Journal Articles
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Item A Review–Unguided Optical Communications: Developments, Technology Evolution, and Challenges(MDPI, 2023) Raj, A.A.B.; Krishnan, P.; Darusalam, U.; Kaddoum, G.; Ghassemlooy, Z.; Abadi, M.M.; Majumdar, A.K.; Ijaz, M.This review paper discusses the complete evolution of free-space optical (FSO) communication, also known as unguided optical communication (UOC) technologies, all the way back to ancient man’s fire to today’s machine-learning-supported UOC systems. The principles, significance, and developments that have happened over the past several decades, as well as installation methodologies, technological limitations, and today’s challenges of UOCs are presented. All the subsets of UOC: FSO communication, underwater optical wireless communication (UOWC), and visible light communication (VLC), with their technology/system developments, potential applications, and limitations are reviewed. The state-of-the-art developments/achievements in (i) FSO channel effects and their mitigation techniques; (ii) radio-over-FSO techniques; (iii) wavelength division multiplexing and sub-carrier multiplexing techniques; (iv) FSO for worldwide interoperability for microwave access applications; (v) space optical satellite communication (SOSC); (vi) UWOC; (vii) photoacoustic communication (PAC); (viii) light-fidelity; (ix) VLC; (x) vehicular VLC (V2LC); and (xi) optical camera communication are reviewed. In addition, the current developments on emerging technologies such as artificial intelligence (to improve the performance of UOC systems), energy harvesting (for the effective utilization of UOC channels), and near-future communication network scenarios (mandatory for secured broadband digital links) are covered. Finally, in brief, to achieve the full potential of UOC systems, challenges that require immediate research attention are summarized. © 2023 by the authors.Item Performance Analysis of Hybrid RF/FSO System Using BPSK-SIM and DPSK-SIM Over Gamma-Gamma Turbulence Channel With Pointing Errors for Smart City Applications(Institute of Electrical and Electronics Engineers Inc., 2018) Krishnan, P.The flourishing technology in wireless communication-free space optics (FSO) offers lots of merits over radio frequency (RF) links due to its license free bandwidth, ease of installation, high security features, and viable cost for short distance communication. It's high speed data rate and immunity against electromagnetic interference makes FSO the emerging technology of today. But, FSO is not always reliable especially during atmospheric conditions, such as fog, rain, mist, and snow. Hence, in account a new technique of hybrid FSO/RF, this includes advantages of both FSO and RF technologies. Through this paper intend to perform an extensive analysis of the error and misalignment effects encountered in line of sight communication. Pointing error and turbulence effects are the main drawback parameters for our analysis. For this purpose I have taken into consideration different modulation techniques-binary phase shift keying-subcarrier intensity modulation, differential phase shift keying-subcarrier intensity modulation communication system with reference to on-off keying (OOK) modulation. The novel expressions for outage probability and BER for both FSO and RF system are derived which uses Rician channel and 16QAM modulation scheme alongside hybrid FSO/RF system for weak, moderate, and strong turbulence regimes using Meijer-G function. © 2013 IEEE.Item Analysis of beam divergence on free space optical link using polarization shift keying technique(SPIE spie@spie.org, 2019) Rajeev, M.; Mathew, G.A.; Krishnan, P.Beam diverging is an indispensable phenomenon on the quality and bit error rate (BER) performance of the free space optical (FSO) communication system. The role of an efficacious modulation technique that mitigates the combined effect of beam diverging and atmospheric attenuation is highly relevant. The weak depolarizing property of the atmosphere advances the use of polarization shift keying (PolSK) modulation. We are subjected to investigate the quality and feasibility of the communication link, considering the impact of beam divergence on the PolSK modulation under various weather conditions. It is shown that PolSK subdues the effects of increase in beam divergence, attenuation factor, and further extends the communication range. A comparison with on off keying and PolSK-based FSO links considering the divergence angle is discussed. Moreover, BER and quality factor graphs are plotted against link distance and beam divergence using PolSK, which improves the performance analysis. © 2019 Society of Photo-Optical Instrumentation Engineers (SPIE).Item Analysis of FSO Systems with SISO and MIMO Techniques(Springer New York LLC barbara.b.bertram@gsk.com, 2019) Krishnan, P.Free space optics (FSO) is a form of line of sight communication technology that uses the help of LASERS and photodetectors to give optical connections from one place to another without the use of an optical fiber. The major hindrance to an FSO communication system comes in the form of atmospheric turbulences characterized by haze, rain, snow, storms among others. In this paper, the bit error rate performance of single input single output and multiple input multiple output based FSO system is analyzed and compared. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Modelling of OFDM based RoFSO system for 5G applications over varying weather conditions: A case study(Elsevier GmbH, 2019) Pati, P.S.; Krishnan, P.From the perspective of spectrum scarcity, Radio-on-FSO (RoFSO)system is the best available option as it operates at unlicensed optical frequencies with the salient features of high bandwidth, cost effectiveness, lower transmit power, immunity to electromagnetic interference and easy deployment. To cater to the high data rate requirement of 5G applications in the near future, RoFSO systems will definitely be the best assisted alternative technology to the conventional wireless systems. However, the atmospheric conditions and non-linearity of the optical link have got a bearing on the efficiency of the system. In this work, we have presented an analytical model for transmitting phase shift keying (PSK)modulated orthogonal frequency division multiplexing (OFDM)signals through free space optics (FSO)links. RL is the load resistor of the photo detector an Considering optical noises, nonlinear distortion resulting from laser diode and the atmospheric turbulence effect modelled by gamma-gamma distribution, we have derived a closed form closed-form bit error rate (BER)and outage probability expression for the aforementioned transmission system. Pointing error is also factored in for more accurate and appropriate analysis of the system performance. Better system performance was observed through use of aperture averaging technique. A case study has been undertaken for analyzing the FSO system performance with reference to BER for various seasons for the year 2016 over IIIT Bhubaneswar area by considering the wind speed and altitude at different floors of the building of the institute. © 2019 Elsevier GmbHItem Performance enhancement of BPSK-SIM- and DPSK-SIM-based FSO downlink over atmospheric turbulence using aperture averaging and receiver diversity(Springer New York LLC barbara.b.bertram@gsk.com, 2019) Krishnan, P.; Jha, G.K.; Walia, A.In this paper, we evaluate the bit error rate (BER) performance of a free-space optical satellite downlink by considering the atmospheric turbulence effects using binary shift keying subcarrier intensity modulation and differential phase shift keying subcarrier intensity modulation (DPSK-SIM). The performance of the link is enhanced using aperture averaging and receiver diversity. The closed form mathematical expressions of BER for BPSK-SIM and DPSK-SIM schemes are derived and analyzed. It is observed that on varying the turbulence level, the performance of the link degrades when the turbulence level increases. The improved BER of 10?12 and 10?10 at signal-to-noise ratio of 30 dB of the link for both BPSK-SIM and DPSK-SIM is obtained by using aperture averaging (aperture diameter, D = 10 cm) and receiver diversity with optimal combining. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Design optimization of a highly sensitive spiral photonic crystal fiber for liquid and chemical sensing applications(Academic Press Inc. apjcs@harcourt.com, 2019) Malavika, R.; Krishnan, P.In this paper, a spiral shaped photonic crystal fiber (PCF)is proposed for liquid sensing applications. The guiding properties of the proposed PCF are analysed. Sensitivity coefficient is analysed by varying different structural parameters for a wider wavelength range. In addition, confinement loss is also studied at the same wavelengths. The relative sensitivity achieved for the optimized parameters are 56.8%, 58.3% and 62.7% for water (n = 1.33), propane (n = 1.34)and propylene (n = 1.36)respectively. With its low confinement loss and high sensitivity the proposed PCF can be used for liquid and chemical sensing applications. © 2019 Elsevier Inc.Item Design and Analysis of a Novel Optical Circulator Based on Photonic Crystal for Photonic Integrated Circuit Applications(Springer New York LLC barbara.b.bertram@gsk.com, 2019) Krishnan, P.; Nasre, D.In this paper, a novel design of a 4-port optical circulator is proposed using two-dimensional square lattice photonic crystal ring resonators. This design is suitable for photonic integrated circuit applications and allows the design to function in both clockwise and anticlockwise mode. It presents the evaluated transmission of over 84%. The nanowires in this structure are made up of silicon having a refractive index of 3.46 surrounded with air. The wide range of wavelength covers the third window of transmission, so the wavelength in this range can pass across the offered design of optical circulator. The transmission range of wavelengths is calculated and analyzed. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Circular-Pattern Photonic Crystal Fiber for Different Liquids with High Effective Area and Sensitivity(Springer New York LLC, 2019) Revathi, R.; Soni, A.; Bir, K.; Senthil, R.; Krishnan, P.A solid core photonic crystal fiber (PCF) is preferred to signify the work of circular-pattern PCF for chemical identifying motive. Finite element method is used to obtain several properties of PCF. Various computations are applied to numerically explore the use of PCF for sensing justifications at different wavelengths ranging between 1.4 and 1.65 ?m. The solid core is filled with liquid glycerol (n = 1.4722), ethanol (n = 1.354), and toluene (n = 1.4968), and on applying various geometric parameters of the fiber, 65.16%, 61.65%, and 64.05% of sensitivity are observed respectively. Transmission of heavy data with high speed depends on effective mode area. For glycerol and toluene, the effective area is observed as 2.81 ?m2 and 3.07 ?m2 respectively. Perfectly matched layer is applied in outer most cladding to overcome reflection. Higher sensitivity is observed by this design operating at different wavelengths. Similarly, properties like confinement loss and effective area are also computed. Design containing core material such as glycerol, ethanol, and toluene has been compared by different properties. The core materials employed in this paper are used to analyze the potential of sensors. This PCF can be used in diverse application of bio sensing or sensing related areas. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.Item Enhanced Optical Wireless Communication System for Bio-signal Monitoring Applications(Springer, 2020) Krishnan, P.; Gopikrishna, S.In this paper, optical wireless communication (OWC) technology based mobile remote care unit is proposed. Since, the existing RF based medical systems are suffer with misdiagnosis due to electromagnetic interference (EMI) and the influence of the radiation field on medical equipment’s. These issues are addressed with the help of OWC links between the medical sensors and receiver fixed in the ceiling of the hospital room. The power efficiency of the proposed system is analyzed. The results show the importance of OWC over the RF communication network in healthcare monitoring systems. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.