Not All Wireless Power Is Created Equal: Retro-directivity vs. Beamforming

The advent of RF wireless power at a distance is upon us. The promise is to power our electronic devices such as home electronics, wearables and even smartphones from many feet away.

Yup, the holy grail of power, sought for over 120 years and the focus of how our devices will be powered in the near (and far) future. This will be the next Wi-Fi if you like, so let’s dig right in to understand the proposed technologies out there.

First, as the title suggests, there are multiple ways to do this and unlike contact pad chargers that rely on the same science behind them, power at a distance is a whole different ballgame.

But if you’ve only educated yourself on beamforming wireless power, you have completely missed out on a different animal altogether: radio-frequency-based (RF) retrodirective wireless power. 

A retrodirective antenna is an antenna that transmits the signal back in the same direction it came from.” 1

RF-based retrodirective wireless power is not beamforming

Beamforming technology was originally invented to create RADAR systems in the late 1930’s by the British armed forces. Unlike the rotating dish that we envision as being RADAR, the original RADAR was a set of antennas that emitted a powerful phased signal, that the signals were directed to different paths into the sky and the system waited to receive a reflected signal. But it was this possibility to create a directed signal that interested the wireless power crowd. This was the origin of Beamforming and is essentially the technology behind it today.

In short: Beamforming is the ability to emit signals from many antennas such that the total signal is a wavefront that is directed at specific path.

Wireless power beamforming works something like this:

  • The transmitter emits a scanning beam to cover all angles within range
  • While beam is scanning, listen to a device respond indicating that this path has a device along it
  • The transmitter notes the angle of the path to that device
  • Once the scanning ends, the transmitter can now shoot a beam towards the device it wants to power.
  • Listen to the device while it is powered, and if the power changes (because device was moved or someone/something blocked the way), stop the power and start scanning (repeat this cycle)
  • Scanning can take from a few seconds to half a minute depending on distance.

RF retrodirective wireless power is based on very different hardware and capabilities. Every antenna in the retro-directive array can receive or transmit signals on demand. The same capability that allows an array to send a beam in a specific direction can send many beams at the same time. To determine the phases (or beams) of the retro-directive array, the receiver sends a small signal called a “beacon” that would travel every available path between the receiver and the transmitter. The transmitter’s antennas detect the beacon signal and thus instantaneously determine the phase for that antenna such that the array now creates all the “beams” that arrived at the transmitter, without executing any algorithm in hardware or software. The transmitter now emits the phases from all its antenna array and creates all the “beams” through the paths they arrived at and thus powering the receiver device. This beacon is repeated a hundred times a second in case the device moves or the paths between the receiver and transmitter have changed; allowing power delivery while the device is in motion.

This retro-directivity is a game changer for wireless power delivery. To help showcase the differences between an RF retrodirective wireless power system and beamforming, we’ve created this chart for you.

Retrodirective Wireless Power Versus Beamforming: A Comparison

Feature 

RF retro-directive wireless power

Beamforming wireless power

Line of sight

No line of sight is required. Multipaths are used by default

Single clear path required for power

Location/ orientation of receiver

Position/orientation of the receiving device does not matter

Receiving device must be pointing at the transmitter and within transitter coverage (around the “boresight” or in front of array)

Motion of receiver

Can power devices in motion at speeds or an object thrown across the room

Receiving device must be stationary for at least several seconds to receive power

Accuracy and RF efficiency

Retro-directive power guarantees maximum RF efficiency between transmitter and receiver under any conditions 

Transmitter does its best case for every scenario, usually the receiver antenna is not tuned for that angle, or polarization – thus reduced efficiency

Continuous operation when humans are around

System does not emit signals through any blocked path, maintains power even when people are around

Transmission must shut down if any motion is happening in the environment.

Computational power needed

No algorithms or computations required. Transmitter knows the exact phases for return signal

Requires complex computations to create beams, scan the environment and listen the powered devices

Shape of transmitter

Transmitters and receivers can be any shape — cylinder, organic, pyramid, or curved surfaces

Beamforming requires flat. transmitters

Safety (1)

Inherently safe to use around people and pets; system constantly readjusts through the repeated beacon its emitted paths, no algorithms needed

Energy transfer must stop when the beam is interrupted by people, pets, or other objects. Signal control requires complex algorithms that make assumptions of the environment, all beamforming systems proposals today include a “turn power off when humans are around” to ensure safety

 

Retrodirective Wireless Technology Provides Maximum Power

Ossia’s Cota wireless power is the only Retro-directive technology in the market today, the ability of the transmitter antenna to send signals exactly back to where it came from, is the main thing that differentiates Ossia’s Cota from other wireless power technologies. When a Cota transmitter sends power back to the receiver, it is matching its acceptance criteria without fail. 

Think of it this way, if you look into a mirror and you can see a reflection of someone’s eyes, you know they can see your eyes, too. With Cota, the transmitting and receiving antenna can “see” each other. It uses all available clear paths, direct and reflected, to deliver maximum power to the receiver.

No Complex Algorithms Translates to Speed, Accuracy, Safety, and Efficiency

It’s important to emphasize that these retrodirective paths are mapped without software or complex algorithms. Software, which is required for beamforming to locate a receiver, is subject to bugs or unexpected fringe cases, which requires engineers to constantly solve for an unending number of real-world situations. 

Beamforming might be the wireless power technology that you have heard of, but it is viable only in limited applications. RF retrodirective wireless power technology overcomes most of the limitations and challenges of beamforming. It does not need to seek out and calculate angle, orientation, polarization, or distance, and it does not require line of sight or for the device to be motionless. Retro-directivity, by definition, solves those engineering problems inherently.

Ossia’s RF-based retrodirective wireless power, Cota, has received FCC certifications in the US, multiple patents, and EU/UK regulatory approval for unlimited distance transmission. It is available for license.

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1 https://en.wikipedia.org/wiki/Retrodirectivity

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