Figure 1 - uploaded by Vahid Ahmadi
Content may be subject to copyright.
Source publication
Free-Space Optical (FSO) communication is reputed for its ability to proffer solution to the access network bottle-neck but when used over long range communication links, it suffers from scintillation caused by the atmospheric turbulence. The scintillation is modelled using the negative exponential distribution. Hereby, we present the outage probab...
Context in source publication
Context 1
... is considered because it does not need adaptive threshold to perform optimally (Popoola, Ghassemlooy and Leitgeb, 2007b), it is resilient to the irradiance fluctuation and can be easily demodulated coherently by using well evolved low phase noise RF oscillator ( Rongqing et al., 2002). Figure 1 shows the block diagram of SIM FSO system. Prior to modulating the laser irradiance, the sub- carrier signal is pre-modulated with the source data d(t) using PSK. ...
Similar publications
Free-space optical (FSO) communication is known for its various impairments such as atmospheric turbulence and misalignment fading. In this paper, we study the performance of an FSO link operating under combined path loss and atmospheric and misalignment fadings. We derive a closed-form expression for the bit error rate (BER) of the FSO link in suc...
The communication between two legitimate peers in the presence of an external eavesdropper is studied from a physical-layer security perspective in the context of free-space optical (FSO) communications. We discuss viable mechanisms to eavesdrop the communication and study the effect of random optical irradiance fluctuations inherent to FSO communi...
Due to the fact that the existing generation of wireless communication cannot possibly keep up with the current traffic explosion and emerging applications, research and development on next-generation (i.e., sixth generation, 6G) wireless technologies is being carried out worldwide. In this regard, it is anticipated that the space-air-ground (SAG)...
Free space optical (FSO) communications have the potential to be one of the most essential technologies for solving the high-bandwidth demands of communications between satellites and ground stations. In this study, we examine the impact of the atmosphere on satellite–ground FSO uplink and downlink communications. To consider diverse atmospheric co...
In recent years and in the ongoing development of all-optical systems, we propose two novel hard-switching based hybrid RF-FSO system architecture in the presence of atmospheric turbulence and pointing error. In the first proposed scheme, the hybrid Relay based RF-FSO link (path 1) is paralleled with a RF link (path 2). In the second scheme, the FS...
Citations
... If the optical radiation at the receiver is anticipated to be a plane wave, hence, the two parameters and which characterize the irradiance fluctuation PDF can be expressed in equations 7 and 8 respectively [15]. ...
Free-space optical (FSO) communication systems have received a lot of attention from academia and industries due to their low cost, high security and data rates for various wireless communication applications. However, factors such as fog and atmospheric turbulence degrade the performance of the FSO communication system. Therefore, this study presents an improved wavelength diversity based free space optical link using a single channel approach. The authors employed both Kim and Al-Naboulsi’s models to model the attenuation-induced FSO link. Furthermore, the Gamma-Gamma turbulent model was utilized to explain the turbulent nature of the FSO communication link. Experimental results showed improved performance in terms of Bit Error Rate (BER) and Quality factor of the FSO link up to a distance of 7km compared to the multiple channel approach that gives a signal output for distances less than 3km only. Hence, the impact of attenuation on the FSO link was reduced and the performance of the system was enhanced under fog and atmospheric turbulence conditions.
... The interaction between THz propagating wavelengths and turbulent medium generates random phase and amplitude variations of the received signal, which results in the performance degradation of the link. Consequently, atmospheric turbulence is a very complex stochastic effect which hinders the propagation of the information-bearing beam and causes the so-called scintillation effect which results, in turn, in random temporal and spatial fluctuations of the received signal intensity, analogous to fading in RF systems [2,20,40,61,62]. Considering therefore that this effect brings about continuous and very rapid fluctuations on the intensity of the signal that arrives at the receiver's side, it should be mainly investigated through statistical methods and processes. ...
In recent years, the THz frequency band (0.3 THz–10 THz) has attracted an increasing research interest for the realization of emerging high-speed wireless communication links. Nevertheless, the propagation of THz signals through the atmospheric channel is primarily subjected to signal attenuation due to free space path loss (FSPL), water vapor, adverse weather conditions along with atmospheric turbulence-induced and misalignment-induced scintillations. Therefore, in this work, a multi-hop line-of-sight THz system that utilizes serially connected decode-and-forward relays is proposed to extend the total THz coverage distance under the presence of fog, rain or clear weather conditions, as well as water vapor, atmospheric turbulence, non-zero boresight pointing errors and FSPL. Under these circumstances, an average bit error rate (ABER) analysis is performed. In this context, novel closed-form ABER expressions are derived. Their analytical results demonstrate the influence of each of the above limiting factors as well as their joint impact on the ABER performance. Finally, the feasibility of extending the total THz link distance through multi-hop relaying configurations is also evaluated.
... In the utterance accuracy measurement, grammatical errors are the standard used. There are three methods for measuring the accuracy of utterance, including the type frequency method, the verb form comparison method, and the proportion of error-free clauses [25,26]. ...
The purpose of this paper is to improve the English teaching levels of colleges and universities. First, the primary attributes of dynamic nonlinear system theory in second language acquisition are analyzed. Second, the influencing factors of English grammar application ability in theoretical dimension and measurement methods are proposed. Six first-year students majoring in English in a university were randomly selected as research objects, who were followed up for a semester, during which three identical oral English tasks were finished (in September, October, and November of 2020). The fluency, accuracy, and complexity of students' utterances are statistically analyzed using SPSS 26.0. Results demonstrate that the fluency of students' utterances has increased from 110 syllables per minute on average in September to 129 syllables per minute on average in November. The proportion of error-free clauses has increased from 0.5 to 0.55, and the number of clauses in the T unit has increased from 1.42 to 1.51. Students P1, P2, P3, P5, and P6 have a gradually increasing accuracy of utterance, while student P4 shows a gradually decreasing utterance accuracy. Students P1 and P5 have a decreasing utterance accuracy, while students P2 and P6 present an increasing utterance accuracy. Besides, the utterance accuracy of students P3 and P4 first decreases and then increases. The performances in utterance complexity of students P2, P3, P4, P5, and P6 have been increasing gradually, while the performance of student P1 has been decreasing gradually. The normal distribution of each student's test scores is different, as well. Hence, any minor changes will affect the overall oral English levels of students.
... During saturation atmospheric turbulence and beyond (strong turbulence), the negative exponential model has been found to be more suitable [12]. The fluctuation of the amplitude of the wave field traversing the turbulent medium has been experimentally verified to follow the Rayleigh distribution. ...
Free space optical (FSO) communication system is a wireless communication technology which possess advantages such as huge license free bandwidth and high data carrying capacity among others. It has proved to be a viable solution towards the radio frequency spectrum crunch and hence has been applied in cellular back haul, earth to satellite links, deep space and defense among others. However, FSO performance is adversely affected by atmospheric turbulence and weather conditions. Different channel models are used to estimate the performance of FSO communication system. Meanwhile, different mitigation techniques have been proposed and analyzed over the years for improvement of FSO communication. Of late, hybrid modulation schemes have proved to be another research direction. In this article, we review the FSO channel models as well as hybrid modulation schemes that have been recently introduced to further improve the FSO performance. The conditional probability of error for the commonly used FSO modulation schemes are presented. Finally, the expressions for the bit error for the hybrid modulation schemes are also given.
... There are various channel models proposed to express the randomly fading FSO links [4] to analyse the performance of the systems. For example, log-normal distribution [5] can be used to model weak turbulence levels, while the strong turbulence conditions can be characterized by the K-distribution [6] and negative exponential [7]. Similarly, Malaga channel [8,9] is another fading model which results in mathematically intractable analysis in many scenarios. ...
Generalized frequency division multiplexing (GFDM) can be a desirable option for free space optical communication systems since it has several advantages such as good spectral efficiency, low peak-to-average power ratio (PAPR) and low co-channel interference. In this paper, we present the performance improvement of GFDM over FSO links by employing maximum ratio transmission (MRT) technique. The asymptotic and closed form expressions for both symbol error probability and outage probability are derived for GFDM over gamma-gamma channel model. The theoretical results are verified with the Monte-Carlo simulations. Moreover, the impact of turbulence levels and weather conditions are also explored.
... When the link length is high, i.e., up to several kilometers, the number of independent scatterings becomes dominant. When the optical signal travels through this turbulent medium, the amplitude fluctuations can be assumed to follow the NE distribution [34]. Thus, the probability density function (PDF) of the NE-distributed channel gain is given by [10] f ...
In this paper, we study the effect of a beamforming scheme for a multiple-input-single-output (MISO) uplink free-space optical (FSO) communication system with nonorthogonal multiple access (NOMA) over negative exponential fading channels. 1-bit feedback about the channel state information is required by the proposed beamforming scheme. The feedback is considered to be error-free in this work; however, an error due to successive interference cancellation (SIC) is taken into consideration. It is inferred by bit error rate (BER) analysis that a higher weight given to the channel with higher channel gain and a slightly lower weight given to the channel with lower channel gain gives the best performance as compared to all arbitrary schemes in a $2 \times 1$ 2 × 1 FSO-NOMA system. Further, it is also analytically shown that using a 1-bit feedback-based beamforming scheme in an FSO-NOMA system suppresses the effect of SIC error; hence the BER performance of both the transmitters (TXs) is the same, which is not the case for a conventional NOMA scheme. A simplified asymptotic upper bound of BER of the proposed scheme is obtained by using the order statistics, and an optimized beamforming vector is found by minimizing this upper bound. It is then established analytically as well as through simulations that the beamforming vector is independent of average signal-to-noise ratio as long as the two channels remain independent and identically distributed. We further compare the proposed scheme with an FSO-NOMA system without feedback and beamforming and with single-input single-output FSO system using 4-ary pulse amplitude modulated signaling. It is shown by simulations that the proposed scheme outperforms both of them and has a huge coding gain advantage. A numerical analysis of this scheme is also provided for gamma-gamma (GG) and log-normal (LN) turbulence regime. The proposed scheme is extended to $3 \times 1$ 3 × 1 and $4 \times 1$ 4 × 1 MISO systems, and it is revealed that the performance of the system degrades as the number of TX increases.
... This has seen free space optical (FSO) communication systems as an alternative means of providing high bandwidth over short to medium range links. 1 Major motivation behind FSO has been the huge license free bandwidth, high transmission rate, increased security, and low installation as well as maintenance costs, among others. 2 Hence, it is rapidly becoming familiar in modern life. ...
... January 2020 • Vol. 59(1) ...
... It arises due to temperature and pressure variations leading to variation in the refractive index. Furthermore, since FSO is a line of sight communication system, misalignment between the transceivers tends to be another challenge towards achieving desirable quality of service provisioning [4]. ...
... From Table 3, we can see that spectrum slicing was used alongside with other modulation techniques such as OOK to improve transmission range of data over free space. However, most works conducted had limited data rate as; 1.56 Gbps in [24,25], 1.25 Gbps in [26], and 2.5 Gbps in [4,5] and 1 Gbps in [7]. Moreover, in [4,5], the geometric losses were not considered and much longer range was observed in [5]. ...
... However, most works conducted had limited data rate as; 1.56 Gbps in [24,25], 1.25 Gbps in [26], and 2.5 Gbps in [4,5] and 1 Gbps in [7]. Moreover, in [4,5], the geometric losses were not considered and much longer range was observed in [5]. The maximum transmission distance was 2.5 km in [24], 3.3 km in [25] and 1.1 km in [26], 4.75 km in [4] and 1 km in [7]. ...
Free space optical (FSO) communication has been a subject of interest for researcher and the corporate world since a few decades ago. This has been motivated by the huge benefits associated with this technology. Meanwhile, the generation and all optical up conversion of millimeter wave (mmw) for propagation in a FSO channel has been an important research direction. In this article, a single side band tone mmw generated by the optical carrier suppression (OCS) method has been modulated by modified duo-binary return to zero (MDRZ) and polarization shift keying (PolSK) modulated baseband data. The resulting optical signal was propagated in a FSO cannel subjected to combined weather and atmospheric turbulence induced fading. Later on, a comparative analysis was carried out between hybrid OCS/MDRZ and OCS/PolSK mmw signals propagated over free space. Using the performance metrics such as the bit error rate, quality (Q) factor and the received power among others, the simulation results indicated that hybrid OCS/MDRZ performs better than hybrid OCS/PolSK.
... Atmospheric turbulence affects optical signals along the propagation path which in return deteriorate the performance of FSO systems [5]. Although several statistical models such as log-normal distribution [6], [7], negative exponential distribution [8], K distribution [9], and Gamma-Gamma distribution [10] have been proposed to model the atmospheric turbulence induced fading effects in FSO communication systems, in recent years Málaga (M) distribution has gained increasing notice as a generalized turbulence model [11] covering many channel models including log-normal, Gamma-Gamma, and K turbulence channel models. ...
In this paper, we investigate the performance of free space optical (FSO) communication systems over imprecise Malaga (M) turbulence induced fading channel, which is the generalized form of the Gamma-Gamma and K turbulence channels as well. In the analysis we first derive the probability distribution function and cumulative distribution function of the channel's fading coefficient in the presence of channel estimation error. Then, we obtain the exact closed-form expressions of average bit error rate (BER), outage probability (OP), and ergodic channel capacity to quantify the performance of the considered system. Additionally, to provide further insights about the system performance in high signal-to-noise ratio (SNR) regime, the asymptotic expressions of BER and OP are obtained. Furthermore, analytic results are successfully validated with Monte-Carlo simulations. The results show that the imprecise channel estimation has detrimental effects on the performance of FSO communication systems over M turbulence channels, e.g., an estimation error of 5% results in approximately 8 dB SNR loss for the BER value of 5×10^(-2).
... Prominent amongst them are lognormal distribution, K-distribution and Gamma-Gamma distribution model. While log normal distribution provides accurate characterization of weak turbulence regime, the K-distribution model on other hand suits very strong turbulent conditions (Khalighi and Uysal 2014;Kaushal et al. 2017;Popoola et al. 2008;Tsiftsis et al. 2009). Gamma-Gamma channel model exhibit superior fit between analytical and experimental results and can be accurately used to model turbulence conditions ranging from weak to strong Khalighi and Uysal 2014;Kaushal et al. 2017). ...
Free space optical (FSO) communication systems have recently gained huge attention as possible last mile solution in delivering high speed data services for terrestrial applications. However the FSO link performance, particularly the link range is significantly limited by the severity of atmospheric adversities affecting the channel. In this paper we advocate the use of multi-hop relay techniques to transmit wavelength division multiplexed (WDM) signal over the FSO channel. Since weather induced impairments in FSO links are distance dependent phenomena, hence relay transmission allows substantial performance enhancement by alleviating channel losses while, WDM provides cost effective solution in improving the transmission capacity. With aggregate link losses as high as 40 dB/km, the proposed 32 channel—10 Gbps (320 Gbps) FSO link has been evaluated by comparing bit error rate (BER) performances and eye patterns over different turbulent regimes. Gain optimized EDFA amplification and conventional electrical amplification have been employed to realize amplify-and-forward (A–F) multi-hop transmission in the proposed link with the former delivering more inspiring BER performance of over the latter. Our simulation results indicate that for receiver SNR of 35 dB, BER improvement up to five orders of magnitude can achieved using triple relay FSO link in contrast to direct link operating under similar conditions. Additionally, it is also observed during the analysis that for target BER of 10⁻⁵, incorporation of triple relay enhances the link range by approximately 1200 m over direct link. However on the flip side, our investigations also revealed that as the number of relay nodes is increased, the SNR gain for specified BER does increase but the magnitude of gain declines. The proposed link was designed and investigated using OptiSystem™ 14.2.
