Author ORCID Identifier

https://orcid.org/0000-0003-0719-8024

Date of Award

28-1-2025

Document Type

Thesis

School

School of Electrical & Electroncis Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.N.R.Raajan

Keywords

Optical Communication, OFDM, Precoding, Companding, Non Linearity

Abstract

Using light-emitting diodes (LEDs) for data transfer, visible light communication (VLC) presents a strong substitute for radio frequency communication. Yet, because of the non-linearity of LEDs, conventional Orthogonal Frequency-Division Multiplexing (OFDM) approaches in VLC are limited regarding spectrum efficiency, peak-to-average power ratio (PAPR), and system linearity. Multi-carrier asymmetrically Clipped Optical OFDM (MADO-OFDM) is introduced in this study. For VLC operations based on Optical Orthogonal Frequency Division Multiplexing (O-OFDM), a model-driven Deep Learning (DL) approach has been presented. Utilizing an Auto Encoder (AE) network technology reduced the non-linearity of the LEDs. A proposed improved ADO-OFDM protocol, called MADO-OFDM, adaptively modifies the number of subcarriers needed for ACO-OFDM and DCO-OFDM transmissions in response to downlinking multiple point-of-service requests. The Generalized Piecewise Linear Compander (GPLD) and Discrete Hartley Matrix Transform (DisHMT) precoder are merged with the proposed MADO-OFDM to enable high-speed data transmission with lower PAPR. Using its unlicensed spectrum, VLC overcomes the limitations associated with radio frequency (RF) communication. VLC is a response to the issue of insufficient bandwidth, driven by the quick development of LEDs

[1]. The use of visible band optical connections in free space is called VLC. VLC is starting to show promise as a future high-capacity interior wireless network. The modulation bandwidths of commercially sold LEDs range from 2 to 20 MHz

[2]. The modest LED modulation bandwidth (2–20 MHz) in VLC makes the adoption of multicarrier modulation (MCM) techniques necessary to provide high-speed data transport. These days, OFDM is frequently used in VLC as an MCM approach

[3]. Compared to single-subcarrier structures, OFDM has a higher optical efficiency and a stronger defense against inter-symbol interference (ISI). Unlike the RF-OFDM system, which employs a bipolar and misleading signal for intensity modulation (IM) and direct detection (DD), the VLC-OFDM method utilizes a unipolar and real signal

[4]. Since the immediate output power of LEDs modulates the time dimension signals transmitted by DD and IM VLC systems, those signals need to be real and non-negative. A without licensing bandwidth, high transmission power, an excellent signal-to-noise ratio, and inexpensive front ends can all be obtained with an LED-based VLC

[5]. With the help of these features, VLC can effectively supplement RF transmission of signals. The use of two types of multi-carrier modulation, Discrete Multi-Tone (DMT) and OFDM modulation, for VLC systems is currently receiving more attention

[6]. However, the impact of LED non-linearity is not taken into account in these studies. The resulting distortion causes the LED nonlinearity to become sensitive, which presents many challenges for the OFDM-based VLC technology

[7]. The biasing current ought to be carefully selected to limit the signal's size to the largest linear range because an LED's driving current has no direct effect on its output optical power. Use a time frame pre-distortion approach to compensate for the nonlinear deformation of OFDM symbols

[8]. It is crucial to possess prior understanding of the LED transfer function, nevertheless. The time-domain form OFDM symbols were divided into many portions in order to control different numbers of LEDs. OFDM is currently a well-established technique because of its many advantages, such as its large bandwidth and absence of electromagnetic interference

[9]. The exponential expansion in bandwidth needs has made research and technological advancement of utmost importance. Over multiple consecutive generations, optical communication has evolved [10]. While many LEDs will increase system complexity, several transmitters can help minimize nonlinearity. A DFE in non-linear feed-forward was used to reduce the LED's non-linearity. Reducing the LED non-linearity of OFDM-based VLC devices requires an effective, albeit somewhat difficult, compensating strategy.

This research presents a comprehensive approach to address the limitations of traditional OFDM techniques in VLC systems. The introduction of MADO-OFDM offers a promising solution to improve spectral efficiency, reduce PAPR, and mitigate LED non-linearity issues. Utilizing a model-driven Deep Learning strategy as an AE network system to minimize LED non-linearity represents a significant contribution. This approach enhances system linearity and improves the overall performance of O-OFDM-based VLC processes. Furthermore, the proposed MADO-OFDM incorporates an adaptive adjustment of subcarrier numbers based on service requests in downlink multiple access scenarios. This adaptability ensures efficient utilization of resources and enables dynamic allocation of subcarriers, thereby enhancing system flexibility and scalability. Additionally, the integration of the DisHMT precoder and GPLD further enhances the performance of MADO-OFDM by reducing PAPR and enabling high-speed data transmission. Looking ahead, future research can explore advancements in DL techniques to further enhance LED non-linearity mitigation and spectral efficiency in VLC systems.

Download

Share

COinS