Cascaded Orthogonal Space-Time Block Codes for Wireless Multi-Hop Relay   Networks
  Distributed space-time block coding is a diversity technique to mitigate the effects of fading in multi-hop wireless networks, where multiple relay stages are used by a source to communicate with its destination. This paper proposes a new distributed space-time block code called the cascaded orthogonal space-time block code (COSTBC) for the case where the source and destination are equipped with multiple antennas and each relay stage has one or more single antenna relays. Each relay stage is assumed to have receive channel state information (CSI) for all the channels from the source to itself, while the destination is assumed to have receive CSI for all the channels. To construct the COSTBC, multiple orthogonal space-time block codes are used in cascade by the source and each relay stage. In the COSTBC, each relay stage separates the constellation symbols of the orthogonal space-time block code sent by the preceding relay stage using its CSI, and then transmits another orthogonal space-time block code to the next relay stage. COSTBCs are shown to achieve the maximum diversity gain in a multi-hop wireless network with flat Rayleigh fading channels. Several explicit constructions of COSTBCs are also provided for two-hop wireless networks with two and four source antennas and relay nodes. It is also shown that COSTBCs require minimum decoding complexity thanks to the connection to orthogonal space-time block codes. 
Rahul Vaze and Robert W. Heath Jr