Reduction of decoding complexity for mimo sphere decoding, qostbc, and ostbc systems

Abstract

In this dissertation, the problem of decoding complexity for sphere decoding, quasi-orthogonal space-time block codes, and orthogonal space-time block codes is considered. First, the thesis investigates the complexity reduction of sphere decoding by reducing the number of computational operations required by the receiver to retrieve the transmitted signal. We show that this reduction in the complexity is made possible by a simple reordering of the real-valued representation of the channel matrix. Making use of the new structure; decoding the real and imaginary parts of each jointly detected symbol can be performed independently and in parallel. A significant complexity gain is achieved while providing the optimal performance. Next, we look at sphere decoding from an implementation perspective. We propose a proper tree search traversal technique that reduces the overall SD computational complexity. We exploit the similarity among the complex symbols in a square QAM lattice representation and rewrite the squared norm ML metric in a simpler form allowing significant reduction of the number of operations required to decode the transmitted symbols. We also show that this approach achieves > 45% complexity gain for systems employing 4-QAM, and that this gain becomes bigger as the constellation size gets larger. Then, the thesis focuses on the decoding complexity of quasi-orthogonal space-time block codes and orthogonal space-time block codes. We propose a reduced-complexity Maximum Likelihood detection algorithm based on the real-valued lattice representation of the channel and QR decomposition. We show that for square L-QAM constellations and N transmit antennas, the proposed algorithm reduces the decoding computational complexity from O(LN /2) for conventional ML to O(L) for systems employing QOSTBCs and from O(L) for conventional ML to O( L ) for those employing OSTBCs without sacrificing the performance. We also show that for QOSTBC systems with rate r = ns/T, where ns is the number of transmitted symbols per T time slots;, the proposed algorithm decomposes the original complex-valued system into a parallel system represented by ns 2 × 2 real-valued components, thus allowing for a simple joint decoding of two real symbols. For OSTBC systems, the channel, as seen by the real and imaginary parts of every transmitted complex symbol, is 1 × 1 real-valued simple channel. This makes the detection of the real and imaginary parts be carried out simply and independently.

Cite this paper

@article{ayanoglu2008reduction,
  title = {{Reduction of decoding complexity for mimo sphere decoding, qostbc, and ostbc systems}},
  author = {Ayanoğlu, Ender and Azzam, Luay},
  year = {2008}
}

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