STBC (Space Time Block Coding)
STBC is a kind of coding scheme for TX diversity or MIMO and I hope you can figure out the meaning of STBC directly from following illustration. Just connect the words in red -:). In short, STBC is a multi Antenna technique in which a multiple symbols are transmitted as a block (2 D array) of multiple Antenna (representing Space) and Symbol Time. There can be different ways on how to arrange the symbols within the block to be transmitted. One of the most common arrangement is as shown below. This is called Alamouti code.
In the following illustration(example), x represents 'bit' and s represent a symbol. You would notice one bit is mapped to one symbol, so this example is for BPSK.
In some material, you may see this matrix represented as shown below. Basically it doesn't matter whether you present as shown above or as shown below, but you have to be careful about writing mathematical process in channel modeling.
Now you may ask "Why they use this kind of specific block (array) ?".. "how do they guarantee that this kind of block transmittion gurantees the improved performance ?" This is the part where you need to go through a thick book and a lot of mathematic formulas to find the answer. I would not go through these topics for now.
Just for now, you can only take it as a proven technology in theory and widely used in the industry.
If you are really interested in the details (theory), you may read Alamouti Code/OSTBC page as an example.