Super receivers for visible light communications

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I. IntroductionConsidering the limited radio spectrum, visible light communication (VLC) using an unlicensed light spectrum nowadays offers an opportunity to create a new wireless infrastructure for data transmission and daily indoor illumination, playing a vital role in the coming Internet of Things (IoT). Compared with conventional radio frequency (RF), VLC based on LEDs has better confidentiality and low-power consumption. It is always immune to electromagnetic interference, especially in places full of electronic equipment. Researching this field will soon contribute to compatible standardization of lighting and telecom infrastructures, and establish an open architecture guide for the upcoming IoT.II. Research QuestionsOnce the new wireless network is constructed directly from existing prototypes, the sensitivity and bandwidth limitations of the optical receivers on terminals is the key bottleneck. Considering the eye safety and standards for interior illumination, the amount of received light is limited. Therefore, the maximum data rate of a VLC system is ultimately determined by the sensitivity of receivers. For receivers, one of the important constraints is the etendue conservation. Increasing the active area of the photodetector can enhance the received optical power but lead to lower bandwidth due to larger capacitance. Once typical concentrators are included for a receiver, there is always a trade-off between optical gain and field-of-view (FoV) because of the etendue conservation. Accordingly, fluorescent concentrators, which first absorb then re-emit as a medium, are proposed to simultaneously provide a high optical gain and a wide FoV for receivers. Nevertheless, most of the current works have limited data rates of 10-40 Mbps due to low SNR and bandwidth. They can hardly work in a complex environment such as underwater conditions or mobile communication. Therefore, it is necessary to design a super receiver solution to access a large number of terminals in the IoT, which ensures the stability and efficiency of optical wireless communications.III. Research MethodologyGiven my past research and knowledge, I will consider optoelectronic devices designs and algorithm optimization to meet the above challenges. a) Fluorescent Concentrators and Silicon PhotomultiplierSince the large lifetime is the main factor limiting the modulation bandwidth of receivers, it is necessary to find a short-lifetime fluorescent material with high quantum efficiency. Meanwhile, extending the active region of PD by films or plastic optical fibers, the light from different directions could be received simultaneously with mitigated alignment criteria. Moreover, a light detector capable of detecting individual photons can be introduced in the receiver to improve the sensitivity. In this way, realizing a large-scale and efficient detector array will be possible, where spatial diversity can greatly enhance the data rate. b) Algorithm Optimization and ExtensionBenefiting from the color converter's concentration performance, we can support multiple users simultaneously without complex lenses and provide a single user with moderate mobility. With the multiple access and MIMO methods, the system could be extended by times, requiring no extra separation by wavelength and polarization states. To obtain better performance, equalization algorithms including pre-leveling, clipping, and complex post-equalization are inevitably utilized to improve the data rate.IV. Alignment to EPSRC's strategies and research areasThis project falls within the EPSRC Optical communications research area.