209 research outputs found
Time-Gated Photon Counting Receivers for Optical Wireless Communication
Photon counting detectors such as single-photon avalanche diode (SPAD) arrays
are commonly considered for reliable optical wireless communication at power
limited regimes. However, SPAD-based receivers suffer from significant dead
time induced intersymbol interference (ISI) especially when the incident photon
rate is relatively high and the dead time is comparable or even larger than the
symbol duration, i.e., sub-dead-time regime. In this work, we propose a novel
time-gated SPAD receiver to mitigate such ISI effects and improve the
communication performance. When operated in the gated mode, the SPAD can be
activated and deactivated in well-defined time intervals. We investigate the
statistics of the detected photon count for the proposed time-gated SPAD
receiver. It is demonstrated that the gate-ON time interval can be optimized to
achieve the best bit error rate (BER) performance. Our extensive performance
analysis illustrates the superiority of the time-gated SPAD receiver over the
traditional free-running receiver in terms of the BER performance and the
tolerance to background light
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
Relay-Assisted Free-Space Optical Communications
The atmospheric lightwave propagation is considerably influenced by
the random variations in the refractive index of air pockets due to
turbulence. This undesired effect significantly degrades the
performance of free-space optical (FSO) communication systems.
Interestingly, the severity of such random degradations is highly
related to the range of atmospheric propagation. In this thesis, we
introduce relay-assisted FSO communications as a very promising
technique to combat the degradation effects of atmospheric
turbulence. Considering different configurations of the relays, we
quantify the outage behavior of the relay-assisted system and
identify the optimum relaying scheme. We further optimize the
performance of the relay-assisted FSO system subject to some power
constraints and provide optimal power control strategies for
different scenarios under consideration. Moreover, an application of
FSO relaying technique in quantum communications is investigated.
The results demonstrate impressive performance improvements for the
proposed relay-assisted FSO systems with respect to the conventional
direct transmission whether applied in a classical or a quantum communication channel
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
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