1,155 research outputs found
Turbo Decoder for Low-Power Ultrawideband Communication Systems
A new method to reduce the computational complexity
of the turbo decoding in ultrawideband (UWB) orthogonal
frequency division multiplexing (OFDM) system is proposed.
Existing stopping techniques for turbo decoding process using
constrained decoding assume fixed signal-to-noise ratio (SNR)
for all the OFDM symbol bits so they fail to yield an acceptable
bit-error rate (BER) performance in multicarrier systems. In
this paper, we propose a bit-level stopping technique for turbo
decoding process based on the constrained decoding method. In
this technique, we combine the cyclic redundancy check (CRC)
with an adaptive threshold on the log likelihood ratio (LLR)
on each subcarrier to detect for convergence. The threshold is
adaptive in the sense that the threshold on the LLR of a bit is
determined by the average SNR of the OFDM symbol and the
channel gain of the transmission subcarrier. Results show that
when the channel state information (CSI) is used to determine
the threshold on LLR, the stopping technique can reduce the
computational complexity by about 0.5–2.5 equivalent iterations
compared to GENIE turbo without degradation in the BER
performance
Turbo Decoder for Low-Power Ultrawideband Communication Systems
A new method to reduce the computational complexity of the turbo decoding in ultrawideband (UWB) orthogonal frequency division multiplexing (OFDM) system is proposed. Existing stopping techniques for turbo decoding process using constrained decoding assume fixed signal-to-noise ratio (SNR) for all the OFDM symbol bits so they fail to yield an acceptable bit-error rate (BER) performance in multicarrier systems. In this paper, we propose a bit-level stopping technique for turbo decoding process based on the constrained decoding method. In this technique, we combine the cyclic redundancy check (CRC) with an adaptive threshold on the log likelihood ratio (LLR) on each subcarrier to detect for convergence. The threshold is adaptive in the sense that the threshold on the LLR of a bit is determined by the average SNR of the OFDM symbol and the channel gain of the transmission subcarrier. Results show that when the channel state information (CSI) is used to determine the threshold on LLR, the stopping technique can reduce the computational complexity by about 0.5-2.5 equivalent iterations compared to GENIE turbo without degradation in the BER performance
ENHANCEMENT OF ITERATIVE TURBO DECODING FOR HARQ SYSTEMS
This paper presents a new method for stopping the iterative turbo decoding. First, a bit-level convergence test using the cross-entropy analyses is used to select non converged bits and establish a simple and effective stopping rule. Next, an adaptive approach is used to compute a scaling factor for normalizing the extrinsic information of the previously selected bits. The extra coding gain obtained from this normalization can compensate for the performance degradation of the stopping rule. The simulation results of the proposed stopping criterion show an interesting application in a hybrid automatic repeat request systems with turbo coding scheme, where the decoding complexity can be fairly reduced.
Simulation results of the proposed criterion, in comparison with previously published stopping rules, were presented for illustrating the adaptive termination according to a changing SNR environment
Iterative Decoding and Turbo Equalization: The Z-Crease Phenomenon
Iterative probabilistic inference, popularly dubbed the soft-iterative
paradigm, has found great use in a wide range of communication applications,
including turbo decoding and turbo equalization. The classic approach of
analyzing the iterative approach inevitably use the statistical and
information-theoretical tools that bear ensemble-average flavors. This paper
consider the per-block error rate performance, and analyzes it using nonlinear
dynamical theory. By modeling the iterative processor as a nonlinear dynamical
system, we report a universal "Z-crease phenomenon:" the zig-zag or up-and-down
fluctuation -- rather than the monotonic decrease -- of the per-block errors,
as the number of iteration increases. Using the turbo decoder as an example, we
also report several interesting motion phenomenons which were not previously
reported, and which appear to correspond well with the notion of "pseudo
codewords" and "stopping/trapping sets." We further propose a heuristic
stopping criterion to control Z-crease and identify the best iteration. Our
stopping criterion is most useful for controlling the worst-case per-block
errors, and helps to significantly reduce the average-iteration numbers.Comment: 6 page
Information-Coupled Turbo Codes for LTE Systems
We propose a new class of information-coupled (IC) Turbo codes to improve the
transport block (TB) error rate performance for long-term evolution (LTE)
systems, while keeping the hybrid automatic repeat request protocol and the
Turbo decoder for each code block (CB) unchanged. In the proposed codes, every
two consecutive CBs in a TB are coupled together by sharing a few common
information bits. We propose a feed-forward and feed-back decoding scheme and a
windowed (WD) decoding scheme for decoding the whole TB by exploiting the
coupled information between CBs. Both decoding schemes achieve a considerable
signal-to-noise-ratio (SNR) gain compared to the LTE Turbo codes. We construct
the extrinsic information transfer (EXIT) functions for the LTE Turbo codes and
our proposed IC Turbo codes from the EXIT functions of underlying convolutional
codes. An SNR gain upper bound of our proposed codes over the LTE Turbo codes
is derived and calculated by the constructed EXIT charts. Numerical results
show that the proposed codes achieve an SNR gain of 0.25 dB to 0.72 dB for
various code parameters at a TB error rate level of , which complies
with the derived SNR gain upper bound.Comment: 13 pages, 12 figure
Approximate MIMO Iterative Processing with Adjustable Complexity Requirements
Targeting always the best achievable bit error rate (BER) performance in
iterative receivers operating over multiple-input multiple-output (MIMO)
channels may result in significant waste of resources, especially when the
achievable BER is orders of magnitude better than the target performance (e.g.,
under good channel conditions and at high signal-to-noise ratio (SNR)). In
contrast to the typical iterative schemes, a practical iterative decoding
framework that approximates the soft-information exchange is proposed which
allows reduced complexity sphere and channel decoding, adjustable to the
transmission conditions and the required bit error rate. With the proposed
approximate soft information exchange the performance of the exact soft
information can still be reached with significant complexity gains.Comment: The final version of this paper appears in IEEE Transactions on
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