In our Laboratory for Innovation and Development on Antennas, Radars, and Electromagnetics (iDARE), we are developing solutions for applied electromagnetic (EM) problems which are relevant for the next generation information and communication technology, security/defence, healthcare, and energy sector.
Our research philosophy attempts to strike a useful balance between three aspects: (a) design of various components (antennas, metasurfaces, circuit topologies), (b) formulation of various computational algorithms and (c) system-level measurements on fabricated prototypes. Some major highlights of the research conducted so far in iDARE, ECE, IISc are focused on the topics:
A. Antennas for Full-Duplex Communication Systems
B. Phase-Gradient Coded Metasurfaces for RIS Realization
C. Theory and Realization of Time-Modulated Transmission Lines and Metasurfaces
D. Antenna Design and Integration Aspects for Automotive MIMO Radars
A. Antennas for Full-Duplex Communication Systems:
Full-Duplex (FD) technology is one of the potential technologies for implementing future 6G communication, due to its potential in doubling the spectral efficiency and significant reduction in latency. However, the main deterrent in the FD implementation is the self-interference (SI) from the system’s own transmitted signal, which is more potent than the desired signal and can cause device saturation. Therefore, to make an FD system work, a typical SI-cancellation (SIC) of more than 100 dB is essential, which is generally achieved jointly by active (analog and digital domain) and passive (antenna domain) SIC techniques. The active SIC techniques require additional circuitry and signal processing blocks, which may increase the power consumption, cost, and size of the system. In addition, ADC dynamic range is one of the major bottlenecks in the system before digital domain cancellation. While having more effective ADC bits helps to decrease quantization errors and enhance the dynamic range, it can also lead to increased processing latency and power usage within the FD communication system.
At iDARE, ECE, IISc we have developed specialized techniques to cancel SI at the antenna domain itself (i.e. well in advance of entering the analog domain), since it is the first component in the RF receiver front end. We have designed three different collinearly polarized two-port microstrip antennas for in-band full-duplex (FD) application, with unique techniques of antenna domain self-interference-cancellation (SIC) between Tx and Rx ports, relying on the manipulation of the surface waves and space-waves that are responsible for the coupling. We started with a two-port microstrip patch antenna system which utilizes DGS (defected ground structure), slightly induced structural asymmetry between radiating patches, and an NFDS (near-field decoupling structure) superstrate to achieve the desired impedance matching and high inter-port isolation. Next, we improved upon the multi-layered design and made it single-layered PCB based two-port multi-radiator (MR) antenna system, where high inter-port isolation is realized by via-rails and DGS. In the next step, we came up with a shared radiator (SR) topology where a central rail of vias along with an LC stub resonator are deployed to realize the impedance matching and high inter-port isolation (> 60 dB). All the proposed designs are initially designed for V2X (Vehicle to Everything)-ITS (Intelligent Transportation Systems) at sub-6 GHz (IEEE 802.11p) band. Furthermore, using two software defined radios (SDRs), we had a real-time demonstration on the capability of our proposed antenna topologies in achieving full duplex communication even in the presence of SI generated by a VNA in our lab-experiment. We received some coverage on this research from the IISc press and a few national media outlets.
Coverage From IISc Press
B. Phase-Gradient Coded Metasurfaces for RIS Realization:
Reconfigurable intelligent surfaces (RIS) have recently emerged as a new paradigm in wireless communications (6G and beyond) to improve energy and spectrum efficiency by controlling the channel environment dynamically. RISs can have potential applications ranging from wireless communication systems to radio frequency energy harvesting, imaging, sensing. At iDARE, ECE, IISc, we have substantially explored the design of such RIS using coded phase-gradient metasurfaces, which achieve anomalous reflection-based beam-steering based on generalized Snell’s law. Such coded metasurfaces can quickly implement complex wave manipulations in simple hardware by strategically altering the coding sequences in an intelligent manner.
To synthesize any RIS, the first step is to design an anomalous reflector that can steer the beam in a desired direction, for any arbitrary incidence angle. Therefore, we obtained the analytical model for the 1-bit and 2-bit coded metasurfaces which provide specific reflection directions for given arbitrary angles of incidence. Next, we create a similar phase profile in CST full-wave solver as generated using the analytical model and observe the corresponding far-field radiation pattern by illuminating it with a plane-wave coming from a specified direction. Finally, the proposed coded metasurfaces are fabricated (Fig. 3), and the anomalously reflected far-field beam is detected by the received power at desired reflection angle and comparing the results with a perfect electric conductor (PEC). While the proposed phase-gradient metasurface based anomalous reflectors are initially synthesized for V2X band, we have also designed scaled version for other frequencies. Furthermore, we have carried out in-depth studies on the impact of metasurface aperture size as well as the number of coding bits on the side lobe level, target deviation error and beamwidth of the scattered radiation.
C. Theory and Realization of Time-Modulated Transmission Lines and Metasurfaces:
Besides the spatially modulated metamaterials and metasurfaces (as described in the previous section), the concept of time-varying media (temporal photonic crystals) is recently finding usage in several exotic applications that involve rich electromagnetic EM phenomena like parametric amplification, frequency conversion, non-reciprocal gain, EM energy accumulation, temporal coating, and temporal aiming (beam-forming). In fact, synthesis of active RIS will also necessitate the analysis and synthesis of time-varying metamaterials and metasurfaces, which makes this area a prime research focus for iDARE, ECE, IISc.
Initially, we carried out a detailed analytical and computational electromagnetic (CEM) treatment of guided electromagnetic (EM) wave propagation in independently time-varying (sinusoidally as well as step-periodically) dielectric medium, using an in-house finite-difference time-domain (FDTD) technique. Besides an ODE (ordinary differential equation) based analytical framework with certain assumptions on the slab thickness and modulation index, 1D FDTD method is deployed to highlight the effects of dielectric modulator parameters like slab-length, modulation index and ratio of modulating frequency with carrier. Both analytical and FDTD approach helps in quantifying interesting EM effects in such time-varying metamaterials, e.g. non-linear phase and amplitude modulation (which often leads to suppression of carrier signal). The concept of time-transitioning state-matrix is further explored, which connects the unusual energy transitions of EM fields in general time-varying media with the exceptional point theory and parity–time symmetric scattering theory.
Moving on from the theoretical explorations, we focus on the PCB realization of such time-varying metamaterials for applications like frequency mixing and translation in microwave/mm-wave bands, using transmission lines and varactor/PIN diodes. However, we found that the conventional commercial full-wave solvers like CST Microwave Studio or Keysight ADS are not capable of handling such time-varying metamaterials. Therefore, we developed our own in-house FDTD codes to model time-varying transmission lines (TVTLs) loaded with time-varying capacitors. TVTLs offer time-varying capacitance, i.e., time-varying permittivity of the media and this concept can be extended to include spatial variation to realize time-varying metamaterials. Using TVTLs with series and shunt time-varying capacitors, we could demonstrate the frequency mixing as observed in the case of EM waves interacting with time-varying dielectric slab. We further developed a novel computational framework using MATLAB Simulink to model such TVTLs and validated the FDTD-results.
In the next step, we fabricated a prototype of the TVTL structure using PCB technology and PIN diodes. Using the measurement setup as illustrated above, we observed agreement between the results from FDTD simulation and measurements, which also corroborated the theoretical and computational treatment of EM wave interaction with time-varying dielectric slab.
Besides the TVTL based scheme for guided EM wave interaction with a time varying metamaterial media, we synthesized polarization-insensitive reconfigurable metasurface tiles using PIN-diodes to experimentally quantify the effect of free-space EM wave interaction with time-varying metasurfaces. Moreover, using the through-wall Radars developed by ARSL Laboratories, ECE, IISc, we have experimentally demonstrated the possibility of Doppler Radar spoofing using these time-varying metasurfaces.
D. Antenna Design and Integration Aspects for Automotive MIMO Radars:
Transcending from cellular and WLAN (Wireless Local Area Network) domain, research on MIMO (Multiple-input Multiple-output) techniques is currently emerging significantly in the context of RADAR (Radio Detection and Ranging) systems as well. Motivated from the discussions with various automotive industry contacts, at iDARE, ECE, IISc we are carrying out computational EM analysis of automotive MIMO Radars, both in terms of antenna design and vehicle integration aspects. First, we proposed some modified binomially tapered series-fed microstrip patch arrays for MIMO Radar applications, to improve the beamwidth, sidelobe level, impedance matching performance and mutual coupling levels. Next, we critically examined the different integration challenges which emanate from the fact that automotive RADAR sensors are mostly integrated behind the bumper, without disturbing the aerodynamic profile and outer body look of the vehicle. Such integration poses significant challenge due to the degradation of MIMO RADAR performance by the resulting EM interactions.
To model such EM effects in a computationally efficient manner in terms of time and system memory (since the bumper and other vehicle parts are electrically large compared to the MIMO Radar), we propose various strategies like use of surface equivalence principle to model the radiating antenna features of MIMO Radar, and equivalent modelling of the stratified paint layers. Using these approaches, we investigate the impact of bumper on the Radar performance by evaluating metrics like AAAF (antenna array ambiguity function) and BLM (bidirectional loss model). Such studies will potentially assist automotive industry personnel in choosing the proper bumper/paint material for achieving robust and uncompromised functionality, besides paving the way for quick design of matching layers to keep the recalibrate the MIMO Radar performance. The research on MIMO Radar antenna design and integration studies have resulted in the publication of multiple conference papers and one invited book chapter.
Furthermore, considering three different MIMO Radar topologies, efficient target detection has been studied by using suitable Radar performance metrics like ambiguity function and field of view (FOV), besides antenna parameters like impedance matching and mutual coupling. As an outcome of this above study, a novel inter-leaved array topology for MIMO Radar antennas to gain significant advantage with respect to state-of-the-art designs in terms to the angular resolution and FOV.
Presently available facilities
♦ Software: CST Microwave Studio + One Acceleration Token, Keysight ADS, MATLAB (Mathworks)
♦ RSA306 USB Real Time Spectrum Analyzer: Quantity = 1, Frequency range = 9 kHz to 6.2 GHz
♦ ADALM-PLUTO Active Learning Module (PlutoSDR): Quantity = 5, Frequency range = 325 to 3800 MHz, can be extended to 6 GHz
♦ Vector Network Analyzer 1: Quantity = 1, Keysight-N9950B, FieldFox 32 GHz Microwave Analyzer (Hand-held, Portable)
♦ Vector Network Analyzer 2: Quantity = 1, LibreVNA, Maximum Frequency = 6 GHz (Supported from AMPL)
♦ Vector Network Analyzer 3: Quantity = 1, NanoVNA, Maximum Frequency = 3 GHz
♦ Mini-Circuits’ ZHL-4240+ Coaxial Amplifier (Medium High power): Quantity = 1, Frequency range = 600 to 4200 MHz
♦ Gain Blocks ADL5610: Quantity = 2, Frequency range = 30 MHz to 6 GHz
Additionally, the following facilities and components are present: (i) Mechanical Toolkit and Drilling Machine, (ii) In-house Body Mimicking Gel Preparation Facility, (iii) DC Sources, Cables, Connectors for Sub-6 GHz as well as mm-wave, attenuators, (iv) Soldering Station. We ensure full compliance with the safety requirements while using the hardware and facilities as mentioned above.
Furthermore, we use MATLAB (Mathworks) and its various toolboxes (RF Toolbox, Antenna toolbox etc) for doing analytical studies and computational analysis of antennas and circuits operating in Radio Frequencies. For the fabrication of PCB Fabrication with smooth finish, we rely on local vendors.
Some additional snapshots of experiments being conducted in iDARE Laboratory, ECE, IISc (besides the ones included in the research highlights section) are as follows:
E1. Wireless power transfer into Implanted Antennas:
E2. SAR Measurement in Tissues using Catheter Antenna:
E3. Transmission through Active Metasurface: