out 0a out 2a out 3a out 1b out 0a out 2a 1st

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Unformatted text preview: 1b last transmitted symbol Rate 1/6 Turbo Codeword out 0a out 1a out 2a out 3a out 1b out 3b out 0a out 1a out 2a out 3a out 1b out 3b ...... out 0a out 1a out 2a out 3a out 1b out 3b out 0a out 1a 1st transmitted symbol out 2a out 3a out 1b out 3b last transmitted symbol Figure 6-3: Turbo Codewords for Different Code Rates k) Turbo Codeword synchronization: An Attached Sync Marker (ASM) shall be associated with each Turbo Codeword (see section 8), and it shall precede the Turbo Codeword. NOTES 1 The ASM is used to achieve Codeword synchronization of the Turbo decoder (i.e., frame synchronizers are normally set to expect a marker at a recurrence interval equal to the length of the ASM plus that of the Turbo Codeword). 2 Differential encoding does not provide benefits with Turbo Codes, and the ASM can also be used to resolve phase ambiguities. In fact, differential encoding before the Turbo encoder cannot be used because the Turbo codes recommended in this document are non-transparent, and differential encoding after the Turbo CCSDS 131.0-B-2 Page 6-8 August 2011 CCSDS RECOMMENDED STANDARD FOR TM SYNCHRONIZATION AND CHANNEL CODING encoder is not advised because it introduces considerable loss of performance. It also would require differential detection, which is more complex with soft symbols. This implies that phase ambiguities have to be detected and resolved before decoding. 3 A diagram of a Turbo Codeword with ASM is shown in figure 6-4. The length of the Turbo Codeword is inversely proportional to the nominal code rate r. Rate-Dependent Attached Sync Marker Turbo Codeword 32/r bits k /r bits 4/r bits r = 1/2, 1/3, 1/4, or 1/6 (nominal code rate) k = Telemetry Transfer Frame Length or Information Block Length Figure 6-4: Turbo Codeword with Attached Sync Marker CCSDS 131.0-B-2 Page 6-9 August 2011 CCSDS RECOMMENDED STANDARD FOR TM SYNCHRONIZATION AND CHANNEL CODING 7 LOW-DENSITY PARITY-CHECK CODING 7.1 OVERVIEW Low-Density Parity-Check (LDPC) codes are binary block codes with large codewords (hundreds or thousands of bits). They may be u...
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