Coherent modulating retroreflector free-space optical system using self-interferometric compensation and self-coherent enhancement

The performance of a free space optical (FSO) communication system is significantly affected by various atmospheric turbulence conditions and pointing errors (PEs) apart from the additive noise, which is assumed to be Gaussian. Optical pre-amplifiers are an essential component of FSO systems for improving the receiver sensitivity. However, optical pre-amplification results in amplified spontaneous emission (ASE), which dominates the receiver thermal and shot noises. The square law photodetection process at the receiver in a FSO system necessitates the consideration of chi-square statistics for the decision variable contrary to the Gaussian approximation that is widely used in the literature. In this paper, we evaluate the bit error rate (BER) performance of a FSO system assuming non-return-to-zero on–off keying modulation in the presence of ASE noise under weak, moderate to strong, and very strong atmospheric turbulence regimes and PEs. We also derive asymptotic BER expressions for the considered FSO system for large values of the signal-to-noise ratio in terms of simple elementary functions. Further insight into the system is provided by performing a diversity analysis.

The use of optical spatial modulation (OSM), which has been recently emerged as a power and bandwidth efficient pulsed modulation technique for indoor optical wireless communication, is proposed as a simple, low-complexity means of achieving spatial diversity in coherent free space optical (FSO) communication systems. In doing so, this paper makes several novel contributions as follows. It presents a generic analytical framework for obtaining the Average Bit Error Probability (ABEP) of uncoded OSM with coherent detection in the presence of turbulence-induced fading. Although the framework is general enough to accommodate any type of models based on turbulence scattering, the focus in this paper is the H-K distribution. Although this distribution represents a very general scattering model valid over a wide range of atmospheric conditions, it is has not been considered in the past in conjunction with FSO systems possibly because of its mathematical complexity. The proposed analytical framework yields exact performance evaluation results for MIMO systems with two transmit- and an arbitrary number of receive apertures. In addition, tight upper bounds are derived for the error probability for OSM systems with an arbitrary number of transmit apertures as well as for convolutionally encoded signals. The performance of OSM is compared to that of well established coherent FSO schemes, employing spatial diversity at the transmitter or the receiver only. Specifically, it is shown that OSM can offer comparable performance with conventional coherent FSO schemes while outperforming the latter in terms of spectral efficiency and hardware complexity. Various numerical performance evaluation results are also presented and compared with equivalent results obtained by Monte Carlo simulations which verify the accuracy of the derived analytical expressions.

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

Nowadays, transmitting and receiving data with high speed and a high level of security are the main demands. So, a new model of spectral amplitude coding optical code division multiple access (SAC-OCDMA) is suggested in this paper, based on a free space optical (FSO) communication system using coherent sources. Three different codes: enhanced double weight (EDW), modified double weight (MDW) and multi-diagonal (MD) codes are assigned to our proposed model with the direct detection (DD) technique. Furthermore, the model is simulated under different weather conditions including clear air (CA), light mist (LM), very light fog, and light fog (LF). The system performance is evaluated through its bit error rate (BER), Q-factor, received power, and signal to noise ratio (SNR). Moreover, classification of the information received by the three different SAC-OCDMA using three different codes (EDW, MDW, and MD) is still challenging. So, two different machine learning (ML) algorithms are used, in this study, to classify the data received using the different codes. Detecting which code is received at the receiver end is important in order to reduce code error detection. Two algorithms: K-Nearest Neighbor (KNN) and Support Vector Machine (SVM) are adopted to classify different codes used for data transmission under four different weather conditions. The ML input dataset consists of the obtained simulation results, including Q-factor, BER, and SNR. Each feature is to be normalized before using ML. The obtained results show that the performance of the proposed FSO model achieved the longest propagation range under CA weather conditions, 7 km, while the shortest is under LF, which is 0.98 km. This is due to the attenuation of fog that causes signal degradation. The FSO system that uses EDW gives the best result under different weather conditions, while the system that uses MD code gives the worst performance. Also, the highest power is achieved when the EDW code is used at 5.5 km. The EDW has a received power of − 21.58 dBm, while the received power is − 22.04 dBm and − 23.8 dBm for MDW and MD codes, respectively. As for classification between the received information coming from three different codes under different weather conditions, both algorithms, KNN and SVM, achieve promising results in most cases. They showed more than 97% of classification accuracy under CA, LM, and LF weather conditions.

In this paper, various link characteristics and bit error rate (BER) performance of a free-space optical (FSO) multiple-input-single-output (MISO) system with repetition coding (RC) is investigated both experimentally and theoretically. The study is performed in the presence of atmospheric turbulence (AT) over a correlated channel with an erbium- doped fiber amplifier employed at the receiver (Rx). It is shown experimentally that the measured AT levels of two parallel links are different and the links experience non-identical fading influence. The analytical BER expression for an optically pre-amplified correlated FSOMISO system with non-identical AT channel is derived. The BER results of the considered system with pre- amplifier is compared with the BER results of an FSO system without pre-amplifier, and it is shown analytically that a 2 X 1 FSO system with pre-amplifier offers better BER performance in terms of coding gain of /spl approx/ 6 dB over the system without pre-amplifier. Moreover, it is also shown that the FSO system with RC gives better performance with identical links as compared to non- identical links. Further, it is also revealed that the diversity order of the considered system is independent of correlation and use of pre- amplifier. All the analytical results are validated by comparing them with Matlab simulations.

Relay-assisted cooperative free space optical (FSO) communication system is a powerful technique, which offers advantages like fading mitigation and significant gain in spatial diversity due to shorter transmission hops. Even so, it can be hampered by jamming activities which in turn degrade the overall system performance. In this paper, the effect of relay jamming in the decode-and-forward (DF) protocol based cooperative FSO communication system, is introduced and studied thoroughly. The probability density function (pdf) of a random variable containing mixture of the negative exponential and Gaussian random variables is derived. The error performance of the considered FSO system is analysed over this new kind of additive mixture noise for different probabilities of jamming. By utilizing the derived pdf, a closed-form expression of the average bit error rate (BER) of the DF FSO relaying is obtained. A comparison of the BER performance of maximum-likelihood (ML) detector and a derived threshold based detector is also provided. It is observed that the derived threshold based detector offers almost similar performance as that of the ML detector. A thorough numerical study is also provided by considering different pointing error parameters of jammer's and user's channels.

The integration of intelligent reflecting surfaces (IRS) into free-space optical (FSO) communication systems is considered as a promising and innovative approach to overcoming existing challenges due to the strict line-of-sight (LoS) requirements faced by conventional FSO setups, thereby ensuring more flexible and controllable links and expanding the communication coverage area. Nonetheless, the configuration of IRS-assisted FSO systems remains relatively novel, requiring investigation into optimizing IRS placement to enhance system performance while mitigating interference and obstacles in dynamic environmental conditions. This work aims to evaluate the improvement of the system performance achieved through optimized IRS positioning while accounting for the main degrading factors inherent in FSO systems and IRS design. We present a comprehensive model for a multilink terrestrial IRS-assisted FSO system, taking into consideration key degrading factors such as pointing errors stemming from both transmitter and IRS jitter, IRS dimensions, and turbulence-induced fading. Furthermore, we derive closed-form expressions for the probability density function of channel coefficients, average bit error rate, and outage probability. Our results indicate that optimal IRS placement is in proximity to the transmitter, taking into account all channel impairments. Furthermore, we showed that optimal IRS placement tends to shift towards the receiver side as the IRS jitter increases. Consequently, the analysis presented offers valuable insights for efficiently designing IRS-assisted FSO systems, particularly regarding IRS placement in the presence of pointing errors.

In this paper, we analyze the average bit error rate (BER) and outage probability performances of a free-space optical (FSO) system. We used M-ary amplitude shift keying modulation (M-ary ASK) scheme. Lognormal atmospheric turbulence with the effect of pointing errors (PE) and, path loss is employed to model the FSO channel. However, to overcome the degradation in BER performance due to pointing errors, the single-input-multiple-output technique with maximal ratio combining diversity is used. For instance, the average BER for M=2 without using SIMO is 1.944×10−6 at 0 dBm transmitted power, while the average BER for M=2 with using SIMO technique with N=2 become 1.512×10−11 at 0 dBm. The asymptotic expression for average BER and the closed-form expression for outage probability are both driven and tested with and without SIMO.

In this paper, the effect of jamming in free space optical (FSO) link, is evaluated by deriving closed-form expressions of the bit error rate (BER) and outage probability (OP) for single-input single-output (SISO) and multiple-input single-output (MISO) FSO systems. The effects of partial-band jamming and broadband jamming over the error performance of the considered FSO systems are also analyzed. The jammer behaves as a random noise source, following the negative exponential distribution-due to the atmospheric turbulence. Therefore, in the presence of jammer, the error performance of FSO systems is governed by the additive negative exponential noise. It is shown by a rigorous analysis that a MISO FSO system can significantly mitigate the effect of jamming. Specifically, we consider a $2\times 1$ FSO system for analysis, and demonstrate the improvement in BER and OP performance an FSO system can gain by using an additional spatial dimension, in the presence of a random jammer. The mitigation of jamming, due to implementation of arbitrary transmit apertures, is also verified through simulation results; which lies in complete agreement with the analytical results. Also, the error performance under the jamming effect is studied numerically over the Gamma-Gamma fading channel incorporated with pointing error effect.

The average bit error rate (ABER) performance of an orbital angular momentum (OAM) multiplexing-based free-space optical (FSO) system with multiple-input multiple-output (MIMO) architecture has been investigated over atmospheric turbulence considering channel estimation and space-time coding. The impact of different types of space-time coding, modulation orders, turbulence strengths, receive antenna numbers on the transmission performance of this OAM-FSO system is also taken into account. On the basis of the proposed system model, the analytical expressions of the received signals carried by the k-th OAM mode of the n-th receive antenna for the vertical bell labs layered space-time (V-Blast) and space-time block codes (STBC) are derived, respectively. With the help of channel estimator carrying out with least square (LS) algorithm, the zero-forcing criterion with ordered successive interference cancellation criterion (ZF-OSIC) equalizer of V-Blast scheme and Alamouti decoder of STBC scheme are adopted to mitigate the performance degradation induced by the atmospheric turbulence. The results show that the ABERs obtained by channel estimation have excellent agreement with those of turbulence phase screen simulations. The ABERs of this OAM multiplexing-based MIMO system deteriorate with the increase of turbulence strengths. And both V-Blast and STBC schemes can significantly improve the system performance by mitigating the distortions of atmospheric turbulence as well as additive white Gaussian noise (AWGN). In addition, the ABER performances of both space-time coding schemes can be further enhanced by increasing the number of receive antennas for the diversity gain and STBC outperforms V-Blast in this system for data recovery. This work is beneficial to the OAM FSO system design.

The high data rate, unlicensed spectrum, and secure communication make FSO system attractive to be used in next-generation wireless networks. However, the atmospheric effects like rain, snow, haze and fog severely degrade the Free Space Optics (FSO) system performance. Out of all weather condition fog heavily attenuates the optical signal. Therefore, before establishing FSO link a complete metrological survey of the area should be done. In this paper, we present a link availability analysis of two Indian cities namely Amritsar and New Delhi under fog weather condition. In literature, various techniques are mentioned that are used to enhance the reliability of FSO system under harsh weather conditions. Orthogonal Frequency Division Multiplexing is widely used in FSO. To achieve high data rate and high link availability under fog, we have designed a coherent detection Orthogonal Frequency Division Multiplexing with spatial diversity FSO system and calculate the link availability for two Indian cities. In literature, only theoretical estimation of link availability is given but in this research, a practical estimation of link availability is presented. At first, the theoretical estimation of link availability is presented on the basis of visibility data. The visibility data is simulated by OPTISYSTEM software and link performance is calculated on the basis of Bit Error Rate and Signal to Noise Ratio. The practical estimation of link availability is calculated and it is concluded that link availability of Amritsar is higher than that of New Delhi. The designed system significantly improves the link availability even under harsh weather conditions like fog.

A computationally efficient expression is presented for evaluating the average bit error rate (BER) of an intensity-modulation and direct-detection free-space optical system with on-off keying signaling technique operating in turbulent atmosphere described by the exponentiated Weibull distribution. The presented numerical results show the effects of aperture averaging on the average BER under weak and moderate turbulence conditions, and are confirmed by Monte Carlo simulations.

The salient features and advantages of free-space optical (FSO) system are very appealing for different applications in a number of communication network sectors. In spite of the advantages and diverse applications of FSO communication, its extensive use is hindered by the atmospheric turbulence-induced fading in real-life scenarios. Spatial diversity technique is one of effective means of mitigating turbulence-induced fading and, consequently, improves the system performance. In this paper, we study the spatial diversity schemes for mitigating turbulence-induced fading in the FSO communication systems using the bit error rate (BER) as a performance metric. The schemes considered are based on orthogonal space–time block codes and repetition codes (RCs). We derive simple approximate closed-form expressions for the error probability of the log-normal FSO links with intensity modulation and direct detection (IM/DD). Furthermore, we also investigate the effects of spatial correlation between the transmit apertures on the system performance. We achieve this using the exponential model for determining the correlations between the apertures. We observe that the proposed BER expressions are able to quantify the effects of spatial diversity schemes and spatial correlations on the system.

Free space optical (FSO) communication system is a groundbreaking technology in the field of communication systems. Non-orthogonal multiple access (NOMA), on the other hand, currently being used in fifth-generation (5G) wireless systems, is a powerful technique for establishing communication for multiple users using the same time and frequency resources. In this paper, the performances of NOMA and orthogonal multiple access (OMA) in an FSO-based communication system are studied. In the FSO communication system, two base station units are considered to be connected to a central unit for FSO backhauling using an uplink-fixed NOMA scheme. Bit error rate (BER), ergodic capacity, and energy efficiency (EE) performances of the NOMA-based FSO system are studied and compared with an OMA-based FSO system. It is found that the NOMA-based system provides approximately 10% of the ergodic capacity gain and increases EE by 37%-60% for a given BER compared to the OMA-based system.

Free-space optical (FSO) communication offers advantages such as high bandwidth and high transmission rates, making it suitable for 6G backhaul networks. However, FSO systems have drawbacks like sensitivity to fog, atmospheric turbulence, and pointing errors. Radio frequency (RF) links are incorporated to achieve more reliable and wider-range communication. RF links are susceptible to rain and have limited bandwidth but are not sensitive to fog or atmospheric turbulence. The complementary characteristics of RF and FSO systems make hybrid RF-FSO systems highly effective in mitigating environmental impairments. Reconfigurable intelligent surfaces (RISs) can also further extend the coverage and enhance the system bit error rate (BER) performance of RF-FSO. The state-of-the-art link selection schemes in RIS-assisted hybrid RF-FSO are normally based on channel state information (CSI). However, there is a chance that the CSI is inaccurate or even unavailable to the receiver. This paper investigates a hybrid RF-FSO communication system where CSI is unknown. In our system, the RF link is modeled using the Nakagami-m distribution, and the FSO link accounts for attenuation, Gamma-Gamma distributed turbulence, and pointing errors. The key concept of our system is the adoption of index modulation (IM) for link switching so that CSI is not needed at both transmitter and receiver, and we call it RIS-IM. Exact closed-form expressions for the signal-to-noise ratio (SNR) of the RIS-RF and RIS-FSO links are derived using multivariate Fox’s H-function without using central limit theorem, including the probability density function (PDF) and cumulative distribution function (CDF). We derive the closed-form expressions for the average BER (ABER) for RIS-IM using the SNR statistics. Furthermore, numerical analyses of RIS-IM are conducted. The numerical results of RIS-IM are given comparing with pure-IM, RIS-FSO, and hard switching as benchmarks. Mont-Carlo simulation results demonstrate the superiority of RIS-IM in terms of ABER. In the future, more analysis including outage probability (OP) and ergodic capacity (EC) await further explorations.