Faculty Publications
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Item Long haul, repeaterless, coherent optical multichannel communication systems employing hybrid multiplexing schemes wherein Wavelength Division Multiplexing (WDM) and Optical Code Division Multiplexing (OCDM) are combined is analyzed in the presence of performance limiting factors such as Inter Channel Interference (ICI), Stimulated Raman Scattering (SRS) and detector shot noise. Error control coding techniques in the form of rate 1/2 convolutional codes are employed in the system to counter these performance limiting factors. It is seen that the use of error control coding technique can enhance the system performance to such an extent that a Bit Error Rate (BER) performance of less than 10-8 is achievable without crossing the SRS power threshold for as many as 1000 to 1200 channels. Advantages of the hybrid multiplexed system over systems relying solely on WDM or OCDM are also discussed along with the advantages of using error control coding compared to optical amplification in countering the performance degrading factors.(Fachverlag Schiele und Sohn GmbH, Coding to enhance performance of hybrid multiplexed coherent optical communication systems in the presence of ICI and SRS) Kumar, M.S.; Bhat, K.N.H.; Umesh, G.2003Item Iterative or Turbo decoding of parallel concatenated binary error control codes in Binary Phase Shift Keyed (BPSK) Communication System corrupted by both Additive White Gaussian Noise (AWGN) and oscillator phase noise is discussed. As an example of application of the discussed theory, iterative decoding technique of Parallel Concatenated block Codes (PCBC) is applied to a coherent Optical Code Division Multiple Access (OCDMA) network in the presence of Multiple Access Interference (MAI), detector shot noise and laser phase noise. Remarkable improvement in performance resulting from the use of turbo decoding in the OCDMA network is also discussed.(Medknow Publications, Performance of iteratively decoded parallel concatenated error control codes in phase noise corrupted BPSK systems) Kumar, M.S.; Bhat, K.N.H.; Umesh, G.2004Item Electrochemical and optical properties of a new donor-acceptor type conjugated polymer derived from 3,4-didodecyloxythiophene(Indian Academy of Sciences, 2010) Hegde, P.K.; Vasudeva Adhikari, A.V.; Manjunatha, M.G.In this communication, we report the synthesis and characterization of a new donor-acceptor type conjugated polymer carrying alternate 3,4-didodecyloxythiophene and (1,3,4-oxadiazol-yl)pyridine moieties and evaluation of its optical and electrochemical properties. The polymer was synthesized through precursor polyhydrazide route. It has well defined structure, stability and it shows good solubility in common organic solvents. Optical and electrochemical properties were studied by UV-visible, fluorescence spectroscopy and cyclic voltammetric studies, respectively. It displays bluish-green fluorescence both in solution and in film state. Cyclic voltammetric studies showed that the polymer P1 possesses a HOMO energy level of -6-01 and LUMO energy level of -3-34 eV. The preliminary studies clearly reveal that the new polymer can have potential application in the fabrication of light emitting diodes. The studies on its device application are in progress. © Indian Academy of Sciences.Item Design and synthesis of a new thiophene based donor-acceptor type conjugated polymer with large third order nonlinear optical activity(2011) Vishnumurthy, K.A.; Vasudeva Adhikari, A.V.; Sunitha, M.S.; Ann Mary, K.A.A.; Philip, R.In this communication we describe the design and synthesis of a new conjugated polymer (P) carrying 3,4-dialkoxythiophene, 1,3,4-oxadiazole and thienylene-vinylene units, from its monomers using Wittig condensation method. The structure of newly synthesized polymer was confirmed by FT-IR, 1H NMR, UV-vis spectral, elemental analysis and gel permeation chromatographic techniques. The polymer exhibited good thermal stability with the onset decomposition temperature around 320 °C in nitrogen atmosphere. Further, its electrochemical, linear and nonlinear optical properties have been investigated. The optical and electrochemical band gap was found to be 2.21 eV. Its third-order nonlinear optical property was investigated by Z-scan and DFWM techniques, using a Q-switched, frequency doubled Nd:YAG laser producing 7 ns laser pulses at 532 nm. Z-scan results reveal that the polymer exhibits self-defocusing nonlinearity. The operating mechanism involves reverse saturable absorption. The polymer shows strong optical limiting behavior due to effective two-photon absorption (2PA). The value of 2PA coefficient was found to be 3.0 × 10-11 mW, which is comparable to that of good optical limiting materials. The fluorescence quantum yield of the polymer in solution was determined using quinine sulfate as standard and it was found to be 35%. © 2011 Elsevier B.V. All rights reserved.Item Optical limiting materials: Synthesis, electrochemical and optical studies of new thiophene based conjugated polymers carrying 1,3,4-oxadiazole units(John Wiley and Sons Inc, 2013) Shivashankar, S.M.; Anantapadmanabha, V.K.; Vasudeva Adhikari, A.V.In this communication we report synthesis of three new donor-acceptor (D-A) type conjugated polymers carrying 1,3,4-oxadiazole moiety and thiophene unit with different side groups at its 3,4 positions (4-methoxybenzyl:P1, 3-methylbenzyl:P2 and 4-nitrobenzyl:P3) through polycondensation route using a series of newly synthesized monomers. The structures of new monomers and polymers were confirmed by NMR, FTIR spectroscopic methods followed by elemental analysis. Further, molecular weight and thermal stability were determined using gel permeation chromatography and thermogravimetric analysis. The linear optical and electrochemical properties of polymers were investigated by UV-vis absorption, fluorescence spectroscopic, and cyclic voltammetric studies. The polymers P1-P3 were found to be thermally stable and their electrochemical band gaps were determined to be 1.98, 2.14, and 2.18 eV respectively. Further their nonlinear optical properties were investigated by Z-scan method using 532 nm, 7 ns laser pulses. The results reveal that they possess good optical limiting behavior due to effective three-photon absorption (3PA) with absorption coefficient 2.5 × 10-24 m-3W2, 1.6 × 10-24 m-3W2, and 1.0 × 10 -24 m-3W2. © 2012 Society of Plastics Engineers.Item BER performance enhancement for secure wireless optical communication systems based on chaotic MIMO techniques(2014) Abdulameer, L.F.; Jignesh, J.D.; Sripati, U.; Kulkarni, M.There has been a growing interest in the use of chaotic techniques for enabling secure communication in recent years. This need has been motivated by the emergence of a number of wireless services which require the channel to provide low bit error rates (BER) along with information security. The aim of such activity is to steal or distort the information being conveyed. Optical Wireless Systems (basically Free Space Optic Systems, FSO) are no exception to this trend. Thus, there is an urgent necessity to design techniques that can secure privileged information against unauthorized eavesdroppers while simultaneously protecting information against channel-induced perturbations and errors. Conventional cryptographic techniques are not designed for protecting information integrity when data is being transferred over a harsh communication medium. Hence, a separate channel coding protocol is often necessary to achieve this goal. Our work indicates that the use of a suitable Chaotic Shift Keying (CSK) map combined with an appropriate Space-Time Code (STC) can allow both requirements to be met. In this paper, we have concentrated on investigating the error rate performance of chaotic-wireless optical communication links operating over atmospheric channel, where the turbulence-induced fading is described by the Gamma-Gamma and log-normal distributions. The main aim of the paper is to assess the feasibility of employing Space-Time Coded chaotic communications over Multiple Input Multiple Output (MIMO) communication channels. Our simulations indicate that the combination of the STC and tent map provides the best BER performance in addition to security when compared to the choice of other maps. © 2013 Springer Science+Business Media Dordrecht.Item Asymptotic bit-error rate analysis of quadrature amplitude modulation and phaseshift keying with OFDM RoFSO over M turbulence in the presence of pointing errors(Institution of Engineering and Technology journals@theiet.org, 2018) Krishnan, K.; Jana, U.; Kumar, 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 Málaga 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.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 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.