Description from "Space-Time Block Coding for Wireless Communication:Performance Results" by:V.Tarokh,H.Jafarkhani,R.Calderbank

space-time block codes which provide a new paradigm for transmission over Rayleigh fading (MIMO fading) channels using multiple transmit antennas.

Data is encoded using a space-time block code, and the encoded data is split into n streams which are simultaneously transmitted using n transmit antennas. The received signal at each receive antenna is a linear superposition of the n transmitted signals perturbed by noise. Maximum likelihood decoding is achieved in a simple way through decoupling of the signals transmitted from different antennas rather than joint detection. This uses the orthogonal structure of the space-time block code and gives a
maximum likelihood decoding algorithm which is based only on linear processing at the receiver.

A space-time block code is defined by a p*n transmission matrix. The entries of the matrix are linear combinations
of the variables x1,x2,...xk and their conjugates. The number of transmission antennas is n.