-hop UOWC system over mixture Exponential–Gamma with two IM-DD fading channels

This paper provides the end-to-end performance of a multi-hop underwater optical wireless communication (UOWC) system using channel-state-information assisted (CSI) or fixed-gain amplify-and-forward (AF) relays, in which the intensity modulation-direct detection (IM-DD) is taken into account for detection. Particularly, not only the IM-DD input-independent but also the IM-DD cost-dependent additive white Gaussian noise (AWGN) channel is considered in such system analyses. In order to focus on multi-hop performance improvement and simplify calculation, the pointing error is not considered in this paper. In our study, the channel irradiance fluctuations are characterized by the mixture Exponential–Gamma (EG) distribution. With Fox H-function, exact analytical expressions for the probability density function (PDF) and the cumulative distribution function (CDF) are derived, respectively. Based on these formulas, with different hop numbers N, the outage probability (OP) and the average bit-error rate (ABER) of various modulation schemes are presented. Furthermore, numerical analyses are provided and to further investigate the effects of the turbulence severity on the performance of multi-hop UOWC system. And simulation results verifies the accuracy of the obtained analytical and asymptotic expressions. The obtained results indicate that the overall system performance enhances with an increase of the number of hops.

Performance Evaluation of a MIMO based Underwater Communication System under Fading Conditions

Performance Evaluation of a MIMO based Underwater Communication System under Fading Conditions

Performance Evaluation of a MIMO based Underwater Communication System under Fading Conditions

Due to the intense importance of underwater applications in military and commercial purposes, Underwater Acoustic Communication (UWAC) is an attracting research area. Very high propagation delay, path loss, and low bandwidth are the factors that challenged the acoustic waves in an underwater environment. Multi-input multi-output (MIMO) techniques are currently considered in UWAC to surpass the bandwidth limitation of the undersea channel. In this paper, the accomplishment of a MIMO in UWAC System highlighting both Line of Sight (LOS), i.e. the Rician fading and Non-Line of Sight (NLOS), i.e. the Rayleigh fading signal propagation, is assessed. Spatial Modulation technique is used, which helps in increasing the data rate in UWAC. It controls the spatial distribution of the energy caused by a signal in such a way that the single ocean channel sustains multiple parallel communication channels. The utilization of Zero Forcing (ZF) equalizer, which estimates the transmitted data proves the success of removing inter symbol interference (ISI). Matlab simulations are done for the UWAC system for values of LOS/NLOS. Because of various scattering effects in NLOS propagation, the error rate is considerably high when compared to that of the LOS propagation. Bit Error Rate (BER) values for the corresponding signal to noise ratio (SNR) are calculated.

Hybrid Cross Layer Mechanism For High Reliability In Under Water Wireless Sensor Network

Underwater sensor network finds lot of applications in sea environment monitoring. Tsunami like conditions can be monitored and reported earlier so that people on shores can be safe. Due to nature of medium, the data delivery ratio in under water sensor networks is very low. The various problems in the underwater sensor networks are crucial delay, Double side spreading, multipath induced fading, bandwidth limitation, Doppler effects. These problems reduce the data delivery ratio, network lifetime and other parameters.In order to increase the performance of the underwater communication system,Reed Solomon encoding technique with hybrid cross layer methodis used in physical, data link and routing layer . The proposed method increase the reliability of underwater wireless sensor networks.

Scintillation index of an optical wave propagating through moderate-to-strong oceanic turbulence.

In this paper, based on the extended Rytov theory that combines modified large-scale and small-scale amplitude spatial-frequency filters, we theoretically derive the scintillation index (SI) of the plane and spherical waves by means of the approximate oceanic refractive-index spectrum with the variable eddy diffusivity ratio propagating through moderate-to-strong oceanic turbulence. The mean bit-error rate (BER) of the underwater optical wireless communication systems is analyzed by use of the predicted SI and the gamma-gamma distribution model. Numerical results show that the obtained SI models are in good agreement with the results in weak-fluctuation regions. Also, we find that in weak turbulence, the mean BER drops sharply with the increase in mean signal-to-noise ratio, while in moderate-to-strong turbulence, the situation is opposite. We also note that the mean BER differs between the cases of unity and the variable eddy diffusivity ratio.

Influence of eddy diffusivity ratio on Gaussian beam under weak to strong oceanic turbulence

ABSTRACTThe oceanic turbulence refraction power spectrum model with a function of the eddy diffusivity ratio is adopted. By using the Rytov method, the scintillation index (SI) is derived using the aperture averaging technique for Gaussian beam under weak to strong oceanic turbulence. Variations of the SI in weak to strong oceanic turbulence are evaluated versus link length and the oceanic turbulence parameters such as the ratio of temperature to salinity contributions to the refractive index spectrum, the rate dissipation of the mean squared temperature, and the rate of dissipation of kinetic energy per unit mass of fluid. Considering the eddy diffusivity ratio of salinity to temperature, the effects of the ratio of temperature to salinity contributions to the refractive index spectrum on the performance of underwater wireless optical communication (UWOC) systems in weak to strong turbulence are analysed.

Diversity-reception UWOC system using solar panel array and maximum ratio combining.

We demonstrated a diversity-reception lens-free underwater wireless optical communication system employing a 2×2 solar panel array as detectors. The respective relationships between solar panel sizes and photocurrents, output voltages, system bandwidths were studied theoretically and experimentally. The signals output from the array were combined via maximum ratio combining in order to improve the signal quality. A 450-nm blue laser was used in the transmitter and a light spot with a size of 20mm×35mm was formed on the receiving plane after a 7-m transmission in tap water. With the solar panel array, a data rate of 84 Mbps was achieved with a bit error rate of 2.17 × 10-3 using 16-QAM OFDM signal. Meanwhile, a single solar panel with the same size as the array only achieved 60 Mbps. Solar panel array can bring about 40% data rate improvement. With the same detection area, the horizontal detection range of detectors increased from 37 mm for single solar panel to 55 mm for solar panel array thanks to the diversity reception. The results show that solar panel array using maximum ratio combining can enhance the transmission data rate as well as the detection range. The impacts of air bubbles, water fluctuation and microscopic particulates suspension on the proposed solar panel array-based system were also investigated. The results show that the received power sensitivity of solar panel array is 5.22 dB higher than single solar panel with the same detection area with a data rate of 60 Mbps and a BER of 10-3.

Binary phase shift keying-subcarrier intensity modulation performance in weak oceanic turbulence

The performance analysis of underwater wireless optical communication (UWOC) system that employs binary phase shift keying-subcarrier intensity modulation (BPSK-SIM) is investigated in weak oceanic turbulence. BPSK-SIM Gaussian beam and finite sized positive-intrinsic-negative (PIN) photodetector are employed at the transmitter and at the receiver, respectively. Bit error rate (BER) is taken as the performance indicator and in the evaluation of the BER, the required signal power and the aperture averaged scintillation index are obtained respectively by the use of the extended Huygens–Fresnel principle and the Rytov theory. Considering various noise types in underwater turbulence, BER variations are examined versus the oceanic turbulence parameters and the photodetector parameters, namely receiver aperture diameter, PIN-responsivity, noise factor, load resistor, and the electronic bandwidth.

A novel cooperative spectrum signal detection algorithm for underwater communication system

In order to further improve the spectrum resource detection probability and increase the spectrum utilization rate in underwater wireless communication systems, this paper designs a novel multi-layer cooperative spectrum sensing algorithm based on compressed sensing, which uses compressed sensing technology to estimate the spectrum to reduce the sampling rate and the overhead of sonar signals. This new algorithm seeks the optimal hyper-parameter through Bayesian model. The multi-layer Bayesian model is introduced into the Dirichlet process to realize the automatic grouping of compressed perceptual data with the information from the non-parametric grouping mechanism, and the optimal super-parameters are selected through the fusion center to determine the spectrum. Simulation results show that the proposed algorithm fully considers the temporal correlation of compressed perceptual data and effectively improves spectral sensing performance of underwater communication system.

Photon-Counting Underwater Wireless Optical Communication by Recovering Clock and Data From Discrete Single Photon Pulses

To realize a long-distance underwater communication by using low-cost light emitting diode (LED) and single photon avalanche diode (SPAD), we proposed a method for recovering clock and data directly from discrete random pulse sequences output by SPAD. On this basis, a new communication model is established, taking into account not only fluctuations in photon flux and quantum efficiency of photon detection in a time-slot, but also the phase difference of the recovered clock caused by it. A photon-counting underwater optical wireless communication (UOWC) system was built. The effects of the system setting parameters such as the number of gating signals, synchronous character length and baud rate on the average bit error rate (BER) were verified by experiments. The experimental results show that under condition of only average 10 photons in one time-slot, photon-counting UOWC with a BER of 3.51 × 10 -4 and a baud rate of 1 Mbps can be achieved.

Wideband and Wide Beam Polyvinylidene Difluoride (PVDF) Acoustic Transducer for Broadband Underwater Communications.

The advances in wireless communications are still very limited when intended to be used on Underwater Communication Systems mainly due to the adverse proprieties of the submarine channel to the acoustic and radio frequency (RF) waves propagation. This work describes the development and characterization of a polyvinylidene difluoride ultrasound transducer to be used as an emitter in underwater wireless communications. The transducer has a beam up to 10° × 70° degrees and a usable frequency band up to 1 MHz. The transducer was designed using Finite Elements Methods and compared with real measurements. Pool trials show a transmitting voltage response (TVR) of approximately 150 dB re µPa/V@1 m from 750 kHz to 1 MHz. Sea trials were carried in Ria Formosa, Faro (Portugal) over a 15 m source—receiver communication link. All the signals were successfully detected by cross-correlation using 10 chirp signals between 10 to 900 kHz.

Performance Enhancement of Underwater Acoustic OFDM Communication Systems

The supported bandwidth of the underwater communication systems is limited to several kilo hertz, which considers as the main challenge for Underwater Acoustic (UWA) communications. Meanwhile, the Bit-Error-Rate (BER) performance of the UWA systems is degrades as a result of water temperature, water salinity, attenuation, and multi-path propagation. In this paper, we present a modification to the conventional Orthogonal Frequency Division Multiplexing (OFDM) based (FFT) using Fast Walsh–Hadamard transform (FWHT) instead of Fast Fourier Transform (FFT). Also, the proposed algorithm is encoded and decoded using Low Density Parity Check (LDPC) coding algorithm. Simulation results show that the proposed algorithm with LDPC coding can improve the BBER system performance than the corresponding traditional one.

Average capacity of a UWOC system with partially coherent Gaussian beams propagating in weak oceanic turbulence.

The average capacity of a single-input single-output (SISO) underwater wireless optical communication (UWOC) system with partially coherent Gaussian beams in a weak oceanic turbulence regime is investigated. An approximate analytical expression of scintillation index is derived mathematically to characterize the impact of oceanic turbulence on the propagation behavior of the partially coherent Gaussian beams. Then, the path loss caused by absorption and scattering in the ocean is numerically simulated with the Monte Carlo method. With consideration for absorption, scattering, and oceanic turbulence, the combined channel fading model is established, and the average capacity of the UWOC system (defined as the maximum mutual information between the input and output) is examined. Results show that the scintillations are reduced by decreases in propagation distance, the dissipation rate of mean-square temperature, and the ratio of the temperature and salinity contributions to the refractive index spectrum. Scintillations are also decreased by increases in source beam width, degree of partial coherence, and the dissipation rate of turbulent kinetic energy per unit mass of fluid. As a result, the average capacity of the UWOC system is enhanced. Moreover, the average capacity of the UWOC system can be promoted with the availability of channel state information at the receiver. This work will benefit the research and development of UWOC systems.