Space time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data transfer.Moreover this is an effective and practical way to approach the demands of MIMO wireless channel.This is a coding technique designed for use with multiple transmits antennas, to increase data rate, capacity and spectral efficiency.As originally introduced, and as usually studied, these are orthogonal in nature but with one most serious drawback that these sacrifice some proportion of their data rate.So, we use quasi-orthogonal STBC s that achieve higher data rates at the cost of inter-symbol interference.