doc.: ieee 802.22-06/0017r0 submission january 2006 patrick pirat, france telecomslide 1...
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January 2006
Patrick Pirat, France Telecom
Slide 1
doc.: IEEE 802.22-06/0017r0
Submission
Duo-binary_Turbo-codes: questions and answersIEEE P802.22 Wireless RANs Date: 2006-01-06
Name Company Address Phone email Martial Bellec France Telecom 4 rue du clos Courtel
35512 Cesson-Sevigné 33299124806 Martial.bellec@francetele
com..com Marie-Hélène Hamon France Telecom 4 rue du clos Courtel
35512 Cesson-Sevigné 33299124806 Marie-
John Benko France Telecom [email protected]
Patrick Pirat France Telecom 4 rue du clos Courtel 35512 Cesson-Sevigné
33299124806 [email protected]
Authors:
Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
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January 2006
Patrick Pirat, France Telecom
Slide 2
doc.: IEEE 802.22-06/0017r0
Submission
Abstract
This set of slides intends to give some answers to the questions that followed the presentation of November 2005
January 2006
Patrick Pirat, France Telecom
Slide 3
doc.: IEEE 802.22-06/0017r0
Submission
• Duo-Binary input: two decoded bit output at a time
– Reduction of latency and complexity per decoded bit (compared to Binary TC)
– Better convergence
– Circular (tail-biting) encoding
– No trellis termination overhead
– Original interleaving scheme
– Larger minimum distances
– Improved asymptotic performances
Duo-Binary Turbo-codeSingle-binary could also be designed to process two bits at once if needed: no advantage. More parellel
sub-blocks could also be used for single-binary.
Complexity (ignoring overhead): duo-binary 8-state decoder require 50% more comparisons per info bit and more than 50% more memory
for extrinsics than single-binary TC. Duo-binary may allow more parallelism than single-binary. LDPC can allow massive parallelism.
Duo-binary is not expected to be better than single-binary. It may be better than some LDPC
implementations.Both duo-binary and single-binary TC implementations will tend to have the similar performance in the waterfall and same convergence performance
with good spread interleavers.
Not unique to duo-binary, should be used also for single-binary.
3GPP standard termination technique is not recommended
because it generate high BER flare.
With a good interleaver design, single-binary gives larger distances and thus better flare performance. Best trade-off to date: single-binary turbo code with Crozier
(dithered relative prime, DRP[1,2]) interleavers: dmin=51 for R=1/3 and K=1504, while duobinary 8-state DVB-RCS gives dmin=33 and with Y. Ould-
Cheikh-Mouhamedou interleaver[3], dmin= 40.As long as a good interleaver
design approach is used, single-binary will tend to give better
distances, and lower flares. Single-binary tends to give better distances because the interleaver is effectively twice
as long as the one for duo-binary.
In general, the main advantages of double-binary Turbo codes apply to single-binary Turbo codes as well (i.e., flexibility, fixed encoder/decoder pair, tail-biting).
January 2006
Patrick Pirat, France Telecom
Slide 4
doc.: IEEE 802.22-06/0017r0
Submission
Internal Interleaver• Algorithmic permutation
–One equation, 4 parameters (P0, P1, P2, P3)
–Parameters selected such that interleaver is contention-free
• Adjusting the TC to a blocksize only requires modification of the 4 parameters
• Quasi-regular permutation (easy connectivity)
• Inherent parallelism
i = 0, …, N-1, j = 0, ...N-1
level 1: if j mod. 2 = 0, let (A,B) = (B,A) (invert the couple)
level 2:
- if j mod. 4 = 0, then P = 0;
- if j mod. 4 = 1, then P = N/2 + P1;
- if j mod. 4 = 2, then P = P2;
- if j mod. 4 = 3, then P = N/2 + P3.
i = P0*j + P +1 mod. N
DVB-RCS standard interleaver.
Better distances have been found with dithered relative prime (DRP) interleavers which are also highly
structured to save memory.
All these features also apply to DRP interleavers .
January 2006
Patrick Pirat, France Telecom
Slide 5
doc.: IEEE 802.22-06/0017r0
Submission
Answers: complexity (1)
• "Raw comparison" of 8-state Duo-Binary TC and 8-state binary TC (UMTS)
Study case:
(54 bytes, rate ½) Binary Duo-binary Ratio
Gate count 13100 24100 + 80%
Memory (bits) 29000 40088 + 38%
Silicon area (0.13 um)
0.23 mm² 0.36 mm² + 50%
Decoded bit per clock cycle
1 2 + 100%
Complexity per decoded
bit(constant clock rate)
1 0.78 - 22%
January 2006
Patrick Pirat, France Telecom
Slide 6
doc.: IEEE 802.22-06/0017r0
Submission
Answers: complexity (2)
• In a first approach, the complexity of an 8-state duo-binary turbo-decoder is about 50% higher than the one of a simple binary decoder
• But, using the same computing clock, a duo-binary decoder processes the data by pairs, and outputs 2 decision data at each cycle. Therefore, using the same clock, a duo-binary turbo-decoder achieves twice the throughput of a binary decoder with only 50% hardware more.
• In the same clock condition and throughput requirements, the hardware of a binary decoder should be duplicated. And then, referring to the complexity per decoded bit, a duo-binary decoder is about 22% less complex than a binary decoder
January 2006
Patrick Pirat, France Telecom
Slide 7
doc.: IEEE 802.22-06/0017r0
Submission
Answers: comparison with single-binary TC
• We believe that Duo-Binary TC represent the best compromise in terms of performance/complexity trade-off(see previous answer). The advantages described in our slides are not limited only to Duo-Binary TC.
• Single-binary TC can also be designed to be parallelized. Duo-Binary TC has an inherent capability to parallelism, enabled by the internal interleaver.
January 2006
Patrick Pirat, France Telecom
Slide 8
doc.: IEEE 802.22-06/0017r0
Submission
Answers: Interleavers
• It is true that well-designed single-binary TC can provide better distances than DVB-RCS standard interleaver, but the Duo-Binary TC can benefit from the two-level permutations (inter-couples and intra-couples)
• We have proposed the interleaver as defined into DVB-RCT/RCS standard, but are open to discussion on other possible interleavers if they represent better alternatives
January 2006
Patrick Pirat, France Telecom
Slide 9
doc.: IEEE 802.22-06/0017r0
Submission
Answers: Performance (1)
• Convergence: Duo-Binary TC show better convergence due to the lower density of erroneous paths– See "The advantages of nonbinary turbo codes", C. Berrou, M.
Jezequel, C. Douillard and S. Kerouedan, Proceedings of Information Theory Workshop, Cairns, Australia, pp. 61-63, Sept. 2001
• Following slides: Performance comparison of Duo-Binary TC and single-binary TC (UMTS) on AWGN for different coding rates and blocksizes
January 2006
Patrick Pirat, France Telecom
Slide 10
doc.: IEEE 802.22-06/0017r0
Submission
Answers: Performance (2)
• Coded blocksize N=864 bits– Information
blocksize K=432 bits for R=1/2
– Information blocksize K=648 bits for R=3/4
• Max-Log-MAP decoding, 8 iterations
January 2006
Patrick Pirat, France Telecom
Slide 11
doc.: IEEE 802.22-06/0017r0
Submission
Answers: Performance (3)
• Coded blocksize N=1440 bits– Information
blocksize K=720 bits for R=1/2
– Information blocksize K=1080 bits for R=3/4
• Max-Log-MAP decoding, 8 iterations
January 2006
Patrick Pirat, France Telecom
Slide 12
doc.: IEEE 802.22-06/0017r0
Submission
Flexibility• Can be easily adjusted to any blocksize
–Storage of the 4 parameters for all blocksizes considered–Possibility of a generic approach (default parameters)
• All coding rates are possible–Through puncturing patterns–Natural coding rate is ½: increased robustness to puncturing
• Performance vs complexity: several adjustments are possible
–Number of iterations, Decoding algorithm, …
• Implementation: interleaver enables different degrees of parallelism
–Can be adjusted to meet complexity/throughput requirements
Most of these features apply to any highly-structured approach.
January 2006
Patrick Pirat, France Telecom
Slide 13
doc.: IEEE 802.22-06/0017r0
Submission
Flexibility
• The number of iterations can be adjusted for a better performance-complexity trade-off
January 2006
Patrick Pirat, France Telecom
Slide 14
doc.: IEEE 802.22-06/0017r0
Submission
Performance• Duo-Binary TC, 8
iterations, Max-Log-MAP decoding
• IEEE 802.16e structured LDPC, BP decoding, 50 iterations
• AWGN, R=1/2, QPSK
• N=576 and 2304 (coded blocksize)
Rather poor performance for LDPC, implementation?
8 iterations for duo-binary TC versus 50 iterations for LDPC!
January 2006
Patrick Pirat, France Telecom
Slide 15
doc.: IEEE 802.22-06/0017r0
Submission
Answers: performance (4)
• The simulation settings used in the previous slides correspond to the ones adopted in IEEE 802.16e standardization group during the selection process of the LDPC code– 50 iterations with BP algorithm
• The results presented correspond to simulation results of the LDPC defined in IEEE 802.16e specification, and not implementation results
January 2006
Patrick Pirat, France Telecom
Slide 16
doc.: IEEE 802.22-06/0017r0
Submission
Short blocksize performance
• Hardware measurements
• Low BER (down to 10-11) are achievable without error floor
Why different block size for BER and FER?
Error flare barely appears. Larger block sizes need to be
used to be more realistic.
Are block lengths of 16 and 18 bytes pertinent to WRAN operation? Even VoIP with 20 ms latency
would likely produce longer blocks.
[1] presented DVB-RCS results for a larger block size (484 bits) and a
lower code rate (1/3) than these cases and shows evidence of flares starting
between PER=1e-3 and 1e-4.
January 2006
Patrick Pirat, France Telecom
Slide 17
doc.: IEEE 802.22-06/0017r0
Submission
Answers: Short Blocksize Performance
• The blocksizes employed in the previous simulations correspond to blocksizes standardized in DVB-RCT
• This figure was included to dismiss some misconceptions that Turbo Codes don't perform well for short block sizes – Please refer to other plots in proposal to see performance for the
larger block sizes.
January 2006
Patrick Pirat, France Telecom
Slide 18
doc.: IEEE 802.22-06/0017r0
Submission
Summary:Gains brought by OQAM and DTC
• OFDM/OQAM brings 10% more bit-rate– When converted in error protection enables to go from ¾ rate to
2/3
– Gain between 1 and 1,5 dB in C/N
• Duo-binary TC offers 3,5 to 4 dB
• When combined the gain is at least 4,5 dB that allows to increase the radius by 7,6 km (17%) with QPSK modulation in a Gaussian channel.
Compared to what?