Please have a look at our “Journey to GSM” series of articles. Our goal here is to introduce and demystify GSM and Software Defined Radio (SDR) technologies. There are only only two parts available so far, but more will come shortly. I’m the author and would be very much appreciate your feedback.

Here's the link to the first part: http://umtrx.org/journey-to-gsm-part-1-introduction/