Faculty Publications

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Subject: search.filters.subject.Atmospheric turbulence

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    BER performance and energy efficiency of Luby Transform codes with varying BPSK thresholds over the Free Space Optical channel
    (2013) Prakash, G.; Kulkarni, M.; Sripati, U.
    In this paper we analyse the Bit Error Rate(BER) performance and energy efficiency of Luby Transform (LT) codes when the signals are Binary Phase shift Keying (BPSK )modulated with varying thresholds and transmitted over the Free Space Optical (FSO)Channel in a wireless sensor network. We model the FSO channel using the Gamma-Gamma distribution function and compare the performance for varied turbulence parameters. We also show how the distribution of the BPSK symbols varies as the distribution varies when the turbulence changes from strong to weak. Use of forward error correction helps in the recovery of the original signal even if some transmitted signals are corrupted during transmission. We analyse the FSO system, which employs Luby Transform Codes, which do not have a fixed code rate. They are considered Rateless, and as many codewords are generated as required to recover all the message bits. We show an increase in SNR and reduced energy per bit for the same BER with LT codes as the BPSK threshold increases. © 2013 IEEE.
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    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.
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    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.
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    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.
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    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 GmbH
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    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.
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    Performance analysis of RoFSO links with spatial diversity over combined channel model for 5G in smart city applications
    (Elsevier B.V., 2020) Kumar, A.; Krishnan, P.
    The smart city concept improves the lives of citizens and optimizes the efficiency of city operations, services through the integration of information and communication technology with the help of the internet of things (IoT) and 5G techniques. The bandwidth demand for 5G, smart city, and IoT applications are fulfilled with wireless optical communications. Particularly, radio over free space optical (RoFSO) communication establishes a very attractive choice for interconnecting central base stations with remote antenna units. In this paper, we consider the transmission of orthogonal frequency division multiplexing (OFDM) radio signals with quadrature amplitude (QAM) modulation format through a free space optical link using spatial diversity mitigation technique. The atmosphere is modeled as the combined channel model which takes into account atmospheric attenuation, turbulence, and pointing errors. The atmospheric turbulence and pointing errors are modeled by Malaga, Beckmann, and Rayleigh distributions. The novel closed form BER expressions are derived for the proposed QAM OFDM RoFSO link with spatial diversity. The results are analyzed and plotted for different weather conditions (clear, haze, light fog), turbulence regimes (weak, moderate, strong), misalignment (weak, enhanced), the order of QAM and number of transceivers. The proposed RoFSO system is highly useful for 5G in smart city applications. © 2020 Elsevier B.V.
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    RoFSO system based on BCH and RS coded BPSK OFDM for 5G applications in smart cities
    (Springer, 2022) Kumar, A.; Krishnan, P.
    The radio over the free space optical (RoFSO) communication system has become a popular research topic in 5G communication in recent years. Atmospheric turbulence typically degrades the performance of the RoFSO system. Multiple input multiple output, aperture averaging, error-correcting codes, and robust modulation are standard mitigation techniques used to reduce the effects of atmospheric turbulence. In this paper, Reed Solomon (RS) and Bose-Chaudhuri-Hocquenghem (BCH) coded binary shift keying (BPSK) orthogonal frequency division multiplexing (OFDM) based RoFSO system is proposed for 5G applications. We introduced RS and BCH coding techniques for the first time in this proposed RoFSO system, and achieved an average bit error rate (ABER) of 10 - 6, at 40 dB, 17 dB, and 4 dB carrier to noise plus distortion ratio (CNDR) for the uncoded, RS coded, and BCH coded systems, respectively, under weak turbulence conditions. That is, when compared to the uncoded system, the proposed RS and BCH coded system provide transmit power gains of 13 dB and 34 dB, respectively. The ABER performance of the proposed coded RoFSO system is investigated and compared to an uncoded system under various turbulence, weather, and pointing error cases. In all turbulence regimes, weather conditions, and pointing error scenarios, the BCH coded system outperforms the RS coded and uncoded systems. The proposed RS and BCH coded system is energy efficient and may be useful in 5G implementation. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
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    Performance analysis of radio-over-free-space optical communication system with spatial diversity over combined channel model
    (Springer, 2022) Kumar, A.; Krishnan, P.
    Radio over Free Space Optical (RoFSO) communication is accepted as one of the promising technologies in communication systems that can fulfill the demands of high bandwidth and high data rate because it has an inherent quality of transmission capacity significantly more than what is provided by radio transmission technologies. It is a low power, high data rate, unlicensed spectrum, and large bandwidth wireless technology. Nevertheless, the full potential of the RoFSO communication system can be utilized only by overcoming the adverse effects of the atmospheric channel, which are scattering, absorption, and turbulence. Pointing error is also another factor responsible for the deterioration of the performance of the RoFSO system. In this paper, spatial diversity at the transmitting and receiving ends is used to improve the performance of the RoFSO system in various turbulence and weather conditions. The Malaga distribution has been used to model atmospheric turbulence. For single input single output (SISO), single input Multiple output (SIMO), multiple input single output (MISO), multiple input multiple output (MIMO) configurations, closed form expressions for average bit error rate (BER) have been estimated. To improve the performance of the proposed system, the two combining techniques, Optimal Combining and Equal Gain Combining, are being considered. The results obtained are compared to various configurations such as SISO, SIMO, MISO, and MIMO cases. The result shows that MIMO offers better average BER performance compared with SISO, SIMO and MISO cases. The 4 × 4 MIMO case has an average BER of 10 - 9 at an average SNR of 25 dB, but the SISO case has an average BER of 10 - 1 at the same average SNR. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.
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    Reconfigurable intelligent surface-assisted free-space optical communication system under the influence of signal blockage for smart-city applications
    (Optica Publishing Group (formerly OSA), 2022) Naik, R.P.; Krishnan, P.; G.D., G.D.G.
    Atmospheric turbulence and pointing errors represent substantial hurdles to free-space optical communications (FSOs), impeding their practical design. The reconfigurable intelligent surface (RIS) is an emerging technology that enables reflective radio transmission conditions for next-generation 5G/6G wireless frameworks by intelligently adjusting the beam in the desired direction using low-cost inactive reflecting elements. In this paper, we proposed an RIS-assisted FSO system for mitigating the effects of atmospheric turbulence, pointing errors, and communication system signal blockage. The probability density function and cumulative distribution functions of an FSO system composed of N-RIS elements are evaluated in a free-space environment that contains obstructions. We derived closed-form expressions for the proposed system's bit error rate (BER), outage probability, and channel capacity. The proposed system's performance is analyzed in terms of BER, outage probability, and channel capacity under various weather conditions, pointing errors, and signal blockage. The results are plotted as a function of number of RIS elements and average signal-to-noise ratio. The proposed system will be beneficial in smart-city applications since it will provide reliable connectivity in urban environments with a high population density and high-rise buildings. © 2022 Optica Publishing Group