I suggest reading about Space-time Block code. There is one paper written by Alamouti, in IEEE. That is available online. If you read about ST Block Code it will be easier to understand other stuffs. **broken link removed** [check the reference section...read Alamouti's paper also Vahid Tarokh's paper] http://publik.tuwien.ac.at/files/pub-et_9815.pdf
Here is Matlab code for Alamouti...may be u can do some simulation your self