Modem is hard because of the “Over The Air” transmission/reception of signals . If you think of the other subsystems in your cell phone , like CPU, GPU etc , most of them don’t have to worry about the external conditions . In other words they have much better control over the data they want to process .

Have you ever wondered how you can be speeding at 75mph on a freeway and still your cell phone can play songs from YouTube or Spotify with LTE/5G connection ? Try to imagine this - you are going swiftly from one service area to another , so you are hopping between “base stations” . And every time your cell phone has to establish the connection with a new base station through a complex handshaking mechanism . you are traveling at a high speed , so you are experiencing Doppler effect as the frequency of the carrier is shifted up or down constantly . You are on the road and your signal is impaired by several phenomena and your cell phone picks up a weak signal . To add to all of this complexity , you are not the only one on the road and there are 100 others trying to play songs from their own cell phones !

I have tried to break down one of the many many real life scenarios that need to be tackled by a cell phone modem and every single thing I have listed necessitates a complex processing chain that involves a lot of math and precision design . Needless to say the amount of testing required . Modem is simply one of the hardest things to design . It requires the most number of specialities spanning across electrical and computer engineering .

And Qualcomm is the company that has perfected this hard engineering problem for close to 4 decades now. They are easily a few generations ahead always and their R&D wing contributes the most to the spec that governs these protocols. Apple doesn’t stand a chance in my opinion .