Single-photon avalanche diode receivers for optical wireless communications
Item statusRestricted Access
Embargo end date03/07/2020
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.