Assuming it's technically possible, that would require on all phones a complete rewrite of the firmware that manages the radio subsystem, which often runs its p

Assuming it's technically possible, that would require on all phones a complete rewrite of the firmware that manages the radio subsystem, which often runs its proprietary OS completely separated from the one running on the phone. To add to the difficulty, the radio subsystem is 100% closed, locked and undocumented hardware/firmware, or possibly documented only after signing a pile of NDAs, which for normal people is the same thing, and manufacturers, carriers and governments would fight hard against any change of this. Moreover, 4G links can cover long distance because towers use more powerful trasmitters, more sensitive receivers and -very important- really good high gain antennas placed in favorable high position that reduces the number of obstacles between them and nearby phones. If the above was possible, phones could never reach more than a fraction of that distance in a direct link. Essentially not doable. Just use the existing infrastructure behind a layer of encryption, possibly on hardware you can trust like a PinePhone or any similar platform.