Unformatted text preview: omponent code and bit-wise interleaved (see
TURBO CODE PERMUTATION) to the second component code. The output is formed by
the parity symbols contributed by each component code plus a replica of the information bits.
Turbo Code Permutation: A fixed bit-by-bit permutation of the entire input block of
information bits performed by a permuter or interleaver, used in turbo codes. CCSDS 130.1-G-1 Page A-3 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE ANNEX B
ACRONYMS AND ABBREVIATIONS
AOS — Advanced Orbiting System
APP — A posteriori probability
ASM — Attached Synchronization Marker
AWGN — Additive White Gaussian Noise
BCH — Bose-Chaudury-Hoquenheim
BER — Bit Error Rate
BPSK — Binary Phase Shift Keying
BSNR — bit SNR
CCSDS — Consultative Committee on Space Data Systems
CRC — Cyclic Redundancy Code
DSN — Deep Space Network
ESA — European Space Agency
FEC — Forward Error Correction
FER — Frame Error Rate
GF — Galois Field
GSFC — Goddard Space Flight Center
JPL — Jet Propulsion Laboratory
MAP — Maximum a posteriori probability
NASA — National Aeronautic and Space Administration
NRZ — Non-Return to Zero
PM — Phase Modulated
PSK — Phase Shift Keying
QAM — Quadrature Amplitude Modulation
RF — Radio Frequency
ROM — Read Only Memory
RS — Reed-Solomon
SNR — Signal to Noise Ratio
SSNR — Symbol SNR
TM — Telemetry
VC — Virtual Channel
WER — Word Error Rate CCSDS 130.1-G-1 Page B-1 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE ANNEX C
RATIONALE FOR TURBO CODE PARAMETER SELECTIONS
C1 GENERAL Because turbo codes can achieve great performance over a wide range of parameter values,
the selection of reasonable code parameters is a major systems issue. The system design must
assess all the parameter-space tradeoffs as they affect both the performance of the code and
systems-related considerations. Turbo codes give the system designer vast flexibility to
choose any desirable combination of parameters without sacrificing performance more than
C2 CODE RATE The code rate of the recommended turbo encoder is selectable from 1/2, 1/3, 1/4, or 1/6.
Lower code rates are also possible to achieve even better performance if the receivers can
work at the correspondingly lower channel-symbol SNR (Eb/N0). The rule of thumb is that
the potential coding gain for using lower code rates pretty much follows the corresponding
gain for the ultimate code-rate-dependent theoretical limits.
For deep-space applications, turbo codes are intended for use with BPSK modulation, with
code rate < 1 bit/channel symbol (spectral efficiency < 1 bit/sec/Hz). The same codes can be
used with QPSK modulation with Gray coding signal assignment to achieve higher spectral
efficiency, as typically required in near-Earth applications. 7
C3 BLOCK SIZE Figure 3-4 shows how some fundamental theoretical lower bounds on the performance of
arbitrary codes on the additive white Gaussian noise channel vary with codeblock length.
Amazingly, this variation is mirrored by the empirically determined dependence on block
length of the performance of a large family of good turbo codes (see also reference ).
Figure C-1 shows simulation results compared to the lower bound for a family of rate-1/3
turbo codes with different block lengths (using the generator polynomials specified in 7.2).
Note that the range of block lengths in this figure, from 256 bits up to 49152 bits, spans both
larger and smaller block lengths than the five specific CCSDS recommended block lengths.
Although there is a 2 dB performance differential between the simulation results for 256-bit
blocks and 49152-bit blocks, the difference between the simulations and the lower bounds
remains approximately the same. The simulation results are about 0.5 dB to 1.0 dB from the 7 Additional turbo codes with matched modulation signal set have been designed for even higher spectral
efficiencies. These codes would require 8PSK or higher level modulations and are not covered in this document. CCSDS 130.1-G-1 Page C-1 June 2006 TM SYNCHRONIZATION AND CHANNEL CODING —SUMMARY OF CONCEPT AND RATIONALE theoretical limits for all code rates ranging from 1/6 to 1/2 and at all codeblock sizes ranging
from 256 to 49152 information bits. Similar results were obtained for turbo codes in the same
family with rates 1/2, 1/4, and 1/6. The significance of these results is that turbo codes appear
to be uniformly good over the entire span of block sizes shown, including all of the CCSDS
recommended block lengths. Threshold Eb/No (dB) 5
Turbo r=1/3 4 Bound r=1/3
10 100 1000
Information Block Size (bits) 100000 NOTE – Bound is calculated for word error rate of 10–4, while turbo code simulations
were for bit error rate of 10–6.
Figure C-1: Comparison of Turbo Code Performance with Blocklength-Constrained
C4 CONSTITUENT CODES Effective turbo codes can be constructed from a wide variety of constituents. Here are some
of the factors underl...
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This document was uploaded on 03/06/2014.
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