Remedies for Different Impairments in Orthogonal Time Frequency Space Systems

Abstract

Embargo up to 15/7/2026 Orthogonal time frequency space (OTFS) has emerged as a promising multiplexing tech nique for future wireless communications. In OTFS-based systems, the information symbols are represented in a two-dimensional (2D) delay Doppler (DD) domain grid. DD-domain representation of the symbols offers three important properties: time invari ancy, compact representation, and sparsity. These three properties make OTFS superior to existing multicarrier multiplexing schemes like orthogonal frequency division multi plexing (OFDM). Time invariancy or slow time variance against wireless channels helps OTFS-based systems to provide reasonably good performance in high mobility; how ever, OFDM fails to do that. Compact representation helps in easy channel estimation in communication systems and easy joint channel and radar parameter estimation in integrated sensing and communication (ISAC) systems. Meanwhile, sparsity helps in easy signal processing for channel estimation, channel equalization, and symbol detec tion. In wireless communication, some impairments, including impulsive noise (IN), jamming attacks, and power amplifier (PA) non-linearities, etc., are prevalent irrespec tive of the multiplexing technique being used. Furthermore, spectrum efficiency (SE) is also one of the main concerns in modern wireless communication due to the scarcity of the available frequency spectrum. Therefore, OTFS-based systems need to handle the above-mentioned impairments and to address these issues, the contributions of this thesis are given below. First, to handle the IN that may arise in power line communication (PLC), indus trial wireless sensor network (IWSN), co-channel and adjacent channel interference in mobile cellular networks, full duplex communication, and under-water communication set-ups, we propose a novel and robust l0-norm constrained maximum Versoria crite rion (l0-MVC)-based channel estimator and maximum Versoria criterion-based message passing detector (MVC-MPD). We derive an adaptive learning method for estimating the optimum sparsity regularization parameter (SRP) to alleviate the need to tune the SRP of the proposed channel estimator in different deployments and time-varying sce narios. Further, to indicate the robustness of the proposed symbol detector under IN, we derive the mean deviation (MD) in the error message and a lower bound (LB) on the bit-error rate (BER) for the proposed symbol detector. The derived MD and LB on BER show better performance than the existing conventional message-passing detector and maximum correntropy criterion-based message-passing detector (MCC-MPD). Next, the issue of jamming attacks in the DD domain is being addressed. The jam ming attack in the DD domain may degrade the performance of the most widely used mean square error (MSE) cost function-based channel estimators and conventional MPD due to the large outliers that restrict MSE-based channel estimators and MPD from converging. Therefore, we propose a recursive Versoria-based soft root sign (RVSR) cost function for channel estimation and a low-complex VSRS-based message passing detector (VSRS-MPD) for symbol detection under jamming attacks. Both the proposed channel estimator and symbol detector are robust under jamming attacks. We derive the MD in the error message and LB on the BER under jamming attack and Nakagami fading channel to justify the superiority of the proposed anti-jammer over existing techniques. The proposed channel estimation scheme requires less pilot overhead than the existing schemes due to the involvement of recursion, which helps in the quick convergence of the algorithm. Further, the proposed anti-jamming schemes are significantly more spec trally efficient than the widely used resource-hopping-based schemes like DD resource hopping (DDRH), frequency-hopped spread sequences (FHSS), and time-hopped spread sequences (THSS). We provide mathematical expressions of the spectrum efficiency (SE) for the proposed anti-jamming scheme to show the improved SE of the proposed anti jammer over existing state-of-the-art anti-jamming techniques. A thorough analysis of the outage probability for non-cooperative and cooperative OTFS-based UAV networks under jamming attacks and a generalized Nakagami-m fading channel. Mathematically derived closed-form expressions of outage probability and simulation results are provided. The effect of UAV location and imperfect channel state information (CSI) on the outage probability is also discussed in detail. Next, we propose a subgrid indexing-non-orthogonal multiple access (SI-NOMA) mechanism in the DD domain. The proposed SI-NOMA can simultaneously support high-rate users (HRU) and low-rate users (LRU). The proposed mechanism also has the capability to serve an unmanned aerial vehicle’s (UAV’s) video and telemetry (V & T) simultaneously by using a single transceiver. We propose a robust and fast converg ing channel estimator, a norm-zero recursive generalized maximum Versoria criterion (l0-RGMVC), and a power threshold-subgrid-dependent generalized maximum Versoria criteria-based message passing detector (PT-SGMVC-MPD) that jointly detects V & T symbols. A complete analytical and simulation study of the proposed scheme has been provided in terms of BER, probability of false detection, spectrum efficiency, and complexity to compare it with the existing state-of-the-art schemes. Next, to address the issue of high peak-to-average power ratio (PAPR) of the existing pilot structure in OTFS-ISAC systems, we propose an all-delay pilot (ADP) OTFS grid structure to get the reduced PAPR. Then, to eliminate the effect of ghost paths and side lobes that arise due to fractional Doppler in the channel, we propose a robust, spectrally efficient, low-complexity, and fast-converging recursive Versoria-aware Champernowne function (VACF)-based adaptive channel estimation algorithm at the communication receiver. At the sensing receiver, we propose an absolute error square-based target range and speed estimator to eliminate the erroneous targets arising from ghost paths. The derived analytical results and simulations indicate that the proposed grid structure, channel estimator, and sensing parameter estimator outperform the existing state-of the-art schemes.