Faculty Publications
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Item 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.Item Adaptive conductance function based improved diffusion filtering and bi-dimensional empirical mode decomposition based image denoising(Springer, 2023) Gupta, H.; Singh, H.; Kumar, A.; Vishwakarma, A.This paper presents a new method for image denoising based on a two-dimensional empirical mode decomposition algorithm and semi-adaptive diffusion coefficient in anisotropic diffusion filter. The proposed model uses a local difference value method to compare and replace some pixels of the noisy image with a pre-processed image that has been passed through a Gaussian filter. A bi-dimensional empirical mode decomposition algorithm is then employed to decompose the noise-contaminated image into its intrinsic mode functions in which high-frequency and low-frequency noise components are removed by applying a diffusion filter. The filter has a semi-adaptive threshold in the diffusion coefficient with parameters like connectivity, conductance function, number of iterations, and gradient threshold. The semi-adaptive threshold for each diffusion is implemented by introducing gradient values in the threshold of the corrupted image. The image is then reconstructed from these denoised intrinsic mode functions. The performance of the proposed method is assessed in terms of peak signal-to-noise ratio, mean square error, and structural similarity index and is compared with the existing methodologies. The results obtained from experimentation indicate that the proposed method is efficient in both feature retention and noise suppression. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item Performance analysis of a RIS-assisted RoFSO communication system over Malaga distribution for smart city applications(Optica Publishing Group (formerly OSA), 2023) Kumar, A.; Krishnan, P.; Raj, A.A.B.Radio over free space optics (RoFSO) is one of the potential technologies that can satisfy the requirements of 5G services in a smart city. However, as RoFSO is line-of-sight (LOS) communication, one of its limitations is the occurrence of a skip zone in the targeted areas. In this work, a reconfigurable intelligent surface (RIS) is proposed as the solution to overcome this connection difficulty, which prevents signal blocking by generating LOS connections. These RIS modules extend the communication channel coverage, making it more intelligent and controllable. The performance analysis based on outage probability, ergodic channel capacity, and bit error rate has been performed using heterodyne detection. Malaga distribution has been used to model atmospheric turbulence. The exact closed-form expressions of the probability density function and cumulative distribution function of the end-to-end signal-to-noise ratio are derived. Exploiting these derived statistics, system performance is investigated through the ergodic channel capacity, outage probability, and average bit error rate for M-ary quadrature amplitude modulation and two binary modulation schemes: non-coherent binary frequency-shift keying and coherent binary phase-shift keying. Numerical results are compared among different turbulence conditions, link lengths, and scattering errors. The results show that the proposed RIS-assisted RoFSO technology has the potential to be effective for 5G smart city applications. © 2023 Optica Publishing Group.Item Variational mode decomposition based image denoising using semi-adaptive conductance function inspired diffusion filtering(Springer, 2024) Gupta, H.; Singh, H.; Kumar, A.; Vishwakarma, A.; Singh, G.K.In day-to-day life, images are the most frequent and casual way of information sharing. These images are susceptible to external disturbances or noise. Thus, to curb noise, image denoising algorithms are utilized. In this paper, the variational mode decomposition, with its concurrent and a non-recursive process for determining the mode functions that also provides a robust method for image denoising, has been introduced. This decomposition process divides the whole spectrum of the signal into a number of sub-bands or mode functions, centered around their respective center frequencies. To these mode functions, spatial filters such as bilateral filter, wiener filter, and modified anisotropic diffusion filter are employed. These filters help in enhancing the yield of the quality assessment metrics; such as mean square error (MSE), peak signal-to-noise ratio (PSNR), and structural similarity index (SSIM), together with the semi-adaptive conductance function in the diffusion filter. The parameters of these respective spatial filters are calibrated, and then selected in order to get the best possible metric scores. The applicability and ability of the algorithm to suppress noise are compared visually and quantitatively for the noisy image using modified variational mode decomposition and other denoising algorithms in both low and high noise levels. The algorithm provides an average decrease of 62% in case of MSE, 28% increase in PSNR, and 110% increase in SSIM when compared with other denoising techniques. The estimated metric score values signify that the proposed method has a better prospect as a denoising algorithm. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item 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.