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Any clue what radio IC they used? I didn't see the hardware mentioned in those links. Can you tune a vanilla LTE baseboard to unlicensed bands? Or did they use a software-defined transmitter with a custom LTE stack?

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I believe most LTE chipsets can be tuned into any band you like, as long as you design your own RF front end (antenna and circuitry to/from the chip) and persuade the chipset manufacturer to give you a firmware update in such a way that they aren't liable for you distributing illegal devices. It's probable quite difficult to find manufacturer who is prepared to work with you if you're only going to buy 50,000 modules. I guess being Cambridge Consultants gives you contacts.

Given that Cambridge Consultants spawned Cambridge Silicon Radio you're probably right.

From the slides linked above they are not using LTE, but something loosely based on it.

So they may be reusing some PHY parts like the OFDM but the rest seems custom.

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