# Bandwidth Density Analysis of Coded Free-Space Optical Interconnects
## 2 Related Work  
Researchers and engineers have developed various techniques to mitigate noise in FSOI channels. These include lenses, advanced coding schemes, adaptive equalization techniques, spatial diversity, and optical beamforming methods. By employing these techniques, the impact of noise can be minimized, allowing for improved data transmission rates, increased link reliability, and enhanced overall system performance. For example,  
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the author of [15] optimized the signal-to-crosstalk ratio in an FSOI system that utilized microlenses. The author considered two scenarios for optimizing the signal-to-crosstalk ratio of the system. One scenario was for an aligned FSOI system, and the other was for a misaligned system. The conclusion was that the optimum signal-to-crosstalk ratio can be obtained by utilizing the detector size as an optimization parameter. In addition, the authors of [18] studied the performance improvement of optical interconnects in terms of data rate and spatial density due to using Reed-Solomon (RS) codes. They highlighted the significant role that coding plays in improving the capacity and misalignment tolerance of FSOI systems. However, the study overestimated the power amount of the crosstalk noise, which led to suboptimal results. The authors of [19] examined the utilization of error correction codes (ECCs), particularly Reed-Solomon codes, along with the fill factor of the receiver array on the bandwidth density. They provided numerical results that demonstrated the significance of using ECCs and the optimum fill factor value in order to enhance the bandwidth density. It was shown that different codeword lengths at different code rate levels provide different gain factors to the system's bandwidth density. Golay code deployment as a forward error correction scheme in optical interconnect systems is considered in [20]. The researchers demonstrated the feasibility of exploiting Golay codes to substitute for erasures, such as misalignment problems, a dead optical transmitter or receiver, or dark fibers, in terms of the bit error rate. In addition, the authors' investigation demonstrated that the use of forward error correction leads to decreasing the amount of optical power consumption in optical communication systems, which, in turn, decreases the requirements of launched power through the system. In [21], the authors discussed the effects of coding and fill factor on the bandwidth density of misaligned FSOI systems under the operation of the fundamental and higher-order transversal modes. The provided simulation and numerical results demonstrated that a significant improvement in the misalignment tolerance and the bandwidth density is achieved upon coding and optimizing the fill factor. The results demonstrated that there is an optimum code rate and an optimum fill factor value for each investigated codeword length. In addition, the authors showed that as the length of the codeword grew, the lateral misalignment tolerance increased. In [22], the authors exploited space-time block codes in FSOI systems to enhance their immunity against noise and misalignment. In particular, the authors expanded the application of space-time coding to a 4 \(\times\) 4 optical channel that employs on-off keying and utilizes real intensity-based signals. The purpose of designing these codes is to improve the system's performance by leveraging the optical crosstalk that occurs in a setup consisting of multiple laser sources and photodetectors. The encoding of data is achieved through the utilization of space-time codes that are derived from orthogonal designs. The encoded data are divided into four different streams, which are transmitted at the same time using four different transmitters operating at the same wavelength. Upon reception, the signal received by the optical receiver is a composite of all transmitted signals, accompanied by the presence of noise. Decision metrics are computed by utilizing the received signals and optical channel gains.