Visible light communication using TDMA optical beamforming

In recent years, light-emitting diodes (LEDs) have emerged as eco-friendly replacements for incandescent light bulbs and fluorescent lamps because of high electric-to-optic conversion efficiency, long lifetime, small size, etc. LEDs also have an interesting feature that the light output can be modulated with high-frequency signals. Using this phenomenon, LED can be used for optical wireless communications, which is usually called visible light communications (VLC) [1–7]. VLC is emerging as a promising alternative for future indoor wireless communication because of its unregulated huge bandwidth (~400 THz), low power consumption, no electromagnetic interference (EMI) generation, etc.

Currently, the performance of VLC cannot surpass the current WiFi technology. Therefore, performance improvement of VLC is one of the key issues for commercialization. To improve the performance of VLC, a lot of techniques have been proposed including electrical and optical domain approaches. Electrical domain approaches include equalization [8], quadrature amplitude modulation (QAM) [9], and orthogonal frequency division multiplexing (OFDM) [10]. Optical domain approaches include blue filtering [8], multiple-input multiple-output (MIMO) [11], wavelength-division multiplexing (WDM) [12, 13], and polarization division multiplexing (PDM) [14].

Recently, one of the optical domain approaches called “optical beamforming” technique has been proposed to enhance the performance of VLC. Optical beamforming is a technique to focus LED light on a desired target so that it can enhance the signal-to-noise ratio (SNR) of the received VLC signal [15, 16]. Since it does not depend on electrical signal formats, it can be widely used in various VLC schemes.

However, the previous demonstration of the VLC using optical beamforming could communicate with only a single target. In the real-life applications, multiple user devices should be accommodated. Therefore, in this paper, we propose and demonstrate a VLC technology using time-division multiple access (TDMA) optical beamforming to accommodate multiple user devices, which focuses LED light on each different target device in each different time slot.

Figure 1 shows the structure and concept of the VLC systems based on ordinary optical beamforming (Fig. 1a) and TDMA optical beamforming (Fig. 1b). Both VLC systems consist of a LED light, a spatial light modulator (SLM), a LED/SLM controller, and user devices. The LED light is covered by a SLM to control the LED light beam. A SLM is a transparent or reflective optical device which can modulate the phase or amplitude of light on each pixel, so can be operated as a dynamic diffractive element controlled by electrical signals [17]. The electrical signal is modulated by the LED and the optical beamforming is controlled by the SLM.

In Fig. 1, a scenario of two user devices is assumed. As shown in Fig. 1a, the VLC system based on ordinary optical beamforming can accommodate only a single target. However, in the real-life applications, multiple user devices should be accommodated. Therefore, we propose a VLC technique based on TDMA optical beamforming, as shown in Fig. 1b, which focuses LED light on each user device in each time slot. For example, the LED light is focused on user device 1 at a time slot 1 and then it is focused on user device 2 at a time slot 2. The SLM focuses the LED light on each user device according to the location and allotted time slot of each device. The location of user devices can be detected by using a location-detecting algorithm based on direction code, which has been proposed previously [15].

Figure 2 shows the block diagram of the experimental setup for the proposed VLC using TDMA optical beamforming. Figure 3 shows the picture of the experimental setup. In the experimental setup, a LED is modulated by a pseudo-random binary sequence (PRBS), non return-to-zero (NRZ) signal. The modulated LED light is passed through a beam expander in order to control the beam size and form it into a parallel ray. The beam expander can be omitted if the optical design is optimized. After the beam expander, the light goes into a spatial light modulator (SLM), which has 800 × 600 translucent liquid crystal pixels with a pixel pitch of 32 × 32 μm. The size of the active area in the SLM is 26.6 × 20.0 mm. The SLM is an optical device that can modulate optical phase on each pixel in 256 levels (8 control bits), so it can operate as a dynamic diffractive element controlled by electrical signals.

A control computer sends a Fresnel lens function to the SLM according the information of the TDMA time slot and location of the target devices. This signal modulates the phase of SLM pixels and the SLM functions like a dynamic lens. After passing through the SLM, the light is beamformed and focused on the optical receivers. OSRAM SFH-213 photodiodes are used for the optical receivers. Either amplifiers or filters are not used in the receiver site.

where L is the focal length, R ₁ is the radius of the first circle in the Fresnel lens function, and λ is the wavelength of the light. The Fresnel lens function is controlled by a control computer according to the information of the time slot and the location of the target devices. In the experiment, two different target devices are considered. In time slot 1 and 2, the Fresnel lens function is controlled to focus the LED light on target 1 (ch.1) and target 2 (ch.2), respectively. The Fresnel lens function for each time slot is shown as a gray display in Fig. 4, which shows the level of phase transition. The separation distance between the two circle centers in time slot 1 and 2 is 14 mm in the SLM. Detailed system parameters of the experiment are summarized in Table 1.

System parameters	Values
Pixel size of SLM	32 × 32 μm
Active area of SLM	26.6 × 20.0 mm
Display resolution of SLM	800 × 600 pixels
Gray level of SLM	256 (8 bits)
# of TDMA receivers	2
Separation distance of channels in SLM	14 mm

We have proposed and demonstrated a VLC using TDMA optical beamforming to accommodate multiple users in the VLC using optical beamforming. TDMA optical beamforming is a technique to focus LED light on each different target device in each time slot. Our results have shown that each user device can enjoy the benefit of the optical beamforming during its time slot with the TDMA optical beamforming. The VLC signal amplitude of each channel increases by 5~10 dB, the optical power density of the VLC signal increases by 2~4 dB, and the transmission distance increases from 110 to 200 cm with the TDMA optical beamforming. Since the proposed technique can increase the performance of VLC efficiently and does not depend on the electrical modulation formats, it can be widely used in various applications.