  A construction of a new family of distributed space time codes (DSTCs) having
full diversity and low Maximum Likelihood (ML) decoding complexity is provided
for the two phase based cooperative diversity protocols of Jing-Hassibi and the
recently proposed Generalized Non-orthogonal Amplify and Forward (GNAF)
protocol of Rajan et al. The salient feature of the proposed DSTCs is that they
satisfy the extra constraints imposed by the protocols and are also four-group
ML decodable which leads to significant reduction in ML decoding complexity
compared to all existing DSTC constructions. Moreover these codes have uniform
distribution of power among the relays as well as in time. Also, simulations
results indicate that these codes perform better in comparison with the only
known DSTC with the same rate and decoding complexity, namely the Coordinate
Interleaved Orthogonal Design (CIOD). Furthermore, they perform very close to
DSTCs from field extensions which have same rate but higher decoding
complexity.
