Linear chirp spread spectrum technique is widely used in underwater acoustic communication because of their resilience to high multipath and Doppler shift. Linear frequency modulated signal requires a high spreading factor to nearly reach orthogonality between two pairs of signals. On the other hand, nonlinear chirp spread spectrum signals can provide orthogonality at a low spreading factor. As a result, it improves spectral efficiency and is more insensitive to Doppler spread than the linear counterpart. To achieve a higher data rate, we propose two variants (half cycle sine and full cycle sine) of the M-ary nonlinear sine chirp spread spectrum technique based on virtual time-reversal mirror (VTRM). The proposed scheme uses different frequency bands to transmit chirp, and VTRM is used to improve the bit error rate due to high multipath. Its superior Doppler sensitivity makes it suitable for underwater acoustic communication. Furthermore, the proposed method uses a simple, low-power bank of matched filters; thus, it reduces the overall system complexity. Simulations are performed in different underwater acoustic channels to verify the robustness of the proposed scheme.

This study investigates acoustic propagation and transmission loss in shallow water at an unexplored site in the northern Arabian Sea near the Pakistan coastline using a normal mode theoretical framework. Sound propagation in shallow water with range-independent bathymetry was analyzed using a customized Kraken C program to compute eigenvalues and eigenfunctions. The sound speed profile and clay silt sediment samples of the northern Arabian Sea, which characterize the water column and ocean bottom, respectively, were determined. Coherent and incoherent transmission losses for frequencies ranging from 50 to 500 Hz were calculated across different ranges and depths. Results indicate significant intensity fluctuations with increasing range, leading to higher transmission loss. Low frequencies (50–225 Hz) exhibit more significant transmission loss, while higher frequencies (230–500 Hz) show reduced loss. Transmission loss is higher for receivers at 19 m depth compared to those at shallower depths (8 m and 12 m) because the receivers are positioned near the layer of bottom sediments. Factors such as source and receiver depth, sediment properties, bottom roughness, and sound frequency significantly influence transmission loss. The novel dataset for the region supports the assessment of sonar performance, underwater communication, navigation, and marine life exploration.

Ocean bottom is considered critical in the computations of Sonar and subsequent performance evaluation. During the detection and classification of submerged targets & threats (i.e. Submarines, Sea-mines, underwater structures, etc.), the scattered Sonar signals by the Ocean bottom may degrade due to interfering reverberation. Thus, it reduces the strength of desired target echo signal. The ocean bottom is usually entirely enclosed by sediments of various types and starting points including any store of insoluble material, mainly soil particles, and rocks. The movement of sediments from land to sea is due to many reasons which include wind, streams and ice. The remaining parts of marine life, results of submarine volcanism substances from seawater, and materials from space that collect on ocean bottom are also in the category of ocean sediments. For acoustic propagation evaluation, specified models of ocean sediments are required which can be extensively arranged by the wave hypothesis utilized (for example fluid, elastic, or poroelastic). In this paper, a study of the acoustic climate, hypotheses of wave spread in sediments, and models got from these speculations mainly focused. The method based on the Biot model with analytical technique to predict the propagation characteristics (velocity of wave and attenuation) of sound propagation in marine sediments are described. Various physical parameters that impact wave speed and attenuation, for example, porosity, grain size, permeability, and effective stress are also numerically analyzed. The simulations and results obtained through the Biot model are evaluated based on various data from the reference literature. Computer simulations are programmed using MATLAB.

The objectives of this paper are to analyze the effectiveness of parameters on sound propagation in a shallow-water environment. The procedure for calculation of transmission loss is only the method to analyze the influence of environmental parameters. The normal mode approach is carried out for the calculation of transmission loss. And it is conducted in the range independent environment Transmission loss for sound propagation in shallow water depends upon many natural variables such as sea surface, water medium, and sea bottom. Analyses are finalized on the results obtained by considering two types of sound channels. The results indicated that acoustic transmission loss in a shallow-water environment is dependent on the source & receiver depths, sea surface, sound speed profile (SSP) in water, sound speed in bottom, density of water & bottom, propagation range and frequency. It is necessary to mention that better transmission was found during the sound velocity increases with depth; whereas the poor transmission occurred in negative gradient channel.