Free-space Optical Wireless Research Articles

A long-reach radio over free space optics (Ro-FSO) system using hybrid orthogonal frequency division multiplexing (OFDM)-multibeam concept with enhanced detection

Abstract This paper focuses on designing of a 10 Gbit/s-10GHz hybrid Orthogonal frequency division multiplexing (FDM) based Radio over Free Space Optics (Ro-FSO) transmission link using optical single sideband modulation format and its performance has been compared using a different number of transmission beams. The proposed link has been simulated and compared using 1-beam, 2-beam, and 4-beam in the system. We show that by using the 4-beam system in the OFDM-Ro-FSO link, a 3000 m range has been achieved reliably, which a notable improvement when compared to previous work is. Also, we show an improved performance of the system by using an enhanced detection mechanism using a Square root module (SRm) at the receiver side.

Enhancing high-speed networks using RGB-based WLAN through Ro-FSO integration in the 5 GHz band

Abstract The limited bandwidth constraints imposed by conventional wireless carriers pose a significant hurdle when it comes to the delivery of high-speed broadband services. In response to this challenge, Radio over Free Space Optics (Ro-FSO) has emerged as a viable and innovative solution, seamlessly amalgamating wireless and optical systems. This integration proves particularly invaluable in sensitive environments, such as hospitals, where the risk of electromagnetic interference disrupting critical medical equipment is a real concern. Ro-FSO offers a disruption-free avenue for high-speed data transmission, positioning it as the ideal choice for broadband services, including Wireless Local Area Networks (WLANs). Within the scope of this study, we introduce a high-speed Ro-FSO link, showcasing the capability to concurrently transmit three independent channels with Red, Green and Blue (RGB) laser respectively, each supporting a robust 1 Gbps data rate. These data streams are skilfully up-converted to the 5 GHz RF bands, encompassing transmission distances of 650 m across the FSO channel. Our numerical simulation findings underscore the successful transmission of all the channels using wavelength division multiplexing (WDM), seamlessly meeting the prescribed Bit Error Rate (BER) and eye pattern criteria, solidifying the Ro-FSO’s standing as a promising solution for high-speed broadband delivery.

Hybrid RoF-RoFSO system for broadband services by incorporating polarization division multiplexing scheme

Abstract The limited bandwidth of traditional wireless carriers presents a challenge for delivering high-speed broadband services. To address this, radio-over-free space optics (Ro-FSO) emerges as a viable solution, seamlessly integrating wireless and optical systems. This integration is particularly valuable in sensitive environments such as hospitals, where electromagnetic interference can disrupt medical equipment. Ro-FSO provides interference-free high-speed data transmission, making it an ideal choice for broadband services, including WLANs. This study presents a high-speed Ro-FSO link capable of simultaneously transmitting two independent channels, each carrying 1 Gbps data up-converted to the 2.4 GHz and 5 GHz RF bands over a 5 km SMF and 4200 m FSO channel. The incorporation of polarization division multiplexing enhances the spectral efficiency of the Ro-FSO link. Our findings demonstrate the successful transmission of both channels meeting the required bit error rate (BER) and eye pattern criteria.

Experimental demonstration of a free space optical wireless video transmission system based on image compression sensing algorithm

The wireless transmission of video data mainly entails addressing the massive video stream data and ensuring the quality of image frame transmission. To reduce the amount of data and ensure an optimal data transmission rate and quality, we propose a free-space optical video transmission system that applies compressed sensing (CS) algorithms to wireless optical communication systems. Based on the Artix-7 series field programmable gate array (FPGA) chip, we completed the hardware design of the optical wireless video transceiver board; the CS image is transmitted online to the FPGA through Gigabit Ethernet, and the video data is encoded by gigabit transceiver with low power (GTP) and converted into an optical signal, which is relayed to the atmospheric turbulence simulation channel through an attenuator and a collimating mirror. After the optical signal is decoded by photoelectric conversion at the receiving end, the Camera-Link frame grabber is d; thus, the image is collected, and it is reconstructed offline. Herein, the link transmission conditions of different algorithm sampling rates, optical power at the receiving end, and atmospheric coherence length are measured. The experimental results indicate that the encrypt-then-compress (ETC) type algorithm exhibits a more optimal image compression transmission reconstruction performance, and that the 2D compressed sensing (2DCS) algorithm exhibits superior performance. Under the condition that the optical power satisfies the link connectivity, the PSNR value of the reconstructed image is 3–7 dB higher than that of the comparison algorithm. In a strong atmosphere turbulence environment, the peak signal-to-noise ratio (PSNR) of the corresponding reconstructed image under different transmission rates at the receiving end can still exceed 30 dB, ensuring the complete reconstruction of the image.

Diffraction-free distance enhancement of Bessel beams based on spatial domain phase modulation

Bessel beams have garnered significant interest due to their unique diffraction-free properties and extensive potential applications. In this work, we propose a spatial domain phase modulation theory to achieve diffraction-free distance enhancement of Bessel beams, overcoming the limitation of the traditional methods due to the inability to infinitely decrease the wave vector angle. The traditional formula for non-diffraction distance is also modified. Simulation results demonstrate that our proposed scheme can significantly increase the maximum diffraction-free distance of zero-order and higher-order Bessel beams by more than two times, while ensuring the self-healing property of Bessel beams. Furthermore, our proposed scheme is not restricted to specific systems or limited to the optical wavelength range. This implies that the results have great applicative potential in long-distance free-space optical communication and wireless energy transmission.

Performance analysis of a RIS-assisted RoFSO communication system over Malaga distribution for smart city applications.

Radio over free space optics (RoFSO) is one of the potential technologies that can satisfy the requirements of 5G services in a smart city. However, as RoFSO is line-of-sight (LOS) communication, one of its limitations is the occurrence of a skip zone in the targeted areas. In this work, a reconfigurable intelligent surface (RIS) is proposed as the solution to overcome this connection difficulty, which prevents signal blocking by generating LOS connections. These RIS modules extend the communication channel coverage, making it more intelligent and controllable. The performance analysis based on outage probability, ergodic channel capacity, and bit error rate has been performed using heterodyne detection. Malaga distribution has been used to model atmospheric turbulence. The exact closed-form expressions of the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio are derived. Exploiting these derived statistics, system performance is investigated through the ergodic channel capacity, outage probability, and average bit error rate for M-ary quadrature amplitude modulation and two binary modulation schemes: non-coherent binary frequency-shift keying and coherent binary phase-shift keying. Numerical results are compared among different turbulence conditions, link lengths, and scattering errors. The results show that the proposed RIS-assisted RoFSO technology has the potential to be effective for 5G smart city applications.

Artificial Bee Colony Based Iterative Heuristic Signal Detection Technique for Optical Wireless Communications

In this paper, we consider a problem of optical signal detection and present a novel artificial bee colony (ABC) based heuristic approach for optical spectrum sensing in a free-space optical wireless network that overcomes channel impairment due to noise uncertainty. In particular, we propose a generic ABC-based optimization framework in conjunction with eigenvalue-based detection (EVD). The uniqueness of the proposed algorithm is that it is executed iteratively by comparing various local solutions until an optimum solution is obtained. The channel is characterized as correlated multivariate fading and the received signal is quantified as a continuous waveform wideband detector. We derive the joint statistic of the correlated multivariate channel fading in the multiple-input multiple-output (MIMO) optical network. Subsequently, the marginal probability density function of the channel coefficients is obtained. For comparison purposes with existing optical spectrum sensing techniques, noise uncertainty is considered. It is demonstrated from simulations that the proposed ABC-EVD spectrum sensing yields performance improvement and the analytical results derived are verified. From the comparative study, it is also found that the proposed scheme outperforms existing optical spectrum sensing techniques, particularly in a regime of low signal-to-noise ratio. When compared to EVD, it is shown that, at a received signal-to-noise ratio of -15 dB, the performance gain achieved in terms of the probability of detection is 15% higher over the entire range of target probability of false alarm. When compared with the ED technique, the proposed technique outperforms it with approximately 50% higher probability of detection at the noise uncertainty of 1 dB and received SNR of -10 dB. The proposed algorithm offers fast convergence, strong robustness, and high flexibility.

Predicting the performance of radio over free space optics system using machine learning techniques

The efficiency and reliability of radio over free space optical communication may be significantly degraded by different weather conditions, thus leading to decrease and increase of signal to noise ratio (SNR) and bit error rate (BER) of the recovered signal respectively. Therefore, real time optical performance estimation is of utmost importance to enable the awareness of channel conditions and for achieving high quality of service. In this work, a terrestrial radio over free space optics (RoFSO) system has been investigated by employing 32-QAM and 64-QAM OFDM modulation schemes under rain, haze and clear weather conditions. Performance of the system is further estimated by employing artificial neural networks (ANN), k-nearest neighbours (KNN) and decision tree (DT) machine learning (ML) techniques with root mean square error (RMSE) and coefficient of determination (R2) as performance metrices. Atmospheric attenuation and different internal system parameters act as input features with BER and SNR of the received signal as the modelling targets. The best performing model has been found to be by applying an ANN approach with lower RMSE values for SNR and BER as 1.87 and 1.26 respectively. The value of R2 predicted by the model are 0.97139 and 0.97802 for SNR and BER data sets respectively.

Absolute Time Delay Measurement Over an Existing Radio Over Free‐Space Optical Link with Sub‐Picosecond Precision

AbstractDistributed RF systems are referred to as the fundamental structure for future wireless communication and sensors. To enable wideband coherent operation, synchronization across the distributed systems with massive parallel absolute time delay (ATD) measurements up to sub‐picosecond precision should be implemented. Traditional ways to achieve precise ATD measurements usually rely on probe signals with broad bandwidth, while that of RF systems is limited. Although an all‐optical solution has high precision, it is complex and not compatible with RF systems. Microwave photonics, which combines the merits of RF and photonics, potentially provides a low‐cost and high‐precision solution that can be deployed in existing RF systems. Here, ATD measurement with sub‐picosecond precision directly over an existing radio over free‐space optical (RoFSO) link, requiring no additional hardware beyond the link, is achieved. The bandwidth of the system is only 10 MHz, while the measurement results meet well with those obtained by optical combs. The measurement errors caused by atmospheric turbulence are well suppressed by a dynamic Kalman filter. The RoFSO links can be stabilized in a closed‐loop measurement. The standard deviation of the timing jitter is 0.28 ps, and the Allan variance is around 1.9 × 10−19 @ 1000 s, which is sufficient to synchronize millimeter‐wave signals.

A novel energy efficient routing technique for SDN-enabled underwater WSNs using free-space optical communication

Abstract Free-space optical (FSO) wireless sensor network is rapidly growing for underwater communication applications. However, the high-energy loss and propagation distance are the key concerns during data transmission in SDN-enabled underwater wireless sensor networks (UWSNs). In addition, long-distance free-space data transmission in UWSNs relies heavily on FSO communication. Thus, FSO communication is integrated with SDN-enabled UWSNs to maximizing the network lifespan called SDN-enabled free-space optical underwater wireless sensor networks (FSO-UWSNs). Furthermore, clustering and routing can effectively balance the network load for energy-efficient data delivery in SDN-enabled FSO-UWSNs. However, choosing the optimal control nodes (CNs) in clustering is considered as an NP-hard problem. Accordingly, self-adaptive genetic approach-based particle swarm optimization (SAGA-PSO) is proposed as a cluster-based routing to optimize the CNs in heterogeneous SDN-enabled FSO-UWSNs. The proposed hybrid model of metaheuristics and genetic mutation, in which the native PSO is amended with the self-adaptive inertia weights and genetic mutation operation to identify the CNs based on genetic diversity dynamically. In addition, a novel fitness function is proposed to balance the cluster size by considering the most significant parameters like energy and distance of network devices. The SAGA-PSO is simulated using the ns-3 simulator, and SDN policies are controlled via the ONOS controller. Moreover, the proposed nature-inspired SAGA-PSO approach outperforms the existing state of arts by considering the performance metrics such as; alive nodes, stability period, average residual energy, the packet transmitted to CS, average delay, and fitness value.

On the Optimization of Integrated Terrestrial-Air-Underwater Architecture Using Optical Wireless Communication for Future 6G Network

Optical wireless communication (OWC) is one of the promising solution to enhance the capacity of the next generation cellular as well as fiber-wireless (FiWi) broadband access networks. The proposed work aims to complement an OWC system by utilizing unmanned aerial vehicles (UAV) to improve the performance of an integrated terrestrial-air-underwater (TAU) communication framework. Specifically, an integrated FiWi based UAV assisted hybrid free-space optical (FSO) and underwater wireless optical communication (UWOC) system is proposed to meet high data rate requirement between a terrestrial ground station and an autonomous underwater vehicle (AUV). The performance of the proposed scheme is compared against the conventional radio frequency (RF) based TAU communication framework. We also optimize the divergence angle of the optical beam of FSO link using a novel COgnition-based divergence angle tracking (CODAT) algorithm such that the outage probability of FSO links is minimized and the data rate requirement of AUV is also satisfied. The impact of the position of over-sea surface (OSS) relay and UAV altitude on the performance of the proposed system is also analyzed. Through the obtained results it has been shown that the proposed OWC based integrated TAU communication framework outperforms the conventional RF based TAU communication framework.

WDM-RoFSO transmission: Demonstration based on IEEE 802.11 WLAN standard

We show two experiment setups to verify the applicability of WDM (wavelength division multiplexing)-RoFSO (Radio-over-Free-Space Optics) system: (i) transmission of images files following the data format of IEEE 802.11 wireless local area network; (ii) robust performance analysis of bit error rate for on–off keying signal at a speed of 10 Gbps. Specifically, it is shown that, for 100 m indoor transmission, our testbed satisfies an error-free condition of BER ≤10−12, achieving a total data rate of 20 Gbps. In this paper, we contribute to the evaluation of the practical FSO measurement data by providing experimental measurements.

Full-duplex transmission without an uplink light source for a millimeter-wave radio over a free-space optical system.

The radio over free-space optical (RoFSO) system is a promising alternative technique for mobile fronthaul networks such as aerial base stations where fiber link is unavailable to deploy. However, the full-duplex transmission imposes challenges on the design of the RoFSO system due to the additional structural complexity when ease-of-installation is a mandatory request. The interplay between the uplink and downlink for a simplified structure has been less investigated in related works. In this paper, we propose a full-duplex RoFSO transmission system structure in which the uplink light source is physically saved. The optical center carrier of the double-sideband (DSB) modulated signal in the downlink is extracted as the light source for the uplink, and thus the system complexity is reduced. In order to mitigate the permanence degradation of uplink transmission due to the attenuated light source, channel estimation and power pre-compensation are implemented without additional training symbols by utilizing the characteristics of the channel correlation between uplink and downlink and the information independence of optical center carrier in the DSB. The proposed channel estimation algorithm is verified theoretically and experimentally. The results show that the difference between uplink and downlink channel attenuation is -0.7dB. In case of the excessive power compensation and the resulting effects, a power control mechanism is introduced to the power pre-compensation. By using the power pre-compensation mechanism, the bit error rate (BER) can reach 1.58×10-3 and 3.8×10-3, respectively, under the channel conditions of medium turbulence and weak turbulence. Owing to the power control mechanism, the power saving rates of downlink and uplink are up to 78% and 67%.

Performance analysis and rain attenuation modelling of RoFSO link for hilly region of India

Abstract Free Space Optics (FSO) is one the most in-demand technology, and is used indoors, on the ground, in space, and even underwater. It is used to broaden communication access to places where bandwidth is insufficient or where difficult topography prevents the use of cable communications. One of the foremost communication challenges that contribute to the degradation of FSO performance is the environmental condition. Hilly regions experience heavy rainfall and have rough terrain which may demean the FSO link communication. In this work, for real-time rain rate analysis of the Meghalaya region two unique rain attenuation models are investigated for monsoon months from the years 2014 to 2020. Considering Marshall and Palmer’s model and by employing 8, 16 and 32-quadrature amplitude modulation (QAM) schemes, radio-over-free space optical (RoFSO) system has been evaluated by varying FSO channel range, power and bit rate.

Performance Comparison of WDM MIMO RoFSO Links for 5G Applications

The ubiquitous deployment of 5G systems is expected shortly. The 5th generation systems are envisioned to deliver a comprehensive selection of services with revolutionary data rates through a single network. Optical fiber networks are going to be the backbone of such 5G systems endowing huge data transfers. Radio over Fiber (RoF) is a promising network architecture that can provide all the provisions envisaged in the development of 5G systems. However, in certain scenarios, these fiber optic networks may encounter empirical problems in deployment. Radio over Free Space Optical (RoFSO) links can be used as an alternate in such environments. To increase the data carrying capacity of the link, Wavelength Division Multiplexing (WDM) communication equipping multiple wavelengths can be used. The free space link may suffer from atmospheric disturbances like rain, fog, and haze. In such adverse conditions, Multiple Input Multiple Output (MIMO) transceivers can be used to increase the link performance. In this work, the performance analysis of such WDM MIMO RoFSO links carrying 5G frequencies is carried out. The work also presents a comparison of single path and MIMO RoFSO links, single channel and WDM RoFSO links, and a combination of these. It is found that, transporting data over WDM channels can increase the capacity of the RoFSO system tremendously while the MIMO link setup can maintain the transmission range and link performance even when high data rates are used.
 HIGHLIGHTS
 
 A quantitative performance comparison of Radio over Free Space Optical links
 WDM increases the capacity while the MIMO links help to maintain the transmission range
 A combination of WDM and MIMO RoFSO links is identified to be the best

Performance analysis of WDM MIMO RoFSO links for 5G applications

The soon to be deployed 5G systems are expected to cater an agglomerate of services at profuse data rates wirelessly. Radio over fiber (RoF) networks are the most suitable to implement such systems. In situations where the practical implementation of RoF systems are non-viable, radio over free space optical (RoFSO) links can be employed. The traditional wavelength division multiplexing (WDM) technology can be incorporated to multiply the data rates. However, the link performance may be degraded due to various atmospheric factors. This can be compensated using multiple input multiple output (MIMO) techniques. In this paper, the performance analysis of such a WDM RoFSO link for 5G frequencies is carried out. The work also presents quantitative performance comparison between single input single output (SISO) and MIMO RoFSO links for 5G systems which will be useful for researchers in this area. It is observed that, while WDM provides additional data capacity, it degrades the link performance under adverse weather conditions which can be counterbalanced using MIMO techniques.

Hybrid MDM-PDM Based Ro-FSO System for Broadband Services by Incorporating Donut Modes Under Diverse Weather Conditions

High-speed data demand in sensitive locations has prompted new wireless technologies to grow in areas like hospitals for bio-sensor data transmission between doctors and patients. However, interference of electromagnetic spectrum or highly sensitive medical equipment in such locations can prevent radio waves which can further compromise the health of patients. Radio over Free Space Optics (Ro-FSO) can fulfil high-speed data demand in such locations without any such interference. However, the Ro-FSO performance is highly influenced by different adverse weather conditions, particularly haze and rainfall, which further cause attenuation in the transmission path of Ro-FSO systems. These atmospheric turbulences mainly affect the transmission link range of Ro-FSO systems. In this work, Ro-FSO system is designed by incorporating hybrid mode division multiplexing (MDM) and polarization division multiplexing (PDM) schemes to deliver four independent channels, each carrying 10 Gbps data upconverted to 40 GHz radio signal, over 3.4 km free space optical link operating under clear weather conditions. In addition to this, the proposed Ro-FSO link is subjected to different weather conditions, particularly partially hazy/rainy and dense fog/very rainy. The reported results indicate the achievement of acceptable bit error rate (BER≈10–3) for all channels up to 3400m FSO link under clear weather conditions, 1000m under partially haze/rain and 620 m under dense fog/heavy rain.

Performance Investigation of a High Data Rate Mode Division Multiplexed-Free Space Optics Link Under Harsh Weather Conditions

The requirement of high data rate information transmission is rising exponentially for supporting different services including social networking, web streaming, and biomedical sensor data transmission. Such services required high channel bandwidth with secure information transmission and immunity to electromagnetic interference. Radio over free space optics (RoFSO) is witnessed as a promising technological solution to provide high data rate transmission over free space channel. We report on the design of a 2×10 Gb/s-10 GHz RoFSO transmission system using the mode division multiplexing technique and evaluate its transmission performance over varying levels of dust weather conditions. The comparison of non-return to zero (NRZ) and return to zero (RZ) binary digital optical modulation techniques is carried out in the proposed system. It is found that the proposed system using NRZ modulation serves 14.5 km transmission range; however, in the case of RZ modulation, it was restricted to 10 km for a target bit error rate (BER) of 10−6, thus resulting in a noticeable link enhancement of 4.5 km. Also, we demonstrate NRZ-based MDM-RoFSO link performance and availability in dust weather conditions using the BER, maximum reachable link range, and eye diagram as key performance parameters. We obtain a reliable transmission of 20 Gb/s-20 GHz data through HG00 and HG01 channels over a link range of 2500–108 m depending on the external dust weather condition. Furthermore, since this investigation shows the feasibility of RoFSO for small size cells, which is an essential feature of 5G mobile network, the proposed system can thus be implemented as a backhaul/fronthaul link for high-band (above 6 GHz) 5G services and for providing secure transmission of biomedical sensor data.

Multi-user accessible indoor infrared optical wireless communication systems employing VIPA-based 2D optical beam-steering technique.

Infrared optical wireless communication system can achieve ultrahigh capacity and high privacy data transmission. However, for using narrow infrared laser beam as carrier to transmit signal, the high-speed data transmission can only be achieved by point-to-point connection. With the rapid number increasement of consumer electronic devices, such connection method puts a heavy burden on the number of transmitters. Thus, the transmitting end with the point-to-multipoint capability or multi-user accessibility is required. In this paper, we present a multi-user accessible indoor infrared optical wireless communication system employing passive diffractive optics based on a virtually imaged phased array (VIPA). Multiple beams can be generated in a point-to-multipoint scheme by using VIPA-based beam-steering antenna (BSA). On the other hand, by tuning wavelength of laser source, fast 2D steering of multiple beams with the same steering trajectory is supported, which can be used for user ends with changing locations. In the experiment, 5 beams are generated by utilizing only one transmitter. All five beams can realize 12.5 Gb/s on-off-keying (OOK) data rate transmission. Free-space optical wireless transmission at 3.6-m communication distance is demonstrated for system performance verification and evaluation. a total 3.44°×7.9° scanning field of view of five beams is achieved.

Millimeter-wave hybrid OFDM-MDM radio over free space optical transceiver for 5G services in desert environment

We propose a novel design of orthogonal frequency division multiplexing-mode division multiplexing (OFDM-MDM) based radio over free space optical (RoFSO) transceiver under the dust effect. Two separate 20 Gbps-40 GHz 4-QAM data bearing optical beams are transported over two distinctive Hermite-Gaussian modes; HG00 and HG01, of a single-frequency laser to realize a single-channel 40 Gbps-80 GHz 5G communication link. The link performance and availability are investigated under the impact of clear climate and under varied dust conditions, including very light dust, light dust, moderate dust, and dense dust using signal-to-noise ratio (SNR) and total power as the key evaluation parameters. Further, we have determined the maximum reachable link distance within acceptable performance criteria, i.e., SNR 20 dB for each desert weather conditions. A higher data rate with tolerable SNR and power requirements for 5G applications has been verified.