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Unformatted text preview: 1b last transmitted
symbol Rate 1/6 Turbo Codeword
3b ...... out
1a 1st transmitted
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.
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
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.
Marker Turbo Codeword
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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- Spring '14