License-free Spectrum Research Articles

Fading model due to scintillation and pointing error on an optical wireless MISO channel

Optical wireless communication (OWC) yields high data rates and a license-free spectrum. However, OWC is drastically affected by scintillation owing to atmospheric turbulence and pointing errors because of misalignment. To alleviate the ramifications of scintillation and pointing error, multiple input single output (MISO) optical wireless channels are studied by several researchers. In this paper, it is proposed to study the performance of a MISO system in terms of the probability of outage and normalized threshold. Gamma–Gamma distribution is used to model scintillation because it is suitable for modeling both weak and strong turbulence conditions. The analytical technique is used to derive the closed-form expressions for the probability of outage of a 2×1 MISO channel. Moreover, under numerous scintillation conditions, the Mellin transform is applied to derive the closed-form expressions for the probability of outage of an optical wireless MISO channel. The closed-form results are validated through simulations.

Controlled alignment imaging optical MIMO communication system based on light spot detection of arrayed light sources.

Visible light communication (VLC) technology has become one of the potential key technologies for 6 G due to its unique features, such as abundant license-free spectrum, high confidentiality, and low electromagnetic interference. In this paper, an optical multiple-input multiple-output (MIMO) communication system based on imaging reception is proposed, which can improve the data transmission rate and reduce interference between different sub-channels. To overcome the problem of optical communication link alignment, in this paper, we established an imaging optical MIMO communication system with a controlled light source array (CLSA) and proposed an optical alignment strategy to address the alignment issue. The scheme decouples the constraint that the light spot cannot be aligned with the detector in the communication system under the influence of optical parameters. Based on the light spot detection CLSA module, the communication system can ensure the stability of the link alignment when the communication distance changes. Experimental results show that when the communication distance is 20-80 m, the bit error rate (BER) of the communication system remains below 10-6. The single-channel communication rate of the system is 5 Mbps, and the total communication rate is 20 Mbps through the four-channel rate multiplexing. Moreover, if the communication distance increases to the range of 80-100 m, the BER increases, but it is always lower than the forward error correction (FEC) threshold.

An Empirical Approach to Rerouting Visible Light Pathways Using an Adjustable-Angle Mirror to Sustain Communication between Vehicles on Curvy Roads

In this paper, a novel method is demonstrated to sustain vehicle-to-vehicle (V2V) communication on curvy roads via the arrangement of the lateral position of a self-angle-adjustable mirror–reflective road sign (SAAMRS) and light-direction-sensing wide-angle complementary photodiodes (CPDs). Visible light communication (VLC) between vehicles attracts attention as a complementary technology to radio-frequency-based (RF-based) communication technologies due to its wide, license-free spectrum and immunity to interferences. However, V2V VLC may be interrupted on curvy roads due to the limited field of view (FOV) of the receiver or the line of sight (LOS) being interrupted. To solve this problem, an experiment was developed using an SAAMRS along with wide-angle light-direction-sensing CPDs that used a precise peak detection (PPD) method to sustain communication between vehicles in dynamic environments by rerouting the incident light with the highest signal intensity level to the receiver vehicle on curvy roads. We also used real images of curvy roads simulated as polynomials to calculate the necessary rotation angles for the SAAMRS and regions where communication exist. Our experimental results overlapped almost completely with our simulations, with small errors of approximately 4.8% and 4.4% for the SAAMRS angle and communication region, respectively.

Performance analysis of 4QAM-OFDM-FSO link under rain weather conditions

Abstract Free space optical communication (FSO) is a technique based on transmission of data by propagating the light in free space. FSO is inexpensive, providing high data rates, high capacity, high security, and low power and uses the license free frequency spectrum. Nevertheless, FSO is still facing many problems such as bad weather conditions, atmospheric turbulence, and multipath fading. In this research work, orthogonal frequency division multiplexing (OFDM) FSO system is analyzed for low to heavy rain fall intensities. Different rain rates have been considered for low, moderate, heavy, very heavy, and torrential rains. For all the rain rates, a direct detection OFDM–FSO system and a coherent detection system have been simulated and results are compared. Analysis has been done on the basis of SNR, received power, and constellation diagrams. According to these parameters, link length of both the systems for different rain rates was found. Coherent detection is applied to improve the system sensitivity compared to conventional direct detection. In tropical regions where heavy rainfall is the main reason for the degradation of the system, the OFDM system with coherent detection can perform better with high data rates. Second, over a 11 km FSO, a 4-QAM wireless signal is successfully transmitted under very heavy rainfall conditions. The distance achieved by moderate and heavy rains is 25 km and 15.5 km with an acceptable SNR of 25 dB and a system sensitivity (received power) of −65.1 dBm.

Performance comparison of Duobinary, AMI, CNRZ and CSRZ for next generation FSO communication system

Abstract FSO, known as free-space optics, is a competent technology in wireless communication for providing license free spectrum, bandwidth, and high efficiency in next-generation networks. In this paper, an experimental approach is used to deploy FSO channel communication using different intensity modulation schemes such as CSRZ, chirped NRZ, duobinary and AMI. The performance of FSO link is adversely affected by weather conditions like haze, rain, and fog. By using different techniques, it is possible to overcome the effect of atmospheric turbulences. Therefore, four different modulation formats are compared and analyzed by testing different parameters like laser power, range, attenuation, and data rates. Quality of received signal is judged by taking Q-factor, bit error rate (BER), and eye-diagrams. An operating wavelength of 850 nm has been used to simulate the system designs. This paper aims to conclude the best intensity modulation scheme for its use in next generation networks such as 5G where direct deployment of optical fiber is not possible or feasible.

Metasurface-driven polarization-division multiplexing of PCSEL for optical communications

Free-space optical communications hold promising advantages, including a large bandwidth, access to license-free spectrum, high data rates, quick and simple deployment, low power consumption, and relaxed quality requirements. Nevertheless, key technical challenges remain, such as a higher transmission efficiency, a lower transmission loss, and a smaller form factor of optical systems. Here, we demonstrate the viability of circular-polarization-multiplexed multi-channel optical communication using metasurfaces alongside a photonic-crystal surface-emitting laser (PCSEL) light source at wavelength of 940 nm. Through the light manipulation with metasurface, we split the linearly polarized incidence into left and right circular polarizations with desired diffraction angles. Such orthogonal polarization states provide a paradigm of polarization division multiplexing technique for light communication. The PCSEL light source maintains a low divergence angle of about 0.373 degrees after passing through an ultra-thin metasurface without further bulky collimator or light guide, making end-to-end (E2E) and device-to-device (D2D) communications available in a compact form. Both light source and modulated polarized light exhibit a − 3 dB bandwidth over 500 MHz, with successful 1 Gbit/s transmission demonstrated in eye diagrams. Our results affirm that metasurface effectively boosts transmission capacity without compromising the light source's inherent properties. Future metasurface designs could expand channel capacity, and its integration with PCSEL monolithically holds promise for reducing interface losses, thereby enhancing efficiency.

A comprehensive survey of free-space optical communication – modulation schemes, advantages, challenges and mitigations

Abstract The demand for large bandwidth and high data rates in communication systems has become the main cause of the upgrade of traditional networks into free space optical (FSO) technology. FSO technology has gained significant popularity due to its easy deployment, high data rates, abundant bandwidth, enhanced security, and license-free spectrum utilization. However, the performance of FSO communication systems can be affected by certain limiting factors, such as changes in weather conditions during data transmission. To overcome these challenges and improve FSO performance, various modulation techniques are employed. This article presents a concise overview of the FSO communication system, highlighting different modulation techniques used to enhance its performance, as well as discussing its advantages, applications, and existing challenges. Some advanced modulation formats which are recently introduced in the field of FSO communication such as QPSK, DP-QPSK, QAM, and OFDM are also made part of this paper.

CPSS using Dynamic Spectrum Accessibility on the Blockchain over Non-Real-Time Data

Abstract: Due to being dependent on wireless transmission through various devices for big data sharing in Cyber-PhysicalSocial Systems (CPSSs), radio spectrum resources are seriously in limited availability. Despite the fact that license-free access to the spectrum offers a great deal of promise to reduce the developing shortage of spectrum resources, competition for spectrum is more fierce due to the laxer access regulations. A dynamic cycle of "competition-verification-synchronization-competition" may be introduced by a blockchain technology to address this problem of competition. In this study, we offer a general architecture for managing license-free spectrum resources in CPSSs using blockchain and smart contracts. We precisely partition the spectrum of a local cell into numerous channels, each of which is equivalent to a blockchain. Next, we propose a blockchain-KM protocol that could expedite transaction processing while maintaining a generic blockchain's core features. Users of the Blockchain-KM protocol must mine or lease wireless spectrum, and the entire private chain is transformed into a multi-ring blockchain. In contrast to the conventional mining method, the prize in our mining process includes both a spectrum access licence and virtual currency. Once a miner has secured a permit for spectrum access, it will use the permit to send messages through wireless networks. If a miner decides not to send messages, it may sell its permit at auction. During the auction, we provide a virtual currency called Xcoin for trading spectrums or other items.

Impact of Scintillation Effect on Clear Weather and Fog Conditions using MDM-WDM Modulation Technique

The 5th generation optical wireless communication technologies have unique features of higher data rate, scalability in bandwidth, license free spectrum, cost effectiveness, easy deplorability and secured transmission. The role of free space optical (FSO) communication becomes important as the applications of next generation technologies are evolving in almost every sector. In this work, the performance of the terrestrial FSO MDM-WDM link has been investigated through a DP-QPSK modulation scheme at a data rate of 1000 Gbps. The link attenuation has been analysed with the help of Gamma- Gamma and Lognormal scintillation models considering clear weather and varying fog conditions. The link performance has been evaluated through SNR and received optical power for a varying distance of up-to 20 Km.

Error Analysis of Free Space Communication System Using Machine Learning

Free space optical (FSO) communication offers huge bandwidth, license-free spectrum and a more secure channel. PIN diodes are normally used for detection, but avalanche photodiodes (APD) are preferred for detecting high-speed FSO signals in many applications. In the case of APD, the noise distribution is input-dependent Gaussian noise (IDGN) rather than input-independent Gaussian noise (IIGN). We investigate the error analysis using on-off keying (OOK) for various detection approaches. This paper proposes a machine learning approach and compares its performance with soft and hard decisions. Soft values in the case of IDGN and IIGN are derived, and the optimum and sub-optimum detection thresholds are evaluated. The proposed novel ML approach shows better performance gains than the other approaches. It is also demonstrated that the IDGN model should have an optimum detection and achieve a gain of 2.5[dB] and about 1[dB] at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> = 0[dB] and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> = 10[dB], respectively. Experimental results are plotted for the FSO channel data, and a model fit curve is plotted using the ML approach.

Investigation of link due to atmospheric turbulence in free space optical communication for optical wireless terrestrial networks

Abstract In recent years, wireless communication systems have exploded in popularity. Optical wireless technology is an excellent alternative to RF wireless, but it has high capacity, data speed, frequency, and license-free spectrum, and it is simple to implement. Optical wireless communication sends data through air using optical beams. Atmospheric turbulence degrades the functioning of free-breathing space photosensitive interaction methods by causing interruptions due to weather conditions like fog, smoke, and different pollutants. In this paper, free-space optical communications using snow and fog attenuations and Numerical Weather Prediction (NWP) method is used to determine the turbulence due to the atmosphere. This method improves the data transmission during unfavorable weather conditions by changing the various parameters like receiver aperture size, the wavelength of the transmitter, etc. Furthermore it shows that decrease in air quality and instability are the double significant causes, that can degrade general usage routine, especially when it’s foggy or raining heavily. The result shows that the aperture size and wavelength should be changed as per the weather condition.

ABER of M-QAM and K-PSK modulations over double-GG fading channel

ABSTRACT OFDM-based Radio on Free space optical (RoFSO) system gained attention of technocrats and researchers due to high data rates, license-free spectrum and ease of installation among the others. However, it is noteworthy to mention that turbulence-induced fading and pointing error are the major challenges of RoFSO system. Although various literatures include pointing error and/or turbulence induced fading to present different performance measures of RoFSO system, none of them includes all the turbulence condition from weak to strong. In contrary, Double Generalized Gamma (DGG) distribution shows perfect fit over the entire range of turbulence conditions from weak to strong. This research work presents various closed form expressions of BER for M-QAM and K-PSK schemes over DGG distribution under weak, moderate and strong turbulence conditions for the system with and without pointing error. Furthermore, as DGG distribution includes different fading channels such as Double Weibull and Gamma-gamma distribution as its special case. Therefore, we reproduce BER of M-QAM and K-PSK modulation schemes with and without pointing error over Double Weibull and Gamma-gamma fading channels. Simulation shows that irrespective of severity of turbulence, pointing error and/or increase in the order of modulation scheme deteriorate the system performance.

Error analysis of L-PPM modulated MIMO based multi-user NOMA-VLC system with perfect and imperfect SIC.

By offering a wide license-free spectrum, visible light communication (VLC) has emerged as a game-changing technology for networks beyond 5G. In this study, the multiple-input multiple-output (MIMO) technology is combined with a non-orthogonal multiple access VLC (NOMA-VLC) system to provide a reliable error-free high data rate network for multi-user scenarios. A generalized L-PPM modulated MIMO based multi-user NOMA-VLC system with M×N line-of-sight connections for each user is presented. Considering a practical scenario of imperfect successive interference cancellation, a novel closed form formulation of error probability for the proposed system is obtained. This study also includes a comprehensive analysis of two-user and three-user L-PPM modulated 2×2 MIMO-NOMA-VLC systems, and exact error probability expressions are derived. With the identical set of parameters, the L-PPM modulated MIMO-NOMA-VLC system totally outperforms the on-off keying modulated MIMO-NOMA-VLC system. As the number of photodetectors per user increases, the error probability of the considered system decreases. An L-PPM modulated 2×2 MIMO based three-user NOMA-VLC system offers the best performance at a power allocation co-efficient of 0.3. The simulation results validate the derived error probability expressions.

BER analysis of dynamic FOV based MIMO-NOMA-VLC system

Visible Light Communication (VLC) has evolved as a breakthrough technology for beyond 5G networks by providing a wide license-free spectrum. In this paper, we build a robust NOMA-VLC Network which exploits the MIMO technique. In particular, a two-user 2 × 2 MIMO-NOMA-VLC system is proposed which considers on–off keying (OOK) modulation and L-pulse position modulation (L-PPM). The closed-form bit-error-rate (BER) expressions for the OOK and L-PPM modulated two-user MIMO based NOMA-VLC system are derived with the consideration of perfect and imperfect successive interference cancellation (SIC). The analysis shows that the proposed system marginally outperforms the single-input single-output SISO-NOMA-VLC system. The error performance of L-PPM modulated system is superior to that of OOK modulated system for L>2. Further, the increment in the number of photo detector (PD’s) at the receiver does not improve the error performance of the system significantly. To address this, two dynamic field of view (FOV) strategies are proposed for attaining the additional enhancement in error performance of OOK and L-PPM modulated MIMO-NOMA-VLC system. The 2 × 2 MIMO-NOMA-VLC system based on dynamic FOV strategy-2 completely outperforms the 2 × 2 MIMO-NOMA-VLC systems based on static FOV and strategy-1 in terms of error performance. The effect of power allocation co-efficient, α, on the error performance of the proposed systems is investigated. The best average error probability of a two-user MIMO-NOMA-VLC system is attained at α = 0.3. The simulation results are in close alignment with those of the analytical outcomes.

Performance Analysis and Evaluation of Inter-Satellite Optical Wireless Communication System (IsOWC) from GEO to LEO at Range 45000 km

The importance of free-space optical (FSO) communication has increased during the last decade, introducing its unique features such as its high data rate, license-free spectrum, ease of deployability, and low power consumption. In the future, communication between low earth orbit (LEO) and geostationary earth orbit (GEO) satellites will be commonly established by using inter-satellite optical wireless communication (IsOWC) systems. That enables GEO satellites to relay information to and from LEO satellites and fixed Earth stations that otherwise cannot transmit/receive data permanently. IsOWC systems are the most effective application of the FSO, and they will be favorited shortly because of their features. In this work, we established an IsOWC between GEO and LEO satellites at a distance of 45000 km to achieve a target bit error rate (BER) equal to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> . Furthermore, we proposed an IsOWC system with suitable operating wavelengths. Physical techniques such as the multi-input multi-output (MIMO) technique and a wave division multiplexing (WDM) technique are employed.

An exact BER analysis of NOMA-VLC system with imperfect SIC and CSI

Visible Light Communication (VLC) has emerged as a promising technology for 5G and beyond networks by offering a wide license free spectrum. In order to develop a high data-rate and error-free network, in this paper, non-orthogonal multiple access (NOMA) technique is combined with VLC technology. In particular, a two user downlink NOMA based VLC system is considered in which the transmitter uses on–off keying (OOK) and L-ary pulse position modulation (L-PPM). The closed form mathematical expressions for bit-error-rate (BER) of OOK and L-PPM modulated NOMA-VLC system is derived in the presence of perfect and imperfect channel state information (CSI). Moreover, imperfect successive interference cancellation (SIC), which is a more practical scenario, is considered for this analysis. The analysis shows that the error performance of L-PPM modulated NOMA-VLC system is superior to OOK modulated NOMA-VLC system for (L > 2). Further, with the far user power allocation coefficient greater than 0.5, the far user completely outperforms the near user. Furthermore, the channel estimation error has an adverse effect on BER performance of the system under consideration. The performance of the considered system is investigated for the parameters of interest, i.e., location of the far user, field of view (FOV) and channel estimation error. The derived analytical BER expressions are verified with the simulation results.

Centroid Error Analysis of Beacon Tracking under Atmospheric Turbulence for Optical Communication Links

Optical satellite communication has received considerable attention as a promising alternative to radio frequency communication because of its potential advantages including higher data rates and license free spectrum. Many studies have conducted performance analyses of optical communication channels, but few have investigated beacon tracking channels under atmospheric turbulence. The centroid accuracy of beacon tracking channels is limited by not only noise sources, but also a finite delay time, which also fluctuates due to atmospheric turbulence. Consequently, the centroid error is an important figure of merit when evaluating the performance of a beacon tracking system. In this study, the closed-form expressions were derived for average centroid error and fade probability, based on received photoelectron counts depending on exposure time, taking into account the log-normal tracking channels. We analyzed the angular positioning performance of beacon tracking detectors onboard small satellites in the presence of atmospheric turbulence, in terms of centroid error and fade probability. We found that an optimal exposure time exists, which minimizes the centroid error, and that fade probability is inversely proportional to the exposure time. These are significant properties to consider in the design of beacon tracking systems.

Attenuation-based hybrid RF/FSO link using soft switching

Due to high data rates, license-free spectrum, and immunity to electromagnetic interference, free-space optical (FSO) links are being considered as a potential candidate to meet the ever-increasing traffic demands of users. The FSO links remain less explored, as their performance depends on environmental conditions such as dust, fog, and clouds. Such conditions do not affect the radio frequency (RF) links in a similar way; however, RF resources are scarce. As an ultimate solution to this performance/scarcity dilemma, we propose a fuzzy logic-based hybrid architecture of FSO and RF links, which can be used to enhance reliability and resource efficiency. We present an intelligent soft switching mechanism between FSO and RF links using a fuzzy inference system to achieve the maximum link reliability and provide heterogeneous wireless services.

A comprehensive hybrid bit-level and packet-level LoRa-LPWAN simulation model

LoRa is a rising LPWAN technology designed to use a simple Aloha-like medium access mechanism exploiting free ISM frequency bands. This makes LoRa packets inevitably exposed to the threat of the same LoRa network interference (intra-system interference). Additionally, they are also exposed to disturbances caused by packets from other networks using the license-free spectrum (inter-system interference). LoRa-LPWAN is actively being deployed in the literature state-of-the-art. The capture effect essentially reflects the joint relation between the physical layer capabilities of LoRa modulation in relation to the LoRaWAN Aloha-based MAC operation. Moreover, most efforts adopted in the literature have ignored the impact of inter-system ISM interference signals on LoRa-LPWAN transmissions. This paper devises a new paradigm for modeling LoRa modulation performance that incorporates the joint impact of SNR and SIR in order to represent the intra- system interference behavior during LoRAWAN operation. The physical layer abstractions are then combined with comprehensive implementation of the LoRaWAN MAC protocol standard in an OMNeT++ module, delivering a hybrid bit-level and packet-level simulation model. The devised LoRa simulation model framework also introduces a representative abstraction for LoRa reception capabilities under ISM interference in accordance with the IEEE 802.15.4 interference model [1] that has been recently introduced in the literature of long-range, low-power communication. The developed OMNeT++ Simulation model acts as a reliable platform to test different potential bottlenecks in LoRa-LPWAN operating in the free, unlicensed frequency bands. The proposed simulation framework has the potential to support high accuracy of error performance prediction in a realistic LoRa-LPWAN deployment.

An Exact Error Analysis of Multi-User RC/MRC Based MIMO-NOMA-VLC System With Imperfect SIC

Visible Light Communication (VLC) has appeared as a breakthrough technology for beyond 5G networks by delivering a broad license free spectrum. In this paper, multiple input multiple output (MIMO) technique is implemented with non-orthogonal multiple access (NOMA-VLC) system in order to develop a robust error-free high rate network for a multi-user scenario. In particular, a generalized multi-user MIMO-NOMA-VLC system with $M\times N$ line-of-sight (LOS) links per user is proposed which adopts on-off keying (OOK) modulation for data transmission. A novel closed form expression of probability of error for the proposed system is derived considering a practical scenario of imperfect successive interference cancellation (SIC). This paper also presents the detailed analysis of a three user $2\times 2$ MIMO-NOMA-VLC system and an exact closed form expression of probability of error of this system is derived. The three user $2\times 2$ MIMO-NOMA-VLC system completely outperforms the single input single output (SISO) NOMA-VLC system with same set of parameters. The error performance of the proposed system enhances as the number of photo detectors (PDs) at each user increases. A three user $2\times 2$ MIMO-NOMA-VLC system ensures the best performance at the power allocation co-efficient of 0.3. The error performance of the proposed system is investigated for the parameters of interest, i.e., distance between the transmitting LEDs and distance between the receiving PDs at each user terminal. The derived probability of error expressions are verified with the simulation results.