# A Roadmap for Gigabit to Terabit Optical Wireless Communications Receivers
## 1 Introduction  
Optical wireless communications (OWCs) and visible light communications (VLCs) systems are being investigated as ways to increase local wireless communications capacity [1]. An important performance parameter for any communications system is the bit error rate (BER), which depends upon the signal-to-noise ratio (SNR) at the receiver's output. An approach to increasing the SNR of an OWCs or VLCs system is to use silicon photomultipliers (SiPMs) in the receiver [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. These devices are arrays of microcells, containing a single-photon avalanche diode (SPAD), that are designed so that an output pulse is generated whenever an avalanche event occurs. Since a single photon can initiate an avalanche, these microcells can detect individual photons. It is this ability to detect photons that means that the sensitivity of a SiPM receiver is expected to be limited by Poisson statistics. When on-off keying (OOK) is used the number of detected photons per bit when a zero is received determines the average number of photons per bits required to achieve a particular BER. Since avalanche events can be initiated in the dark, at a rate known as the dark count rate (DCR), existing SiPMs are particularly suited to data rates of more than 100 Mbps. Consequently, SiPM receivers have been shown to operate within a few photons per bit of the noise floor determined by Poisson statistics [5]. Unfortunately, after a microcell has detected a photon, the avalanche event has to be quenched by reducing the voltage across the avalanche photodiode (APD) in the microcell. The microcell then has to be recharged, and this means that the SiPM has a non-linear response [4]. The performance of receivers containing commercially available SiPMs can be determined experimentally [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Ideally, the results of these experiments could be used to inform the selection of the best SiPMs from amongst those that have different characteristics. Unfortunately, the transmitter can have a significant impact on any experimental results. Furthermore, it can be very difficult to separate the impact of SiPMs' bandwidths and their non-linear responses. Recently, these problems led to the development of a Monte Carlo simulator [19]. The parameters in this simulator were obtained from either a relevant data sheet or experimental results. The results of the simulator were then validated by comparing them with the results of experiments on the impact of ambient light on the performance of receivers [19]. The aim of this paper is to use this Monte Carlo simulator to compare the impacts of SiPMs' non-linearity and bandwidth on their performance in receivers. The results of this comparison are then used to estimate the performance of commercially available SiPMs when the data are represented by OOK. Guidelines on the selection of SiPMs for use in receivers have been published previously [20; 21]; however, both of these papers assumed that the modulation scheme was orthogonal frequency division multiplexing (OFDM), which is not as energy-efficient as OOK [22]. Furthermore, it was assumed that it is possible to increase the areas of SiPMs without changing any other parameters. In contrast, this paper concentrates upon OOK. In addition, the data sheets of SiPMs show that increasing the areas of SiPMs has a significant negative impact on the width of SiPMs' fast output pulses and hence their bandwidths. Fortunately, there is a method of combining SiPMs so that they act in parallel but retain their bandwidth. The results of the simulation are therefore used to investigate the impact of using multiple SiPMs in parallel in a receiver. Finally, a review of recently developed technology leads to the conclusion that using two stacked integrated circuits to make SiPMs will dramatically increase the data rates that can be supported. This paper is organized as follows: Section 2 contains a list of the important characteristics of commercially available SiPMs.