@Jan Audio:
1) What do you want to achieve, really? Why do you want to build a 4G cell phone from scratch? I ask, because maybe you don't really need/want to start from scratch and could use a different approach, i.e. as already mentioned, using a mobile phone module (SoC), using a cheap phone and rip off the required parts (for a one timer) or, like what they do with expensive luxury phones, getting a premium smartphone, redesign its case (i.e. in pure gold and covered in diamonds - for inspiration see here:
http://www.goldvish.com/single-post/2016/04/22/guinness-world-records-most-expensive-phone-for-goldvish) and modifiying the firmware (see XDA Developers forum).
2) When you ask how difficult it is to make a cell phone (4G), you need to add your starting point! How difficult it would be for Nokia, based on their 3G technology or for you based on existing chips, or based on existing chip layouts, based on existing knowledge, based on existing manufacturing processes, etc. This can be taken to the extreme: imagine you are able to carry the latest Samsung Galaxy S21 Ultra plus a network cell and all required hardware to operate it and travel back to 1960. How much time would be needed to replicate the mobile phone technology of 2021? My guess is that it would basically take more or less the same time! Why? Because the world would not be able to deliver rare earth metals, battery technology, RF knowledge, xx nanometer chip design, software technology, etc.
So if you would want to start from scratch to develop a 4G cell phone, you would need to develop:
- software
- chips
- manufacturing processes
- materials engineering
- etc.
If you "cheat" and say you would start with FPGA and what not, you would still have to develop more than you could achieve in a lifetime. Think of compression algorithms, modulation algorithms, antenna design, etc.
3) Your question does not specify how many people are to be involved! Is it just you, 10 people, 100 people or 1000 people? How "smart" and "educated" are the people on your team? Do they cover all required technological aspects?
4) Your question does not specify how much money you are prepared to invest. As others mentioned, the certification and NDA's will cost you a 6 figure number alone!
If you are really committed into developing your own 4G cell phone and I have no doubt that there are people smart enough to put this off, your way to go is this:
1) Get yourself a propper SDR, like this:
https://www.ettus.com/all-products/un210-kit/2) Learn how to program it using the SDR, FPGA, etc.
3) Install GNU Radio
4) Learn how to use GNU Radio, how to develop in Python and C++ for GNU Radio
5) Build the required blocks (many are already available)
6) Build a 4G implementation using the blocks
7) Publish the results of your software defined 4G cell phone and you will be a star! I mean that seriously.
Regards,
Vitor
PS: It has already been done (partially) -
https://github.com/kit-cel/gr-lte