Even a RPI won't help you unless you can build all of the software for the microprocessors which drive the wireless stack. Even then, vendors (e.g. Qualcomm) will already have their software on the chip when you get it.

A completely open spec, open source set of components is what the community has desired for a long time. As standards get more complex and evolve faster, 4G and beyond, it becomes less possible to keep up in the open.