Are you suggesting some 3G RF hardware can be retrofitted with software patches to be compatible with 4G networks ?

This is theoretically possible (SDR) but it's very expensive.

(Before anyone replies about $20 SDRs, those aren't even close to 4G/5G capable.)

Indeeed. Last time I looked these chips were $1300 each in quantities of 1000. Too expensive for even a luxury car.

I think the answer here is an internal USB modem or something. Easy to upgrade. Just up to the car manufacturer to commit to writing new drivers. (I assume that they don't want to do this, so are probably happy when technology changes out from under them. Everyone has to buy a new car!)

What makes this silicon so expensive?

- Car stuff has to be certified in order to be road legal (in Europe, ECE marked)

- Car stuff has to deal with a way wider temperature range than normal consumer stuff (-40 to +125 °C). That is the thing that makes silicon so expensive.

- Car stuff has to deal with vibrations and mechanical stress, meaning more expensive PCBs, less options for solder (remember the infamous NVIDIA solder issue of ~2010?) and connectors

- Car voltage regulation is a hot mess, at least for ICE vehicles - IIRC during engine start, voltages can drop to something like 6-ish volts and after that (or during battery disconnects/other load dumps) into 120-ish volts, and the voltage has an awful lot of ripple because usual generators are AC that's driven through a diode rectifier, with the battery acting as stabilizer.

- Most stuff in cars requires hours of work to replace it - Tesla were to my knowledge the first ones who did software updates outside of shop visits at all.

Regarding the temperature range; this procides a bit more detail and nuance:

https://electronics.stackexchange.com/questions/301826/how-t...

Future-proofing puts you on the wrong side of Moore's Law. Let's say 4G requires 10x the transistors of 3G. In reality that's not a problem because 4G came out 10 years after 3G so transistor costs came down 10x or more. But if you wanted to build a 4G-upgradeable radio years early you'd have to pay for that 10x performance upfront using older and more expensive technology. This same argument applies to future-proofing other tech like video codecs as well.

High complexity, low volumes, I would imagine. A few of the answers below talk about it being a car part but SDRs that can do 3G/4G are targeted at people developing cheaper chips. Electronic test gear is much more expensive than cars (at least at the bleeding edge), so the pricing doesn't preclude it from being used there.

the radio cannot be software defined (and user modifiable) for type acceptance and regulatory reasons

In what country? The FCC certainly allows SDR to be certified. "User modifiable" is sufficiently broad as to be nearly meaningless.

Here's the actual text on restrictions for SDR: https://www.law.cornell.edu/cfr/text/47/2.944

The FCC defines SDR here extremely broadly, any radio which "frequency range, modulation type or maximum output power (either radiated or conducted)" can be changed.

This describes a substantial portion of radios - including many if not most WiFi access points. They are legal and many US companies design, manufacture, and/or sell these types of radios (I work for one of them).

Absolutely they are legal to sell, but you have to go through type acceptance under part 15, and then you need to make it so the user cannot modify them.

“ The rules require any manufacturer certifying a device under the new process to take steps to prevent “unauthorized” changes to the software on the device that might alter its radio frequency and power parameters in a way that takes it out of compliance with the regulations known as FCC Part 15 regulations.”

Test equipment is exempt because it is not licensed as an intentional emitter under part 15.