The Li-Fi access network supported by WDM of FSO communications using modified duobinary signaling

Optical wireless communication (OWC) is a promising technology for future mobile access networks in smart cities, offering advantages such as higher speed, increased data rates, enhanced security, reliability, and superior overall performance. However, achieving these benefits requires overcoming key challenges in ensuring efficient and stable transmission access. To address this, we propose the use of wavelength division multiplexing (WDM) over free-space optical (FSO) communications as a high-speed backbone to support light-fidelity (Li-Fi) access networks. Additionally, a modified duobinary signal (DBS) generator is introduced to modulate the WDM frequency channels. The integration of DBS in WDM-based FSO links enhances performance and improves the resilience of signal delivery for Li-Fi access. DBS is designed with a precoder to shift the bit stream, an encoder to convert non-return-to-zero (NRZ) signals, and a dual Mach–Zehnder modulator: The first modulator generates a half-width signal period, while the second introduces a phase shift to the signal. Simulation results indicate that DBS achieves superior performance compared to standard DBS (S-DBS) and conventional NRZ in FSO channels subject to atmospheric turbulence. Performances of WDM channels at total capacity of 1 Tbps are 37 dB and 10-27 for signal-to-noise ratio (SNR) and bit error rate (BER), respectively. Additionally, in the 39th WDM channel under weak turbulence, DBS achieves BER in the range of values of 10-31 to 10-22 for capacities from 100 Gbps to 600 Gbps, respectively. DBS is also capable of delivering high-quality signals over distances ranging from 300 m to 1500 m. Moreover, Li-Fi performance is primarily influenced by the quality of signal transmission in the FSO channel, as turbulence effects within the Li-Fi channel itself are relatively minor. Consequently, Li-Fi performance closely mirrors that of the FSO link. Experimental results further confirm that Li-Fi achieves optimal network performance at installation heights of 3–4 m.