Wireless Research Areas
Crowded Spectrum: Dynamic Sensing and Sharing
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.
Crowded Devices: RF Coupling and Multiple Antennas
As wireless devices shrink and the features expand, more sensors, transmitters, and receivers are forced to share a very small area. This antenna and radio frequency (RF) coupling typically leads to undesirable results, such as signal interference and increased electromagnetic radiation exposure. At the Notre Dame Wireless Institute, our research approach is to challenge conventional wisdom. In other words, we are seeking to turn a perceived disadvantage into an effective advantage by researching the benefits of antenna coupling and exploiting the presence of multiple transmitters for remote sensing applications, increased device performance and decreased electromagnetic radiation exposure. Our research into exploiting the traditional drawbacks of MIMO (multiple-in, multiple-out) systems has promising implications, not only for handheld wireless devices but also for larger mobile platform scenarios, ranging from military vehicles in the battlefield to aircraft carriers at sea.
Further, this research effort advocates the design and analysis of low-complexity high-bandwidth multiport matching networks to compensate for coupling in radio-frequency transmitters, receivers, and circuits. The ideal multiport matching network inserted between independent sources and coupled loads compensates for the coupling by eliminating both reflected power and also power transferred from one source through a load to another source.
Among the design limitations of such networks, complexity and bandwidth are usually the most important, especially for compact wideband wireless communication devices. These limitations have not been well studied, and this effort includes an integrated two-pronged exploration of these issues in microwave and millimeter wave matching networks. The first prong uses network-theoretic analyses to explore systematic, unified, design methods that work for any load structure. The effort also seeks standardized limits against which network performance in both complexity and bandwidth can be measured. The second prong couples an experimental program to both validate as well as inform the modeling choices made in the design and optimization of the matching networks.
Practical issues such as undesired parasitic coupling and electromagnetic discontinuities in distributed circuit implementations are examined. Of particular interest are the microwave (2.4 GHz) and millimeter wave (60 GHz) frequencies with both lumped and distributed radio-frequency components and an emphasis on simple module layouts. The effort is transformative because it offers a comprehensive unified set of metrics, design criteria, and methodologies for complexity and bandwidth of multiport matching networks applicable to compact radio-frequency devices.
Low complexity, Low cost, Low power RF
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 3D (spherical) gradient index 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.
Recently, the method of transformation optics has been presented as a means of physically distorting an electromagnetic structure and maintaining its functionality by spatially varying the permittivity throughout the structure. This has enabled the design of flat lenses but they require so-called gradient index optics. Over the past year, we have developed a process for manufacturing gradient index optics targeting the millimeter wave bands from 30-150 GHz. The approach is known as perforated media.
Millimeter Wave Wireless
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 the 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.