Wireless channels are prone to large fluctuations. Diversity is a technique that provides the receiver with multiple, possibly less-attenuated copies of the transmitted signal and thus helps to mitigate fading. Diversity can be provided in time, frequency or space. Spatial diversity is preferred as it is both rate and bandwidth efficient. Coding the information across transmit antennas in the absence of channel knowledge such that the receiver can suitably extract the information and exploit spatial diversity (possibly while providing coding gain) is called Space Time Coding (STC).

 Two main categories of STC are 

• Space Time Trellis Codes
   

• Emphasis: Encoding procedure at the transmitter
   

• Goal: Diversity advantage, Coding gain
   

• Complexity: High (MLSE Decoder)
   

• Space Time Block Codes
   

• Emphasis: Encoding procedure at the transmitter
   

• Goal: Diversity advantage
   

• Complexity: Low (ML Decoder, Symbol based)