# Highly Sensitive SPAD-Based Receiver for Dimming Control in LiFi Networks +
## 1 Introduction  
The exponential growth in data capacity and demand for transmission bandwidth has worsened the scarcity of the radio-frequency (RF) spectrum, making it increasingly challenging to provide access to transmission bandwidth in wireless communication networks, particularly in the fifth generation (5G) and upcoming sixth generation (6G) [1]. To overcome this dilemma, a possible solution has been proposed by shifting towards the terahertz (THz) range, around 10 THz [2]. However, this approach has its own set of difficulties, including power demands, device complexity, and the cost of components such as sources, power amplifiers, antennas, and subharmonic mixers. Alternatively, the ongoing strain on RF-based wireless technologies could be reduced by using the free visible spectrum (380-780 THz) to provide visible light communications (VLCs) using energy-efficient light-emitting diode (LED)-based lighting fixtures in indoor and potential laser diodes (LDs) to outdoor environments [1]. In fact, compared to RF systems, visible and infrared wireless communications may be the most practical technology for illumination, data communications, and localization in most indoor environments because they are low in cost, have high security, and are immune to electromagnetic interference [3]. Future Internet of Things (IoT) applications will have a wide range of performance requirements, such as high data rates and dependable time-sensitive networking. This has led to an increased interest in alternative networking technologies. One such technology is indoor light fidelity (LiFi), which uses light waves to transmit data instead of radio waves [4]. In recent years, researchers have made significant progress in developing LiFi technology that can provide high data rates and low latency. However, one of the key challenges in LiFi networks is maintaining consistent high network performance regardless of lighting level. To address this challenge, researchers have proposed various techniques, such as adaptive modulation and coding schemes, to ensure that LiFi networks can operate reliably under different lighting conditions. In addition, researchers have also explored the potential applications of LiFi technology. For example, LiFi can be used for indoor localization, where the location of a device can be determined based on the strength of the received light signal, in electromagnetic-sensitive industries, or in crowded spaces filled with a lot of RF communication waves [5; 6; 7; 8]. Other studies have investigated the potential of deep learning-based LiFi systems to address challenges and maintain high network performance in indoor environments with varying geometrical configurations and user behavior, including device orientation and blockage. Results have shown that these systems offer superior performance compared to conventional channel estimation techniques [9]. Introducing dimming capabilities into a VLC/LiFi system will result in significant energy savings and offer end users complete control over the light output, making the entire system suitable for commercial usage. However, dimming support was identified as one of the key challenges in VLC by the IEEE 802.15.7 task group as dimming may affect communication performance to some extent. One efficient method of enhancing the performance of optical wireless communication (OWC) devices under weak power reception due to the dimming control is by utilizing highly sensitive photon-counting receivers, such as a single-photon avalanche diode (SPAD). A SPAD is an avalanche photodiode (APD) that is biased beyond the \"Geiger\" region's reverse breakdown voltage. For each detected photon, a SPAD launches billions of electron-hole pair creations that result in a big current detection, and so may be described as a single photon counter. Poisson statistics can be used to represent the photon detection process of an ideal photon counter or in low photon arrival rate regimes [10].