IJCNC 04

NOISE MITIGATION METHODS
FORDIGITALVISIBLE LIGHT COMMUNICATION

Wataru Uemura 1 and Takumi Hamano 2
1 Faculty of Advanced Science and Technology, Ryukoku University, Shiga, Japan
2 Faculty of Science and Technology, Ryukoku University, Shiga, Japan

ABSTRACT

Visible Light Communication (VLC) using Light Emitting Diodes (LEDs) has gained attention due to its low power consumption, long lifetime, and fast response. However, VLC suffers from optical noise generated by ambient light sources such as fluorescent lamps, which leads to waveform distortion and increased bit error rates (BER). In this paper, we propose two noise reduction methods for Digital Visible Light Communication (DVLC) systems. The first method exploits the periodic nature of interference caused by AC-powered-line illumination and reduces interference by subtracting sampled noise waveforms from the received signal. Second, inspired by Active Noise Control (ANC) techniques, an additional photodiode is introduced for noise reception, and subtraction circuits are employed to attenuate noise in real time. Experimental results show that both methods improve BER performance compared with conventional receivers, with the ANC-inspired approach achieving superior performance under all tested conditions

KEYWORDS

Visible Light Communication, Active Noise Control, Noise Reduction Methods

1. INTRODUCTION

Visible Light Communication (VLC) has emerged as promising wireless communication technology that exploits the visible spectrum between 380 nm and 780 nm. Unlike radio frequency (RF) systems, VLC provides clearly defined communication areas determined by illumination and can be seamlessly integrated into existing lighting infrastructure. However, the performance of VLC can be significantly degraded by optical noise originating from ambient light sources such as fluorescent lamps [1 – 5]. Digital Visible Light Communication (DVLC) refers to VLC systems that employ digital modulation schemes such as OOK, PPM, or I-PPM to transmit binary data through visible light.

In DVLC systems, the receiver typically distinguishes HIGH and LOW states of the signal by applying a threshold to the received voltage. When optical noise is superimposed on the signal, this thresholding process can lead to incorrect symbol detection, thereby degrading communication reliability.

In this paper, we address the problem of optical noise in VLC and propose two methods that enable robust demodulation in noisy environments. Section 2 provides an overview of the fundamentals of VLC, while Section 3 discusses the problems caused by noise in VLC. Two general categories of noise are considered: periodic and non-periodic. In Section 4, we focus on periodic noise caused by power-line interference, which depends on the frequency of the AC power supply (typically 50 Hz or 60 Hz worldwide). For example, in western Japan the frequency is 60 Hz, and all experiments in this paper are conducted under such conditions. To mitigate this interference, we propose a subtraction method in which one cycle of the noise waveform is sampled and removed from the received signal, thereby reducing the noise amplitude.

In Section 5, we focus on non-periodic noise. An auxiliary photodetector is introduced to capture only the noise component, excluding the VLC signal. This reference noise signal is then subtracted from the received waveform in real time using a subtraction circuit.

The effectiveness of both proposed methods is evaluated by comparing the relationship between the energy-per-bit-to-noise ratio (Eb/N0) and the bit error rate (BER) against a conventional method without noise mitigation. Finally, Section 6 presents the conclusions of the paper.

2. VISIBLE LIGHT COMMUNICATION (VLC)

Visible Light Communication (VLC) is a wireless communication technology that uses electromagnetic waves in the visible spectrum, with wavelengths ranging from 380 nm to 780 nm [6 – 10]. In practical systems, white light-emitting diodes (LEDs) are commonly used as transmitters, either by combining multiple wavelengths across the visible band or by mixing blue and yellow light. In digital VLC using pulse modulation, the ON state of the light source is assigned to HIGH, while the OFF state corresponds to LOW. Information is conveyed through a sequence of time slots, with each slot represented as either HIGH or LOW.

LEDs have been extensively investigated as transmitters in VLC. Compared with conventional lighting devices such as incandescent bulbs, LEDs provide distinct advantages: lower power consumption, longer operational lifetime, and much faster response speed. This rapid response enables high data rates in VLC. Furthermore, their ability to switch ON and OFF at high frequencies effectively suppresses perceptible flicker, allowing LEDs to function simultaneously as illumination and communication devices.

On the receiver side, photodiodes are widely used as detectors because their response time is faster than that of phototransistors. Since VLC employs light as the transmission carrier, ordinary illumination devices can also act as transmitters. Moreover, because communication is confined strictly to the illuminated area, the coverage of VLC is well- defined, in contrast to radio frequency (RF) communication, where the boundaries of the communication range are less distinct. This spatial confinement also provides security advantages, as eavesdropping from outside the illuminated area is inherently difficult [11, 12]. Table 1 summarizes the features of VLC.

Figure 1. On-Off keying which assigns the HIGH state to “1” and the LOW state to “0”

Table 1 .Features of Visible Light Communication (VLC)

Figure 2.An example where four slots are used for 4PPM.

2.1. Flicker in VLC

Flicker occurs when the duty ratio of HIGH and LOW states within one cycle of a transmission slot is not constant. In such cases, the human eye perceives rapid and irregular fluctuations in brightness, rather than simple high-speed ON-OFF switching. Continuous exposure to flickering light can have adverse effects on human health, such as dizziness and nausea. Therefore, in VLC systems where lighting devices are also used as transmitters, modulation methods must be carefully designed to avoid flicker.

2.2. Modulation Methods for VLC

This subsection introduces several modulation schemes for VLC: On-Off Keying (OOK), Pulse Width Modulation (PWM), Pulse Position Modulation (PPM), and Inversed Pulse Position Modulation (I-PPM). Among these, OOK and PWM are prone to flicker and are therefore not suitable when VLC is used simultaneously for illumination. In contrast, PPM and I-PPM maintain a constant duty ratio within each cycle, thereby preventing flicker. For this reason, these two modulations have been the focus of much recent VLC research.

2.2.1. On-Off Keying (OOK)

As shown in Figure 1, OOK transmits binary data by assigning the HIGH state to “1” and the LOW state to “0” (yellow indicates HIGH and white indicates LOW in the figure). OOK is the

Figure 3.An example where four slots are used for I-4PPM.

Figure 4.The relationship between the brightness (number of bits) and the transmission efficiency

simplest digital modulation scheme to implement. However, since it does not maintain a constant duty ratio, flicker can occur. Consequently, OOK is generally unsuitable for VLC systems that rely on lighting devices as transmitters.

2.2.2. Pulse Position Modulation (PPM)

As shown in Figure 2, PPM encodes information by changing the position of a pulse within a predefined number of slots [13]. The example in the figure uses four slots. In PPM, the duty ratio is constant across all symbol assignments, so flicker is inherently avoided. For this reason, PPM is suitable for VLC systems.

However, the ratio of HIGH-to-LOW is fixed at 1:N (where N is the number of slots minus one). As the number of slots increases, the proportion of LOW states becomes larger, making the illumination appear dimmer to the human eye. Furthermore, increasing the number of slots reduces transmission efficiency, since the maximum duty ratio cannot exceed 50%. Therefore, when VLC transmitters are also used for illumination, PPM may not be the most practical choice.

2.2.3. Inverse Pulse Position Modulation (I-PPM)

As depicted in Figure 3, I-PPM encodes binary information by shifting the position of the LOW state within a fixed number of slots. While conventional PPM which uses the HIGH state as the information carrier, I-PPM instead assigns information based on the LOW state. Figure 3 shows an example with four slots. Like PPM, I-PPM maintains a constant duty ratio and thus voids flicker, making it well-suited for VLC.

In contrast to PPM, the HIGH-to-LOW ratio in I-PPM is N:1 (where N is the number of slots minus one). As the slot count increases, the position of HIGH states becomes larger, producing brighter illumination. For this reason, I-PPM is often preferred in VLC systems where both communication and lighting requirements must be satisfied.

The relationship between the number of bits and the number of slots in I-PPM is expressed in Eq. (1), and the corresponding transmission efficiency is given in Eq. (2). The calculated values are summarized in and illustrated in Figure 4.

Table 2. Information transmission efficiency of I-PPM.

As shown in Figure 4, increasing the number of slots —thereby producing brighter illumination—inevitably reduces transmission efficiency. A common countermeasure is to shorten the symbol duration, which makes it possible to increase the slot count without sacrificing efficiency. However, as the modulation order increases, although the communication rate improves, the time allocated per bit becomes shorter, making the system increasingly susceptible to noise. Therefore, the slot number must be carefully managed with respect to both transmission efficiency and robustness against noise.

2.3. Digital Visible Light Communication

2.3.1. Transmitter

Transmission process begins with data provided from a terminal device, such as a computer, to a microcontroller that performs modulation. Based on the input data, the microcontroller devices the LED to emit modulated optical signals. The specific modulation scheme is predetermined by the program implemented on the microcontroller.

2.3.2. Receiver

On the receiver side, a photodiode converts incident light into corresponding voltage variations. These signals are classified into HIGH and LOW levels using a comparator, which functions as a specialized operational amplifier. The comparator determines the logic level by comparing the voltage at its non-inverting input with a reference threshold applied to its inverting input. If noninverting voltage exceeds the threshold, the comparator outputs the positive supply voltage, interpreted by the microcontroller as HIGH; otherwise, it outputs the negative supply voltage, interpreted as LOW. The resulting sequence of HIGH and LOW signals is then demodulated by the microcontroller, according to its programmed algorithm, and the recovered information is transferred back to a terminal device such as a computer

3.NOISE IN DIGITAL VISIBLE LIGHT COMMUNICATION

This section discusses the challenges posed by noise in digital VLC systems and explains its sources and effects..

Figure 5. Fluorescent lamps generate noise waveforms with distortions

Figure 6. When the noise is superimposed on a digital

3.1. Overview of Noise

In digital VLC, light sources other than the intended transmitter act as interference. A typical example is fluorescent lighting, which produces distorted noise waveforms, as illustrated in Figure 5. When such noise is superimposed on a digital VLC signal, the received waveform becomes distorted, as illustrated in Figure 6. The origin of this distortion lies in the rectification process of alternating current (AC) power, which is examined in the next subsection

3.2. Rectification of AC Voltage

Rectification is the process of converting alternating current (AC) into direct current (DC), performed by a rectifier circuit. A common implementation is the diode-based full-wave or bridge rectifier, which makes use of both the positive and negative halves of the AC cycle. In such circuit to produce a quasi-DC waveform. However, the resulting signal is not perfectly constant due to transient phenomena. In addition, the frequency of the rectified output becomes twice that of the original AC input.

3.3. Transient Phenomena and Signal Distortion

In VLC receivers, HIGH and LOW states are determined relative to a threshold voltage. In Figure 7 illustrates an example in which the threshold (red line) is set near the center of the signal shown in Figure 6. As indicated, the segment marked with a red circle should represent a LOW signal, but noise raisesit above the threshold, leading to a false HIGH detection. Conversely, the segment