What sort of radio signal are telephone calls and SMS texts?


I have been studying how radio signals work lately trying to get my ham radio license. I then realized that phone calls and texts are essentially radio signals. What sort of signal are they? What frequencies do cell phones calls operate on? Is there a way to intercept and decode sms text messages using some sort of reciever? How are they encoded and decoded? This is just for hypothetical reasons. I have no interest in trying to intercept phone calls or anything.

In: 5

Up to about thirty years ago it was all very simple. The phones were essentially walkie-talkies with a microprocessor to automatically tune them to the right frequency on command from a tower. They used ordinary narrow band FM modulated signals.

Since then the systems have [gradually evolved](https://en.wikipedia.org/wiki/Comparison_of_mobile_phone_standards) to great complexity. The modulation, encoding, data transmission protocols are all quite elaborate, and the books describing them are thousands of pages long. It would take a motivated effort to learn and to understand how exactly any particular standard is implemented. Needless to say, modern systems are built with data protection in mind, and casual eavesdropping is not possible.

Most LTE cell signal is in the 700MHz band. Within this band there are channels of varying bandwidth between 5 and 20MHz. These channels work similar to how CB and GMRS channels work (since you are learning about amateur radio) but they won’t sound like FM signals if you were just listening to them. Most of them are a digital mode. Analog modes went away with 2G.

There are other 4G and 5G frequencies, the 700 band is just a popular one.

Yes, you can *sniff* for call/text data using devices that mimic a cell phone tower — essentially it’s just a radio receiver tuned to the cell phone frequency bands. Law enforcement uses Stingray devices as a sort of ‘man-in-the-middle’ attack to surveil targets. They have to have a warrant, though.

End to end encryption apps make data a bit harder to read, even if intercepted.

I don’t think that the voice cell signal is encrypted by default, though.

Radio plans vary by continent:



Protocol signaling, encoding, duplexing, multiplexing and modulation varies by technology. On 4G and 5G both TDD and FDD duplexing schemes are used, OFDM multiplexing schemes are used to share radio resources amongst many users.