In depth: NASA’s passive Wi-Fi saves 80% power

NASA has revealed a technique which dramatically cuts the power consumption of Wi-Fi comms, at least at the remote terminal.

Key is modifying the Wi-Fi base station to allow the transmitter of the remote terminal to be replaced by a modulated passive reflector.

“In a Wi-Fi radio, 70-80% of power is consumed generating Wi-Fi signal. If you only reflect, you save the transmit power,” Adrian Tang of NASA’s Jet Propulsion Laboratory told Electronics Weekly. Tang is working with Frank Chang at UCLA.

What returns to the basestation is not some pale shadow of Wi-Fi.

“You get PHY, header, and everything; you get real Wi-Fi comms back,” said Tang.

The scheme works like this:

The basestation is modified to emit a 20dBm continuous-wave (CW) sinewave at the Wi-Fi fundamental frequency, while the remote terminal has an antenna connected to a variable phase shifter and a load.

Even without the phase shifter, the remote terminal can amplitude-modulate reflected signals by switching the load between matched and short-circuit.

With the right phase shift options, the signal reflected back from the antenna can be a clean Wi-FI signal with modulation up to 16-QAM – and phase shifters can be as simple as switches connecting the antenna to transmission lines of various lengths.

Tang and Chang have implemented such a modulator, offering QPSK and ASK as well as covering 2.4 or 5.83GHz, on a CMOS chip – its 200µm2 footprint is small enough to be added to a baseband SoC.

“At the remote terminal, there is no synthesiser, no power amplifier, just a modulator; and the modulator is just a bunch of switches,” said Tang.

For the demonstrator, the chip also includes a pseudo-random number generator.

The basestation has a conventional Wi-Fi receiver chip, but it needs some help.

Its receive antenna gets the smaller-than-usual modulated, swamped by reflections of the original CW signal from the local environment – putting reception well outside the dynamic range of conventional Wi-Fi chip front-ends.

To get over this, Tang and Chang have created a second chip (see photo) from 65nm CMOS which sits between the basestation receive antenna and the conventional Wi-Fi chip.

At the receive antenna, all the local CW reflections add to a single CW signal of arbitrary phase and amplitude.

The second chip takes a sample of the transmitted CW signal and, via a variable phase shifter and a variable attenuator (right in the photo), adds it to the received signal.

With the correct phase shift and attenuation, most of the incoming CW signal can be nulled, leaving the Wi-Fi signal. And the correct values are controlled by feedback loops in an on-chip signal processor which updates the phase and amplitude settings every 100µs.

“We can get about 60dB of suppression. We don’t actually need it all. We only need to stop the receiver from compressing,” said Tang. “10-20dB is good enough for a normal Wi-Fi chip.”

In the photo, the central block is the signal processor. The right-hand circuit samples the CW transmit signal. Its two horizontal structures are the variable phase shifter above the variable attenuator. On the left hand side is the antenna interface circuit.

So far, the scheme has been tested at up to 6m, and 330Mbit/s has been achieved at 2.5m range.

One fly-in-the-ointment is that Wi-Fi basestations are not permitted to transmit CW signals.

“We are working to make it 100% Wi-Fi standard compatible,” said Tang.

Patents have been applied for, and applications in wearables are expected. “There are agreements in place for the commercialisation of the technology,” said NASA.

The technology came out of a NASA project to eliminate mechanically-steered antennas in space – whose pivots have a habit of seizing, according to Tang.

One option is beam-forming using a fixed planar array of antennas, each driven by a separately phase-shifted signal through its own power amplifier, or low-noise amplifier (LNA) in the receive case.

As an alternative, NASA is experimenting with a single power amplifier and fixed antenna, bouncing its energy off a fixed planar array of reflective antennas, each with its own variable phase-shifter to steer the beam. If it works, it will cut out the need for multiple power amplifiers or LNAs.

steve bush

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