1,025 research outputs found
On chip interconnects for multiprocessor turbo decoding architectures
International audienc
CONVERGENCE IMPROVEMENT OF ITERATIVE DECODERS
Iterative decoding techniques shaked the waters of the error correction and communications
field in general. Their amazing compromise between complexity and performance
offered much more freedom in code design and made highly complex codes, that were
being considered undecodable until recently, part of almost any communication system.
Nevertheless, iterative decoding is a sub-optimum decoding method and as such, it has
attracted huge research interest. But the iterative decoder still hides many of its secrets,
as it has not been possible yet to fully describe its behaviour and its cost function.
This work presents the convergence problem of iterative decoding from various angles
and explores methods for reducing any sub-optimalities on its operation. The decoding
algorithms for both LDPC and turbo codes were investigated and aspects that contribute
to convergence problems were identified. A new algorithm was proposed, capable of providing
considerable coding gain in any iterative scheme. Moreover, it was shown that
for some codes the proposed algorithm is sufficient to eliminate any sub-optimality and
perform maximum likelihood decoding. Its performance and efficiency was compared to
that of other convergence improvement schemes.
Various conditions that can be considered critical to the outcome of the iterative decoder
were also investigated and the decoding algorithm of LDPC codes was followed
analytically to verify the experimental results
Constellation Shaping for WDM systems using 256QAM/1024QAM with Probabilistic Optimization
In this paper, probabilistic shaping is numerically and experimentally
investigated for increasing the transmission reach of wavelength division
multiplexed (WDM) optical communication system employing quadrature amplitude
modulation (QAM). An optimized probability mass function (PMF) of the QAM
symbols is first found from a modified Blahut-Arimoto algorithm for the optical
channel. A turbo coded bit interleaved coded modulation system is then applied,
which relies on many-to-one labeling to achieve the desired PMF, thereby
achieving shaping gain. Pilot symbols at rate at most 2% are used for
synchronization and equalization, making it possible to receive input
constellations as large as 1024QAM. The system is evaluated experimentally on a
10 GBaud, 5 channels WDM setup. The maximum system reach is increased w.r.t.
standard 1024QAM by 20% at input data rate of 4.65 bits/symbol and up to 75% at
5.46 bits/symbol. It is shown that rate adaptation does not require changing of
the modulation format. The performance of the proposed 1024QAM shaped system is
validated on all 5 channels of the WDM signal for selected distances and rates.
Finally, it was shown via EXIT charts and BER analysis that iterative
demapping, while generally beneficial to the system, is not a requirement for
achieving the shaping gain.Comment: 10 pages, 12 figures, Journal of Lightwave Technology, 201
High-Rate Space-Time Coded Large MIMO Systems: Low-Complexity Detection and Channel Estimation
In this paper, we present a low-complexity algorithm for detection in
high-rate, non-orthogonal space-time block coded (STBC) large-MIMO systems that
achieve high spectral efficiencies of the order of tens of bps/Hz. We also
present a training-based iterative detection/channel estimation scheme for such
large STBC MIMO systems. Our simulation results show that excellent bit error
rate and nearness-to-capacity performance are achieved by the proposed
multistage likelihood ascent search (M-LAS) detector in conjunction with the
proposed iterative detection/channel estimation scheme at low complexities. The
fact that we could show such good results for large STBCs like 16x16 and 32x32
STBCs from Cyclic Division Algebras (CDA) operating at spectral efficiencies in
excess of 20 bps/Hz (even after accounting for the overheads meant for pilot
based training for channel estimation and turbo coding) establishes the
effectiveness of the proposed detector and channel estimator. We decode perfect
codes of large dimensions using the proposed detector. With the feasibility of
such a low-complexity detection/channel estimation scheme, large-MIMO systems
with tens of antennas operating at several tens of bps/Hz spectral efficiencies
can become practical, enabling interesting high data rate wireless
applications.Comment: v3: Performance/complexity comparison of the proposed scheme with
other large-MIMO architectures/detectors has been added (Sec. IV-D). The
paper has been accepted for publication in IEEE Journal of Selected Topics in
Signal Processing (JSTSP): Spl. Iss. on Managing Complexity in Multiuser MIMO
Systems. v2: Section V on Channel Estimation is update
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