Overview

Portable wireless devices, especially smartphones, provide connectivity by integrating processing power, touch screens, cameras, and motion sensors with multiple radios, many of which can run simultaneously. The human body in close proximity to these devices absorbs some of the electromagnetic radiation. Each device is required to comply with the Federal Communications Commission’s limits on the ‘specific absorption rate’ (SAR), measured in Watts/kg, of radiation that is considered safe for the body. Many portable wireless devices sold today already operate near the accepted human SAR limit. With ongoing improvements in the batteries, size, features, and processing power of phones, there is a need to be creative in how the phone’s technology can still work within the SAR radiation limits.

Performance problems with cell phones are often caused by the uplink, or how much information a portable wireless device can transmit to the base station. The device being held either in hand or next to the head means that the transmission of the uplink information to the base station can run into SAR limits. There is a need to manage the tension between improving uplink performance, power, and limiting SAR. This project examines how multiple transmitters on a portable wireless device can be modeled, analyzed, and used while subject to a SAR constraint. The research looks at how to model the constraint, and how to realize the reliability and high data-rate advantages of multiple antennas subject to the constraint.

Improved Phone Design

The portable devices being generally small, the transmitters are physically close to one another and coupled electromagnetically. Hence, the research requires a blended effort in three distinct areas: (i) electromagnetics and near-field exposure modeling with multiple transmitters, (ii) wireless communication system far-field information-theoretic performance evaluation with a near-field SAR constraint, (iii) the design of SAR codes for optimum performance.

Cell phone design generally uses a single transmitter, which creates a ‘hot spot’ of radiation where the antenna and amplifier are located. The ‘Improved Phone Design for SAR constraints’ project is researching the idea of replacing the single transmitter used in today’s cell phones with multiple transmitters at different points within the device. By using multiple transmitters, the project aims to disperse the radiation and reduce SAR. The multiple transmitters must be used properly to both improve performance as a wireless communication device and to simultaneously reduce SAR. Early results for the new hardware structure show great promise.

To measure whether the electromagnetic radiation is within the limits, the SAR Measurement Device Lab uses a half-head mannequin with an operational dissected cell phone attached to its ear. The half-head was specially made to mimic the flesh and fluids of a real human being and has sensors inside to measure the electromagnetic radiation that the phone projects into the half-head.

Besides the half-head, a second testing method involves a full-head mannequin in Wireless Institute’s Anechoic Chamber Lab. Here the external signals are sealed out and the internal reflections of electromagnetic waves are absorbed completely. This chamber is used to test the long-range performance of the newly constructed, multi-transmitter phones.

Of particular interest, are next-generation devices that plan to use millimeter-wave signals above 30 GHz, where exposure to electromagnetic radiation is not well modeled or understood.

Sponsors and Collaborators

The ‘Improved Phone Design for SAR constraints’ project started in June 2014 and is sponsored by an NSF award of over one million dollars. This is a Wireless Institute multi-PI project and is being worked by Notre Dame’s Bertrand Hochwald (Lead PI) and Patrick Fay, Purdue University’s David Love, and University of Illinois’ Jianming Jin.

Former graduate students involved in this project include Arash Ebadi Shahrivar (Qualcomm), Ding Nie (Apple).

To realize the vision of millimeter waves for commercial wireless we are examining cross-disciplinary approaches to meet the beam-agile, wideband requirements while maintaining sufficiently low power and cost to be deployable in small cells and mobile devices. This problem requires a “full-stack” solution from the network layer to the physical layer so we approach the problem through a collaboration of systems analysis, circuit design, and integration of emerging materials and devices. In the Advanced Millimeter-Wave Lab, we are able to design, fabricate, and measure novel transceiver architectures and RF front-ends (including beam steering) for millimeter-wave wireless communications as well as evaluate new materials and devices offering unique capabilities at millimeter waves.

Overview

One of the key challenges of millimeter wave systems is the increased path loss at high frequencies compared with current sub-6 GHz wireless networks. A proposed solution is to employ high-gain antenna arrays which compensate for the path loss but have a corresponding narrow beamwidth. In order to maintain a wireless link with a mobile device, beam-steering is employed. Traditional beam-steering uses active phased arrays which incur a high cost and power dissipation. An alternative to active phased array beam steering antennas is the passive Luneberg lens which boasts zero power dissipation, low-loss, and high gain. However, the Luneberg lens is a bulky 3D (spherical) gradient index (GRIN) lens traditionally requiring elaborate machining of concentric shells of dielectric. Due to the difficulties of fabrication this lens has traditionally only been realized in bands below 20 GHz.

Lens antennas

With recent advances in additive manufacturing of high-performance and low-loss RF materials, such GRIN lenses can be realized with a low-profile while maintaining their wideband performance. However, the reduction in volume results in unavoidable degradation in beam-scanning performance, reducing the field-of-view and limiting its use in modern mobile wireless networks. To address this Professor Chisum and his team have proposed several novel architectures including compound and hybrid lens architectures comprising collections of several lenses or combinations of sparse phased-arrays and lenses, respectively. Professor Chisum is sponsored by the Office of Naval Research and has collaborations with leading 3D-printing companies in industry in order to prove the methods are viable in the current and future bands for 5G and 6G wireless networks. 

In order to realize GRIN lenses we have developed a suite of manufacturing processes for gradient index media targeting the millimeter wave bands from 8-100+ GHz. One method, based upon a perforated dielectric, mixes air and dielectric in varying ratios throughout the volume of the lens in order to sculpt the electromagnetic fields as they propagate through the lens. This method is highly reliable, extremely wideband, and very low loss. It can currently be realized by drilling standard high-frequency PCB substrates or through 3D printing. A lower-cost alternative is based upon a metallo-dielectric mixture in which small metal inclusions provide strong localized polarization. These methods are amenable to standard PCB printing and are thus very low-cost. While their frequency response is traditionally narrowband due to material resonances, we have been exploring methods to push the material resonances out of the bands of interest. This method currently supports operation up to 40 GHz.

One of the advantages of a GRIN lens is the many degrees of freedom available for design. This permits a designer to simultaneously pursue multiple design objectives such as bandwidth, efficiency, wide field-of-view, and even low sidelobe levels. However, such a multi-objective optimization requires a rapid and accurate numerical solver. Current state-of-the-art electromagnetic solvers using high-performance computing (HPC) clusters still require hours to solve a single iteration of the lens optimization. Therefore we have developed in-house codes which combine curved-ray geometrical optics with diffraction theory in order to solve the radiation pattern of a lens candidate in less than one minute, and with ~0.1dB accuracy compared to the full-wave electromagnetic solvers.

A new networking paradigm

The hardware is now mature and capable of offering high-performance millimeter-wave beam-scanning antennas for mobile wireless base-stations, or for tracking of low-earth-orbit (LEO) satellite communications. The next phase of this research is to collaborate with other WI researchers in order to demonstrate network-level properties including beam identification and tracking, search and track, make-before-break capabilities, and even the ability to operate with a deployed 5G NR waveform.

Sponsors and Collaborators

The GRIN lens antenna project is currently funded by the Office of Naval Research (ONR) and the Department of Homeland Security (DHS) with past support ship from the Satcom industry. We are collaborating with industry-leading additive manufacturing companies including Fortify and NanoDimension, as well as MIT Lincoln Laboratory.

Publications

N. Garcia, J. Chisum, “Compound GRIN Fanbeam Lens Antenna with Wideband Wide-angle Beamscanning“ IEEE Transactions on Antennas and Propagation, Jun. 2022

N. Garcia, W. Wang, J. Chisum, “Feed Corrective Lenslets for Enhanced Beamscan in Lens Antenna Systems“ Optica Express, Apr. 2022

W. Wang, N. Garcia, and J. Chisum, “The Systematic Design of Non-commensurate Impedance Matching Tapers for Ultra Wideband Gradient Index (GRIN) Lens Antennas“ IEEE Transactions on Antennas and Propagation, Jul. 2021

N. Garcia, J. Chisum, “Reduced dimensionality optimizer for efficient design of wideband millimeter‐wave 3D metamaterial GRIN lenses“ Microwave and Optical Technology Letters, May 2021

N. Garcia, J. Chisum, “High-Efficiency, Wideband GRIN Lenses With Intrinsically Matched Unit Cells“ IEEE Transactions on Antennas and Propagation, Apr. 2020

W. Wang, J. Chisum, “Hybrid Geometrical Optics and Uniform Asymptotic Physical Optics for Rapid and Accurate Practical GRIN Lens Design“ 2022 IEEE/MTT-S International Microwave Symposium, Jun. 2022

Efficient use of spectrum is key to allowing government and commercial wireless technologies to flourish in the future. The RF spectrum below 6 GHz, in particular, has become extremely valuable as wireless communications technologies supplant wired solutions. As everyone looks for “1000x” improvements over the existing generation of wireless communications, one of the main challenges has become how to better utilize frequencies at 6 GHz and below.

Notre Dame’s Wireless Institute is actively engaged in researching and developing system models, sensing algorithms, and signaling schemes for utilizing spectrum more efficiently. The vision is to create localized spectrum efficiency maps, possibly in coordination with national geo-location databases, to enable wireless devices to dynamically find under-utilized spectrum, while minimizing interference.

To accomplish this, the RadioHound project has been initiated to develop a new spectrum efficiency monitoring platform – a system with a network of high-resolution, low-cost distributed overlay and compare local, real-time spectrum usage and efficiency to the FCC/NTIA radio spectrum sensors, a central data store, and an interface for analytics and visualization that could frequency allocation chart. The spectrum monitors would also be sufficiently sensitive, portable, programmable and low cost to provide an invaluable resource for mapping out spectrum efficiency across the United States. A complete understanding of the space-time efficiency utilization of spectrum is the key to improving it and will enable the regulated sharing of any inefficiently utilized spectrum resources.