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Millimeter-wave Quasioptical Beamsteering Antennas

Current military and future commercial wireless communications systems are relying more and more on multi-antenna arrays (e.g., AESAs, MIMO, massive MIMO) to realize the benefits of spatial reuse, interference mitigation, and higher gain.

Traditionally, a beam-steering antenna array employs active microwave circuits (variable gain amplifiers and phase shifters or true time delay units) in each element (or sub-array) of the array to achieve beam steering and pattern/sidelobe control. These active components, in addition to requiring significant power, are large and expensive. Modern fully-digital phased arrays employ full bandwidth DAC/ADC data processors at each element (or sub-array) to form many beams from one aperture (up to the number of elements in the array) but this approach require significant power and cost, especially as channel bandwidth increases (in the GHz) in future systems. One approach to realize the flexibility of digital beamforming while lowering the power and cost is to use hybrid beamforming methods in which part of the beam is formed by a digital baseband and part is realized with traditional VGAs and phase shifters. This incremental approach to the problem of increasing data rates is only a stop-gap measure.

We are investigating new fully analog solutions to the problem of multi-beam, wideband, millimeter-wave antennas for both base stations and mobile devices. Modern fabrication methods such as 3D printing and microfabrication make it possible to easily fabricate gradient index lenses such as the Luneberg lens. This fabrication approach, combined with the new theoretical frameworks in transformation optics provide a means for designing flat, on-wafer gradient index (GRIN) lenses. We have developed perforated dielectrics with engineered permittivities ranging from 0.1-1.0 of the background permittivity using various polygon cells and lattices.

[Relative permittivity of flattened Luneberg lens (left) and original Luneberg lens (right)]

Our approach is to combine microfabrication and wafer bonding to create fully passive, low-loss, wideband, multi-beam, beam-steering lens antennas. These lenses are fabricated on wafer so they can easily be combined with active electronics to form a completely integrated multi-beam front-end for millimeter-wave communication systems.