Date Added: Jul 2010
Because of their orthogonality properties, unitary matrices are an important class of matrices that are used in mathematics, physics, control, communications and others. In Multiple-Input Multiple-Output (MIMO) communication systems, there are two main applications that use unitary matrices: Differential Space-Time Modulation (DUSTM) and precoding. DUSTM is used when the Channel State Information (CSI) is not available for both transmitter and receiver, while unitary precoding is used when complete or partial CSI is available for both sides. For DUSTM and limited feedback MIMO systems, a codebook of unitary matrices should be designed. Conventionally, design parameters are optimized based on a cost function depending on the application.