## 1 Introduction
Additionally, FSO links can play a pivotal role in establishing redundant communication pathways, enhancing overall network resilience. Figure 1: Schematic of the FSO communication link coopted into MDM-based optical distribution networks (ODNs) contributes to a high data rate. MDM-FSO is a promising approach for increasing broadband penetration while providing cost-effective solutions for transmitting multiple RF signals over a high-speed optical carrier using inexpensive fiber cabling. This hybrid approach integrates the advantages of MDM systems and FSO technology, offering a robust, scalable, and efficient alternative for high-capacity communication networks [12; 13]. MDM-FSO systems present a license-free RF solution, making them particularly attractive for areas with regulatory or financial constraints. Unlike conventional RF communication, which often requires frequency licensing, FSO links transmit signals through the atmosphere rather than optical fibers, effectively eliminating the high costs associated with fiber installation in densely populated urban environments. This feature enhances their feasibility in urban settings while reducing overall deployment costs [2]. In regions where wireless radio technology faces challenges--such as mountainous terrains or areas with limited access to traditional infrastructure--FSO links can be integrated with existing mobile cellular radio networks. This combination enables rapid and reliable deployment of pervasive wireless systems, addressing connectivity gaps in hard-to-reach locations [7]. Furthermore, FSO technology offers a higher degree of security due to its narrow beamwidth and immunity to electromagnetic interference, making it ideal for sensitive applications. By combining the spectral efficiency of MDM systems with the flexibility and cost-effectiveness of FSO technology, MDM-FSO systems hold significant potential to revolutionize broadband connectivity, addressing challenges in urban, remote, and rugged terrains alike. Although MDM-FSO-based systems have the potential to guarantee high-speed data rates, several critical challenges must be addressed to optimize their performance. These challenges include power sharing among mode channels, optical filter optimization, and the effects of atmospheric turbulence and scintillations [14]. These factors significantly impact the signal-to-noise ratio (SNR) of the MDM-FSO link, necessitating the development of advanced solutions to mitigate these issues [3; 4]. By integrating MDM with FSO technology, referred to as MDM-FSO, a seamless and cost-effective pathway for enhancing the capacity of existing wireless networks can be realized. This integration enables continuous collaboration with RF wireless networks, offering a scalable approach to meet the demands of modern communication systems [5]. In addition to conventional binary modulation techniques such as non-return-to-zero (NRZ) and return-to-zero on-off keying (RZ-OOK), duobinary modulation is increasingly recognized as an effective format for overcoming bandwidth and multipath challenges [15; 16; 17; 18]. Duobinary modulation reduces bandwidth requirements compared to OOK for a given data rate by intentionally introducing controlled inter-symbol interference (ISI) via pulse-shape filters at the transmitter [19]. Adopting MDM-based duobinary modulation with FSO links can achieve medium- and long-haul transmissions with higher data rates, making it a promising solution for high-capacity optical communication networks [6; 7; 8; 9; 10; 11]. This paper examines the performance of duobinary (DB) and modified duobinary (MDB) schemes for intensity modulation/direct detection (IM/DD) MDM-FSO systems. The adoption here is because of their power efficiency, high receiver sensitivity, and low complexity in equalization [20; 21]. Key performance metrics include mode power sharing, system component optimization, and bit error rate (BER) at varying transmission distances. Experimental results compare various MDB schemes using the proposed link.