Is it legal in the United Stated to operate a mobile/cellular network in the "unlicensed" 915 MHz ISM frequency band? I mean given I have all other permissions needed (mobile network service license etc.) except the frequency spectrum license.
The technology used may be GSM/UMTS/LTE. The mobile network frequencies (2G/3G/4G) partially overlap the 902-928 MHz ISM band so this should be technically possible (channels in 902-915 + 925-928 MHz) even though naturally this limits the network bandwidth while respecting the maximum ISM band radio power.
However, there's a potentially better choice, the Citizens Broadband Radio Service. This is a "lightly" licensed service, with cheaper and free licenses that are coordinated in shared spectrum. Companies run private CBRS LTE and you can buy small base stations like Wi-Fi access points. More importantly, many commercial handsets support this frequency band, like newer iPhones.
So it seems you could try either the ISM route or the Ham route. If you adhere to the rules for amateur radio, you could consider yourself an amateur radio operator and as such should be allowed to use the band.
You need an amateur radio license
There's a 1500W power limit
Specifications must be public (not a problem for GSM)
No cryptography allowed
There are RF exposure limits (but you're not likely to hit those)
Your station must identify itself every 10 minutes
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