Open AccessArticle

Spatial Multiplexing Holography for Multi-User Visible Light Communication

Chaoxu Chen

^1,2,3,4,

Yuan Wei

^1,2,3,

Haoyu Zhang

^1,2,3,

Ziyi Zhuang

^1,2,3,

Ziwei Li

^1,2,3,5,

Chao Shen

^1,2,3,5

Junwen Zhang

^1,2,3,5,

Haiwen Cai

⁴,

Nan Chi

^1,2,3

and

Jianyang Shi

^1,2,3,*

Key Laboratory for Information Science of Electromagnetic Waves (MoE), Department of Communication Science and Engineering, Fudan University, Shanghai 200433, China

Shanghai Engineering Research Center of Low-Earth-Orbit Satellite Communication and Applications, Shanghai 200433, China

Shanghai Collaborative Innovation Center of Low-Earth-Orbit Satellite Communication Technology, Shanghai 200433, China

⁴

Zhangjiang Laboratory, Shanghai 200433, China

⁵

Peng Cheng Laboratory, Shenzhen 518055, China

Author to whom correspondence should be addressed.

Photonics 2025, 12(2), 160; https://doi.org/10.3390/photonics12020160

Submission received: 18 January 2025 / Revised: 14 February 2025 / Accepted: 15 February 2025 / Published: 17 February 2025

(This article belongs to the Special Issue Editorial Board Members’ Collection Series: Optical Wireless Communication)

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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. "> Figure 2
Application of power allocation holography in projection. "> Figure 3
(a) Process of generating SMH for power allocation; (b) Process of generating SMH for power coverage; (c) Process of generating SMH for OAM secure communication. "> Figure 4
Schematic diagram of precise transmission space sampling. (a) Transmission process without pre-control; (b) Transmission process with pre-control. "> Figure 5
(a) Simulation results for power allocation; (b) Convergence line of GA; (c) Simulation results for power coverage under MF method; (d) Simulation results for power coverage under SMH. "> Figure 6
(a) Simulation of optical field at receiver plane under SMH and conventional method; (b) Power trend of user encoded with OAM modes l = 20 under different OAM keys; (c) Simulation results for OAM secret communication. "> Figure 7
Experimental setup. (a) Experiment platform; (b) Signal processing. "> Figure 8
Experimental results for power allocation. (a) Data rate curve with ROP; (b) One frame of 4K video received under MPH and SMH; (c) BER of each row in the first frame; (d) Mean BER of each second of the video; (e) Working point test for User 1; (f) Working point test for User 2; (g) Working point test for User 3. "> Figure 9
Experimental results for power coverage. (a) User 1’s ROP trend with XoY offset; (b) User 2’s ROP trend with XoY offset; (c) User 3’s ROP trend with XoY offset; (d) Coverage area diameter of 3 users under MF and SMH; (e) Data rate trend with XoY offset of three users; (f) CCD recording at User 1’s plate under different methods; (g) CCD recording at User 2’s plate under different methods; (h) CCD recording at User 3’s plate under different methods; (i) MSE and SSIM of recordings under different methods. "> Figure 10
Experimental results for OAM secure communication. (a) ROP of three users with different OAM keys; (b) Data rate of three users with different OAM keys; (c) CCD recording of three users; (d) One frame of a 1080P video received by User 3 under different OAM keys. ">

Versions Notes

Abstract

Given the burgeoning necessity for high-speed, efficient, and secure wireless communication in 6G, visible light communication (VLC) has emerged as a fervent subject of discourse within academic and industrial circles alike. Among these considerations, it is imperative to construct scalable multi-user VLC systems, meticulously addressing pivotal issues such as power dissipation, alignment errors, and the safeguarding of user privacy. However, traditional methods like multiplexing holography (MPH) and multiple focal (MF) phase plates have shown limitations in meeting these diverse requirements. Here, we propose a novel spatial multiplexing holography (SMH) theory, a comprehensive solution that overcomes existing hurdles by enabling precise power allocation, self-designed power coverage, and secure communication through orbital angular momentum (OAM). The transformative potential of SMH is demonstrated through simulations and experimental studies, showcasing its effectiveness in power distribution within systems of VR glasses users, computer users, and smartphone users; enhancing power coverage with an 11.6 dB improvement at coverage edges; and securing data transmission, evidenced by error-free 1080P video playback under correct OAM keys. Our findings illustrate the superior performance of SMH in facilitating seamless multi-user communication, thereby establishing a new benchmark for future VLC systems in the 6G landscape.

Keywords:

digital holography; visible light communication; orbital angular momentum

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 1a. The progression 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 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. Experimentally, SMH ensures precise power allocation, enabling seamless video streaming across different resolutions for VR glasses, computers, and smartphones. Additionally, it achieves over 100% target power coverage with a significant 11.6 dB improvement at the coverage edge compared with the multiple focal (MF) method from wavefront shaping, and it secures communication through accurate OAM key matching, facilitating error-free 1080P video playback.

This study positions SMH as a pioneering approach to designing VLC systems optimized for multi-user environments, laying the foundation for next-generation communications in the 6G era.

2. Design Principle of Spatial Multiplexing Hologram

2.1. Spatial Multiplexing Hologram for Multi-User Power Allocation

In VLC systems, it is essential to accommodate the diverse optical power demands of users who utilize different communication devices and engage in varied applications. To optimize resource utilization and minimize optical power wastage, the VLC system should be capable of dynamically allocating power among users. Conceptually, this allocation process can be likened to projecting an image of users onto different imaging plates, each situated at unique locations. For example, as shown in Figure 2, the little bunny object has been partitioned into multiple layers to create multiplexing holograms. By employing power allocation holography, we can enhance the heart region of the projected image.

The creation of SMH for effective power allocation is depicted in Figure 3. A significant challenge in this process is the beam’s diffraction transmission, which results in the expansion of the light plane and potential drift of the target position as the transmission distance increases [19,20]. To mitigate this issue during the preprocessing stage, it is imperative to precisely sample the transmission space as shown in Figure 4. The source plate is defined as an SLM plate, characterized by a bilateral pixel count of

N_{x} \times N_{y}

and an initial pixel pitch of

Δ x_{0}

. The pixel intervals at a distance

z

from the source, as determined by Fresnel diffraction, are

Δ x = z λ / (N_{x} Δ x_{0})

and

Δ y = z λ / (N_{y} Δ x_{0})

for horizontal and vertical directions, respectively. Consequently, the light field’s horizontal and vertical magnification factors,

α_{x} = Δ x / Δ x_{0}

and

α_{y} = Δ y / Δ x_{0}

, can be computed. For a given user located at position

(x, y, z)

, these magnification coefficients are initially calculated based on the distance

z

. Thus, the target’s position within the source field can be accurately described as

x_{0} = x / α_{x}

and

y_{0} = y / α_{y}

A user’s information can be represented as an electrical field

E (x, y, z)

, where

x

y

, and

z

denote spatial coordinates. For multiple users, we define a vectorial representation as follows:

E = |\begin{matrix} E_{1} & E_{2} & \dots & E_{n} \end{matrix}|

(1)

In reducing inter-hologram crosstalk, we employ Fourier holography complemented by a random phase vector

ϕ

. The expression for the resulting hologram vector is [14,21,22,23]:

H = \iint E \exp (i ϕ) \exp [i (ξ x + η y)] d x d y

(2)

where

ξ

and

η

respectively represent the spatial electrical field distribution on SLM. Assuming the electrical fields

E_{1}

and

E_{2}

of two users are added as different random phase vectors

ϕ_{1}

and

ϕ_{2}

, the resulting holograms can be expressed as

\{\begin{matrix} H_{1} = \iint E_{1} \exp (i ϕ_{1}) \exp [i (ξ x + η y)] d x d y \\ H_{2} = \iint E_{2} \exp (i ϕ_{2}) \exp [i (ξ x + η y)] d x d y \end{matrix}

(3)

The orthogonality of

H_{1}

and

H_{2}

can be verified by a conjugate multiplication integral:

\begin{matrix} \iint H_{1} H_{2}^{*} d ξ d η = \iint E_{1} E_{2} \exp [i (ϕ_{1} - ϕ_{2})] d ξ d η \\ = \{\begin{matrix} \iint E_{1} E_{2} d x d y & ϕ_{1} = ϕ_{2} \\ 0 & ϕ_{1} \neq ϕ_{2} \end{matrix} \end{matrix}

(4)

As the difference between

ϕ_{1}

and

ϕ_{2}

increases, the degree of orthogonality between two holograms also increases [14].

To accommodate users at different distances

z

, we generate Fresnel lens plates with a corresponding focal length

f

, described by the following equation [24]:

F (ξ, η, f) = \exp [- \frac{j π}{λ f} (ξ^{2} + η^{2})]

(5)

Introducing an initial weight column vector

w

for these electrical fields, the SMH can be articulated as

H_{m u x} = (H \cdot F) \times w

(6)

Leveraging the SMH for reconstructing user information in three-dimensional space via Fresnel diffraction, the projection at a z-plane is given by

E_{R e s} (x, y, z) = \frac{\exp (\frac{j 2 π}{λ z})}{j λ z} \iint H_{m u x} \exp \{\frac{j π}{λ z} [{(x - ξ)}^{2} + {(y - η)}^{2}]\} d ξ d η

(7)

The relative power received by a user at the z-plane,

p (z)

, is then determined by

p (z) = \iint_{s} E_{R e s} (x, y, z) E_{R e s}^{*} (x, y, z) d S

(8)

where

S

represents the area of the user’s receiver aperture. From this, we can derive a vector representing the real power received by all users at different locations, normalized as follows:

p = \frac{[\begin{matrix} p_{1} & p_{2} & \dots & p_{n} \end{matrix}]}{\max [\begin{matrix} p_{1} & p_{2} & \dots & p_{n} \end{matrix}]}

(9)

Owing to diffraction effects and optical losses, the initial weights

w

may fail to achieve the intended power distribution. Consequently, we employ an optimization strategy to identify the optimal initial weight vector

w

, which satisfies the target power distribution at the users’ receivers. The optimization is formulated with objectives and constraints as follows [25]:

\begin{matrix} \min \frac{1}{N} \sum_{i}^{N} {(p_{i} - p_{t, i})}^{2} \\ \{\begin{matrix} 0 \leq w_{1}, w_{2}, \dots, w_{n} \leq 1 \\ 0 \leq p_{t, 1}, p_{t, 2}, \dots, p_{t, n} \leq 1 \end{matrix} \end{matrix}

(10)

In this study, we utilize genetic algorithm (GA) to achieve precise power allocation among users, demonstrating the efficacy of our proposed method in a VLC system setting. The GA method is implemented using MATLAB’s Global Optimization Toolbox in our work.

2.2. Spatial Multiplexing Hologram for Multi-User Power Coverage

Given the compact apertures of receivers in VLC systems, which are optimized for a high speed and sensitivity, signal transmission often involves long distances and substantial optical losses. These factors can lead to substantial pointing errors, potentially jeopardizing the success of communications. To mitigate these challenges, VLC systems must be capable of customizing the coverage area of beams at the receivers’ plates, thereby augmenting communication robustness [15].

Figure 3 illustrates the methodology for adjusting beam coverage. In the preprocessing stage, location correction is performed as described in Section 2.1. Moreover, the size of the beam’s coverage area must also be predetermined. Utilizing the previously determined magnification factors,

α_{x}

and

α_{y}

, the desired radius

r

of the coverage area at a distance

z

from the source plate is adjusted to

r_{0} = r / \sqrt{α_{x}^{2} + α_{y}^{2}}

As described in Section 2.1, users’ information is translated into images. Following Equations (2)–(6), the SMH is synthesized. It is important to note that this section primarily concentrates on ensuring adequate power coverage on the received plate, thus obviating the need for weight optimization specific to power allocation. Additionally, larger power coverage is not always better. While setting a larger light spot at the receiver plane can ensure power coverage, power dispersion remains a concern. This dispersion can lead to a lower data rate or even reduce the power below the receiver’s sensitivity threshold.

2.3. Spatial Multiplexing Hologram Using OAM for Multi-User Secure Communication

Ensuring secure communication is of utmost importance in VLC systems. One novel approach to enhance security involves manipulating the OAM of light beams during the generation of SMH. This method leverages the unique property of OAM, wherein only specific OAM modes can reconstruct the optical power intended for specific users effectively [18,26].

The core concept is that incorporating an OAM phase into the beam’s structure acts as a form of encryption—akin to a key. Incorrect or false OAM keys cause the resultant optical field to assume a toroidal (doughnut-shaped) configuration, leading to the dispersion of optical power. This characteristic ensures that only receivers equipped with the correct OAM decoding capabilities can successfully focus the light into a usable signal, thereby enhancing the overall security of the communication within the VLC system.

SLM-based OAM multi-user communication has traditionally been classified into two main categories: mode-division multiplexing (MDM) and space-division multiplexing (SDM). In MDM, OAM modes are transmitted coaxially, with different users using different OAM keys at similar locations to decode the signals. In SDM, OAM modes are modulated in different directions through a single SLM, allowing different users to receive signals from different forward directions but at similar depths. However, a significant drawback of these methods is the inability to control the forming depth of OAM, requiring users to be positioned on the same plane, which is uncommon in realistic scenarios. Additionally, the enlargement of OAM rings over distance results in substantial power dispersion. By utilizing SMH, these challenges can be addressed effectively.

In addition to the previously outlined steps, the OAM phase within the complex amplitude domain is represented as

P = \exp (i l θ)

(11)

Here,

l

denotes the mode number of OAM, while

θ

refers to the angular component of the holograms, which are generated by Equation (2) in polar coordinates. Accordingly, the array of OAM modes corresponding to different users can be articulated as

P = |\begin{matrix} P_{1} & P_{2} & \dots & P_{n} \end{matrix}|

. Thus, we can construct the OAM-enriched SMH through the following summation:

H_{m u x} = \sum_{i = 1}^{n} H_{i} F_{i} P_{i}

(12)

In the reconstructed light field, users typically observe ring-shaped light spots, which may lead to diminished received optical power. To counteract this issue and enhance power reception, it is essential to incorporate the correct inverse OAM modes. By applying these inverse modes before reception, the ring-shaped illumination can transform into concentrated, solid light, thereby significantly increasing the received optical power. To further explain the theory of this method, we take a two-user scenario for example.

Assuming that an SMH multiplexes two users with OAM keys

l_{1}

and

l_{2}

, respectively, the OAM part of the optical field can be expressed as

E = E_{0} [\exp (i l_{1} θ) + \exp (i l_{2} θ)]

(13)

At the location of User 1, we use the inverse OAM phase to decode. The OAM of the optical field at the receiver plane can be expressed as

\begin{array}{l} E_{1} = E_{0} [\exp (i l_{1} θ) + \exp (i l_{2} θ)] \exp (- i l_{1} θ) \\ = E_{0} + E_{0} \exp [i (l 2 - l 1) θ] \end{array}

(14)

The optical intensity is given by

\begin{array}{l} I_{1} = \iint E_{1} E_{1}^{*} d x d y \\ = \iint {|E_{0}|}^{2} + 2 {|E_{0}|}^{2} \exp [i (l 2 - l 1) θ] + {|E_{0}|}^{2} \exp [i 2 (l 2 - l 1) θ] d x d y \\ = {|E_{0}|}^{2} + δ_{1,2} \end{array}

(15)

It is noteworthy that the part

E_{0} \exp [i (l 2 - l 1) θ]

is eliminated when calculating intensity due to the orthogonality of OAM. Here,

δ_{1,2}

represents the crosstalk between the two users, which is minimal. If an OAM key other than

l_{1}

l_{2}

is used to decode the optical field, the intensity would only retain the crosstalk part, indicating a failure to decode properly.

This approach underscores the importance of precise OAM mode alignment in enhancing the efficiency and security of VLC systems.

3. Simulation Results

In this section, we simulate a scenario in which five users are positioned within the forward transmission space, spanning from 0.6 m to 0.8 m, with an equal spacing of 0.05 m between them. The spatial sampling parameters for this simulation are set as follows: the light wavelength is defined as 635 nm, aligning with the experimental setup described in the subsequent section. The sampling matrix is configured to 1024 × 1024 points, with the sampling interval varying according to the transmission distance. At the source plate, this interval is set to 8 μm, consistent with the parameters of the SLM utilized in our experiments.

For the power allocation simulation, we assign target normalized power levels to the five users as 0.2, 0.4, 0.6, 0.8, and 1, respectively. In the context of power coverage simulation, the corresponding coverage areas for these users are designated as 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, and 1 mm, respectively. Regarding the simulation for secure communication using OAM, we encode distinct OAM modes for each user, ranging sequentially from 1 to 5. The details of these parameters and their respective values are summarized in Table 1.

3.1. Simulation Results for Power Allocation

Following the methodology outlined in Section 2.1, the SMH for a five-user VLC system is successfully generated. The convergence line of GA during optimization is illustrated in Figure 5b. The population size is set to 100, with a migration factor of 0.5. The normalized central power for each user, as depicted in the bar chart of Figure 5a, demonstrates the efficacy of the SMH in terms of power distribution.

When compared to the multiplexing holography (MPH) method, which is detailed in Ref. [15], the power allocation achieved by the SMH is notably closer to the intended targets. The Mean Square Error (MSE) between the actual and targeted power distributions for the SMH is only 4.37 × 10⁻⁴, representing a substantial improvement over the MSE of 0.2149 observed with the MPH approach for the same five users. This discrepancy is attributed to the expansion of the light field in the MPH method. Consequently, the SMH method exhibits an improvement of 99.8% in power allocation accuracy compared to the MPH, highlighting its superior performance in maintaining the integrity of power distribution within VLC systems.

3.2. Simulation Results for Power Coverage

Subsequently, we conducted a power coverage experiment employing the principles outlined in Section 2.2. This experiment was designed to compare our method against the MF method detailed in Reference [15]. The results of this comparison are illustrated in Figure 5c,d. These results demonstrate the effectiveness of our approach in providing targeted power coverage, as opposed to the benchmarks established by the MF method. The graphical representation in Figure 5c,d highlights the differences in power distribution and coverage area between the two methods, providing a clear visualization of the comparative advantages and potential limitations inherent in each approach. It is evident that users employing the MF method cannot achieve precise power coverage, and the coverage ratio diminishes as the transmission distance and target power coverage increase. In contrast, SMH enables users to attain a high-power coverage ratio, approaching 100% at short distances. Although this value declines at greater distances due to diffraction effects, it remains significantly higher than that achieved with the MF method.

Figure 5c displays the power coverage areas for five users utilizing the MF method. It is evident that there is minimal variation in the power coverage, which places considerable demands on the precision of receiver alignment. A slight misalignment can lead to communication failures due to inadequate power reception. Conversely, Figure 5d illustrates that the SMH method enables precise power coverage within a three-dimensional transmission space. This ensures that signals are receivable over a larger area, significantly reducing the likelihood of communication failures. The contrast between the two methods underscores the superior adaptability and reliability of the SMH approach in maintaining effective power distribution in variable spatial configurations.

3.3. Simulation Results for OAM Secure Communication

In terms of secure communication, the users’ information undergoes dual encryption following the principles outlined in Section 2.3. Firstly, as shown in Figure 6a, we demonstrate the dispersion resistance and arbitrary location OAM-forming ability of SMH. We created 5th- and 10th-order OAM multiplexing holograms using SMH, the conventional SDM method [17], and the MDM method [16]. The SDM method splits the forward space and forms two OAM modes in different directions, but the depths of the two OAM modes are similar, and their locations change as the distance increases. In the MDM method, two OAM modes are coaxially added, and users can only decode the signal at the same place. In contrast, SMH forms the two OAM beams at 0.6 m and 0.8 m, respectively, with smaller rings, indicating minimal dispersion. This highlights the superiority of SMH. Then, Figure 6b illustrates the relationship between power variation and OAM keys to highlight the critical role of accurate OAM decoding. Specifically, for user encoded with OAM modes l = 20, only the application of the inverse OAM phase l = −20 yields maximal power reception. Moreover, the power reception quality incrementally improves as the decoding OAM modes more closely align with the correct values. This is because light spots decoded with OAM modes closer to the correct value tend to assume a Gaussian distribution.

Figure 6c displays the power distributions for five distinct users across five separate plates within the forward transmission space. Each user is encoded with unique OAM modes ranging from 1 to 5. The figure demonstrates that when the correct OAM key is applied to decoding at the appropriate plate, the respective user experiences high-power reception. Conversely, other plates, which encounter incorrect OAM keys, exhibit significantly lower power levels. Furthermore, the disparity between the intended and applied OAM keys directly influences the power received: the greater the discrepancy in the OAM keys, the lower the power that is captured. This pattern underscores the precision required in matching OAM keys for optimal power distribution and effective secure communication within VLC systems.

4. Experimental Setup

4.1. Experimental Platform

Figure 7a presents the experimental platform utilized in this study. The signal generation process begins with a bit-power-loading discrete multi-tone (BPL-DMT) signal, which is fed into an arbitrary waveform generator (AWG, Tektronix AWG710B, Tech Technology, Shanghai, China) capable of a maximum sampling rate of 4.2 GSa/s. These signals are subsequently amplified using a Mini-Circuits ZFL-2500VH+ amplifier (Mini-Circuits, New York, NY, USA) and passed through an adjustable attenuator (SMA, KT3.0-30/1S-2S, C&T RF Antennas Inc., Dongguan, China) to mitigate nonlinearity effects.

The enhanced signals are directed into a laser diode mount (Thorlabs LDM9T/M, Thorlabs, Newton, NJ, USA), which is integrated with temperature-electric coolers (TECs) and controllers. A 638 nm laser diode (Mitsubishi, ML501P73, Tokyo, Japan) is employed to generate the visible light signals. These signals undergo polarization adjustment via a polarizer (insertion loss 0.03 dB @ 632 nm) to ensure compatibility with the spatial light modulator (SLM, Meadowlark, 1920 × 1200 XY Phase Series, insertion loss 1 dB @ 632 nm, Meadowlark Optics, Frederick, MD, USA). After polarization, the light is focused through a lens onto a 50 μm diameter pinhole, serving to eliminate extraneous spatial modes. Subsequently, the light is expanded by the 4-f system (total insertion loss ≈ 3 dB @ 632 nm) to increase the light spot size.

For modulation, visible light is directed through SLM1, which loads the SMH for spatial region transmission. The light then traverses free-space to SLM2. In the context of OAM secret communication, SLM2 decodes the signal using inverse OAM modes. While, for experiments focusing on power allocation and coverage, SLM2 functions similarly to a mirror, reflecting the signals. The transmission distance is calculated from SLM1 for power allocation and coverage experiments, while SLM2 serves as a reflective mirror that does not affect the transmission. In OAM secure communication, SLM2 is positioned 0.1m before the APD to ensure the complete decoding of the OAM modes.

Finally, the signals are received by a photodetector (Thorlabs, APD430x), converted into electrical signals, and fed into an oscilloscope (Agilent Technologies, MSO9254A, Agilent Technologies, Shanghai, China). When it comes to observing the images, the CCDs replace the APD at the same location. The oscilloscope outputs the data to a computer for subsequent signal processing.

Three users are positioned in a forward free space following SLM1, located at distances of 0.6 m, 0.7 m, and 0.8 m, respectively. For the power allocation experiment, the target power ratios for the three users were set to 0.15, 0.35, and 1, corresponding to the required data rates for different types of video transmission. In the experiment focusing on power coverage, the targeted diameters for the power coverage areas were established as 0.2 mm, 0.4 mm, and 0.6 mm, respectively. This variation in size aimed to demonstrate the system’s capability to adapt to different spatial power distribution requirements. For the OAM secure communication experiment, the users were encoded with distinct OAM modes: 10, 20, and 30, respectively.

4.2. Signal Processing

Figure 7b illustrates the BPL-DMT modulation and demodulation process, integral to the experimental setup. The BPL process is bifurcated into two main stages: signal-to-noise ratio (SNR) estimation and bit power loading. Initially, QPSK (quadrature phase shift keying) modulation signals are dispatched to ascertain the channel’s SNR. This involves mapping all subcarriers with the QPSK signal, followed by DMT modulation application.

In accordance with a predetermined SNR table, the bit allocation for each DMT-modulated subcarrier is computed. The total count of subcarriers stands at 1024, which includes 512 signal carriers along with their conjugate symmetries, thereby establishing Hermitian symmetry. To accommodate the low-frequency portion of the signal, zero-padding is employed, involving eight subcarriers. Subsequently, the signal undergoes upsampling by a factor of two.

Employing the Levin–Campello (LC) algorithm, we optimize the allocation of bits to each subcarrier, ensuring that the bit error rate (BER) remains within the acceptable range for the 7% forward error correction (FEC) BER threshold (3.8 × 10⁻³) under a predefined power budget. This optimization strategy aims to maximize spectral efficiency and data rate while adhering to stringent error rate constraints.

In this experiment, multiple user profiles were simultaneously loaded, forming a frequency-division multiplexing communication system through the use of OFDM. However, time-division multiplexing is also feasible if the refresh rate of the SLM can meet the requirements of time-varying signal transmission.

5. Experimental Results

5.1. Experimental Results for Power Allocation

Within VLC systems, we consider three distinct types of users: VR glasses users, PC users, and smartphone users, each with unique video quality requirements. VR glasses users typically engage in viewing 4K virtual reality movies, necessitating the highest data rate. Conversely, PC users generally require sufficient bandwidth to watch 1080p videos, while smartphone users often consume content at lower resolutions, such as 720p shorts, which demands a comparatively lower data rate.

To quantify these requirements, we calculated the necessary data rates for each video resolution category. To examine the impact of received optical power (ROP) on these data rates, we employed neutral-density filters (NDs) to systematically vary the ROP. The resulting relationship between data rate and ROP is illustrated in Figure 8a. This analysis aids in determining the optimal power allocation strategy, facilitating the creation of SMH tailored to the diverse needs of VR glasses, PC, and smartphone users.

As depicted in Figure 8a, distinct user requirements necessitate varying data rates for satisfactory video quality. For VR glasses users aiming to watch 4K videos at 40 FPS, a minimum data rate of 2.47 Gbps is required, with a corresponding ROP of 2 mW. In contrast, PC users viewing 1080p videos at 75 FPS with 10 bit real color require a lower data rate of 1.45 Gbps, associated with an ROP of 0.7 mW. Meanwhile, smartphone users, typically watching 720p shorts at 60 FPS, necessitate an even more modest data rate of 0.41 Gbps, with a corresponding ROP of 0.3 mW.

From these specifications, we established the target power allocation ratio as 0.15, 0.35, and 1 for smartphone, PC, and VR glasses users, respectively. In our experimental setup, we employed ND to adjust the optical power received by the user positioned at 0.8 m—specifically, the VR glasses user—to meet the required ROP. Subsequently, we conducted experiments to adjust the power for the other two user types, aligning with their respective data rate requirements.

Figure 8b illustrates a single frame of a 4K video as received by the VR glasses user, comparing the results under MPH and SMH methodologies. Notably, the MPH lacks effective power allocation capabilities. Consequently, VR glasses users, positioned at the most distant plates in the forward transmission space, typically receive the least power. This insufficiency hinders their ability to achieve the required data rate for streaming 4K video, leading to a high BER and, subsequently, a diminished viewing experience. In contrast, the SMH method significantly enhances the ROP for the VR glasses user by strategically reducing the ROP allocated to the other two user types. This optimization allows the VR glasses user to view 4K videos seamlessly, without any bit errors after post-FEC. Figure 8c demonstrates the BER of each row when reconstructing the first frame of the video. It is evident that MPH has a high BER, which remains above the threshold even after post-FEC correction. In contrast, SMH consistently maintains the BER below the threshold. Figure 8d depicts the mean BER of each second of the video, showing significant fluctuations in BER with MPH methods. Conversely, SMH remains stable and consistently below the threshold.

Figure 8e–g present the data rate curves for different users as a function of the waveform voltage (peak-to-peak voltage, Vpp) of the signals. All users achieved their highest data rates at a Vpp of 200 mV. This pattern is characterized by an initial increase followed by a decline after reaching the peak, attributable to the non-linear effects inherent in the transmission and reception equipment. This trend underscores the critical balance required between signal strength and the limitations imposed by system non-linearities. The graphs also delineate the performance comparison between the SMH and MPH methods in terms of achieving target data rates: 0.5 Gbps for User 1, 1.5 Gbps for User 2, and 2.5 Gbps for User 3. It is evident that users under the SMH approach closely match their respective target data rates, demonstrating the method’s effective power allocation and reduced disparity. Conversely, the MPH method exhibits significant power mismatch, failing to meet the targeted data rates across the user spectrum.

5.2. Experimental Results for Power Coverage

In the power coverage experiment, the diameters for the power coverage areas of the users were set at 2 mm, 4 mm, and 6 mm, respectively, corresponding to their spatial locations. We assessed the variation in power as each receiver moved along the x-axis and y-axis. Due to the circular nature of the light spots, we averaged the values from the negative and positive axes for both x and y, yielding a comprehensive measure of the X or Y (abbreviated as X/Y) plane’s offset values, as depicted in Figure 9a–c. In Figure 9a, we observe that within User 1’s coverage area, the optical power loss experienced under the SMH method is only 0.81 dB. In contrast, under the MF method, this loss significantly increases to 10.21 dB. Consequently, the improvement from MF to SMH at the edge of the target coverage area is notable, registering at 8.4 dB. Similarly, Figure 9b reveals that within User 2’s designated coverage area, the optical power loss under SMH is minimized to 1.14 dB, compared to a much larger loss of 15.66 dB under the MF method. This results in an enhancement from MF to SMH at the boundary of the coverage area by 11.6 dB. Lastly, Figure 9c shows that for User 3’s coverage area, the power loss under SMH is recorded at 1.34 dB, significantly less than the 12.39 dB loss under the MF method. The improvement from MF to SMH at the coverage area’s edge is, therefore, 8.9 dB. In Figure 9d, we define the power coverage area as the region where the power loss is less than 1 dB compared to the central power. For User 1, the power coverage achieved under the SMH method was 113.6% of the target region, significantly surpassing the mere 9.8% achieved under the MF method. Similarly, User 2 experienced an 84.8% coverage with SMH compared to only 3.6% with MF. Meanwhile, User 3’s coverage area was 69.3% under SMH, which is substantially higher than the 6.98% provided by MF. These results clearly demonstrate that the SMH method significantly outperforms the MF method in terms of maintaining adequate power coverage. This indicates the superior performance of SMH in ensuring extensive and uniform power distribution across different users within the targeted regions.

Figure 9e displays the variations in data rate relative to the central data rate as a function of the X/Y offset. This trend mirrors the pattern observed for ROP in relation to the X/Y displacement. Under the MF method, all users exhibit a pronounced decline in data rate immediately beyond the central area. In contrast, for users under the SMH method, the rate of data loss aligns more closely with the predefined coverage area, indicating a more uniform distribution of data rates within the targeted region. Figure 9f,g feature images captured by a CCD at the users’ respective plates. The size of the light spot closely matches the intended target size under SMH, whereas the light spot is much smaller than the target size under MF methods. Specifically, using Mean Square Error (MSE) and Structural Similarity (SSIM) to evaluate the imaging results, SMH demonstrates better stability in both metrics, while MF exhibits significant fluctuations. These images further illustrate that the smaller the spot, the greater its brightness, indicative of the ROP’s intensity.

5.3. Experimental Results for OAM Secure Communication

In the OAM secure communication experiment depicted in Figure 10, we evaluate the ROP for each user when subjected to various OAM keys. Figure 10a illustrates the scenario under the specific condition of the APD’s received aperture size. It is observed that utilizing keys that are proximate to the correct modes marginally reduces the diameter of the optical rings, thereby concentrating the power. Due to the compact size of OAM rings when decoding with closely matched OAM keys and the large photosurface of the VLC receiver, which directly receives optical signals in free space, the isolation rate between modes is relatively low compared to other couplings to fiber works [27,28]. Nonetheless, the peak optical power is achieved using the correct OAM keys, confirming the anticipated trend.

Figure 10b presents a bar chart illustrating the data rates achieved under different OAM keys for three users. The OAM modes encoded in the holograms for these users increase from 10 to 30, from User 1 to User 3, respectively. Correspondingly, the isolation rate between different modes increases. This phenomenon can be attributed to the physical characteristics of the light rings associated with higher OAM modes; specifically, the higher the mode, the larger the diameter of the resultant OAM rings. This results in the failure of the APD to adequately receive signals under incorrect keys due to the dispersion of optical power.

Figure 10c shows images captured by a CCD demonstrating the effect of using no OAM keys versus the correct OAM keys. Without the correct OAM keys, the light forms large rings, leading to significant ROP dissipation. Additionally, these rings increase in size with the diffraction distance. In contrast, when the correct OAM keys are applied, the optical light spot contracts to a Gaussian-type distribution at the target distance, greatly enhancing the efficiency of signal reception.

In Figure 10d, User 3 serves as the focal point for an experiment transmitting a 1080P, 10 bit, 60 FPS real-color video under three distinct OAM keys. When the correct OAM key is used, the video transmits and plays seamlessly, with a BER below the threshold required for post-FEC, ensuring clear and uninterrupted viewing. However, incorrect OAM keys lead to decreased ROP and subsequently lower data rates, resulting in an elevated BER, distorted video playback, and a degraded viewing experience.

6. Conclusions

In this study, we developed an SMH framework to facilitate a multi-user system in visible light communication (VLC). Our approach was designed to fulfill the unique requirements of the VLC system, offering a high degree of user customization including self-designed power allocation, adjustable power coverage areas, and secure communication facilitated by optical keys. Utilizing holographic theory, we demonstrated the efficacy of SMH through both simulations and experimental validations.

In simulations, we configured a five-user scenario to evaluate the performance of SMH in terms of power allocation, coverage, and OAM secure communication, witnessing superior outcomes compared to the traditional approaches. Experimentally, SMH enabled precise power distribution among users, ensuring seamless video streaming quality across different resolutions—from 4K to 720P—unlike the conventional MPH method.

Furthermore, SMH showcased remarkable efficiency in power coverage experiments, achieving over 100% target area coverage and enhancing edge power by up to 11.6 dB. In terms of secure communication, our findings revealed that higher OAM modes enhance the isolation rate between modes, bolstering the security of transmitted data. Specifically, during the transmission of 1080P videos, only the correct OAM key facilitated clear and uninterrupted streaming, underscoring the potential of SMH in safeguarding communication.

To the best of our knowledge, this represents the first application of digital holography for enabling multi-user communication in a VLC system. This approach innovatively addresses distinct self-design needs for various system participants. With its versatility and high performance, the SMH framework paves a new path for VLC system development and holds the potential to shape the evolution of the 6G era.

Author Contributions

Conceptualization, C.C. and J.S.; methodology, C.C.; software, C.C and Z.Z.; validation, C.C., Y.W. and H.Z.; formal analysis, C.C.; investigation, Z.Z.; resources, N.C.; writing—original draft preparation, C.C.; writing—review and editing, J.S., J.Z., Z.L., H.C. and C.S.; project administration, J.S.; funding acquisition, N.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China (2022YFB2802803) and Natural Science Foundation of China Project (No. 61925104, No. 62031011, No. 62201157).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data underlying the results are all presented in this paper.

Conflicts of Interest

The authors declare no conflict of interest.

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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.

Figure 2. Application of power allocation holography in projection.

Figure 3. (a) Process of generating SMH for power allocation; (b) Process of generating SMH for power coverage; (c) Process of generating SMH for OAM secure communication.

Figure 4. Schematic diagram of precise transmission space sampling. (a) Transmission process without pre-control; (b) Transmission process with pre-control.

Figure 5. (a) Simulation results for power allocation; (b) Convergence line of GA; (c) Simulation results for power coverage under MF method; (d) Simulation results for power coverage under SMH.

Figure 6. (a) Simulation of optical field at receiver plane under SMH and conventional method; (b) Power trend of user encoded with OAM modes l = 20 under different OAM keys; (c) Simulation results for OAM secret communication.

Figure 7. Experimental setup. (a) Experiment platform; (b) Signal processing.

Figure 8. Experimental results for power allocation. (a) Data rate curve with ROP; (b) One frame of 4K video received under MPH and SMH; (c) BER of each row in the first frame; (d) Mean BER of each second of the video; (e) Working point test for User 1; (f) Working point test for User 2; (g) Working point test for User 3.

Figure 9. Experimental results for power coverage. (a) User 1’s ROP trend with XoY offset; (b) User 2’s ROP trend with XoY offset; (c) User 3’s ROP trend with XoY offset; (d) Coverage area diameter of 3 users under MF and SMH; (e) Data rate trend with XoY offset of three users; (f) CCD recording at User 1’s plate under different methods; (g) CCD recording at User 2’s plate under different methods; (h) CCD recording at User 3’s plate under different methods; (i) MSE and SSIM of recordings under different methods.

Figure 10. Experimental results for OAM secure communication. (a) ROP of three users with different OAM keys; (b) Data rate of three users with different OAM keys; (c) CCD recording of three users; (d) One frame of a 1080P video received by User 3 under different OAM keys.

Table 1. Target location in simulation.

User	Depth (m)	X-Y Coordinate (m)	Power Allocation	Power Coverage	OAM Modes
1	0.6	(0.032,0.032)	0.2	0.2	1
2	0.65	(0.035,0.035)	0.4	0.4	2
3	0.7	(0.035,0.044)	0.6	0.6	3
4	0.75	(0.014,0.044)	0.8	0.8	4
5	0.8	(0.037,0.057)	1	1	5

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Chen, C.; Wei, Y.; Zhang, H.; Zhuang, Z.; Li, Z.; Shen, C.; Zhang, J.; Cai, H.; Chi, N.; Shi, J. Spatial Multiplexing Holography for Multi-User Visible Light Communication. Photonics 2025, 12, 160. https://doi.org/10.3390/photonics12020160

AMA Style

Chen C, Wei Y, Zhang H, Zhuang Z, Li Z, Shen C, Zhang J, Cai H, Chi N, Shi J. Spatial Multiplexing Holography for Multi-User Visible Light Communication. Photonics. 2025; 12(2):160. https://doi.org/10.3390/photonics12020160

Chicago/Turabian Style

Chen, Chaoxu, Yuan Wei, Haoyu Zhang, Ziyi Zhuang, Ziwei Li, Chao Shen, Junwen Zhang, Haiwen Cai, Nan Chi, and Jianyang Shi. 2025. "Spatial Multiplexing Holography for Multi-User Visible Light Communication" Photonics 12, no. 2: 160. https://doi.org/10.3390/photonics12020160

APA Style

Chen, C., Wei, Y., Zhang, H., Zhuang, Z., Li, Z., Shen, C., Zhang, J., Cai, H., Chi, N., & Shi, J. (2025). Spatial Multiplexing Holography for Multi-User Visible Light Communication. Photonics, 12(2), 160. https://doi.org/10.3390/photonics12020160

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Spatial Multiplexing Holography for Multi-User Visible Light Communication

Abstract

1. Introduction

2. Design Principle of Spatial Multiplexing Hologram

2.1. Spatial Multiplexing Hologram for Multi-User Power Allocation

2.2. Spatial Multiplexing Hologram for Multi-User Power Coverage

2.3. Spatial Multiplexing Hologram Using OAM for Multi-User Secure Communication

3. Simulation Results

3.1. Simulation Results for Power Allocation

3.2. Simulation Results for Power Coverage

3.3. Simulation Results for OAM Secure Communication

4. Experimental Setup

4.1. Experimental Platform

4.2. Signal Processing

5. Experimental Results

5.1. Experimental Results for Power Allocation

5.2. Experimental Results for Power Coverage

5.3. Experimental Results for OAM Secure Communication

6. Conclusions

Author Contributions

Funding

Institutional Review Board Statement

Informed Consent Statement

Data Availability Statement

Conflicts of Interest

References

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