1) The document discusses space-time coding techniques used in wireless communication systems to improve reliability of data transmission using multiple transmit antennas.
2) It describes space-time block codes (STBC) such as Alamouti codes and orthogonal designs which transmit redundant copies of data across antennas without loss of data rate.
3) It also discusses space-time trellis codes (STTC) which provide coding gain but have higher complexity than STBCs.