Faculty Publications
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Item Non-orthogonal space–frequency block codes from cyclic codes for wireless systems employing MIMO-OFDM with index modulation(Elsevier B.V., 2019) Raghavendra, R.; Shripathi Acharya, U.S.Space–frequency codes (SFC) having error correcting structure can be used to enhance the bit error rate (BER) performance of modern wireless systems (5G and beyond) employing multiple-input multiple-output (MIMO) and multi-carrier communication. In this work, the construction of non-orthogonal space–frequency block codes (NSFBC) from (n,k) cyclic codes has been proposed. In which, n represents the number of symbols in the codeword and k represents the number of symbols in the information sequence. The performance of proposed codes has been evaluated in MIMO systems employing orthogonal frequency division multiplexing and index modulation (MIMO-OFDM-IM). We initially obtained (n,k) full rank cyclic codes for any 1 q m . Further, NSFBCs are obtained from full rank codes using Rank preserving maps. In a 2 × 2 system and a 10-path MIMO channel, the proposed full rank NSFBC with rank-preserving IM mapping (FR-NSFBC-IM), over F 5 2 , provides he similar BER performance when compared to MIMO-OFDM-IM system with Rate-1 Alamouti code and QPSK. Moreover, it provides an improvement in spectral efficiency of about 0.9 b/s/Hz. When compared to the MIMO-OFDM-IM with BPSK, FR-NSFBC-IM codes over F 5 2 provide an asymptotic SNR gain of about 1 dB and also the spectral efficiency has been improved by about 0.6 b/s/Hz. In the 4 × 4 scenario, full rank NSFBCs over F 5 4 with rank deficient IM mapping (RD-NSFBC-IM) provide an improvement in spectral efficiency of about 1.3 b/s/Hz. However, BER performance is similar to that of MIMO-OFDM-IM with BPSK. © 2019Item Spatially Modulated Non Orthogonal Space Time Block Code: Construction and design from cyclic codes over Galois Field(Elsevier B.V., 2019) Godkhindi Shrutkirthi, G.S.; G.D., G.S.; Shripathi Acharya, U.S.A new class of non-binary Spatially Modulated Non-orthogonal Space Time Block Code designs (SM-NSTBC) has been proposed. These designs employ full rank, length n,(n|qm?1,m?n) cyclic codes defined over GF(qm). The underlying cyclic code constructions have the property that the codewords when viewed as m×n matrices over GF(q) have rank equal to m (Full rank). These codes are punctured to yield m×m full rank matrices over GF(q). Rank preserving transformations are used to map the codewords of full rank codes over a finite field to full rank Space Time Block Codes. The proposed scheme can be generalized to handle any number of transmit antenna greater than two. Due to the characteristics of Full rank cyclic codes employed, a coding gain of approximately 1.5 dB to 5 dB is obtained over conventional STBC-SM and SM-OSTBC schemes. This is demonstrated for spectral efficiencies of 4, 5, 7 and 8 bpcu. Analytical as well as Monte-Carlo simulations show that proposed SM-NSTBC outperforms STBC-SM and its variants. The upper bound on average bit error rate has been derived and the computation complexity for ML detection has been estimated. © 2019Item Generalized designs for precoded receive spatial modulation derived from non-orthogonal space time block codes(Springer, 2022) Shashikant, S.G.; G.D., G.D.G.; Shripathi Acharya, U.S.In this paper, a new MIMO scheme termed as precoded Spatially Modulated Non-orthogonal Space Time Block Code (precoded SM-NSTBC) is proposed. The primary concept of precoded SM-NSTBC is to activate a subset of receive antennas in a pre-defined manner and choose specific activated patterns to represent information symbols. We have synthesized schemes derived from full rank Cyclic codes defined over Galois field Rank preserving transformations are used to map the full rank codewords over a finite field to full rank Space Time Block Codes. Due to the characteristics of full rank Cyclic codes employed, a performance improvement of approximately 2 dB to 7 dB is observed. This advantage is realized when the performance of these schemes is compared with precoded SM-OSTBC and precoded STBC-SM. The improvement due to the coding gain is observed in both uncorrelated as well as correlated Rayleigh fading environments. An upper bound on the average bit error rate (ABER) is derived. Close correspondence between Monte-Carlo simulations and analytic values are observed. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item Performance investigation of underwater wireless optical system for image transmission through the oceanic turbulent optical medium(Springer, 2022) Naik, R.P.; Shripathi Acharya, U.S.; Lal, S.; Krishnan, P.The importance of resources contained in the sea and on the sea floor is increasing with each passing day. Hence, exploration of the sea and sea floor has become a very important requirement. Underwater imaging is a science that has gained importance over the past two decades. Underwater images indicate the state of sea floor and transmitting such images through the harsh and turbulent oceanic medium can cause deterioration of the information contained in the image due to diminished color reproduction, low contrast and blur. In this paper, we have performed the simulation studies to understand perturbations induced during the transmission of sea floor images using high-speed optical signaling through the underwater channels. The transmitted irradiance often suffers from underwater turbulence and beam attenuation. The bit error rate (BER) of the system proposed to transmit information through channels has been determined through analytic means and validated through Monte-Carlo simulation. Comparison between the transmitted and received images in the presence of turbulence and attenuation have been presented. The BER performance of the proposed system is evaluated in the presence of beam attenuation and underwater turbulence. The turbulence induced errors are minimized using the transmit/receive diversity and multiple input multiple output (MIMO) techniques. In addition to the diversity techniques, median and adaptive median filters used to minimize the distortion in the received image. The BER results show that the 4 × 5 MIMO system gains 19.50 dB of transmit power at BER of 10 - 5, when compared with the single input single output system. Similarly, an improvement of at-least 18 dB peak signal to noise ratio obtain using the adaptive median filter based system over the un-filter based system. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item Performance analysis of underwater vertical wireless optical communication system in the presence of weak turbulence, pointing errors and attenuation losses(Springer, 2023) Savidhan Shetty, S.C.S.; Naik, R.P.; Shripathi Acharya, U.S.; Chung, W.-Y.In this paper, we have investigated the performance of underwater vertical wireless optical communication (UVWOC) link employing on–off key modulation in the presence of underwater turbulence, pointing errors and attenuation losses. The turbulence of the medium (assumed to be weak turbulence) has been modeled by employing the hyperbolic tangent log normal (HTLN) distribution. Temperature, pressure and salinity are parameters which can bring about variation of optical transmission characteristics with respect to depth of the ocean/sea. An in-depth study of optical transmission through vertical oceanic links requires the the underwater medium to be modeled as comprising of non-identical turbulent layers. Each of these independent and non-identical turbulence layers are modeled using the HTLN distribution function. The pointing error due to misalignment between source and detector is modeled using Rayleigh displacement pointing error. A novel closed-form expression to quantify the average bit error rate (BER) has been derived for single input single output (SISO) communication link. This expression has then been further generalized to make it applicable to the case of receive diversity schemes such as selection combining, majority logic combining and maximum ratio combining. The expression for the average BER associated with the UVWOC link for different pointing errors, different data rates and different types of ocean water has been determined. Novel closed-form expressions quantifying the outage probability and ergodic channel capacity have been derived for SISO and SC receive diversity schemes. The accuracy of all of the closed-form expressions derived in this paper have been validated using Monte-Carlo simulations. © 2022, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Item On the Design of SSRS and RS Codes for Enhancing the Integrity of Information Storage in NAND Flash Memories(Institute of Electrical and Electronics Engineers Inc., 2023) Achala, G.; Shripathi Acharya, U.S.; Srihari, P.The revolution in the field of information processing systems has created a huge demand for reliable and enhanced data storage capabilities. This demand is being met by advances in channel coding algorithms along with upward scaling of the capacities of hardware devices. NAND Flash memory is a type of non-volatile memory. Scaling of the size of flash memories from Single Level Cell (SLC) devices to Multilevel cell (MLC) devices has increased the storage capacity. However, these multi-bit per cell architectures are characterized by significantly higher Raw Bit Error Rate (RBER) values when compared with SLC architectures. The requirement of low Undetected Bit Error Rate (UBER) values has motivated us to synthesize powerful channel codes for enhancing the integrity of information Storage in multi-level NAND Flash Memory devices. This paper describes the synthesis of novel Subfield Subcodes of Reed Solomon Codes (SSRS) and Reed-Solomon (RS) codes which are matched to multi-bit per cell architectures. UBER values have been calculated for each of the synthesized codes described in this paper. This allows the determination of the performance and the improvement in data storage integrity brought by using these codes. We have shown that the synthesized SSRS and RS codes can provide very low UBER even when the corresponding RBER values are appreciable. As RS codes permit the detection and correction of a greater number of errors for a given code length, their performance is superior to that of SSRS codes. This improved performance is obtained at the cost of greater complexity of encoding and decoding processes. © 2013 IEEE.Item Performance analysis of underwater vertical wireless optical communication links using selection combining(Optica Publishing Group (formerly OSA), 2023) Savidhan Shetty, S.C.S.; Naik, R.P.; Shripathi Acharya, U.S.In this paper, we have investigated the performance of underwater vertical wireless optical communication (UVWOC) links employing on–off key modulation for selection combining based receive diversity schemes in the presence of underwater turbulence, pointing errors, and attenuation losses. Due to variations in temperature and salinity along the ocean’s depth, turbulence-induced fading affects the performance of the underwater vertical wireless optical link. The vertical link of the underwater medium influenced by a strong turbulence regime is modeled using cascaded gamma–gamma distribution considering multiple non-identical layers along with attenuation losses and pointing errors. We have derived closed-form expressions for the average bit error rate (BER) and outage probability (OP) for the link employing multiple detectors at the receiving end. The accuracy of all of the closed-form expressions derived in this paper has been validated using Monte Carlo simulations. © 2023 Optica Publishing Group.Item Performance Analysis of MIMO-EGC System for the Underwater Vertical Wireless Optical Communication Link(Institute of Electrical and Electronics Engineers Inc., 2023) Savidhan Shetty, S.C.S.; Naik, R.P.; Shripathi Acharya, U.S.; Chung, W.-Y.In this paper, we have investigated the performance of an underwater vertical wireless optical communication (UVWOC) link employing multiple input-multiple output (MIMO) operating in conjunction with equal gain combing (EGC) techniques perturbed by weak and strong turbulence in the presence of pointing errors and attenuation losses. Vertical underwater turbulence, which varies from layer to layer due to temperature and salinity variation connected to depth, is modeled using hyperbolic tangent log-normal (HTLN) distribution in the case of weak underwater turbulence and gamma-gamma (GG) distribution in the case of strong underwater turbulence. Novel closed-form expressions quantifying the average bit error rate (BER) have been derived for the UVWOC MIMO EGC system for weak and strong turbulence regimes. The expression for the average BER associated with the UVWOC link for different values of pointing error, differing vertical layer depth, modulation types, and differing numbers of sources and detectors have been determined. In addition, closed-form expressions for the outage probability (OP) and ergodic channel capacity (ECC) have been derived for the UVWOC MIMO EGC system. The accuracy of all closed-form expressions derived in the paper has been verified using Monte Carlo simulations. © 2013 IEEE.