 
Summary: 1
RealValued Maximum Likelihood Decoder for
QuasiOrthogonal SpaceTime Block Codes
Luay Azzam, Student Member, IEEE, and Ender Ayanoglu, Fellow, IEEE
Center for Pervasive Communications and Computing
Department of Electrical Engineering and Computer Science
University of California, Irvine
Abstract
In this letter, we propose a low complexity Maximum Likelihood (ML) decoding algorithm for quasi
orthogonal spacetime block codes (QOSTBCs) based on the realvalued lattice representation and QR
decomposition. We show that for a system with rate r = ns/T, where ns is the number of transmitted
symbols per T time slots; the proposed algorithm decomposes the original complexvalued system into
a parallel system with ns 2 × 2 realvalued components, thus allowing for a simple joint decoding of
two real symbols. For a square QAM constellation with L points (LQAM), this algorithm achieves full
diversity by properly incorporating twodimensional rotation using the optimal rotation angle and the
same rotating matrix for any number of transmit antennas (N 4). We show that the complexity gain
becomes greater when N or L becomes larger. The complexity of the proposed algorithm is shown to
be linear with the number of transmitted symbols ns.
Index Terms
Quasiorthogonal spacetime block codes, pairwise symbol decoding, multipleinput multipleoutput
