## 2 Related Work  
**Background. ** In our prior work [5], we introduced a new architecture for VLC that uses a high-speed photodetector and an LCD shutter acting as a programmable image sensor aperture. In this work, our measurement studies proved that noise and interference can be separated spatially using our VLC receiver to improve the Signal to Noise Ratio (SNR) and Signal to Noise Interference Ratio (SINR) significantly. Through a proof-of-concept experimentation, in our previous work [5] we have studied the feasibility of noise and interference reduction by manually selecting one of the shutter pixel apertures for higher signal reception. In this paper, we relax the assumption (considered in our prior work) of the a priori knowledge of which shutter should be opened (or closed), and advance the design by proposing a novel automated shutter control to help differentiate LED signals on the photodiode. This paper leads and consolidates the idea of multiple access in VLC by adopting a shutter controlling algorithm in the receiver. We evaluate the system for high speed reception (up to 2 MHz data transmission frequency) for multiple access from 2 LEDs and feasibility of automation under controlled mobile settings. In the rest of this section, we discuss some of the existing works that are closely related to the challenges targeted in our system. However, we emphasize that no prior work has shown the capability of achieving high-speed multiple access in VLC using a single photodiode receiver, which remains the key focus of this paper's contribution. **Existing VLC technologies. ** Recent years have seen active development of VLC research prototype systems. However, these systems have specific limitations owing to the customizations in their hardware/software design. PureLiFi [6] devices have shown the capability of Mbps-Gbps data rates, but their performance is limited to static VLC settings and carry high hardware overheads, complex signal processing, thus requiring a huge cost for the product. OpenVLC1.3 [7] is an open-source embedded VLC prototype based on simpler protocols that offers data rates up to 400 kbps. The customized hardware and requirement of modifying the operating system kernel makes this option very challenging to generalize. With a 1.4 kbps data rate, LocalVLC [8] presents a low-cost VLC prototype for indoor IoT applications based on morse code modulation, however, fails to motivate its usage for typical indoor IoT applications where higher throughput is typically required. Other VLC systems including Purple VLC [9; 10; 11], have similar challenges as discussed above. With achieving high data rate remaining a key challenge for existing VLC systems, the addition of multiple access requirement can either require significant modifications to these designs or may not be feasible at all. Our proposed design can potentially address high-data rate, multiple access and mobility all at once through a unified design. By using the shutter control mechanism efficiently, signals can be differentiated from noise and interference, thus enabling cleaner signal reception or higher Signal-to-Noise (SNR) and/or Signal-to-Interference-and-Noise Ratios (SINR). With a cleaner signal, the signal modulation mechanisms can be kept simple with more focus laid on achieving multiple access, high-throughput reception and mobility. **Non-Orthogonal Multiple Access in VLC. ** Recent works have proposed non-orthogonal multiple access (NOMA) schemes for VLC [12; 13; 14] to improve spectral efficiency and enable multiple access in VLC. However, the common challenge in these designs is the reliance on channel state information (CSI) and the requirement for the transmitter and receiver to be informed a priori about CSI. For example, the work in Reference [12] introduces a power domain based multiple access protocol so that the users can use the entire bandwidth during the communication session, but requires the CSI and only works in small indoor environments.