Complexity reduction of filtered-OFDM systems for high-speed visible light communications

Contemporary wireless communication infrastructures face inherent limitations in radio frequency (RF) bandwidth allocation. As global requirements for enhanced data throughput intensify, systemic challenges, such as spectral congestion and diminished data transmission efficiency, assume critical importance. Within the framework of emerging 5G and 6G architectures, scholarly and industrial focus has pivoted toward leveraging the optical spectrum as a viable alternative to circumvent these constraints. Notably, visible light communication (VLC) systems have recently emerged as an innovative paradigm for augmenting traditional RF-dependent wireless networks. Among these advancements, the Filtered-Orthogonal Frequency Division Multiplexing (F-OFDM) modulation scheme has positioned itself as a promising solution for mitigating OFDM's inherent limitations of excessive out-of-band radiation and consequent spectral inefficiency. In direct detection and intensity modulation (DD/IM), LEDs require real-valued and non-negative signals, achieved through the inverse fast Fourier transform (IFFT) with Hermitian symmetry (HS). While this enables noncomplex-valued signals, it halves the electrical bandwidth. Larger IFFT/FFT sizes increase processing demands, chip area, energy consumption, and hardware costs. The HS constraint complicates the design by requiring duplicate FFT/IFFT blocks, raising computational complexity. Additionally, increasing subcarriers worsens the peak-to-average power ratio (PAPR), adding further challenges. Legacy optical OFDM methods employed to transform bipolar OFDM signals into unipolar formats include DC-biased (DCO-OFDM) and asymmetrically clipped (ACO-OFDM) techniques. However, these adaptations entail compromises: DCO-OFDM elevates power requirements, whereas ACO-OFDM diminishes spectral efficiency. This thesis primarily concentrates on optimizing spectral ‎efficiency and reducing the complexity of VLC systems. Therefore, the effort offered ‎in this thesis introduces a different methodology to eliminating the reliance on ‎HS. Two methods, NHS-DCO-F-OFDM and NHS-Flip-F-OFDM, are proposed to generate real F-OFDM signals by rearranging the real and imaginary components of complex F-OFDM signals in the time domain. These methods reduce system complexity by 50%, avoid PAPR issues, and enhance power efficiency contrasted to established HS-based F-OFDM. The NHS-filtered OFDM approach also improves bandwidth efficacy and cuts out-of-band emission power by 115 dB, offering better spectral localization than conventional OFDM.