Ccsds 1301 g 1 page a 3 june 2006 tm synchronization

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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 intrinsically necessary. 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 [16]). 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 3 2 1 0 -1 10 100 1000 10000 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 Lower Bound 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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