Integrated Free-Space Optics and Fiber Optic Network Performance Enhancement for Sustaining 5G High Capacity Communications

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Introduction
Looking ahead, it is evident that we are steadily going towards new technologies like the Internet of Everything, driverless vehicles, mixed reality, and ultra-high resolution video conferencing. However, the throughput, reliability, and latency requirements will have to meet increasingly demanding criteria due to these new uses. Due to recent advancements in sub-terahertz (sub-THz) technology, researchers are pushing the 5G new radio (NR) to a higher band, particularly the sub-THz spectrum stretching between 100 to 300 GHz, as mentioned in Figure 1 [1][2][3]. 5G NR connection in the sub-THz band is gaining popularity due to its high data rates. Te sub-THz range has the high frequencies required to greatly boost data speeds. Due to its high-frequency properties, the sub-THz band is well suited for many new 5G applications that require huge data speeds. Te 5G NR sub-THz communication's high-frequency characteristic allows for the delivery of large amounts of data over short distances [4,5,6]. Te wireless transmission range is however constrained by the substantial air loss transmission window of 5G NR sub-THz communication. 5G is suitable for densely populated areas because of its sub-THz communication constraints. Since 5G sub-THz connectivity cannot transmit a signal through long-range wireless transmission, it is not the best solution in sparsely populated areas. Given the growing growth of MMW communications, researchers have naturally explored for additional high-frequency bands, particularly the sub-terahertz (sub-THz) band, which is located above the millimeter-wave (MMW) band. Due to their vast bandwidth, the sub-THz spectrum can be utilised in a variety of developing applications that require high access data rates [7,8]. Due to its characteristics, MMW and sub-THz communications can provide high data speeds across short distances. Te limitations of 5G NR communications in the MMW and sub-THz bands make them better suited for short-range wireless transmission than long-range wireless transmission. A diferent kind of wireless transmission technique is called free-space optical (FSO) communication. FSO communication has drawn a lot of interest as a potential solution to the problem of short-range 5G wireless communications [8][9][10]. A promising approach to achieving the goals of high transmission capacity and longhaul transmission is to aim the transmission rates at many tens of Gb/s using an integrated fber optics and FSO-based 5G NR system for the simultaneous transmission of 5G MMW and sub-THz signals. Te integrated fber optic and FSO-enabled 5G NR system has the capabilities of high transmission capacity, high access data rates, long-haul transmissions, and vast service regions by combining long-reachfber-FSO convergence with short-range 5G wireless extension. Tis paper proposes and experimentally validates the integrated fber optics and FSO-based 5G NR model for the simultaneous transmission of 5G MMW and sub-THz signals. We simultaneously broadcast signals in the 125, 150, 175, and 200 GHz sub-THz bands using 64quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM).

Previous Work and
Background. Diferent algorithms and strategies to enhance FSO performance have been presented by numerous research organisations from all over the world. Te authors demonstrate recent advancements in FSO technology and the variables that afect the interpretation of the results in [11]. Te study in [12] assesses the efectiveness of the FSO system across a range of geographic regions. Digital signal processing (DSP) and orthogonal frequency division multiplexing (OFDM) techniques are used to reduce the channel-induced restrictions. Full duplex and all FSO transceivers were recommended by Li et al. [13], who also assessed the performance of the system. In [14], the quality factor and electrical power for FSO lines were explored, and simulation assessments were carried out using OptiSystem. In [15], the bit error rate (BER) of an intensity modulation and direct modulation (IM/DD) FSO link was examined. Te OFDM mode division multiplexing (OFDM-MDM)-based FSO transceiver was described in cite 10. Signal to-noise-ratio (SNR) and total power are used as the main measuring variables to quantify the dust efect. For FSO linkages, the efect of sandstorm conditions was examined in [16]. Te performance of the backhaul network, which was introduced in [17] for a 5G-based FSO system is assessed and contrasted with that of a traditional FSO link. Reference [18] looked into the function of the FSO framework in the nextgeneration satellite communication system. Te COVID-19 epidemic, on the other hand, has ofered an increase to online application and marketing services, overtaxing the already deployed FSO setups. To increase capacity and transmission accuracy, the combined optical network and FSO structure are presented in this study utilizing 5G NR and sub-THz based 64QAM-OFDM signals.
Te structure of this manuscript is as follows. Te analytical inquiry is presented in Section 2, the proposed experimental setup is described in Section 3, the experimental measurements and analysis are covered in Section 4, and the combined fber-FSO model is summarised in Section 5.

Theoretical Investigations of Proposed Fiber-FSO System
Te mixed FSO and fber link system is introduced in this paper, purposing to minimize the nonlinear issues and FSO related impairments like RF alignment issues, FSO pointing errors, and cochannel interference. Tis section includes the analytical calculations for the proposed FSO and optical systems. Te channel model of FSO is defned [14,15] as follows:  where the F ch presents the FSO channel, ψ a , is the atmospheric turbulence loss, ψ l is the geometric loss, and the ψ p is the FSO pointing issues. Tree components are considered for optical signal transmission [16]. (1) Line of sight component.
(2) Line of sight coupled with the scattered component. (3) Independent scattered component. Te power distribution function (pdf ) for free space is expressed [17][18][19][20] as follows: and B means where α m is the large scale scattering process, β m is the fading parameter, c is the independent scattering component, and φ(.) represents gamma function. Te p ′ is defned as follows: where the p is the power for the frst line of sight component and 2τb 0 is the coupled line of sight and scattered component. Te parameter a n is further explain [21] as follows: where β m is the fading element, α m is the efective number of large scale scattering process. Te FSO system performance is conditioned by the transceiver and structural ways; this leads to FSO pointing impairments, and it is calculated in terms of PFD [22,23] as follows: where A 0 is integrated optical power function, u is related to jitter deviation and equal to ω z /2σ 2 . Te width of the data carrying laser beam is denoted by ω z . Te statistical analysis of FSO pointing errors, turbulence fading, and cochannel interference is expressed [24,25] as follows: In equation (6) the f F ch /ψ a (F ch /ψ a ) declares the conditional probability. By substituting equation (1) to equation (5) in equation (6), the CDF of the N channel is defned as follows: where G 3,1 2,4 is the Meijer's G function. On the transmitter side, multipulse position modulation (MPPM)-based intensity modulation direct detection system is used for the FSO system. Te electrical flter is installed on the receiver side to remove unwanted signals from the original signals. Te output of the fltered signal is calculated as follows: Te average received optical power, R is the photodetector responsivity, k(t) is the additive white Gaussian (AWG), and C n is the signal time slot. Te transmitter and receiver telescope gains are expressed [18,26,27] as follows: G t is the transmitter gain, G r is the receiver gain, and d is the diameter. Te receiver signal to noise ratio (SNR) of the FSO system is estimated as follows: σ is the variance of channel noise, η is the efciency, Mod is the modulation order, and M is the number of transceivers. Te conditional probability error of the presented integrated optical network and FSO system is calculated as follows: where erfc is the error function. Te outage probability of the fading channel is calculated as

Experimental Setup
Tis section discusses the experimental background of the presented model, which is depicted in Figure 2. On the output side of the 5 m RF system, a mixer is connected to downconvert the sub-THz signals into a lower frequency range. In the next process, the downconverted signals are further purifed through low-noise amplifcation (LNA), which consists of a 3.8 dB noise fgure and an 18-40 GHz frequency range. In the fnal step, the 64QAM-OFDM demodulation is linked to the end side of the presented fber FSO system. Tis block contains all the key parameters, such as P/S and S/P converters, QAM demapper, CP removal, FFT, and ADC. Table 1 includes the list of elements applied for analyzing the performance the proposed fber-FSO system.

Experimental Measurements and Analysis
Te presented joint model of giber and FSO is evaluated experimentally in this section. Te features of the suggested model are extracted in graphical presentation and described, along with the performance of the 5G NR sub-THz-based fber-FSO system as compared to current approaches. Figure 3 introduces the results estimation regarding received optical power and BER, which compares the outlets among 5G NR sub-THz fber-FSO system and conventional fber-FSO system using clear and poor weathers conditions. It is clarifed that the efcacy of the presented 5G NR-based fber FSO framework is far better than the conventional model, even in poor weather conditions. If we investigate the difference among proposed model and traditional fber-FSO model in terms of BER points, the achievements of 5G NR based fber-FSO system in poor weather are 5 points reliable than the achievements of standard fber-FSO system even in clear weather situations. Te results of the performance of UTC-PD in terms of relative response (dB) and frequency (GHz) are discussed in Figure 4, showing that the relative response of the presented model decreases with increasing the frequency range. Te high-bandwidth UTC-PD has attractive performance at high-frequency response for a combined fber-FSO system. Te bandwidth above 250 GHz is considered suitable for  International Journal of Optics UTC-PD to act as an optical detector and convert the optical signal into electrical form at the receiver side. Furthermore, the proposed 5G NR based fber-FSO system calculations are investigated for diferent frequency ranges of sub-THz (125, 150, 175, and 200 GHz), as highlighted in Figure 5, applying 22 Gbps data rate speed with EVM and SNR measuring parameters. It is shown that by enhancing the range of 64QAM-OFDM signal frequency, the performance of the system is disturbed as compared to 125 and 150 GHz signal frequencies. Te EVM 14% range is observed by 125 GHzbased 64QAM-OFDM signal earlier (19 dB) than 150, 175, and 200 GHz signal frequency (21.2, 21.8, and 24 dB, respectively). Tis performance of the presented fber-FSO system was studied at 25 km SMF length, 500 m FSO volume, and 0.5 m RF cross-media range. In Figure 5, the experimental investigations are done for 20 and 25 GHz bandwidth and 100, 150, and 200 GHz 64QAM-OFDM signal frequencies at a 22 Gbps data rate. Te results show fruitful achievements at 25 GHz bandwidth; the EVM threshold line is touched at −30 dBm. On the other side, it is evaluated that with increasing the signal frequency range, the performance of fber-FSO is degraded because of high-capacity signals; however, at maximum output power, the position of fber-FSO can be balanced against high-data rate transmission, as shown in Figure 6. In addition, three types of 64QAM-OFDM signal frequencies (100, 150, and 200 GHz) are tested in Figure 6 at 22 Gbps, and 25 km SMF length, which describes the efcient outlets for all frequency ranges and bandwidths. Similarly, in Figures 7 and 8 the exhibitions of the 100 GHz signal-based channel of the fber-FSO system and the actions of the generated 64QAM-OFDM signals, respectively.
In fnal discussion of the experimental analysis, Figure 9 presents the constellation diagram measurements of the proposed 5G NR based fber-FSO system and conventional fber-FSO system, where Figure 9(a) declares the constellation analyzer of the input 64QAM-OFDM signals, Figure 9

Conclusion
Te 5G NR sub-THz-based integrated fber and FSO system is designed and evaluated for high capacity range transmission, including 500 m FSO and 0.5 RF mediums. Te presented model is carried out using mathematical structure using the valid and real-based parameters like input power range, signal frequency of 64QAM-OFDM, and received power and length of SMF. After the detailed discussion on the mathematical framework, the experimental analysis are then measured and compared with the conventional fber-FSO system. Te experimental results are studied using various signal frequency ranges like 125,150, 175, and 200 GHz, received power, relative response of UTC-PD, 500 FSO medium, and 0.5 m RF wireless medium. In correlation with the conventional fber-FSO system it is concluded that the results investigations that the presented model has effcient and reliable outlets even at high capacity channel transmissions. In future studies, we can add the updated machine learning model to further smooth the fow of signals and minimize high-order noises including phase and nonlinear impairments.

Data Availability
Te data are available in this paper.

Conflicts of Interest
Te authors declare that they have no conficts of interest.