Browsing by Author "Krishnan, P."

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A design for an ultrafast all-optical full subtractor based on two-dimensional photonic crystals
(Springer, 2021) Abhishek, J.; Krishnan, P.; Robinson, S.
A novel passive all-optical full subtractor device is realized using a two-dimensional (2-D) photonic crystal. The proposed structure is realized using the concept of beam interference in a sequence of T- and Y-shaped line-defective waveguide structures made of a linear material such as silicon with air as the background medium. The major advantage of this design approach is the avoidance of nonlinear materials and semiconductor optical amplifiers. In addition to the actual input ports, some reference input ports are also utilized to achieve the desired full subtractor functionality. The different input combinations to the full subtractor are given and optimized via several simulations. The full subtractor functionality is validated using the Opti-FDTD tool. The simulation results for the full subtractor are supported by the E-field distribution and power analysis at the output ports for different input logic values at a wavelength around 1550 nm. The proposed structure shows a fast response time of 0.16 ps, a bit rate of 6.25 Tbps, and a contrast ratio of 3.97 dB, and occupies an area of 465 µm2. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.
A Highly Sensitive Photonic Crystal Fiber Gas Sensor for the Detection of Sulfur Dioxide
(Springer Science and Business Media B.V., 2022) Elizabeth Caroline, E.C.; Nizar, S.M.; Krishnan, P.
Sulfur dioxide (SO2) is one of the most prevalent contaminants in the atmosphere. It is mostly generated as a byproduct of the burning of sulfur-containing coal and oils, as well as the smelting of various ores. SO2 contributes to the development of major diseases such as asthma and chronic bronchitis. In this paper, we proposed a Wheel Structured circular air hole Photonic Crystal Fiber (WS-PCF) based gas sensor to detect SO2 gas. The proposed WS-PCF gas sensor consists of a four-layer-thick circular cladding air hole. The diameter of the first layer varies throughout the optimization procedure, while the diameters of the succeeding three layers remain constant. The numerical investigation on the sensor parameters such as numerical aperture, effective area, non-linearity, confinement, loss and the relative sensitivity of the proposed sensor are extensively analyzed in a wavelength range of 0.9 µm to 1.2 µm. The proposed WS-PCF gas sensor offers the highest relative sensitivity of 83.64% and a lower confinement loss of 6.34 × 10 - 9dB/ km. The proposed sensor is simple and offers 14% high sensitivity and very low confinement loss (10-3reportedinliterature) compared with the exist literature. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.
A low loss hexagonal six-port optical circulator using silicon photonic crystal
(Springer, 2023) Sangeetha, S.; Vani, D.; Gupta, P.; Krishnan, P.
A 6-port optical circulator using silicon photonic crystals has been designed and proposed in this paper as an essential component of an optical communication system. The proposed 6-port circulator has greater isolation values between the input and isolated ports and lower insertion loss values between the input and output ports. The proposed circulator achieves maximum isolation of 38.7 dB and minimum insertion loss of 0.0029 dB. The proposed designs are extremely useful in the telecommunications industry, wavelength division multiplexing and photonic integrated circuits applications due to their low insertion and high isolations. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
A Low-Complexity Solution for Optimizing Binary Intelligent Reflecting Surfaces towards Wireless Communication
(Multidisciplinary Digital Publishing Institute (MDPI), 2024) Janawade, S.A.; Krishnan, P.; Kandasamy, K.; Holla, S.S.; Rao, K.; Chandrasekar, A.
Intelligent Reflecting Surfaces (IRSs) enable us to have a reconfigurable reflecting surface that can efficiently deflect the transmitted signal toward the receiver. The initial step in the IRS usually involves estimating the channel between a fixed transmitter and a stationary receiver. After estimating the channel, the problem of finding the most optimal IRS configuration is non-convex, and involves a huge search in the solution space. In this work, we propose a novel and customized technique which efficiently estimates the channel and configures the IRS with fixed transmit power, restricting the IRS coefficients to (Formula presented.). The results from our approach are numerically compared with existing optimization techniques.The key features of the linear system model under consideration include a Reconfigurable Intelligent Surface (RIS) setup consisting of 4096 RIS elements arranged in a 64 × 64 element array; the distance from RIS to the access point measures 107 m. NLOS users are located around 40 m away from the RIS element and 100 m from the access point. The estimated variance of noise (Formula presented.) is 3.1614 (Formula presented.). The proposed algorithm provides an overall data rate of 126.89 (MBits/s) for Line of Sight and 66.093 (MBits/s) for Non Line of Sight (NLOS) wireless communication. © 2024 by the authors.
A performance study of an inter-UAV-based free space optical (FSO) system in the maritime environment
(Walter de Gruyter GmbH, 2025) Nallagonda, V.R.; Nagaraju, N.S.; Bhavani, D.; Gogineni, R.; Suneel, S.; Krishnan, P.
Recent advances in optical communication technology have enabled users to access high-data services on maritime links using the unmanned aerial vehicles (UAVs)-based free space optical (FSO) system. However, ensuring reliable data communication for maritime links is a significant challenge due to harsh weather conditions. To address the growing demand for maritime links, we have proposed a maritime UAV-based FSO communication system that can support high-speed data rates and provide extended communication coverage. This paper investigates various aspects of maritime UAV-based FSO systems, including the basic architecture, different channel characteristics, and application scenarios. The paper also includes an analysis of the outage probability of the Inter UAV-based FSO communication system, considering the lognormal distributed channel model for the maritime Inter UAV-based FSO link with heterodyne detection (hd). Additionally, it presents an accurate analytical channel model between UAVs. It compares analytical and simulation results, considering maritime turbulence, pointing error, and hovering UAV fluctuations. © 2025 Walter de Gruyter GmbH. All rights reserved.
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.
All-Optical UAV-Based Triple-Hop FSO-FSO-VLC Cooperative System for High-Speed Broadband Internet Access in High-Speed Trains
(Institute of Electrical and Electronics Engineers Inc., 2023) Aarthi, A.; Bhargava Kumar, L.B.; Krishnan, P.; Natarajan, R.; Jayakody, D.N.K.
In this paper, we proposed an unmanned aerial vehicle (UAV) based all optical triple hop mixed free space optical-free space optical-visible light communication (FSO-FSO-VLC) system for broadband internet access in high-speed train applications. The system consists of triple hops from gateway to the UAV, UAV to train and train to the end user. Two decode and forward relays are mounted on the UAV and train respectively to transmit the data between the gateway to the end-users. The first hop between gateway to UAV consist of FSO link which follows M-distribution. The second hop between the UAV to train consists of FSO link and is modelled using Gamma-Gamma distribution which takes into account both atmospheric turbulence and pointing errors due to position/orientation deviation. The third hop between the train and end user is connected using VLC link and it is mathematically modelled using Lambertian emission distribution. The relays map the incoming signal on the FSO links and send it down to the user inside the train via the VLC downlink. We derived the closed form expressions for average bit error rate and outage probability of the proposed system. This paper investigates the effects of atmospheric turbulence, field of view, beam divergence angle, displacement deviation variance, optical concentrator gain, number of access point, and modulation schemes on system performance. First time, we proposed all-optical system which offers high data rate and low transmission delay. © 2013 IEEE.
Analysis and Enhancement of Spectral Efficiency of UWOC System for IoUT Applications
(Institute of Electrical and Electronics Engineers Inc., 2023) Mitra, A.; Kumar, A.; Krishnan, P.
Underwater wireless optical communication (UWOC) has engrossed significant attention in diverse fields, such as defence, military applications, environmental monitoring, and scientific research. The growing number of interconnected devices in underwater environments has increased the importance of UWOC capacity analysis. It will be instrumental in underwater optical wireless sensor networks (UOWSNs) and the Internet of Underwater Things (IoUT). Understanding the concept of underwater channel capacity is crucial as it determines the maximum volume of reliable information that can be sent through the underwater communication channel. This work focuses on analyzing the performance of a UWOC system that facilitates communication between a surface source (ship) and an autonomous underwater vehicle (AUV) as the intended recipient. The investigation considers explicitly heterodyne detection and models the channel with Exponential Generalized Gamma (EGG) distribution. The study presents closed-form expressions for average channel capacity, employing Meijer's G function and derived average spectral efficiency (ASE) based on the derived channel capacity. Additionally, the paper delves into the influence of various factors, including pointing errors, bubble levels, water types, beam waist, and aperture radius, on the system's overall performance. Â© 2023 IEEE.
Analysis and optimization of uniform FBG structure for sensing and communication applications
(Springer, 2020) Divya Shree, M.; Sangeetha, A.; Krishnan, P.
A uniform fiber Bragg grating sensor is sketched and inspected by the finite-difference time-domain method in furtherance of obtaining ultimate transmission and reflection spectra by optimizing the FBG parameters like refractive index, grating height, grating width, wafer width, wafer length. The maximum transmission power spectrum is achieved as ? 7 dB for the refractive index of 3.005, and the maximum reflection spectra are obtained as 6 dB for the grating height of 1 ?m which is enhanced nine times than the precedent work. The proposed FBG is a simple, light-weight, low-cost uniform structure, and it offers high reflectivity and ease of handling. Therefore, it is highly useful in sensing and communication applications. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.
Analysis of beam divergence on free space optical link using polarization shift keying technique
(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).
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).
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.
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).
Asymptotic bit-error rate analysis of quadrature amplitude modulation and phaseshift keying with OFDM RoFSO over M turbulence in the presence of pointing errors
(2018) Krishnan, P.; Jana, U.; Ashokkumar, B.K.
In case where installation of optical fibre is not practically reliable, the new generation wireless communication popularised as free-space optics (FSO) systems can be used to transmit radio-frequency (RF) signals. FSO technology can efficiently transfer multiple RF signals, and this new technique is commonly referred as radio-on-FSO (RoFSO). The major challenges in this technology are atmospheric turbulence and pointing errors. In this study, the authors analysed the performance of RoFSO links employing the orthogonal frequency division multiplexing (OFDM) scheme under both modulation formats namely quadrature amplitude modulation and phase shift keying in the newly developed M laga or M distribution turbulent channel model. The performance is evaluated with consideration of both scintillation and pointing errors in M distribution channel for higher values of the carrier-to-noise plus distortion ratio. M distribution channel serves as an effective generalised model which unifies all other available statistical channel models. The novel expressions of average bit-error rate has been derived and performance comparison has been done between the two modulation formats. This study identifies the crucial parameters that deteriorate the efficiency of the OFDM signal over FSO link which can be helpful in designing an optimal system. The Institution of Engineering and Technology 2018.
BER analysis of a full-duplex relay-assisted BPSK-SIM based VLC system for indoor applications
(Optica Publishing Group (formerly OSA), 2023) Bhargava Kumar, L.B.; Naik, R.P.; Choudhari, D.; Krishnan, P.; G.D., G.D.G.; Jagadeesh, V.K.
This paper contemplates a relay-assisted visible light communication (VLC) system, where the light source (table lamp) acts as a relay node and cooperates with the main light source. Following the IEEE 802.15.7r1 VLC reference channel model, we assume that there are two different light sources present in an office room. The first one is the source terminal present on the ceiling and another one is the desk lamp that serves as the relay station, which works in a full-duplex method. Because of the loop interference channel, we model the VLC relay terminal using ray tracing simulations. We have analyzed the bit error rate (BER) performance of the relay-assisted VLC system using a binary phase shift keying–subcarrier intensity modulation (BPSK-SIM) technique. The proposed method outperforms existing phase shift keying (PSK) and square M-quadrature amplitude modulation (M-QAM) techniques. The proposed VLC system using the BPSK-SIM technique achieves a BER performance of 10−12 for an SNR of 20 dB. The results of the proposed full-duplex and half-duplex relayed VLC systems are evaluated using equal power allocation (EPA) and optimum power allocation (OPA) techniques over three different modulation schemes, which are 2-PSK, square M-QAM, and BPSK-SIM. © 2023 Optica Publishing Group.
Bit error rate analysis of ground-to-high altitude platform free-space optical communications using coded polarization shift keying in various weather conditions
(Springer, 2022) Nallagonda, V.; Krishnan, P.
High altitude platforms (HAPs) aided free-space optical (FSO) communication, a future emerging technology for next-generation communication systems. HAP aided FSO communication systems, contributing significantly to data hunger applications. Weather conditions, angle of arrival fluctuations, blockages, and pointing error loss due to the HAP’s hovering state are some of the limitations to establishing an efficient link. In this paper, we proposed for the first time a Ground-to-HAP FSO communication system based on polarization shift keying to improve performance under hovering fluctuations. We also improved the proposed system’s performance by employing BCH and repetition coding schemes. The proposed system’s average bit error rate performance is expressed in closed form, and the results are analysed under various weather conditions such as rain (light and heavy) and fog (light and moderate). The results for coded and uncoded cases are compared. The achieved coding gain is 28.5 dB. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
Bit error rate analysis of polarization shift keying based free space optical link over different weather conditions for inter unmanned aerial vehicles communications
(Springer, 2021) Nallagonda, V.; Krishnan, P.
The increasing availability of unmanned aerial vehicles (UAVs) is an exciting part of future emerging technology with advanced scientific and industrial interests. Free space optical (FSO) communications’ ability to offer very high data rates and the mobility of unmanned aerial vehicle (UAV) flying platforms make the delivery of Fifth-Generation (5G) wireless networking services appealing to FSO-UAV-based solutions. UAVs play a greater role in end-to-end delivery in next- generation wireless networking systems, serving as a base station, capacity enhancement, high data access, and other disaster management systems. To establish a link between unmanned aerial vehicles and ground stations, FSO can be applied. But, the different weather conditions liken rain, fog effects on the performance of the FSO link, contributing to the loss of the signal. In this paper, we proposed polarization shift keying (POLSK) modulated FSO link based UAV–UAV communication system for 6G beyond applications. We examine the effect of different weather conditions such as rain, fog on the bit error rate (BER) performance of the proposed system. Novel closed-form expressions for UAV–UAV based FSO propagation channel are derived, and BER performance is investigated under different weather conditions. Fog and rain are the main limiting factors mitigated in this paper by suitable mitigation techniques by increasing receiver field of view. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
CAR-BRAINet: Sub-6 GHz aided spatial adaptive beam prediction with multi head attention for heterogeneous vehicular networks
(Institute of Physics, 2025) Menon, A.G.; Krishnan, P.; Lal, S.
Heterogeneous Vehicular Networks (HetVNets) play a crucial role by integrating different communication technologies, such as sub-6 GHz, mm-wave, and DSRC, to meet the diverse connectivity requirements of 5G/B5G vehicular networks. HetVNet helps address humongous user demands, but maintaining a steady connection in highly mobile, real-world conditions remains challenging. Though ample studies have been conducted on beam prediction models, a dedicated solution for HetVNets has been sparsely explored. Hence, developing a reliable beam prediction model, specifically for HetVNets, is necessary. This paper introduces a lightweight deep learning-based model termed ‘CAR-BRAINet’, which consists of convolutional neural networks with a powerful multi-head attention (MHA) mechanism. Existing literature on beam prediction is primarily studied under a limited, idealised vehicular scenario, often overlooking the real-time complexities and intricacies of vehicular networks. Therefore, this study aims to mimic the complexities of a real-time driving scenario by incorporating key factors, such as prominent MAC protocols (3GPP-C-V2X and IEEE 802.11BD), the effect of Doppler shifts under high velocity and varying distance, and SNR levels, into three high-quality dynamic data sets for urban, rural, and highway vehicular networks. CAR-BRAINet achieves a steady improvement of 11.6467% in spectral efficiency, with a 93.1638% lighter architecture compared to existing methods, resulting in a 94.7103% reduction in prediction time. Therefore, demonstrating a precise beam prediction across all vehicular scenarios, with minimal beam overhead. Thus, this study justifies the effectiveness of CAR-BRAINet in complex HetVNets, offering promising performance without relying on mobile users’ location, angle, and antenna dimensions, thereby reducing redundant sensor latency. © 2025 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.
Channel capacity enhancement of RIS-assisted FSO communication system for high-speed trains access connectivity
(Elsevier B.V., 2025) Kumar, A.; Dewangan, K.; Krishnan, P.
The rising utilization and technological progress of high-speed train (HST) systems, combined with the requirement for uninterrupted and dependable internet connectivity, have made it essential to provide broadband internet services onboard HSTs. Free space optics communication (FSOC) is a promising technology for providing broadband Internet access to HSTs. Technological innovations such as reconfigurable intelligent surfaces (RISs) are being explored to enhance the performance of FSOC systems. This study proposes an Optimal Rate Adaptation (ORA)-based RIS-assisted FSOC system. We investigated the statistical properties of RIS-assisted and direct FSO channels under moderate to strong turbulence conditions with and without pointing errors. Considering the aforementioned fading conditions, we subsequently formulated the average signal-to-noise ratio and the average channel capacity of the system using the ORA technique. The results show that the proposed RIS-assisted FSOC system achieves an average spectral efficiency (ASE) of 4 bits/sec/Hz at 24 dB average SNR for a 500 m link length. © 2025 Elsevier B.V.
Circular-Pattern Photonic Crystal Fiber for Different Liquids with High Effective Area and Sensitivity
(2019) Senthil, 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.