2.1 Concept and evolution of FSO

Fast data transfer rates, license-free operation, energy efficiency, adaptability, and a high level of security are all characteristics of FSO (Free Space Optical Communication). Due to the increased demand for high speed internet services, FSO systems without license may be a feasible alternative to currently available data communications technologies and to bandwidth intensive wireless technologies. Unlike cable communication, FSO modulates data using a narrow laser beam travelling through the atmosphere or free space, and the protocol is made suitable for terrestrial FSO communications. Because of the intricacy of the activity and composition of the atmosphere, the channel has a significant effect on transmitting the laser light contained within. Due to molecule and aerosol absorption and scattering of atmosphere, as well as scintillation due to atmospheric turbulence, the BER of the communication system is significantly larger than that of wired communication systems. Fundamentally, the disadvantage of FSO technology is that it is very dependent on a variety of weather occurrences, limiting the availability of FSO lines.

Jeon et al. ^[1] demonstrates FPGA based real time high resolution transmission of video signal through the emulator of FSO channel, that accurately moulds the FSO channel particularities like scintillation, turbulence, and power attenuation. The bit rate is 200 kbps and 48 kbps respectively, and the sample rate is 22,050 Hz for video and audio. For future work, high turbulence distortion removal scenarios can be concentrated and methods can be made using deep learning techniques. Poliak and Kubicek ^[2] summarizes the applications and fundamental principles of free space optical links. Additionally, a demonstration of experimental FSO setup is also discussed. Its primary objective is to illustrate the value of FSO as a solution to last mile interconnect problem, particularly for end users. The primary disadvantage of FSO technology is its reliance on various effects of the atmosphere. This is partially overcome practically through the combination of transmitters and receivers (TX/RX), link margin, special coding techniques, or the use of a blended RF/FSO link (backup microwave link).

Gu et al. ^[3] investigated and assessed the capability of Gardner's timing error detector (TED) and the feedback timing recovery algorithm (TRA) for FSO signals, which put forward a multiplication free blind TED method with constant output characteristics and lower computational complexity. The performance of TRAs is contingent upon the accuracy of the timing error detector (TED). As a result, simulations can be performed using a variety of TED schemes in future. Htay et al. ^[4] tested a real time free space optical link of 10 Gbps under various conditions of atmosphere. To validate the experiment, tiny form-factor pluggable transceivers placed on an FPGA were employed as an FSO transmitter and receiver. Under turbulence and fog circumstances, the performance of a high-speed single FSO connection is evaluated using a specialized indoor atmospheric laboratory. This task can be completed and evaluated with a real-time outdoor context in future. Huang et al. ^[5] presents a hybrid optical wireless network interconnections based on heterogeneous FSO / VLC for integrated space air ground ocean (SAGO) communication architectures. They are to be used in future particularly in security required or radio frequency sensitive environments. Future work will focus on the enhancement and evaluation of network-layer mechanisms, as well as the development of experimental platforms such as acquisition-pointing-tracking (APT) modules. A novel transmitting - receiving protocol is proposed by Guo et al. ^[6] for a communication system based on FSO and is discussed in terms of both the receiving and transmitting protocols. To reduce system cost and enhance system integrity, the transceiving protocol can be programmed into one FPGA chip. New protocols will be capable of operating at higher bit rates in future.

Touati et al. ^[7] were the first to examine a hybrid free space optics/radio frequency (FSO/RF) transmission system for the atmospheric turbulences effects. Due to the sensitivity of FSO technology to atmospheric turbulences, this study demonstrates the technology's operational capabilities while promoting a better understanding of the technology. Measures should be taken during the installation of the FSO link to mitigate the factors such as high temperature and solar irradiance in the future. Lu, et al. ^[8] introduces field illustration of a 1.87 kilometer real time Raptor coded FSO communication urban link. This work represents an important first step toward implementing Raptor coded hybrid RF / FSO links using FPGAs. Advance research will concentrate on matrix inversion algorithm optimizations, as it utilizes a significant amount of hardware resources. A compact Free Space Optical Communication System is introduced by Wang et al. ^[9], complete with hardware and a physical layer protocol specified by the hardware. The rapid synchronization method is demonstrated in detail to elaborate how it can be used to reduce the required handshake bandwidth. Over time, various coding schemes have evolved to allow for greater control of atmospheric disturbances. An optical communications system utilizing offline digital signal processing and intradyne reception over a 10.45 km link through the atmosphere, is evaluated by Conroy et al. in ^[10]. The transmission of 40 GBauds in the C band is elaborated and it is compared to measurements in lab. Future optical coherent GEO (Geostationary Equatorial Orbit) feeder links communications systems will require significant optimization to support the faster data rates at which they work. Intradyne reception is highly capable for future communications because of technology's resistance to turbulence. Modulation formats like QPSK which is a higher order format may improve the data throughput-to-size ratio. Millimeter waves, when combined with FSO, provide an excellent performance and ease of deployment for radio over free space optics (Ro FSO) systems. The purpose of the work by Murugan et al. ^[11] is to analyze the effectiveness of a proposed system in fog weather conditions. The proposed system is capable to transmit data at a rate of 10 Gbps and consists of four independent channels. These channels are transmitted over a 40 GHz radio signal at a 1000 meter free space distance. For 5G applications, the proposed system can be enhanced with Dense Wavelength Division Multiplexing, UDWDM, and other modulation schemes in conjunction with the Ro-FSO system. The purpose of the research by Kiasaleh ^[12] was an APD based FSO communications affected by turbulent state of the channel. This paper addresses the issue of detection of optimum signal in channel aided FSO communications. Since channel assisted receivers will have wider error floor free signal ranges than ordinary receivers, they can be employed instead of standard receivers. In the optical domain, the all optical amplify and forward (OAF) based technique can filter and amplify attenuated optical signals, and thus distance of transmission of FSO communication systems can be increased. For an OAF assisted dual hop system, the RSL (receiving sensitivity loss) of the system was studied by Huang et al. in ^[13] using emulations and investigations on FSO systems with on-off keying (OOK) and differential phase shift keying (DPSK) signals. The OAF relaying node can be put between two equal distances in the OOK dual-hop system, with the reception sensitivity remaining essentially unchanged. To improve semiconductor optical amplifier (SOA) based scintillation alleviation, the study by Hong et al. ^[14] suggests using polarization shift on-off keying (PS-OOK) transmission in FSO communication. In a vertical FSO link, the scintillation effect is crucial. This technology has a lot of potential for future vertical FSO communication systems, and the performance of RAMAN and EDFA amplifiers can be examined for better performances.

Communication systems using FSO are prone to atmospheric disturbances such as fog, rain, scintillation, turbulence, beam wander and scattering. These effects can be mitigated in practice by utilizing link margin, combining transmitters and receivers (TX/RX), using unique coding techniques such as raptor coding, or utilizing a backup microwave link (hybrid RF/FSO link). In the future, experimental platforms such as acquisition-pointing-tracking (APT) modules and adaptive modulation and coding techniques can be developed, as well as performance measurements in a variety of turbulent situations can be done. Using Raptor codes to join RF and FSO channels is a promising option for boosting the reliability of such communication lines. Table 1 shows the comparison of data rates and transmission distance of different FSO communication systems.

Fig. 1 below shows the progress of transmission distance and data transmission rate using various FSO techniques.

Fig. 1. The enhancement of transmission distance and data transmission rate using FSO techniques.

Table 1. Comparison of FSO based data transmission rate and transmission distance results.

2.2 Various Modulation techniques and FPGA based Optical Communication Systems

Since the inception of VLSI technology, researchers have been attempting to reduce the power required, optimize the area, and obtain a shorter propagation delay through the use of diverse algorithms. Line coding is critical in communication systems because it converts 1s and 0s to a sequence of voltage and current pulses that may be communicated through a physical media such as coaxial cable or optical fiber. Typically, a digital baseband signal is used to specify the spectral properties of a pulse train. NRZ and RZ are the most commonly used line codes. They are all encoded in either unipolar or polar format. Digital modulation is critical in modern communications because it is simpler, more secure, and more efficient than analogue modulation at large distances. Numerous papers in the literature discuss applications of various digital modulation techniques, such as BASK, BPSK, BFSK, OFDM and detectors, transmitters, and baseband processors that use BPSK modulators, digital receivers that use QPSK or QAM.

In ^[15] by Zhang et al., the differential phase shift keying (DPSK) signal is generated in the following sequence; first generating PRBS23 data, then encoding this random data differentially, up sampling this data, band limiting the data using a raised cosine filter, and multiplying this band limited data with sine wave and DPSK signal is finally attained. For more efficient communication systems, LEDs in this can be replaced by strong LASER sources. The focus of the study of He et al. ^[16] is in data center networks; switching, modulation, coding schemes, add-drop multiplexer, OFDM, cross-layer design, system analysis, detection techniques, control and management, free space optics, and optics. The main reason for this study is to refresh and extend understanding of recent breakthroughs in optical communications, as well as to stimulate more research and deployment of new technologies in networks. For point to point data transfer between PCs, a software/ hardware model of a visible light communication system based on System on Chip that make use of DCO OFDM modulation is presented by Adiono and Putra in ^[17]. The developed system utilizes an ARM microprocessor in conjunction with an FPGA. Faster data transfer is enabled by DMA and data buffering between processing elements.

Safi and Bazuin ^[18] discusses FPGA based novel techniques for realizing various types of ASK (Amplitude Shift Keying) and QAM (Quadrature Amplitude Modulation) signal modulators. In view of this, other digital modulators like higher order QAM and 8PSK, 16PSK etc. can be implemented in the future. Once successful implementations are achieved, a whole set of modules can be housed on a single FPGA. Singh and Mishra ^[19] details the development of several line coding methods using VHDL on the Xilinx FPGA platform for the aim of security and area optimization, as well as the ability to provide efficient digital communication in a variety of channel environments. The line code to be used is determined by the presence or absence of a DC level, the power spectral density, the required bandwidth, the BER (Bit Error Rate), the simplicity by which the clock signal may be recovered, and the presence or absence of an intrinsic error detecting property. The designed architecture is simple to implement and efficient for all encoding schemes used in digital communication for the transmission of baseband signals. Yu et al. ^[20] primarily discusses a high-speed hardware architecture for an OFDM (Orthogonal Frequency Division Multiplexing) transmitter using an FPGA. As a result, this article focuses on an FPGA based high speed OFDM transmitter that transmits 3.5 Gbps OOFDM (Optical OFDM) signals via regular SMF (Single Mode Fiber). Additionally, the modulation format can be increased to 128QAM if the frequency offset can be contained more precisely by out-of-band signaling or an upper layer protocol. The authors demonstrate implementations of FPGA based BASK, BFSK, and BPSK digital modulators in ^[21]. The proposed designs are intended to be used educationally in a course on digital communication by amalgamation with additional useful components of the Altera Development and Education (DE2) FPGA board, which use the fewest possible blocks to achieve BASK, BFSK, and BPSK modulation. Also, in software defined radio systems, the digital baseband modulation part of it is based on FPGA designs. Over the last decade, in faster optical systems, Quasi Cyclic Low Density Parity Check (QC-LDPC) codes have been identified as the most feasible solution for soft decision FEC (SD Forward Error Correction) codes. Chang et al. ^[22] provides the results of emulation of a single QC-LDPC code with 20% redundancy based on FPGA for use in 100 Gbps optical transmission systems. Flawless transmission is possible at BERs of 10${}^{-15\ }$with a Q factor of 5.9dB. The proposed coding scheme's implementation complexity is cut in half and non concatenated QC-LDPC code is regarded to be a viable alternative for 100 Gbps systems. Djordjevic and Zou ^[23] describes a rate adaptive Forward Error Correction method based on LDPC codes and its software configurable integrated FPGA architecture. Non binary LDPC codes can be used to bridge the chasm between different bits in higher order modulation formats. FPGA based rate adaptive QC-LDPC codes along with a variety of modulation techniques are one of the most suitable configurations for the coming generation optical communication systems. Prakash and Sadashivappa ^[24] discusses the intricacies of DWDM networks, their design characteristics, and the need for enhancing the system's capacity. This paper analyzed several approaches for mitigating data loss during optical fiber transmission. Additionally, it discusses how to calculate the Quality factor, eye height, and BER which are the measures of the system performance. Thus, this article gives an outline of methods for increasing the data carrying capacity of DWDM (Dense Wavelength Division Multiplexing) systems used for long-haul transmission.

FPGAs can be used to implement the role of fiber and terminal equipment; compensating techniques other than fiber bragg grating (FBG) and dispersion compensation fiber (DCF) can be used to decrease dispersion; and appropriate modulation formats such as OFDM can be used to withstand nonlinearity during long haul transmission. To enhance security of physical layer of OFDM- PON (Passive Optical Network), the work by Zong et al. ^[25] proposes and demonstrates a real time protected transmission system employing chaotic data encryption. The FPGA boards are used to encrypt and decrypt OFDM signals at OLT (optical line terminal) and ONU (optical network unit). OFDM transmission based on digital chaos which is implemented in real time will be an ideal contender for the future generation secure PON. Design and development of a first time Volterra DFE (parallel pipelined Volterra decision feedback equalizer) hardware structure for four level pulse amplitude modulation transmission is discussed in ^[26] by Chen et al. The developed FPGA architecture is prospected for resource consumption, and its throughput is demonstrated to exceed 24 Gb/s, which is critical for real time optical communication systems. It is possible to attempt to resolve the issue outlined above by utilizing dispersion compensating fibers. A simple IMDD (Intensity Modulation and Direct Detection) system applying a DFB (directly modulated distributed feedback) laser attains the highest speed ever 11.25 Gbps real time end to end OOFDM transmission for the first time with a high electrical spectral efficiency over 500m multimode fibers is described by Giddings et al. ^[27]. Real time experimental investigations can be conducted to inspect the possibility of the OOFDM scheme for enhancing MMF (Multi Mode Fibers) based LANs capacity greater than 20Gbps. A real time base band OFDM transceiver with channel estimation scheme and a basic symbol synchronization on a single off the shelf commercial FPGA is set up by Chen et al. ^[28]. By employing an economical DFB laser, experiment possess successfully proven a real time OOFDM long reach PON system with direct detection using a SSMF across 100 km. The power penalty of this transmission was only 1.1 dB at a BER of $1\times {\rm e}^{-3}$. The power penalty and BER could be improved even more. Mohamed et al. ^[29] presented the constitution and execution of a baseband OFDM receiver - transmitter using FPGA hardware. This paper discusses what an OFDM system is, how it is implemented, and how the simulation results are analyzed. This OFDM system is capable of implementing a variety of M-QAM (BPSK, 4 QAM, 8 QAM, 16 QAM, and 64 QAM) modulation methods. They are used in the multicarrier OFDM transmitter/receiver subsystem.

Sousa et al. ^[30] describes experimental work on hybrid coherent ultra dense WDM (UDWDM)-PON FSO system that is long reach, gigabit capable and aiding several applications running on the same fiber network in MBH (mobile backhaul) networks. Additionally, for the first time, dual polarization QPSK (DPQPSK) signals is illustrated over standard SMF (SSMF) and FSO links in real time reconfigurable DSP transmission/reception. To obtain a sufficient data rate for 5G mobile wireless networks, it enables the future generation coherent optical access networks.

Nanou et al. ^[31] examines the application of a MIMO Volterra decision feedback (VDFE) equalizer in the backdrop of a direct detection (DD) optical communication channel using NRZ DQPSK. Additionally, a simplified VDFE is launched, and the performance of both the schemes are compared using simulations. Modern FPGAs are an ideal platform for developing such high quality and complex equalizers with high outturns. A field example of 100 Gbps real time coherent optical OFDM (CO-OFDM) detection using polarization division multiplexed QPSK modulation is reported by Kaneda et al. ^[32]. For such a CO-OFDM back to back transmission experiment, this paper details one of the smallest BER at $5\times 10^{-9}$, as well as the successful transmission of 495.2 kilometers over a mix of fiber types that are field installed. The field test is carried out in a dispersion controlled system of WDM with adjacent 10 Gbps ethernet links. Future systems may benefit from other bit loading techniques, higher order modulation, higher resolution ADCs and as well as real time signal processing transceivers. Emeretlis et al. ^[33] proposes an IMDD optical communication link of 40Gbps utilizing NRZ DQPSK signaling and FPGA implementation of a DFE for electronic compensation of impairments. The proposed equalizer is used to compensate electronically the residual chromatic dispersion (CD) across the installed optically compensated paths. Modern FPGA devices are an appropriate technology for such electronic equalization techniques which need high data processing rates and computationally intensive optical communication systems. On FPGA hardware, Mecwan and Shah ^[34] explains the execution and constitution of a baseband OFDM receiver and transmitter. Modules for mapping and de-mapping 64-QAM data, as well as parallel-to-serial and serial-to-parallel converters, are included in the work. Additionally, the design incorporates inverse fast Fourier transform (IFFT) and 64 point FFT as modules of primary processing.

For the first time, Zhang et al. ^[35] presents an innovative energy efficient clock gating based technology for selective downlink reception of frames of OFDM in ONUs in OFDM PONs. BUFGCE is a module used for easy clock gating in the proposed scheme. Based on the identification of frame outputs it controls the clock of a demodulation module. This technique can also be simply collaborated with other low-complexity FPGA DSP energy saving solutions. Li et al. ^[36] researched on a 100 GS/s fourth order single bit sigma-delta modulator (SDM) in real time high frequency band without using the analog or optical up conversion for an all digital radio over fiber (RoF) transmission. This is the first and the fastest real time demonstration of sigma-delta modulated RoF above 24 GHz reported so far. Additionally, this all-digital SD RoF transmission enables the development of more advanced wireless concepts. Since the same clock source is shared by each sigma delta modulated serial stream, multiple streams can be generated centrally. This certainty can be exploited to enable digital beam forming, reduce inter cell interference, and even enable distributed MIMO. Cannon et al. ^[37] came up with a photonic crystal fiber with concurrent polarization insensitive phase transmultiplexing and multicasting of a single 10 Gbaud carrier suppressed RZ on off keying (CSRZ OOK) signal and four 10 Gbaud RZ BPSK signals using cross phase modulation and birefringence. Future technologies are used to generate a variety of high-performance QPSK systems with a variety of modulation formats.

Silveira et al. ^[38] theoretically evaluates various optical modulation patterns under three transmission constraints: self-phase modulation, chromatic dispersion, and spectral narrowing caused by filter cascading and operating at 40 Gbps. Additionally, this article compares the performance of various patterns at 40 Gbps and at a standard BER of $10^{-12}$ in terms of the optical signal to noise ratio (OSNR) penalty obtained after going through ten optical filters in succession and a standard fiber. Software defined optical transmission (SDOT) is a concept that utilizes reassembly of optical transponders that can cope up with modulation formats and/or multiple line rates, similar to software defined radio (SDR). Optical network capacity expansion can be done after governing the estimate required for each format. Yao et al. ^[39] investigates and compares a recently proposed receiver side duobinary shaping Nyquist wavelength division multiplexing (RS-DBS Nyquist WDM) technique. It is thought as a favorable candidate for the next generation of long distance, greater data rate, large capacity, optical transmission systems due to its economical nature. This study can be extended to other techniques such as WDM and multiple modulation techniques. Jia et al. ^[40] suggest and speculatively validates the creation of a RZ-DPSK at 10Gbps using a directly modulated chirp managed laser (CML). The CML's driving signal is in two level inverse return to zero (IRZ) format rather than a three level RZ format. This investigation can be expanded to include additional laser sources and optical phase modulators. Chi and Lin researched the RZ transmission of data and pulse quality performance of two of the most accepted optical pulse carriers ^[41] created by the self-started OEO are measured and compared. The two different sets of RZ pulse carrier producers are a distributed feedback laser diode (DFBLD) that is either externally modulated via a nonlinearly biased electro absorption modulator (EAM) or gain switched directly with a microwave power amplifier. Using other carrier modulation methods, similar comparisons can be made and the findings can be analyzed. In a greatly nonlinear single fiber, using bidirectional cross phase modulation, the work by Wen and Feng ^[42] suggests and experimentally demonstrates the conversion method of an all optical modulation format. Successful transformation to a polarization division multiplexed RZ QPSK signal of 50 Gbps from four 12.5 Gbps NRZ OOK signals are done with a marginal power penalty in this experiment. The proposed converter's practicability and performance are studied analytically and numerically. For all optical future communication systems, the proposed converter format could be used at nodes in PON networks. The research by Li et al. ^[43] presents an FPGA based communication strategy for the traditional channel of a faster quantum key distribution (QKD) system. Here the channel is fiber, and a lightweight protocol for communication that is optimized for the QKD post processing system (QKDPPS) is developed. For a faster QKD system, the results reveal that this technique can maintain a consistently reliable communication at 1.66 Gbps for the classical channel for a very long time period and it is possible to increase the transmission distance further in future. Other strategies can be used to enhance the BER, OSNR, and distance of transmission. Real time detection of a polarization multiplexed square 16 QAM signal at 40 Gbps is evidenced in ^[44] by Pfau et al. using an intradyne coherent receiver based on an FPGA. For a data rate of 40 Gbps, the symbol rate attained is 5 Gbaud. The minimal BER of $3.3\times10^{-5}$ in back-to-back 40 Gbps transmission data rate is achieved.

This study shows that OFDM based PONs may provide allocation of dynamic bandwidth in both frequency and time domains. On the data carrying OFDM symbol sub carriers, applying QAM like modulation formats of high level is one of the least expensive methods to improve transmission capacity and reduce bandwidth requirements for electronic components at both transmitter and receiver.