arXiv subjects

Ambuj Kumar Mishra

Publications and source records attributed to Ambuj Kumar Mishra.

7 recordsLinked to original sources

arXiv2025

3D-HQAM Constellation Design and Performance Evaluation under AWGN

This paper proposes a simple and effective method for constructing higher-order three-dimensional (3D) signal constellations, aiming to enhance the reliability of digital communication systems. The approach systematically extends the conventional two-dimensional hexagonal quadrature amplitude modulation (2D-HQAM) constellation into a 3D-HQAM signal space, forming structured lattice configurations. To address the increased decision complexity resulting from a larger number of constellation points, a dimension reduction (DR) technique is introduced, allowing the derivation of closed-form symbol error probability (SEP) expressions under additive white Gaussian noise (AWGN) conditions. Theoretical SEPs closely match simulation results, validating the accuracy of the proposed method. The minimum Euclidean distance (MED) of the 3D constellations shows a minimum increase of 12.14% over 2D constellation for 8-HQAM, reaching up to 160.81% for 1024-HQAM constellations. This significant improvement in MED leads to enhanced error performance. Therefore, the proposed 3D constellations are promising candidates for high-quality and reliable next-generation digital communication systems.

eess.SP↗

arXiv2023

Yukawa-Casimir wormholes in f(Q) gravity

Casimir energy is always suggested as a possible source to create a traversable wormhole. It is also used to demonstrate the existence of negative energy, which can be created in a lab. To generalize, this idea, Yukawa modification of Casimir source has been considered in Remo Garattini (Eur. Phys. J. C 81 no.9, 824, 2021). In this work, we explore the Yukawa Casimir wormholes in symmetric teleparallel gravity. We have taken four different forms of $f(Q)$ to obtain wormhole solutions powered by the original Casimir energy source and Yukawa modification of the Casimir energy source. In power law form $f(Q)= αQ^2 + β$ and quadratic form $f(Q)= αQ^2 + βQ + γ$, where $α, β, γ$ are constants and $Q$ is non-metricity scalar, we analyze that wormhole throat is filled with non-exotic matter. We find self-sustained traversable wormholes in the Casimir source where null energy conditions are violated in all specific forms of $f(Q)$, while after Yukawa modification it is observed that violation of null energy conditions is restricted to some regions in the vicinity of the throat.

physics.gen-ph↗

arXiv2021

Traversable wormhole solutions with non-exotic fluid in framework of $f(Q)$ gravity

The presence of exotic matter for the existence of the wormhole geometry has been an unavoidable problem in GR. In recent studies researchers have tried to deal with this issue using modified gravity theories where the WH geometry is explained by the extra curvature terms and NEC's are not violated signifying the standard matter in the WH geometry. In the present article we are trying to find the solutions of traversable wormholes with normal matter in the throat within the framework of symmetric teleparallel gravity $f(Q)$ where $Q$ is the non metricity scalar which defines the gravitational interaction. We will examine the wormhole geometries for three forms of function $f(Q)$. First is the linear form $f(Q)=αQ$, second a non -linear form $f(Q)=αQ^2 + β$ and third one a more general quadratic form $f(Q)=αQ^2 + βQ + γ$ with $α$, $β$ and $γ$ being the constants. For all the three cases the shape function is taken as $b(r) = {\frac{r_{0}\ln(r+1)}{\ln({r_0}+1)}}$ where $r_0$ is the throat radius. A special variable redshift function is considered for the discussion. All the energy conditions are then examined for the existence and the stability of the wormhole geometry.

physics.gen-ph↗

arXiv2021

Non-exotic wormholes in 4-D Einstein-Gauss-Bonnet gravity

In the present paper, we investigate wormholes in 4D-Einstein-Gauss-Bonnet gravity without the requirement of exotic matters. We have taken the radial dependent red-shift function $ϕ=\ln \left( {\frac {r_{0}}{r}}+1 \right)$ and shape function $b(r)={\frac{r_{0}\ln(r+1)}{\ln({r_0}+1)}}$ as well as anisotropic matter sources through equation of state (EoS) $p_r(r) = ωρ(r)$. We examine the energy conditions, flaring out condition, throat condition and anisotropic parameter. The volume integral quantifier is also analyzed to validate the existence and stability of the WH solution. We find, for -ve branch wormhole solutions satisfy the null energy conditions (NEC) throughout the entire space time and +ve branch reduces exactly to Morris-Thorne of GR.

gr-qc↗

arXiv2020

Diagnosing the R$\acute{e}$nyi Holographic Dark Energy model in a flat Universe

In this paper, we have examined the R$\acute{e}$nyi holographic dark energy (RHDE) model in the framework of an isotropic and spatially homogeneous flat FLRW (Friedmann- Lema$\hat i$tre-Robertson-Walker) Universe by considering different values of parameter $δ$, where the infrared cut-off is taken care by the Hubble horizon. We examined the RHDE model through the analysis of the growth rate of perturbations and the statefinder hierarchy. The evolutionary trajectories of the statefinder hierarchy $S_3^1$, $S_3^2$ $S_4^1$, $S_4^2$ versus redshift $z$, shows satisfactory behaviour throughout the Universe evolution. One of the favourable appliance for exploring the dark energy models is the CND (composite null diagnostic) $\{ S_3^1 - ε\}$ and $\{ S_4^1 - ε\}$, where the evolutionary trajectories of the $\{ S_3^1 - ε\}$ and $\{ S_4^1 - ε\}$ pair show remarkable characteristics and the departure from $Λ$CDM could be very much assessed.

gr-qc↗

arXiv2020

A new shape function for wormholes in $f(R)$ gravity and General Relativity

In the present work, a new shape function is proposed inside a modified $f(R)$ gravity and General Relativity in wormhole (WH) geometry. The shape function obeyed all the desired conditions of WH geometry. The equation of state (EoS) parameter, anisotropy parameter and the energy conditions are computed. The tangential null energy conditions and the weak energy condition are validated, as well as the radial energy conditions, which demonstrates the nonappearance of exotic matter due to modified gravity allied with such a new proposal.

physics.gen-ph↗

arXiv2019

Traversable Wormholes in $f(R,T)$ Gravity

In the present article, models of traversable wormholes within the $f(R, T)$ modified gravity theory are investigated. We have presented some wormhole models, developed from various hypothesis for the substance of their matter, i.e. various relationships with their components of pressure (lateral and radial). The solutions found for the shape functions of the wormholes produced complies with the required metric conditions. The suitability of solution is examined by exploring null, strong and dominant energy conditions. It is surmised that the normal matter in the throat may pursue the energy conditions yet the gravitational field exuding from the adjusted gravity hypothesis support the appearance of the non-standard geometries of wormholes.

physics.gen-ph↗