I don't think that is the case of any smartphone SOC. Even for questions about power management you tend to implement radio function with a dedicated hardware,

I don't think that is the case of any smartphone SOC. Even for questions about power management you tend to implement radio function with a dedicated hardware, so that for example the CPU can go to sleep and be waked up when a phone call arrives (for examaple). It would be too expensive to have the main CPU implement the 4G radio in software, they don't do so, it would also require precise timing that a non real time OS cannot provide. Typically you have the modem that has its own microcontroller inside that runs its own firmware, that is encrypted. On Android phones you have a partition for the radio firmware, that you should really never touch (since doing so you can brick your device). Of course there will be a shared memory area between the radio and the main CPU to talk, but that is only for communication, then the radio microcontroller has its own RAM to implements its functions.