Receiver design for SPAD-based VLC systems under Poisson-Gaussian mixed noise model

Single-photon avalanche diode (SPAD) is a promising photosensor because of its high sensitivity to optical signals in weak illuminance environment. Recently, it has drawn much attention from researchers in visible light communications (VLC). However, existing literature only deals with the simplified channel model, which only considers the effects of Poisson noise introduced by SPAD, but neglects other noise sources. Specifically, when an analog SPAD detector is applied, there exists Gaussian thermal noise generated by the transimpedance amplifier (TIA) and the digital-to-analog converter (D/A). Therefore, in this paper, we propose an SPAD-based VLC system with pulse-amplitude-modulation (PAM) under Poisson-Gaussian mixed noise model, where Gaussian-distributed thermal noise at the receiver is also investigated. The closed-form conditional likelihood of received signals is derived using the Laplace transform and the saddle-point approximation method, and the corresponding quasi-maximum-likelihood (quasi-ML) detector is proposed. Furthermore, the Poisson-Gaussian-distributed signals are converted to Gaussian variables with the aid of the generalized Anscombe transform (GAT), leading to an equivalent additive white Gaussian noise (AWGN) channel, and a hard-decision-based detector is invoked. Simulation results demonstrate that, the proposed GAT-based detector can reduce the computational complexity with marginal performance loss compared with the proposed quasi-ML detector, and both detectors are capable of accurately demodulating the SPAD-based PAM signals

A comparison of APD and SPAD based receivers for visible light communications

Visible light communications (VLC) is an alternative method of indoor wireless communications that requires sensitive receivers. Ideally, single photon avalanche detectors (SPADs) could be used to create more sensitive receivers. However, the dead-time, finite output pulse width and photon detection efficiency of existing SPAD arrays limits their sensitivity and bandwidth. In this paper an accurate equation for the impact of dead-time on the sensitivity of a SPAD array is presented. In addition the impact of the width of the output pulses on the on-off keying (OOK) data rate is investigated. Finally, a comparison between receivers containing an APD and a large array of SPADs shows that although the receiver containing the SPAD is more sensitive in the dark the APD-based receiver is more sensitive in normal operating condition. However, the models that predict the performance of both receivers suggest that newer SPAD arrays will enable significant improvements in receiver sensitivity

Single-photon avalanche diode receivers for optical wireless communications

Single-photon avalanche diodes (SPADs) have been widely applied in many applications over the past few decades thanks to their high sensitivity, high photon detection efficiency and high timing resolution. Nowadays, they are drawing particular attention in the field of optical wireless communication (OWC), resulting in wider and deeper studies among the scientific research community. Compared with positive-intrinsic-negative (PIN) diodes and avalanche photodiodes (APDs), SPADs provide much higher internal gains and sensitivities, thereby easily overcoming the thermal noise and enabling the detection of individual photons without the need for transimpedance amplifiers (TIAs). However, upon detecting a photon, the SPAD is unable to respond to subsequent incident photons for a certain period of time, called dead time. This dead time is caused by the quenching circuit, which is of two principal modes: active quenching (AQ) and passive quenching (PQ). Depending on the structure of this circuit, the dead time can be constant or variable, in any case, it degrades the photon counting performance of the SPAD. In this thesis, a comprehensive analytical approach is presented for modelling the counting statistics of SPAD detectors in the presence of dead time. To the best of author’s knowledge, this is the first in-depth study of the impact of dead time in the context of OWC. Using the concepts of arrival processes and renewal theory, the exact photocount distributions and the count rate models are derived for AQ and PQ single SPADs. It is shown that, unlike ideal photon counting detectors, in AQ and PQ single SPADs, the photocounts do not follow a Poisson distribution. The results confirm that AQ single SPADs generally exhibit less counting losses and therefore, higher count rates compared to PQ single SPADs and the count rate gap in high photon rate regimes is substantial. It is also shown that the photocount distribution of a SPAD array can be well approximated by a Gaussian distribution, for which the mean and variance are dead time dependent. The numerical results suggest that as the size of the array increases, the gap between the photon counting performance of AQ and PQ SPAD arrays tends to vanish. Furthermore, in this thesis, the bit error performance of SPAD-based OWC systems with AQ single SPADs, PQ single SPADs and AQ SPAD arrays are evaluated. The results show that the SPAD dead time significantly degrades the bit error ratio (BER) of the systems. The system with an AQ single SPAD exhibits better BERs compared to the system with a PQ single SPAD. The effect of dead time is mitigated to some extent when an array is employed. The analytical and Monte Carlo simulation results are provided for various dead time values, background count levels and SPAD array sizes. From a communication theory point of view, the dead time also limits the achievable data rate of SPAD-based systems. In this thesis, the information transfer rate of SPAD detectors is also investigated. To this end, the SPAD is modelled as a communication channel. Using an information theoretic approach, the channel capacity and the capacity-achieving input distributions for AQ single SPADs and AQ SPAD arrays are obtained for various dead time values, background count levels, and array sizes

The Bit Error Performance and Information Transfer Rate of SPAD Array Optical Receivers

In this paper the photon counting characteristics, the information rate and the bit error performance of single-photon avalanche diode (SPAD) arrays are investigated. It is shown that for sufficiently large arrays, the photocount distribution is well approximated by a Gaussian distribution with dead-time-dependent mean and variance. Because of dead time, the SPAD array channel is subject to counting losses, part of which are due to inter-slot interference (ISI) distortions. Consequently, this channel has memory. The information rate of this channel is assessed. Two auxiliary discrete memoryless channels (DMCs) are proposed which provide upper and lower bounds on the SPAD array information rate. It is shown that in sufficiently large arrays, ISI is negligible and the bounds are tight. Under such conditions, the SPAD array channel is precisely modelled as a memoryless channel. A discrete-time Gaussian channel with input-dependent mean and variance is adopted and the properties of the capacity-achieving input distributions are studied. Using a numerical algorithm, the information rate and the capacity-achieving input distributions, subject to peak and average power constraints are obtained. Furthermore, the bit error performance of a SPAD-based system with on-off keying (OOK) is evaluated for various array sizes, dead times and background count levels

Analysis of Photon Detection Efficiency and Dynamic Range in SPAD based Visible Light Receivers

We investigate the photon detection efficiency (PDE) and the dynamic range for digital silicon photomultipliers (dSiPMs) over a selection of design parameters: dSiPM unit cell dead time, PDE, unit cell area and fill factor, number of cells, and total dSiPM active area. Two receiver scaling scenarios are con-sidered: varying the number of cells for 1) a fixed unit cell area or 2) a fixed total dSiPM area. Theoretical and simulated results are confirmed with experimental data from a selection of dSiPMs realised on a test chip in130-nm CMOS process

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

Visible light channel modeling for gas pipelines

In this paper, we explore the use of visible light communication as a means of wireless monitoring in gas pipelines. In an effort to shed light on the communication limits in the presence of gas, we create a three-dimensional simulation platform where the pipeline size/shape, the reflection characteristics of the interior coating, gas specifications (i.e., temperature, density, refractive index, transmittance, etc.) and the specifications of the light sources and detectors (i.e., field of view, lighting pattern, etc.) are precisely defined. Based on ray tracing, we obtain channel impulse responses within the gas pipeline considering the deployment of different colored LEDs with various viewing angles. We further investigate the maximum achievable link range to ensure a given bit error rate.NPRP award under the Qatar National Research Fund (a member of the Qatar Foundation)Publisher versio