# Bandwidth Density Analysis of Coded Free-Space Optical Interconnects
###### Abstract  
The performance of free-space optical interconnects (FSOIs) system is significantly influenced by noise, similar to any wireless communication system. This noise has a notable impact on both the bandwidth density and data rate of FSOIs system. To address these challenges, this study proposes the utilization of vertical-cavity-surface-emitting laser (VCSEL) arrays on the transmitter side and photodetector arrays on the receiver side for FSOIs. The study investigates the bandwidth density of the system with and without coding while maintaining a specific bit error rate. An analysis is conducted in the presence of higher-order modes in the laser beams of the FSOIs system and a fundamental Gaussian operating mode. The presence of the higher-order modes leads to degradation in the performance of the FSOIs system in terms of bandwidth density. In addition, we examine the impact of the signal-to-noise ratio (SNR) on the system's bandwidth density for each considered operating mode. The provided simulation results clearly demonstrate that coding significantly enhances the bandwidth density of the systems, with the extent of improvement being closely tied to the employed code rate and codeword length. c 2023 12387  
accommodate growing data requirements and adapt to evolving business needs without significant infrastructure overhauls [3; 4]. FSOIs are also known for their lower power consumption compared to traditional wired interconnects. By utilizing optical signals through free space, FSOIs significantly reduce the power requirements associated with data transmission. This translates to lower energy consumption and reduced operational costs; therefore, FSOI systems can be considered to be environmentally friendly and cost-effective solutions for data-intensive applications. Furthermore, the reduced power consumption contributes to improved energy efficiency and sustainability, aligning with organizations' green initiatives and overall environmental responsibility. Flexibility and mobility are additional advantages of FSOIs. These interconnects offer unparalleled flexibility in terms of positioning and alignment. Unlike physical cables, FSOIs can be easily adjusted, realigned, or repositioned; this provides greater flexibility in network configurations and layouts. This adaptability is particularly valuable in scenarios where dynamic network setups are required, such as in wireless communication between mobile devices, satellite communication, or unmanned aerial vehicles (UAVs). FSOIs enable efficient and reliable communication in environments where physical connections are impractical or challenging to implement. Enhanced security is yet another benefit of FSOIs. Due to the highly directional nature of the optical beam used in FSOIs, the chances of eavesdropping or interception are significantly reduced. The narrow beam confines the transmission within a specific path, minimizing the risk of data breaches or unauthorized access. This makes FSOIs an attractive choice for secure communication in sensitive environments, including military applications, government communications, and financial institutions, where data confidentiality is of utmost importance [5; 6]. Various technologies of optical interconnects--to include FSOIs, waveguides, and fibers, which can be compared across many aspects, as demonstrated in Table 1--can be assessed in terms of bandwidth density. That is identified as the time-space density product assessed at a fixed value of the operating power [7; 8]. A considerably large optical capacity can be realized upon using an array of parallel spatial channels of FSOIs [9; 10]. Therefore, the use of FSOI arrays has witnessed large growth in many technologies, including optical sensing, illumination, data communications, and laser printing [11; 12; 13; 14]. However, there are some limiting factors, including noise sources, that affect the performance of FSOIs. FSOI channels, like other wireless communication channels, are not immune to the detrimental effects of noise.