3 research outputs found
On Interleaving Techniques for MIMO Channels and Limitations of Bit Interleaved Coded Modulation
It is shown that while the mutual information curves for coded modulation
(CM) and bit interleaved coded modulation (BICM) overlap in the case of a
single input single output channel, the same is not true in multiple input
multiple output (MIMO) channels. A method for mitigating fading in the presence
of multiple transmit antennas, named coordinate interleaving (CI), is presented
as a generalization of component interleaving for a single transmit antenna.
The extent of any advantages of CI over BICM, relative to CM, is analyzed from
a mutual information perspective; the analysis is based on an equivalent
parallel channel model for CI. Several expressions for mutual information in
the presence of CI and multiple transmit and receive antennas are derived.
Results show that CI gives higher mutual information compared to that of BICM
if proper signal mappings are used. Effects like constellation rotation in the
presence of CI are also considered and illustrated; it is shown that
constellation rotation can increase the constrained capacity.Comment: 20 pages, 11 figures, uses IEEEtran.cl
SISO APP Searches in Lattices with Tanner Graphs
An efficient, low-complexity, soft-output detector for general lattices is
presented, based on their Tanner graph (TG) representations. Closest-point
searches in lattices can be performed as non-binary belief propagation on
associated TGs; soft-information output is naturally generated in the process;
the algorithm requires no backtrack (cf. classic sphere decoding), and extracts
extrinsic information. A lattice's coding gain enables equivalence relations
between lattice points, which can be thereby partitioned in cosets. Total and
extrinsic a posteriori probabilities at the detector's output further enable
the use of soft detection information in iterative schemes. The algorithm is
illustrated via two scenarios that transmit a 32-point, uncoded
super-orthogonal (SO) constellation for multiple-input multiple-output (MIMO)
channels, carved from an 8-dimensional non-orthogonal lattice (a direct sum of
two 4-dimensional checkerboard lattice): it achieves maximum likelihood
performance in quasistatic fading; and, performs close to interference-free
transmission, and identically to list sphere decoding, in independent fading
with coordinate interleaving and iterative equalization and detection. Latter
scenario outperforms former despite the absence of forward error correction
coding---because the inherent lattice coding gain allows for the refining of
extrinsic information. The lattice constellation is the same as the one
employed in the SO space-time trellis codes first introduced for 2-by-2 MIMO by
Ionescu et al., then independently by Jafarkhani and Seshadri. Complexity is
log-linear in lattice dimensionality, vs. cubic in sphere decoders.Comment: 15 pages, 6 figures, 2 tables, uses IEEEtran.cl