Li-Fi concept in terms of modulation techniques

Li-Fi concept in terms of modulation techniques

Meher Afroj
Department of Computer Science and Engineering, Bangladesh University of Business and Technology
DOI: https://doi.org/10.51584/IJRIAS.2023.8601
Received: 17 May 2023; Revised: 01 June 2023; Accepted: 05 June 2023; Published: 30 June 2023

Abstract – Li-Fi (light fidelity) is a bidirectional wireless system that transmits data via LED or infrared light. Li-Fi technology only needs a light source with a chip to transmit an internet signal through light waves. The system has a receiver to pick up light signals and transmitter to send light signals back to the lamp using infrared light. The technology has high data rate as well as high spectral efficiency and also robust against inter symbol interference. Different forms of OFDM (a multicarrier modulation technique) are being used for Li-fi scheme. This paper reflects the theoretical study of various modulation techniques of Li-fi technology.

Keywords— light fidelity; orthogonal frequency division multiplexing; machine learning; dimming;

I. Introduction

With the escalating need for wireless data communication, the radio spectrum below 10 GHz has proven inadequate to meet the demand. As a result, the wireless communication industry has started exploring the use of radio spectrum frequencies above 10 GHz to address this challenge. Li-Fi, also known as Light-Fidelity, represents a development in this trend towards utilizing higher frequencies within the electromagnetic spectrum [1], [2].Li-Fi utilizes light-emitting diodes (LEDs) for high-speed wireless communication and employs various modulation techniques to encode the signal. Common single-carrier modulation (SCM) schemes used in Li-Fi include on-off keying (OOK), pulse position modulation (PPM), and pulse amplitude modulation (PAM) [9]. However, as the data rate requirements increase in Li-Fi networks, SCM schemes such as OOK, PPM, and PAM may encounter issues like non-linear signal distortion at the LED front-end and inter-symbol interference caused by frequency selectivity in dispersive optical wireless channels. To overcome these challenges and achieve high-speed optical wireless communication, multi-carrier modulation (MCM) techniques are being explored. MCM is more bandwidth-efficient but less energy-efficient compared to SCM. One widely adopted MCM technique in Li-Fi networks is Orthogonal Frequency Division Multiplexing (OFDM) [10], [11]. In OFDM, parallel data streams are transmitted simultaneously using a set of orthogonal subcarriers, eliminating the need for complex equalization. An OFDM modulator can be implemented using an inverse discrete Fourier transform (IDFT) block, which is efficiently realized using the inverse fast Fourier transform (IFFT), followed by a digital-to-analog converter (DAC). As a result, the OFDM signal generated is inherently complex and bipolar. However, to comply with the intensity modulation and direct detection (IM/DD) requirements imposed by commercially available LEDs, certain modifications to conventional OFDM techniques are necessary in Li-Fi systems. These modifications ensure compatibility with the characteristics and limitations of the LED-based transmitters used in Li-Fi technology. To ensure a real-valued output signal after the inverse fast Fourier transform (IFFT), a commonly used approach is to enforce Hermitian symmetry on the subcarriers. Additionally, in Li-Fi, where light intensity cannot be negative, the signal needs to be unipolar. Several methods exist for obtaining a unipolar time-domain signal. One method is Direct Current biased optical OFDM (DCO-OFDM) [12], which employs a positive direct current (DC) bias to generate a unipolar signal. Although this approach increases the total electrical power consumption, it does not result in a loss of spectral efficiency. Another technique is Asymmetrically Clipped Optical OFDM (ACO-OFDM) [13], where only odd subcarriers are used for data transmission, while even subcarriers are set to zero, imposing Hermitian symmetry. As a result, the spectral efficiency of ACO-OFDM is halved. ACO-OFDM requires only a small DC bias, making it more energy-efficient compared to DCO-OFDM. Asymmetrically Clipped Direct Current biased OFDM (ADO-OFDM) [14] combines the DCO-OFDM scheme for even subcarriers and the ACO-OFDM scheme for odd subcarriers. In certain scenarios, ADO-OFDM demonstrates superior power-efficiency compared to both DCO-OFDM and ACO-OFDM. Another modulation scheme called Pulse-Amplitude-Modulated Discrete Multitone Modulation (PAM-DMT) [15] also clips the negative signal as in ACO-OFDM. However, PAM-DMT modulates only the imaginary parts of the signal on each subcarrier, ensuring that signal distortion resulting from asymmetric clipping affects the real component, which is orthogonal to the information-carrying signal. A hybrid optical OFDM scheme known as Asymmetrically Hybrid Optical OFDM (AHO-OFDM) [16] combines ACO-OFDM and PAM-DMT by utilizing both odd and even subcarriers for information transmission. These different modulation schemes, such as DCO-OFDM, ACO-OFDM, ADO-OFDM, PAM-DMT, and AHO-OFDM, offer various trade-offs in terms of power-efficiency, spectral efficiency, and signal distortion management, providing flexibility in adapting Li-Fi systems to different requirements and scenarios.

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