Upon missing a tail sequence the decoder remains in

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Unformatted text preview: l and cannot recognize the next CLTU's START sequence which follows immediately. Although the Start Sequence of the next CLTU is NOT part of a codeblock and so has a high probability of causing CODEBLOCK REJECTION, this attribute is of no help at this point, since the start sequence has already passed. As a result, the entire CLTU which follows is missed. The probability of missing a tail sequence when using the TED decoding mode is the probability that 2 or more changes are introduced into the tail sequence during transmission: PTA = 1 - [(1 - p)n + np(1 - p)n-1] [EQ. G] where n = length of tail sequence in bits. Issue 6 Page D-16 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE The probability of missing a tail sequence when using the SEC decoding mode is the probability that one or more changes are introduced into the tail sequence during transmission: PTB = 1 - (1 - p)n [EQ. H] It should be noted that Equations G and H are conservative approximations, since changes in certain locations may not result in a missed tail. Values are tabulated in Table D-7. Table D-7: Probability of Missing Tail Sequence, PTA (TED) and PTB (SEC) ___________________________________________________________________________ Probability of missing a tail, Tail Sequence Decoding for a Channel BER of -4 10-5 10-6 length, bits mode 10 ___________________________________________________________________________ 1.95x10-5 1.95x10-7 1.95x10-9 64 TED (PTA) SEC (PTB) 6.28x10-3 6.30x10-4 6.30x10-5 56 TED (PTA) SEC (PTB) 1.48x10-5 5.49x10-3 1.48x10-7 5.50x10-4 1.48x10-9 5.50x10-5 48 TED (PTA) SEC (PTB) 1.08x10-5 4.69x10-3 1.08x10-7 4.70x10-4 1.08x10-9 4.70x10-5 TED (PTA) 7.39x10-6 7.41x10-8 7.41x10-10 SEC (PTB) 3.89x10-3 3.90x10-4 3.90x10-5 ___________________________________________________________________________ 40 The probability of rejection of the last frame in a CLTU which follows another CLTU (without reacquisition) may be evaluated as the probability of missing the tail of the preceding CLTU (PTA or PTB) OR the probability of rejecting the last frame of the subject CLTU. That is, <Prob. of frame rejection in subsequent CLTU, TED decoding> = PF2A = PTA + (1-PTA)PFA = PTA + (1-PTA)[(PSA + (1-PSA)PCA] [EQ. I] where PFA refers to the frame performance as previously calculated in Equation E (independent of a previous CLTU); and Issue 6 Page D-17 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE <Prob. of frame rejection in subsequent CLTU, SEC decoding> = PF2B = PTB + (1-PTB)PFB = PTB + (1-PTB)[(PSB + (1-PSB)PCB] [EQ. J] where PFB refers to the frame performance as previously calculated in Equation F (independent of a previous CLTU). Given equal length CLTUs, one can see that each subsequent CLTU will have a higher probability of rejection than the previous CLTU. Example. Two maximum length frames could be packaged into two separate but contiguous CLTUs in an organization as shown in Figure D-5. The probability of rejection of the first (PF1) and second (PF2) frames in this organization may be compared...
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This document was uploaded on 03/06/2014.

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