A clever millimeter-wave lens enables a high-speed, backscatter-powered GHz-band link.
Wireless system designers are often asked to deliver on seemingly incompatible and contradictory goals such as supporting ultrahigh wireless data rates, and do so at ultralow power. Accomplishing this, even if possible, is a challenge which may require lots of technical “tricks” including advanced techniques, custom components, and more.
Now, a team at Georgia Institute of Technology (better known as Georgia Tech) has demonstrated a what they call a first-of-its-kind lens-enabled backscatter system capable of multi-gigabit data rates. At the same time, this backscatter-powered system operates using only a fraction of the power required by conventional wireless devices, therefore bringing high-speed connectivity to disbursed systems.
In general, due to power constraints, backscatter has typically been used only to send small amounts of data, most often in simple identification and sensing systems. However, the researchers say that backscatter doesn’t have to be slow and can operate at gigabit‑per‑second speeds while remaining ultra‑low power—with the right architecture. They foresee applications such as battery-free sensors embedded throughout smart cities and with digital infrastructure for a localized IoT arrangement.
Their lens-enabled backscatter system is capable of multi-gigabit data rates, reaching up to 4 gigabits per second (Gbps) (Figure 1). This dielectric lens focuses incoming millimeter-wave energy (such as from 5G systems) onto an array of tiny antenna elements, allowing both wireless energy capture and high‑speed backscatter communication within the same system.
Figure 1 A close‑up view of the device displays an array of tiny antenna elements positioned behind the lens, each modulating reflected wireless signals to enable high‑speed communication with minimal energy use. (Image source: Georgia Tech School of Electrical and Computer Engineering)
Signals at these frequencies are highly directional and sensitive to alignment; even a small misalignment can break the link. Their lens overcomes that constraint by enabling high gain and wide angular coverage simultaneously, without the need for active beam steering.
The system that can communicate over a ±55-degree field. In their tests, the researchers achieved data rates of up to 4 Gbps with sustained gigabit communication at distances of up to 20 meters, using high-order modulation schemes like those used in modern cellular networks. The system is extremely efficient and requires just 0.08 picojoules per bit. The link-budget analysis projects 1 Gbps back-scatter ranges up to 2.6 km under the 75 dBm effective Isotropic radiated power (EIRP) that is permitted in 5G millimeter-wave systems.
At the core of the millimeter-wave identification (mmID) is a broadband, cross-polarized antenna designed to operate across the full 26–30 GHz band. A broadband element is essential to sustain gigabit-level backscatter, since narrow- band operation would constrain throughput and increase distortion under high-order modulation. Cross-polarization is critical at mmWave, as a co-polarized backscatter would be masked by strong transmitter-receiving coupling from the reader.
To meet these requirements, they implemented a single-layer, capacitively coupled patch antenna designed in CST Microwave Studio and fabricated on Rogers 3003 (εr = 3:00, tan δ = 0:0013), with thickness of 0.254 mm (Figure 2).
Figure 2 a) Layout of the cross-polarized capacitive-coupled patch antenna with dimensions W = 2.85 mm, LS = 1.1 mm, and GC = 0.12 mm. b) Measured vs. Simulated S11 results of the broadband antenna. c) Layout of the FET-based mmWave modulator with dimensions R1 = 1.11 mm and R2 = 1.24 mm. d) Measured vs. Simulated S21 results of the mmWave modulator network. e) Layout of the pixel backscatter element, comprised of the broadband antenna and FET-based wireless mixer. (Image source: Nature Communications)
Gigabit backscatter at mmWave frequencies requires an antenna module that delivers both high gain and wide angular coverage. A dielectric lens provides an efficient solution, acting as a passive focusing element that concentrates incident energy onto the pixel. A key contributor to this long-range performance is the PTFE dielectric lens, which passively concentrates incident mm-wave energy onto the pixel element in a manner analogous to an optical lens. To extend the single pixel design into a practical mmID with wide angular coverage, a 25-element broadband cross-polarized pixel array was implemented, arranged in three concentric rings with a central element (Figure 3).
Figure 3 a) Proposed broadband cross-polarized mmID featuring 25 antenna elements with dimensions L1 = 26 mm, L2 = 52 mm, L3 = 78 mm, W = 90 mm, S = 13 mm, and R3 = 1.35 mm. b) Proposed PTFE lens with dimensions labeled D1 = 74 mm, D2 = 120 mm, and h = 25 mm. (Image source: Nature Communications)
The team performed extensive tests spanning a range of frequency bands, data formats, modulation types, and more, with detailed quantitative results summarized in various tables (Figure 4). They have shown that it is possible to extract GHz-range ambient-RF energy effectively using a printed lens-like antenna.
Figure 4 a) Experimental setup of the proposed lens-based mmID at incidence angles of 0∘ and 55∘ from the PoC reader. b) Block diagram of the PoC reader transmitting and receiving chain to interrogate the lens-based mmID and demodulate the gigabit per second data-rate backscatter. (Image source: Nature Communications)
The detailed project is a fascinating investigation and exploration into RF-based energy harvesting and ultralow-power systems design, without speed compromise. It is described in detail in their readable paper “Broadband multi-beam lens-assisted mmID enabling multi-gigabit backscatter data rates for next-generation wireless networks” published in Nature Communications.
What’s your view on the innovation and cleverness of this project? Is it as impressive as they maintain, or just a well-crafted and analyzed implementation of existing ideas? Is it yet another attention-getting energy-harvesting scheme with added gigahertz connectivity, or does it represent a genuine advance?
—Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors and signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing, and he also developed significant mechanical-engineering insight while designing control electronics for large materials-testing systems.
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