Browsing by Author "Singhal, Rahul"

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720-Mbps 64-QAM-OFDM SCM transmission over RGB-LED-based FSO communication system
(IEEE, 2016) Singhal, Rahul
The paper proposes a 720 Mbps visible light communication (VLC) system using 64-QAM-OFDM signals. Transmission of signals is achieved by modulating red, green and blue (RGB) light emitting diodes (LEDs) and transmitting all three wavelengths over free space optical (FSO) channel after subcarrier multiplexing (SCM) of 64-QAM-OFDM signals on two RF carriers. The performance of the proposed scheme is evaluated for a free space distance under the effect of weak as well as strong scintillations. Data transmission with bit error rates under pre-FEC limit (3.8×10 -3 ) is achieved up to 1.2 meters through the proposed VLC system
All-polymer Electro-optic switch
(IEEE, 2012) Singhal, Rahul
An Electro-optic switch based on Poly(3,4-ethylenedioxythiophene)-Poly(stryene sulfonate) (PEDOT-PSS) electrodes is designed and presented in the paper. An easy-to-fabricate structure of single-mode electro-optic (EO) polymeric waveguide switch based on a Nonlinear Optical (NLO) Polymer is used and analyzed. Electro-optically active waveguides are designed in a photo-definable polymer SU-8 doped with NLO chromophore tri-cyano-vinylidene-di-phenyl-aminobenzene (TCVDPA).
Beam Convergence in Antenna Pattern using Conical and Hemispherical Dielectric Lens
(IEEE, 2018) Singhal, Rahul
In this paper, squinted beams generated due to presence of higher mode in hexagonal patch antenna at 5.5 GHz are converged to form a fundamental mode radiation pattern using conical and hemispherical dielectric lens. The objective of this work is to converge the diverged antenna pattern beams and direct them towards antenna boresight. Antenna loaded with conical and hemispherical dielectric lens improves the radiation pattern by changing its radiation pattern shape from monopole to dipole like radiation pattern while enhancing the boresight gain. Thus, analysis through different simulations on application of two dielectric lenses i.e. conical and hemispherical is presented here. It was observed that dielectric lens shape, radius of its base and height of lens are critical parameters to achieve desired objective.
Bidirectional 64-QAM OFDM-over-fiber transmission on single carrier PON
(IEEE, 2015) Singhal, Rahul
An Orthogonal Frequency Division Multiplexing (OFDM) over fiber based Passive Optical Network (PON) is proposed to cater high data rates to densely populated users in an organization such as a university campus. Performance of a single-carrier fiber-optic bidirectional transmission link carrying Quadrature Amplitude Modulated (16/64-QAM) symbols on OFDM carriers is investigated and demonstrated through simulation. The bidirectional link is based on coherent detection and is designed for carrying 10 Gbps QAM-OFDM signals over a 5-km standard Single Mode Fiber (SMF) between a central office and 32 wireless Access Points (APs). Same optical carrier is utilized for downstream and upstream transmission assuming that a dynamic bandwidth allocation scheme is applied through an ad-hoc network constituted among wireless APs at the distribution end of the network. Bit-error-rates (BERs) of order of 10−3 to 10−5 are achieved when performance of the OFDM-over-fiber is analyzed for electrical signals at wireless APs.
Characteristics Analysis of a Square Loop-based Artificial Magnetic Conductor
(IEEE, 2019) Singhal, Rahul
This paper demonstrates the effect on equivalent circuit modeling of a square loop artificial magnetic conductor (AMC) when strip width of one of the edge of the loop is varied. The unit cell dimensions, the inter-element spacing, the substrate properties and wave incident angles are considered for circuit model simulations and variations are explored. The square loop with different strip widths is chosen, in order to analyze the effect on TE and TM incident wave. Different correction factors for permittivity are required to validate modeling with measurements in TE and TM wave propagation through AMC. Rejection bands obtained for TE and TM wave are different and do not overlap with each other. The results obtained from the circuit simulations are compared and validated with the electromagnetic full wave simulations results and measurements done with compact UWB antenna in an anechoic environment.
Coaxially Fed Hexagonal Patch Antenna for C- and X-Band Applications with Reduced-Ground Plane
(ECTI Association, 2019-08) Singhal, Rahul
The consequence of reduced ground plane in a coaxially-fed hexagonal antenna, on impedance and radiation is analyzed and presented here. Measured and simulated impedance results for a designed antenna are compared with the equivalent circuit model in this paper. The reduction in ground plane excited frequencies in upper C-band and lower X-band. The designed hexagonal antenna uses a substrate with a dimension of 32×33.5 mm2 along with a reduced ground plane of area, 10.44×24.44 mm2. The proposed antenna has a 285 MHz and 380 MHz impedance bandwidth in C-band and X-band respectively. The designed antenna with reduced ground plane works well for C-band and X-band applications.
Comparative Study of Translated Cross Dipole and Square Loop Frequency Selective Surfaces for Band Stop Characteristics in X-Band
(IEEE, 2020) Singhal, Rahul
In this article, two types of band stop frequency selective surfaces (FSSs) based on infinitely large array of translated crossed dipole and square loop (SL) unit elements are presented and further, compared. The impact of translating the unit elements by half of the periodicity along horizontal and vertical directions is compared. Through full-wave electromagnetic simulation, it is found that the transmission characteristics of FSS array based on conventional unit cell and translated unit cell are almost similar primarily because geometry of FSS array do not get affected by translation of unit cell. Although when compared, the square loop based FSS array exhibits a band stop bandwidth of around 202.14% larger than exhibited by crossed dipole based FSS array.
Design of Frequency Reconfigurable Hexagonal Antennas for WLAN Applications
(IEEE, 2019) Singhal, Rahul
In this paper, design methodology of a frequency reconfigurable probe-fed hexagonal patch antenna with a hexagonal annular ring has been discussed using a varactor diode as the tuning element. The proposed technique is applied to two separate antennas; first antenna is capable to switch between different frequency modes covering the WLAN channels (5.2 GHz - 5.825 GHz) while second frequency-reconfigurable antenna covers Wi-Fi channels (2.4 GHz - 2.5 GHz) by means of varying the reverse bias capacitance of the diode via changing the bias voltage. Based on CST MWS simulation results, both the antennas are found to be radiating at their fundamental mode in all the switching bands with same and matched impedance. A gain of around 1.9 dB is observed during simulation in both antenna designs. The structure of both antennas is easy and economical to implement as only one varactor diode is required to achieve reconfigurability. The antennas find their usefulness dynamic frequency selection (DFS) within WLAN 5 GHz band and 2.4 GHz Wi-Fi channel selections.
Design of Ultra Wideband Cubic Dielectric Resonator Antenna Surrounded by Dielectric Micropillars for THz Applications
(IEEE, 2021) Singhal, Rahul
In this paper, a cubic dielectric resonator antenna (CDRA) with patterned structure fed through a microstrip line using gold is proposed. Geometry-induced resonant frequency is synthesized with cuboid periodic pattern through micro pillars grown around the DRA. Following the idea of frequency excitation, a synthesized frequency inherits and combines the modal features from several conventional ones of regular DRAs. Antenna radiation patterns have fine complementation, meanwhile, they are excited by a shared feed. To further demonstrate the mechanism of frequency excitation within ultra-wideband (UWB), the field distribution of the proposed CDRA with micro pillars around it is analyzed by varying the height of the pillars. The radiation patterns and surface fields around the core frequencies are also demonstrated here. The simulation results suggest that periodic geometric structures can be utilized to excite a frequency within UWB band for THz applications.
Design, fabrication and characterization of thin-film polymeric optical splitters
(BITS Pilani, 2013) Singhal, Rahul
Dual carrier multiplexed M-QAM signals modulated optical SSB transmission over fiber
(IEEE, 2016) Singhal, Rahul
In this paper, a simple hybrid radio-over fiber network that can transmit a different quadrature amplitude modulated (QAM) signals simultaneously over a 50-km fiber link is investigated. Simulation results indicate that a maximum of 162 Mbps-64QAM and 108 Mbps-16QAM signals can be sent simultaneously using Optical Single Side Band (OSSB) generation technique without any dispersion compensation. The proposed dual carrier multiplexing (DCM) architecture may be used for the multi-service transmission in heterogeneous networks. In presence of centralized control and processing, subcarrier multiplexing (SCM) techniques over fiber may provide economical and protocol-transparent solutions for the next-generation hybrid networks.
Dual rhombic loop-based frequency selective surfaces for X-band applications
(Wiley, 2022) Singhal, Rahul
Dual rhombic loop (DRL) based low-cost X-band frequency selective surfaces (FSSs) with stopband characteristics are presented in this study. Orthogonally oriented arrays of rhombic loop pairs are printed back-to-back (BTB) on both sides of dielectric substrate to achieve wide stopband characteristics with stable response under oblique incidences. The nonlinearities in the reflection phase of BTB DRL printed FSSs are compensated by printing orthogonally oriented rhombic loop pairs on the same side of the substrate. Merging orthogonally oriented rhombic loop pairs results in a single-layer wide stopband FSS with linear reflection phase under normal incidence. Both polarization dependent merged DRL (MDRL) and independent BTB DRL FSSs are developed and experimentally validated. As per measurements, the BTB printed FSS array exhibits 4.3 GHz wide stopband with transmission nulls at 7.9 and 10 GHz while the single side printed FSS exhibits 3.7 GHz wide stopband with transmission null at 8.3 GHz
Edge perturbation effect in polygonal patch antennas
(IEEE, 2015) Singhal, Rahul
The effect of edge perturbation in vertex-fed polygonal patch antennas is studied and analyzed. A polygram shape is obtained by perturbing the edge of polygon. Here, edges of a pentagon and a hexagon are perturbed to obtain a pentagram and hexagram respectively. Pentagram and hexagram patch designs are simulated for different angles (θ) formed when midpoint (vm) of edge of polygon moves towards the center of polygon between adjacent vertices to understand the effect of edge perturbation. The characterization of antenna designs to observe the effect of change in patch design on scattering, impedance and gain characteristics of antennas is performed through simulation on CST MWS®. It is observed that wideband characteristics between 12 to 20 GHz of pentagonal and hexagonal patch antennas are retained with pentagram and hexagram geometries till angle θ is greater than 165.63° and 163.15° respectively. For lower values of θ, multiband behavior is observed for polygram patch antenna designs. Analyzing results of all patch geometries reflects that the performance of the pentagonal patch antenna is better in terms of gain compared to others within the operating band.
Effect of reduced ground plane in a vertex-fed hexagonal C-band antenna
(IEEE, 2017) Singhal, Rahul
Vertex-fed hexagonal planar antennas excite higher order modes. Monopole like radiation characteristics may be achieved through ground plane reduction. The paper presents the effect of reduced ground plane (RGP) which may be used as a technique to excite a higher-order mode and suppress spurious frequency radiation, if any, due to probe feeding. The designed antenna structure has a hexagonal slot at the centre of a hexagonal patch fed through a probe at a vertex of the hexagon. To understand the effect of RGP on antenna performance, an equivalent circuit model is analyzed and discussed. A study on surface current distribution over patch surface confirms radiation of TM 12 mode. The proposed C-band antenna exhibits a gain of 3.84 dB at 5 GHz and a bandwidth of around 600 MHz that makes it suitable for indoor WLAN applications.
Effect of Residual Resist on Performance of Single-Mode 1 × 4 Optical Splitter in Photosensitive Polymer
(The SAO/NASA Astrophysics Data System, 2010-11) Singhal, Rahul
Polymer residues are generally left in the Y-junctions of the conventional splitters. Besides increased insertion loss, the Y-junction residue results in asymmetric distribution of power at device outputs. An analysis of the device performance in the presence of junction residue is presented and a design to overcome the non-uniformities in output power distribution brought about by the presence of the residue is proposed.
Electromagnetic Scattering Analysis of Conventional Frequency Selective Surfaces for Terahertz Applications
(IEEE, 2023) Singhal, Rahul
Conventional designs of arrays of metallo-dielectric frequency selective surface (FSS) for terahertz (THz) frequency range such as square conducting pads (Pixels), metallic mesh, mesh-loop (ML), square loop (SL) and complementary square loop (CSL) are analyzed and compared through simulations. Pixel FSS demonstrates 0.58 THz wide stop-band and 0.14 THz wide pass-band centered at 2.96 THz and 3.35 THz respectively. By complementing, the pixel array becomes a mesh array that appears as a band-pass FSS from 1.89 THz to 3.45 THz. ML array is formed by removing a square ring from the mesh array and provides a 0.32 THz wide pass-band and 0.3 THz stop-band centered at 0.82 THz and 1.15 THz. After eliminating the outer mesh of ML array, SL array yields a 0.32 THz wide band-stop FSS centered at 1.22 THz. Later, the CSL array is formed when SL array is complemented, and the array demonstrates a 0.6 THz wide pass-band at 1.14 THz. The pixel array provides a high signal rejection capability and the widest −10dB stop-band in the frequency range of 2.58-3.16 THz. This analysis and comparison is significant for reduction of EM coupling or interference in THz based integrated circuits.
Excitation Amplitude/Phase Sensitivity in Circularly Polarized Hexagonal S-Band Antenna
(IEEE, 2019) Singhal, Rahul
A probe-fed circularly polarized hexagonal patch antenna with dual feed configuration is designed and presented for analyzing amplitude and phase sensitivity. The hexagonal patch antenna is excited with a fundamental frequency of 2.45 GHz through two feeds separated by an angle of 90° and 90° phase shift. The probe feed positions are optimized along the X- and the Y-direction of the patch antenna plane with and without simultaneous excitation. The sensitivity of circular polarization purity towards change in amplitude and phase of excitation signal is studied for a dual port hexagonal S-Band antenna.
Fabrication of monomode channel waveguides in photosensitive polymer on optical adhesive
(SAO/NASA Astrophysics Data System, 2011-09) Singhal, Rahul
We present fabrication of optical waveguides in a photosensitive polymer. The process to fabricate monomode polymeric channel waveguides using simple direct ultraviolet (UV) photolithography is optimized. Channel waveguides of width ~2 to 2.4 μm and height ~1.8 to 2.2 μm are obtained on development after a crosslinkable negative tone epoxy SU-8 2002 polymer is exposed to UV through a photomask. Norland Optical Adhesive 61 (NOA 61) was used as under- and overclad. The average insertion loss obtained at chip-level is ~11 dB (TE00) and ~12 dB (TM00) for 2-cm waveguide at 1550 nm.
Fabrication of single-mode Y-branch waveguides in photosensitive polymer with reduced Y-junction residue
(Elsevier, 2012-11) Singhal, Rahul
Single-mode small-core (∼2 μm × 2 μm) Y-branch waveguide structures in photosensitive polymer have been fabricated. Y-branch waveguides are designed by the beam propagation method and Y-branch waveguides are obtained on development after a cross-linkable negative tone epoxy SU-8 2002 polymer is exposed to UV through a photomask. Optical Adhesive NOA 61 is used as under- and over-clad. The fabrication process is optimized to avoid polymer residue at the Y-junction. The average insertion loss obtained for a 7.2 mm 1 × 2 device at chip-level is ∼13 dB at 1550 nm.
Field-Assisted Sensitivity Amplification in a Noble Metal Nanoparticle Decorated WO3/GO Hybrid FET-Based Multisensory Array for Selective Detection of Breath Acetone
(Wiley, 2021-12) Ajmera, Pawan K.; Singhal, Rahul; Hazra, Arnab
The current work shows noble metal (Au, Pd and Pt) nanoparticles, WO3 and graphene oxide based ternary hybrid field effect transistor structured sensor array for the selective detection of breath acetone. Accurate acetone selectivity was achieved by using phase space entire based feature extraction techniques. Field-assisted sensitivity amplification was responsible for the detection of ppb to lower ppm level detection of acetone.