# Spatial Multiplexing Holography for Multi-User Visible Light Communication
## 1 Introduction  
In the advent of the 6G era, visible light communication (VLC) emerges as a pivotal technology, promising high-speed, efficient, and interference-resistant wireless communication [1, 2, 3]. Its applicability spans across the Internet of Things [4], smart industries [5], and indoor navigation [6], marking its significance in the future of communication technologies. In an indoor wireless scenario, through metro systems, massive messages and information are processed by the central office and then distributed to various areas and buildings via optical fibers to enable indoor access [7], as illustrated in Figure 0(a). Theprogression towards large-scale indoor VLC systems accommodating multiple users [8; 9; 10] is an inevitable trend driven by the demands of the 6G era. The development of multi-user systems requires a tailored approach to effectively accommodate the diverse needs of users [11]. This involves ensuring accurate power allocation to mitigate the inherent power dissipation in wide-coverage VLC systems, enabling self-design power coverage to counteract the high probability of alignment errors due to small receiver apertures, and securing communication to protect user privacy. Traditional methodologies fall short in addressing these comprehensive needs. Wavefront shaping techniques, utilizing spatial light modulators (SLMs) to modulate Fresnel zones and alter the wavefronts of optical beams, enable focusing at specific spatial locations, thereby enhancing the optical power received by designated users and improving communication rates [12; 13; 14]. Despite these advancements, this method has a fundamental limitation: it cannot dynamically allocate power based on the varying demands of different users. Consequently, all users receive similar power levels, leading to uncontrollable power fluctuations and an inability to tailor the coverage area according to specific needs. On the other hand, solutions based on digital holography involve recording the spatial positions of users within an environment and generating phase holograms for holographic reconstruction. While this method potentially offers control over the coverage area [15], it still falls short in providing flexible power allocation among users. Additionally, it lacks mechanisms for ensuring secure communication, leaving user data potentially exposed to breaches. In this work, we propose a novel theory of spatial multiplexing holography (SMH), presenting an innovative framework that systematically tackles these challenges by leveraging digital holography to enable a multi-user VLC system. Figure 1b-d depict the transformative impact of SMH: enabling users with varying power requirements to achieve targeted power allocation (Figure 1b), facilitating the self-regulation of power coverage (Figure 1c). Additionally, SMH can be effectively integrated with Orbital Angular Momentum (OAM) for secure communication. Existing research on OAM for optical wireless multiuser communication has primarily focused on mode-division multiplexing (MDM) and space-division multiplexing (SDM) techniques [16; 17; 18]. However, these techniques modulate OAM modes transmitting coaxially within limited distances due to the dispersion of OAM  
Figure 1: (**a**) Scenario of multi-user system in VLC; (**b**) Sketch of power allocation; (**c**) Sketch of power coverage; (**d**) Sketch of OAM secure communication. rings without considering the possibility of users in different three-dimensional locations. Enhanced by SMH's ability to perform beam steering in 3D space, we achieve secure multiuser communication using OAM in three dimensions. This system enhances security by using optical keys, ensuring that system access is restricted to those with the correct keys (Figure 1d). Our findings, validated through both simulation and experimental studies, demonstrate the superior performance and practical applicability of SMH. The simulations highlight its effectiveness in achieving the intended system functions over conventional methods.