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Author: search.filters.author.Bitragunta, SainathSubject: search.filters.subject.Fading channels

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    Relay Selection, Eavesdropper-Aware Relaying, PHY-Secrecy Capacity Analysis of Cooperative Wireless System over Hybrid Fading Channels
    (IEEE, 2023) Bhatia, Ashutosh; Bitragunta, Sainath
    Physical layer (PHY) security (PLS) exploits the randomness of wireless fading channels and offers better secrecy capacity. Cooperative and eavesdropper-aware relays are useful in establishing reliable and energy-efficient communication links between the source transmitter and destination receiver and enhancing PHY secrecy. We consider a four-node, two-hop cooperative PHY-security model with one eavesdropper node. For it, we propose relay selection probabilistically and relaying in the presence of hybrid fading channels. We derive closed-form expressions for probabilities with which the regenerative relay is selected. Further, we develop an analysis of PHY-secrecy capacity and gain useful insights. We evaluate and validate the performance of the proposed strategy and present different numerical results. The proposed model with the relay selection and energy-efficient relaying strategy is a useful potential benchmark for more complex power-adaptive cooperative PHY-secure systems and networks.
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    Fading Averaged-Spectral Efficiency Analysis of Average Interference-Constrained, Cooperative Device-to-Device Communication System
    (IEEE, 2017) Bitragunta, Sainath
    We consider a two hop cooperative device-to-device (D2D) communication system. In it, a non-regenerative relay is subject to a tight constraint on the average interference it to the base station (BS). We derive interference-aware, spectral efficiency optimal relay amplification policy as a function of its local fading gains. We then derive expressions for fading-averaged spectral efficiency and its upper bound, and, present numerical results wherein we compare optimum amplification policy with fixed-amplification policy, and, also with a non-cooperative link. In all performance plots, we show that the optimal policy delivers superior performance when compared to fixed-amplification policy, and, non-cooperative link.
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    Fading-Averaged Symbol Error Probability Analysis of Full Duplex Amplify-and-Forward Relaying over Rayleigh Fading Channels
    (IEEE, 2018) Bitragunta, Sainath
    Half-duplex amplify-and-forward (AF) relay based cooperation has been studied in the literature given its simplicity and practicality. In this paper, we consider a full-duplex relay-assisted AF cooperation model. For it, we analyze the fading-averaged symbol error probability (FA-SEP). Specifically, we derive an expression for exact FA-SEP. To get more insights, we derive tractable upper bound for the exact FA-SEP. Furthermore, we extend our analysis and derive asymptotic FA-SEP in a scaling regime. Finally, we present numerical results to validate our analysis. Our analysis of FA-SEP provides a theoretical benchmark and is useful for analyzing more complex cooperative systems employing multiple full-duplex AF relays.
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    Energy Harvesting Cooperative Wireless Systems: Probabilistic Modeling and Statistical Analysis
    (IEEE, 2019) Bitragunta, Sainath
    In this work, we consider a two-hop cooperative wireless system that consists of multiple energy harvesting (EH) relays. The system uses a time-switching based relaying protocol for EH and information processing at the amplify-and-forward (AF) relay nodes. For it, we formulate an order statistics problem of finding the distribution of maximum energy harvested among several EH relay nodes by considering two different fading models, namely, path loss and Rayleigh fading model, path loss and shadow fading model. For these models, we derive analytical expressions for the mean value of maximum harvested energy distribution. Furthermore, we also present an insightful outage analysis and numerical results to validate the analytical expressions. The analysis that we present lead to an essential application of the best EH relay selection in EH cooperative wireless systems and networks.
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    Doubly Constrained Underlay Cognitive Radio System: Optimization and Analysis
    (IEEE, 2019) Bitragunta, Sainath
    In this paper, we consider a doubly constrained underlay cognitive radio (CR) system. In it, a secondary user (SU) transmitter (Tx) is subject to average interference constraint, and spectral efficiency constraint. For the underlay system, we investigate both average energy efficiency and average spectral efficiency. Specifically, we develop an algorithm that determines the suboptimal operating point of transmit power with which the SU-Tx must operate for maximum energy efficiency. We derive an analytical expression for the fading-averaged spectral efficiency assuming Rayleigh fading with path loss and shadowing. To validate the analytical results, we simulate the model in MATLAB using Monte-Carlo simulations and investigate the system performance. The analysis that we present is useful for constrained cognitive radio systems and networks.
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    Comparative Performance Investigation of MIMO-OTFS and MIMO-OFDM using Deep Neural Network Modeling
    (IEEE, 2021) Bitragunta, Sainath
    We consider two popular wireless physical layers (PHYs), namely, multi-input multi-output (MIMO)-orthogonal frequency division multiplexing (OFDM) and MIMO-orthogonal time frequency and space (OTFS) modulation. For them, we develop and implement a deep neural network (DNN) model for fading channel estimation and investigate the mean square error (MSE) performance. While MIMO offers spatial multiplexing to enhance data rates, OFDM provides diversity to combat intersymbol interference. However, OFDM has limitations in high Doppler environments. The OTFS modulation, a newly developed two-dimensional modulation scheme, overcomes OFDM’s limitations. The OTFS modulation technique is helpful in high Doppler fading channels and offers several advantages. In the high Doppler scenario, we find that the OTFS outperforms the OFDM modulation scheme. The added benefit of OTFS over OFDM lies in exploiting channel diversity better in both the time domain and frequency domain when we employ the technique with appropriate equalizers. We show via simulations that DL-enabled MIMO-OTFS DNN exhibits a minimum MSE of value 0.4246, which is less than that of MIMO-OFDM DNN, which is 0.5355 for fading scenarios with high Doppler. Furthermore, we compare our DNN model with the existing linear interpolation technique for channel estimation in MIMO-OTFS at high Doppler.
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    Hybrid RIS-assisted Cooperative Communication Model Design and Performance Analysis
    (IEEE, 2022) Bitragunta, Sainath
    The spectral efficiency and energy efficiency are well-studied performance measures of the next-generation wireless systems and networks. Recently, researchers found dynamically controlled electronic meta-surfaces or reconfigurable intelligent surfaces (RIS) as useful means to enhance these performance measures, specifically, energy efficiency, in the absence of a direct transmission path. Passive meta-surfaces can only reflect their received signals without amplifying, whereas active meta-surfaces can amplify and reflect. This paper proposes a hybrid, dual RIS-assisted cooperative wireless system (CORISS) with two non-Line of sight paths, one via active RIS and the other via passive RIS. We also propose a novel performance objective function (POF) that depends on each path’s spectral efficiency and energy efficiency with an average power constraint. We investigate the proposed hybrid CORISS system via numerical simulations. Our simulations show that the energy efficiency of the active RIS path is inferior to the passive RIS path in high SNR scenarios.
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    Generalizing the Amplify-and-Forward Relay Gain Model: An Optimal SEP Perspective
    (IEEE, 2012-11) Bitragunta, Sainath
    Two models for AF relaying, namely, fixed gain and fixed power relaying, have been extensively studied in the literature given their ability to harness spatial diversity. In fixed gain relaying, the relay gain is fixed but its transmit power varies as a function of the source-relay channel gain. In fixed power relaying, the relay transmit power is fixed, but its gain varies. We revisit and generalize the fundamental two-hop AF relaying model. We present an optimal scheme in which an average power constrained AF relay adapts its gain and transmit power to minimize the symbol error probability (SEP) at the destination. Also derived are insightful and practically amenable closed-form bounds for the optimal relay gain. We then analyze the SEP of MPSK, derive tight bounds for it, and characterize the diversity order for Rayleigh fading. Also derived is an SEP approximation that is accurate to within 0.1 dB. Extensive results show that the scheme yields significant energy savings of 2.0-7.7 dB at the source and relay. Optimal relay placement for the proposed scheme is also characterized, and is different from fixed gain or power relaying. Generalizations to MQAM and other fading distributions are also discussed.