OpenBTS is an OpenSource project with the aim of implementing and sharing the BTS protocol stack. This project has been going on for several years. I think this project has inspired many other people and has been important trigger for many of SDR Network Simulator solutions we see today.  You can the detailed documentation and source code from OpenBTS Home page. You can also see many of the demonstration, workshops, presentation on YouTube. Following is one example YouTube video of implementation based on OpenBTS (It is not me in the video :), I just grabed this video from YouTube and put it here).  As far as I know, only GSM BTS has been implemented with this project and I haven't seen any other access technology (e.g, WCDMA, LTE etc) based on this project.