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The special thing about mm wave antenna arrays is that you don't need to have a phone on you to be identified by the unique fingerprint of your gait. There's enough information density in that spectrum to resolve your identity accurately even if you're not carrying a phone. And there are papers demonstrating how to do just that.

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&as_vis...

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Just noting that some of those articles are about mm wave devices explicitly designed as radars. But if you dig through you'll find articles geared towards "mm wave antenna arrays", which is a basically a keyword for 5G radio hardware.

[perihelions]

That's astonishing!