1,014 research outputs found
MIMO Underwater Visible Light Communications: Comprehensive Channel Study, Performance Analysis, and Multiple-Symbol Detection
In this paper, we analytically study the bit error rate (BER) performance of
underwater visible light communication (UVLC) systems with binary pulse
position modulation (BPPM). We simulate the channel fading-free impulse
response (FFIR) based on Monte Carlo numerical method to take into account the
absorption and scattering effects. Additionally, to characterize turbulence
effects, we multiply the aforementioned FFIR by a fading coefficient which for
weak oceanic turbulence can be modeled as a lognormal random variable (RV).
Moreover, to mitigate turbulence effects, we employ multiple transmitters
and/or receivers, i.e., spatial diversity technique over UVLC links.
Closed-form expressions for the system BER are provided, when equal gain
combiner (EGC) is employed at the receiver side, thanks to Gauss-Hermite
quadrature formula and approximation to the sum of lognormal RVs. We further
apply saddle-point approximation, an accurate photon-counting-based method, to
evaluate the system BER in the presence of shot noise. Both laser-based
collimated and light emitting diode (LED)-based diffusive links are
investigated. Since multiple-scattering effect of UVLC channels on the
propagating photons causes considerable inter-symbol interference (ISI),
especially for diffusive channels, we also obtain the optimum multiple-symbol
detection (MSD) algorithm to significantly alleviate ISI effects and improve
the system performance. Our numerical analysis indicates good matches between
the analytical and photon-counting results implying the negligibility of
signal-dependent shot noise, and also between analytical results and numerical
simulations confirming the accuracy of our derived closed-form expressions for
the system BER. Besides, our results show that spatial diversity significantly
mitigates fading impairments while MSD considerably alleviates ISI
deteriorations
Atmospheric channel effects on terrestrial free space optical communication links
Abstract. This paper illustrates the challenges imposed by the atmospheric channel on the design of a terrestrial laser communication link. The power loss due to scattering effect is described using the Kim/Kruse scattering model while the effect and the penalty imposed by atmospheric turbulence is highlighted by considering the bit error rate (BER) of an On-Off Keying modulated link in an optical Poisson channel. The power loss due to thick fog can measure over 100 dB/km while snow and rain result in much lower attenuation. We show that non-uniformity in the atmospheric temperature also contributes to performance deterioration due to scintillation effect. At a BER of 10-4, for a channel with a turbulence strength of>0.1, the penalty imposed by turbulence induced fading is over 20 photoelectron counts in order to achieve the same level of performance as a channel with no fading. The work reported here is part of the EU COST actions and EU projects.
Single-Photon Avalanche Diodes in CMOS Technologies for Optical Communications
As optical communications may soon supplement Wi-Fi technologies, a concept known as visible light communications (VLC), low-cost receivers must provide extreme sensitivity to alleviate attenuation factors and overall power usage within communications link budgets. We present circuits with an advantage over conventional optical receivers, in that gain can be applied within the photodiode thus reducing the need for amplification circuits. To achieve this, single-photon avalanche diodes (SPADs) can be implemented in complementary metal-oxide-semiconductor (CMOS) technologies and have already been investigated in several topologies for VLC. The digital nature of SPADs removes the design effort used for low-noise, high-gain but high-bandwidth analogue circuits. We therefore present one of these circuit topologies, along with some common design and performance metrics. SPAD receivers are however not yet mature prompting research to take low-level parameters up to the communications level
Statistical Modeling of Single-Photon Avalanche Diode Receivers for Optical Wireless Communications
In this paper, a comprehensive analytical approach is presented for modeling the counting statistics of active quenching and passive quenching single-photon avalanche diode (SPAD) detectors. It is shown that, unlike ideal photon counting receiver for which the detection process is described by a Poisson arrival process, photon counts in practical SPAD receivers do not follow a Poisson distribution and are highly affected by the dead time caused by the quenching circuit. Using the concepts of renewal theory, the exact expressions for the probability distribution and moments (mean and variance) of photocounts in the presence of dead time are derived for both active quenching and passive quenching SPADs. The derived probability distributions are validated through Monte Carlo simulations and it is demonstrated that the moments match with the existing empirical models for the moments of SPAD photocounts. Furthermore, an optical communication system with on-off keying and binary pulse position modulation is considered and the bit error performance of the system for different dead time values and background count levels is evaluated
Fuzzy logic, edge enabled underwater video surveillance through partially wireless optical communication.
Underwater surveillance is inherently tricky to achieve. Even in the clearest waters, the visibility tends to be in the range of tens of meters. Normally, tethered Remotely Operated Vehicles (ROVs) with underwater cameras are used for underwater imaging at closer ranges. Currently, detailed visible light imaging can be achieved utilising green laser technology, and this is limited to close ranges due to the inherent properties of light attenuation in water. The alternative is to utilise sonar based imaging which is capable of visualising distances, however, this technique is vulnerable to noise that interferes with the operating frequency, rendering the applications somewhat limited. The emergence of high data-rate, wireless, optical communication could allow for dense placement of short-range imaging equipment to monitor areas of strategic interest to extend the range, however, there needs to be a reliable method of wirelessly communicating this data to the sea surface regardless of the localised environmental conditions that may interfere with a visible light transmission. This paper proposes a fuzzy logic, edge computing enabled routing algorithm for optical networks that utilises a wired connection among source nodes to "pass" video data around among themselves to decide which seafloor node is best placed to transmit the data according to relative local turbidity, light intensity and sea-life activity, the main factors that hamper a well-considered wireless optical network. From there, a selected node can theoretically transmit the data from the source to the sea-surface through the wireless optical relay network implemented above. This mechanism shows promise in improving link reliability and throughput compared to alternative systems
SPAD-Based Optical Wireless Communication with Signal Pre-Distortion and Noise Normalization
In recent years, there has been a growing interest in exploring the
application of single-photon avalanche diode (SPAD) in optical wireless
communication (OWC). As a photon counting detector, SPAD can provide much
higher sensitivity compared to the other commonly used photodetectors. However,
SPAD-based receivers suffer from significant dead-time-induced non-linear
distortion and signal dependent noise. In this work, we propose a novel
SPAD-based OWC system in which the non-linear distortion caused by dead time
can be successfully eliminated by the pre-distortion of the signal at the
transmitter. In addition, another system with joint pre-distortion and noise
normalization functionality is proposed. Thanks to the additional noise
normalization process, for the transformed signal at the receiver, the
originally signal dependent noise becomes signal independent so that the
conventional signal detection techniques designed for AWGN channels can be
employed to decode the signal. Our numerical results demonstrate the
superiority of the proposed SPAD-based systems compared to the existing systems
in terms of BER performance and achievable data rate
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