## 1 Introduction  
Visible Light Communication (VLC), is an emerging wireless communication technology that operates unregulated in the visible-light band (400-800 THz frequencies or 380-780 nm wavelengths) of the electromagnetic spectrum, and is enabled by light emitting elements such as light emitting diodes (LED) and light receiving elements such as photodiodes (PD). Due to directionality of light beams, VLC is a line-of-sight (LOS) technology that requires the light transmitter and receiver to be within each others field-of-view (FOV) [1]. The LOS requirement provides novel opportunities for efficient space and time reuse in VLC where multiple light emitting transmissions could be multiplexed. Traditional VLC [2] that operates using a single non-array photodiode receiver based reception, requires to incorporate specific multiple access mechanisms to enable reception from different light emitters. By leveraging the directionality of optical signals and that light emitters can be spatially differentiated, it is possible to multiplex signals by combining space division multiple access (SDMA) with time/frequency/code (TDMA/FDMA/CDMA) division access schemes, however, the nature of photoreceptors to collectively add all the detected photons within its FOV limits makes differentiation of multiple transmissions and from ambient noise very challenging. This limits VLC to the effective communication using only one light beam (or transmitter) at each instance of time. The key challenge in using multiple access mechanisms in single-photodiode non-array VLC receiver systems is that the incoming signals, through may be spatially and temporally separated, but once they reach the receiver collector (lens), the signals are mixed (leading to interference) with each other and thus cannot be differentiated. Unlike radio-frequency communication, where polarity of signals and thus representing signals as complex numbers is possible, in optical wireless, the received signals are essentially the positive-sum of all photons, which carry no polarity. With stringent constraints that there cannot be an extra channel and that the signals must be easily identified from each LED on the photodiode, multiple access in VLC is very challenging without the use of array receivers or side information. Therefore, unless there is extra information regarding the signals (possibly through an extra control channel) or custom detection mechanisms incorporated into the system, the ability to spatially and temporally differentiate optical signals using a single non-array photodiode receiver remains an open challenge. Multiple-Input Multiple-Output (MIMO) architectures for VLC that have been proposed and designed before [3; 4] require multiple photoreceptor elements, for example, as in photodetector arrays and image sensing arrays or cameras. The use of array elements in a receiver limits the sampling bandwidth (photodiode sampling frequency or camera frames-per-second) of the receiver hence limiting the achievable throughput of the system.Therefore, enabling multiple-access while retaining the high-speed sampling capacity of photodiodes is the other key open challenge that remains to be addressed. To address the challenges presented above, this paper explores the use of spatial filtering mechanisms using a new hardware design for VLC receivers, to enable multiple access. In essence, this work presents a proof-of-concept study of using liquid crystal displays (LCD) to potentially enable multiple access in single non-array photodiode receivers. We propose a receiver design that uses LCDs to differentiate multiple LED transmissions and enable multiplexed communication using a single photodiode in the receiver. To this effect, we build our system over a baseline architecture from our prior work [5], where a liquid crystal display (LCD) panel was used as a digital gate or _shutter_, to allow or disallow signals onto a photodiode receiver.