2 for the first k bit times the input switches shall

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Unformatted text preview: be in the lower position (as indicated in figure 6-2) to receive input data. 3) For the final four bit times, these switches shall move to the upper position to receive feedback from the shift registers. CCSDS 131.0-B-2 Page 6-6 August 2011 CCSDS RECOMMENDED STANDARD FOR TM SYNCHRONIZATION AND CHANNEL CODING NOTE – This feedback cancels the same feedback sent (unswitched) to the leftmost adder and causes all four registers to become filled with zeros after the final 4 bit times. Filling the registers with zeros is called terminating the trellis. 4) During trellis termination the encoder shall continue to output nonzero encoded symbols. In particular, the ‘systematic uncoded’ output (line ‘out 0a’ in the figure) shall include an extra four bits from the feedback line in addition to the k information bits. NOTE – In figure 6-2, the encoded symbols are multiplexed from top-to-bottom along the output line for the selected code rate to form the Turbo Codeword. For the rate 1/3 code, the output sequence is (out 0a, out 1a, out 1b); for rate 1/4, the sequence is (out 0a, out 2a, out 3a, out 1b); for rate 1/6, the sequence is (out 0a, out 1a, out 2a, out 3a, out 1b, out 3b). These sequences are repeated for (k+4) bit times. For the rate 1/2 code, the output sequence is (out 0a, out 1a, out 0a, out 1b), repeated (k+4)/2 times. This pattern implies that out 1b is the first to be punctured, out 1a is the second, and so forth. The Turbo Codewords constructed from these output sequences are depicted in figure 6-3 for the four nominal code rates. CCSDS 131.0-B-2 Page 6-7 August 2011 CCSDS RECOMMENDED STANDARD FOR TM SYNCHRONIZATION AND CHANNEL CODING Rate 1/2 Turbo Codeword out 0a out 1a out 0a ...... out 1b out 0a out 1a out 0a out 1b 1st transmitted symbol last transmitted symbol Rate 1/3 Turbo Codeword out 0a out 1a out 1b out 0a out 1a out 1b ...... out 0a out 1a out 1b out 0a out 1a 1st transmitted symbol out 1b last transmitted symbol Rate 1/4 Turbo Codeword out 0a out 2a out 3a out 1b out 0a out 2a out 3a out 1b ...... out 0a out 2a out 3a out 1b out 0a out 2a 1st transmitted symbol out 3a out...
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This document was uploaded on 03/06/2014.

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