Faculty Publications

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Author: search.filters.author.Krishnan, P.Subject: search.filters.subject.Turbulence

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    Experimental demonstration and analysis of underwater wireless optical communication link: Design, BCH coded receiver diversity over the turbid and turbulent seawater channels
    (John Wiley and Sons Inc. P.O.Box 18667 Newark NJ 07191-8667, 2020) Naik, P.N.; Udupi, S.A.; Krishnan, P.
    In this article, we demonstrate an experimental underwater wireless optical communication (UWOC) system in the presence of air bubbles and weak turbulence for varying turbidity levels of the aquatic optical medium. The major limiting factors of the UWOC system are: absorption, scattering, and beam fluctuations; these effects can be mitigated by employing transmitter/receiver diversity schemes and channel codes. In this proposed system, we have employed receiver diversity (selection combining (SC), majority logic combining (MLC), and equal gain combining (EGC)) techniques augmented with Bose-Chaudhuri-Hocquenghem (BCH) codes to improve the performance of on-off keying modulated UWOC system. The bit error rate (BER) expressions are derived for the proposed system and results are validated using analytic and experimental means. The results show that the proposed system, that is, the receiver employing SC, MLC, and EGC receiver combining techniques augmented with the BCH code provides a transmit power gain of 4, 6, and 8 dB respectively, when compared with the uncoded single-input single-output system, at a BER of 10?5. © 2020 Wiley Periodicals, Inc.
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    Co-operative RF-UWOC link performance over hyperbolic tangent log-normal distribution channel with pointing errors
    (Elsevier B.V., 2020) Naik, P.N.; Udupi, S.A.; Krishnan, P.
    In this paper, we have proposed an analytic model and determined the outage probability and average bit error rate (BER) performance of a co-operative radio frequency ? underwater wireless optical communication (RF?UWOC) system. In recent years, UWOC has attracted attention as a useful enabler of underwater activities such as climate and ocean monitoring, surveillance, ocean exploration, underwater wireless optical sensor networks (UWOSN) and internet of underwater things (IoUT) because of its high speed, ease of deployability and wide bandwidth availability which is free of licensing fees. The proposed co-operative RF–UWOC system is designed to establish a connection between an underwater vehicle inside the ocean to a terrestrial ground station using decode?forward and amplify?forward relays. The RF link between the terrestrial ground station to relay is modeled as a Rayleigh distributed channel. The UWOC link between the relay to the underwater vehicle is modeled as being perturbed by the hyperbolic tangent log-normal (HTLN) distribution. To the best of our knowledge, it is for the first time that the perturbations due to weak oceanic turbulence have been modeled using HTLN distribution. This distribution is a member of the class of log-normal distributions derived from hyperbolic tangent distribution. Novel closed-form expressions have been derived for the outage probability and average BER for various modulation techniques that can be employed in this system. The analytical results are evaluated and validated with Monte-Carlo simulations in the presence and absence of pointing errors. The results show that the impact of pointing errors in the RF-UWOC system is to impose an additional SNR penalty of at-least 10 dB to obtain a BER of 10?6 when compared with the system operating without pointing errors. © 2020 Elsevier B.V.
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    Asymptotic bit error rate analysis of convergent underwater wireless optical communication-free-space optical system over combined channel model for different turbulence and weather conditions with pointing errors
    (SPIE, 2020) Bhargava Kumar, B.K.; Krishnan, P.
    The differential phase-shift keying-based dual-hop underwater wireless optical communication-free-space optics (UWOC-FSO) convergent system is proposed for UOWSNs and Internet of Underwater Things (IoUT) applications. In the proposed system, the collected sensor data are transmitted to a decode-and-forward relay using underwater optical wireless communication links modeled as gamma-gamma distribution. The relay transmits the signal to the terrestrial destination using free-space optical link modeled as Malaga distribution. The end-to-end performance of the system (novel expression for asymptotic bit error rate) is derived and analyzed over combined channel model (including the effects of attenuation, turbulence, and pointing errors for both FSO and UWOC channels). The in-depth study is carried out for different weather conditions of FSO (attenuation - very clear, haze, rain, and fog; turbulence - weak and strong; and pointing error - weak and strong based on the g values 1, 2, and 6) and UWOC (attenuation - clear, coastal ocean, and turbid harbor; turbulence - weak, moderate, and strong; and pointing error - weak and strong based on the g values 1, 2, and 6), respectively. The proposed system is highly useful in coastal environments, where the climate is changing adequately as clear, rain, haze, and fog. © 2020 Society of Photo-Optical Instrumentation Engineers (SPIE).
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    Performance enhancement using multiple input multiple output in dual-hop convergent underwater wireless optical communication-free-space optical communication system under strong turbulence with pointing errors
    (SPIE, 2021) Bhargava Kumar, B.K.; Naik, R.P.; Krishnan, P.
    For the first time, we propose a dual-hop multiple input multiple output (MIMO)-based convergent underwater wireless optical communication (UWOC)-free-space optical (FSO) system. The UWOC and FSO links are Gamma-Gamma (GG) distributed. Closed-form expression for the average bit error rate (ABER) is derived for the proposed MIMO-based dual-hop UWOC-FSO convergent system using the GG cumulative distribution function. The end-to-end system performance analysis is carried out by considering the turbulence, attenuation, and pointing error effects for UWOC and FSO links. For the UWOC link, different oceanic conditions, such as the clear ocean, coastal ocean, and turbid harbor, are considered. Various atmospheric effects, such as clear air, fog, rain, drizzle, and haze, are considered for the FSO link. The analytical results of the proposed MIMO-based convergent system are compared with single-input single-output (SISO) system. As a result, it is observed that the proposed MIMO 2 × 3 scheme offers an improvement of 35 dB in the average signal-to-noise ratio compared with the SISO system at ABER of 10-5 in the case of weak pointing error. © 2021 Society of Photo-Optical Instrumentation Engineers (SPIE).
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    Wireless-optical-communication-based cooperative IoT and IoUT system for ocean monitoring applications
    (The Optical Society, 2021) Naik, R.P.; G.D., G.D.G.; Krishnan, P.
    This paper proposes the idea of a new cooperative communication between the Internet of Things (IoT) and the Internet of Underwater Things (IoUT) using wireless optical connectivity for ocean monitoring applications.We considered IoT communication using a hybrid radio frequency (RF)/free space optical (FSO) link and IoUT using a underwater wireless optical communication (UWOC) link. Channel models for RF, FSO, and UWOC links are considered to be Rayleigh,Malaga with pointing errors, and hyperbolic tangent log-normal distributions, respectively. The outage probability and the bit error rate (BER) expressions for the proposed system are derived over the combined channel model, which includes the effects of attenuation, turbulence, and pointing errors. The BER results are plotted for various binary digital modulation schemes such as on-off keying, binary phase-shift keying, binary frequency-shift keying, and differential phase-shift keying over UWOC, hybrid RF/FSO and RF-UWOC, FSO-UWOCwith end-to-end systems.BERresults are extended for various turbulence regions and pointing errors of theFSOlink.MonteCarlo simulation results authenticate the correctness of the results. © 2021 Optical Society of America.
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    RIS Assisted Triple-Hop RF-FSO Convergent With UWOC System
    (Institute of Electrical and Electronics Engineers Inc., 2022) Bhargava Kumar, L.B.; Naik, R.P.; Krishnan, P.; Raj, A.A.B.; Majumdar, A.K.; Chung, W.-Y.
    The convergence of wireless optical communication (WOC) and radio-frequency (RF) systems is a promising technology that overcomes the shortcomings of standalone communication systems. By incorporating reconfigurable intelligent surfaces (RISs) on top of these WOC and RF communication systems, it is possible to circumvent the connection challenges associated with standard line of sight (LOS) communication links. Wireless communication systems with RIS assistance are a promising and evolving technology that enables more efficient and reliable link performance over long distances. The performance of the triple-hop RIS-assisted RF-FSO convergent with the underwater wireless optical communication (UWOC) system is investigated in this article. We considered the fading channel Nakagami-m over the RIS-RF connection and the fading channel Gamma-Gamma (GG) over the RIS-FSO and UWOC links. Then, the average bit error rate (ABER) and outage probability are determined using closed-form expressions. The ABER and outage probability performances of the triple-hop communication system is analysed by varying parameters such as turbulence, misalignment fading, and the number of RIS elements. The obtained results demonstrate an improvement in performance for low turbulence, low pointing error, and an increasing number of RIS elements. Additionally, the data demonstrate the accuracy of the analytical results. © 2013 IEEE.
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    Performance investigation of underwater wireless optical system for image transmission through the oceanic turbulent optical medium
    (Springer, 2022) Naik, R.P.; Shripathi Acharya, U.S.; Lal, S.; Krishnan, P.
    The importance of resources contained in the sea and on the sea floor is increasing with each passing day. Hence, exploration of the sea and sea floor has become a very important requirement. Underwater imaging is a science that has gained importance over the past two decades. Underwater images indicate the state of sea floor and transmitting such images through the harsh and turbulent oceanic medium can cause deterioration of the information contained in the image due to diminished color reproduction, low contrast and blur. In this paper, we have performed the simulation studies to understand perturbations induced during the transmission of sea floor images using high-speed optical signaling through the underwater channels. The transmitted irradiance often suffers from underwater turbulence and beam attenuation. The bit error rate (BER) of the system proposed to transmit information through channels has been determined through analytic means and validated through Monte-Carlo simulation. Comparison between the transmitted and received images in the presence of turbulence and attenuation have been presented. The BER performance of the proposed system is evaluated in the presence of beam attenuation and underwater turbulence. The turbulence induced errors are minimized using the transmit/receive diversity and multiple input multiple output (MIMO) techniques. In addition to the diversity techniques, median and adaptive median filters used to minimize the distortion in the received image. The BER results show that the 4 × 5 MIMO system gains 19.50 dB of transmit power at BER of 10 - 5, when compared with the single input single output system. Similarly, an improvement of at-least 18 dB peak signal to noise ratio obtain using the adaptive median filter based system over the un-filter based system. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
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    Experimental Demonstration and Performance Analysis of Free Space and Underwater Optical Wireless Communication Systems
    (Springer, 2024) Kannan, A.; Bhavana, V.; Younus, S.M.; Rehaman, S.M.; Krishnan, P.
    This research paper presents the development and implementation of a real-time communication system that utilizes LEDs for transmitting text, image, and audio through Free Space Optical Communication (FSOC) and Underwater Wireless Optical Communication (UWOC) channels. The impact of atmospheric and oceanic attenuation and turbulences are considered for the performance analysis of the proposed system. The various atmospheric weather conditions like light fog, dense fog, and heavy rain are considered for FSOC while in the case of UWOC, effects like pure water, and seawater with turbulence and without turbulence are considered. The outcomes of the experiments and simulations, including quality factors, eye diagrams, and bit error rates, are plotted and discussed in the results. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024.