  Distributed Orthogonal Space-Time Block Codes (DOSTBCs) achieving full
diversity order and single-symbol ML decodability have been introduced recently
by Yi and Kim for cooperative networks and an upperbound on the maximal rate of
such codes along with code constructions has been presented. In this paper, we
introduce a new class of Distributed STBCs called Semi-orthogonal Precoded
Distributed Single-Symbol Decodable STBCs (S-PDSSDC) wherein, the source
performs co-ordinate interleaving of information symbols appropriately before
transmitting it to all the relays. It is shown that DOSTBCs are a special case
of S-PDSSDCs. A special class of S-PDSSDCs having diagonal covariance matrix at
the destination is studied and an upperbound on the maximal rate of such codes
is derived. The bounds obtained are approximately twice larger than that of the
DOSTBCs. A systematic construction of S-PDSSDCs is presented when the number of
relays $K \geq 4$. The constructed codes are shown to achieve the upperbound on
the rate when $K$ is of the form 0 or 3 modulo 4. For the rest of the values of
$K$, the constructed codes are shown to have rates higher than that of DOSTBCs.
It is shown that S-PDSSDCs cannot be constructed with any form of linear
processing at the relays when the source doesn't perform co-ordinate
interleaving of the information symbols. Simulation result shows that S-PDSSDCs
have better probability of error performance than that of DOSTBCs.
