We have seen an explosion of radio services, from cellular 2G/3G/4G to wireless LAN, Bluetooth, and GPS. They have provided the fundamental capabilities for communication as well as sensing and location information in today’s modern wireless world. Amazingly, all of these functions are concentrated in a narrow region of the electromagnetic spectrum - between 800MHz and 6GHz. Throw in the classic (or legacy, if you prefer) AM and FM radio bands, and the lower end extends into the kilohertz region. Similarly, the fiber optic backbone of the internet operates in the upper end of the infrared and visible light spectrums. In between these two utilized bands lies a vast, basically unused part of the electromagnetic spectrum called the millimeter-wave (30 - 300GHz) and terahertz (300GHz – 10THz) bands (see below).
Lying in between microwave and visible light, mm-wave and THz signals can combine attractive properties from both of these regions. Signals can be generated and processed coherently using conventional electronics but with small wavelengths and antenna size. Huge swaths of bandwidth can be combined with narrow beams to provide directive communication and imaging and sensing of features sizes that are a millimeter and below. If properly harnessed, mm-wave and THz systems could enable new ways of seeing the world, whether it’s inspecting the density of packaging materials, finding cracks in aircraft fuselages, ensuring quality of pharmaceutical products, creating the world’s best through-wall stud finder, or simply communicating all of this plus multiple streams of uncompressed quad-HD video wirelessly. All of these capabilities have been known for decades, but still, the deployment of millimeter-wave systems has been virtually non-existent, limited to only a few applications (airport screening, automotive radar) with limited volume orders of magnitude smaller than the microwave and optical systems prevalent today.
It’s not lack of desire that has prevented this area from gaining traction but rather lack of capability. If you were to crack open one of the few available systems operating between 100-1000GHz, you would not find anything that looks like the electronics that have dominated the consumer and industrial spaces. Instead, you will find “gold bricks” connected together by rectangular waveguides, and perhaps a large laser or two. The relevant numbers here are around a cubic meter in volume, thousands of watts of power dissipation, and hundreds of thousands of dollars. No wonder this hasn’t gotten widespread traction.
Starting in 2009, I was fortunate to be part of a small team in TI’s Kilby Labs that looked to leverage the increasing speed of modern silicon processes, such as deep submicron CMOS. CMOS has the unquestioned capability for high volume and compact integration; however, like any other technology, it rapidly loses gain and output power at higher frequencies. We had set out to overcome these challenges and to create the most integrated and compact millimeter-wave system out there, setting our sights on an imaging/sensing system with centimeter resolution, and sub-millimeter accuracy.
The initial system specs were driven by the ability to detect hands and fingers at short distances, enabling a new type of proximity sensing and gesture recognition. With all of the processing on a single chip and radiated directly from its package, these mm-scale devices could be sprinkled across the surface of a tablet or smart phone, each taking independent pictures of the surrounding environment, and, at low speeds, combine these together into a picture of the environment.
After developing new techniques to design, model, simulate, and characterize devices and circuits, and building up the system expertise to create a full single-chip imager, the initial results are out with a paper by the Kilby Labs THz Signaling Team: Vijay Rentala, Eunyoung Seok, Srinath Ramaswamy, Swami Sankaran, Baher Haroun and me. (Click here to read the paper that was published at this year’s Symposium on VLSI Circuits in Kyoto, Japan). The paper describes the transmit portion of a 160GHz radar that uses 100ps pulses. This leverages the capabilities of CMOS to support single-chip coherent arrays and shows how the integration of this with antennas-in-package has led to record performance of output power from a planar radiator in silicon. Combined with receivers on the same die, this represents the most sophisticated integration to date above 100GHz, with the signals generated and digitized directly on silicon and only low-speed electrical connections on the board. While still a ways to go until we start seeing these devices in our household, there is no doubt that highly integrated millimeter-wave systems are making rapid progress.
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