As an example table d 6 shows the probability of

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Unformatted text preview: n of the LAST frame for a single CLTU containing 1, 2, 3, and 4 maximum length frames. Figures are derived only for the 64bit codeblock (n=63). Table D-6: Probability of Last Frame Rejection in a Multiple Frame CLTU Using Maximum Length Frames and 64-Bit Codeblocks ___________________________________________________________________________ No. of Total Total DeProbability of rejection of last maximum- number No. of coding frame, with channel BER of length of codemode 10-5 10-6 frames bits blocks 10-4 ________________________(N)________________________________________________ 1 2048 37 TED SEC 2.09x10-1 7.21x10-4 2.32x10-2 7.24x10-6 2.34x10-3 7.24x10-8 2 4096 74 TED SEC 3.74x10-1 1.44x10-3 4.57x10-2 1.45x10-5 4.67x10-3 1.45x10-7 3 6144 110 TED SEC 5.04x10-1 2.16x10-3 6.77x10-2 2.17x10-5 6.98x10-3 2.17x10-7 TED 6.07x10-1 8.92x10-2 9.30x10-3 SEC 2.88x10-3 2.89x10-5 2.89x10-7 ___________________________________________________________________________ 4 D-4.3 8192 147 Multiple CLTUs While in theory any number of frames may be sent in one CLTU, very long CLTUs tend to be counter-productive: that is, the probability of frame rejection increases as the CLTU is made excessivey long. In such cases it may be preferable to divide a large command load into a number of CLTUs sent sequentially. These CLTUs may either be: 1) sent as independent entities (modulation is dropped between CLTUs and the entire resynchronization process begins anew as in the single CLTU case), or 2) strung together in sequence while maintaining bit sync on the channel. Issue 6 Page D-15 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE If CLTUs are separated by dropping modulation, each CLTU will perform as an independent entity and may be analyzed as shown in Sections D-4.1 or D-4.2. However, if the CLTUs are strung together tail-to-start with an uninterrupted bit stream as shown in Figure D-5, some further considerations are necessary. CLTU 1 ACQ CLTU 2 START TAIL START TAIL INFO P INFO P INFO P INFO P INFO P FRAME 1 FRAME 2 Figure D-5: Multiple Frame CLTU Organization (Contiguous CLTUs) With this organization, the CLTU Tail Sequence becomes important, since the Tail Sequence must be relied upon to declare a CODEBLOCK REJECTION and force the decoder into SEARCH STATE at the end of each CLTU. ONLY IF THE DECODER IS IN SEARCH STATE WILL IT BE ABLE TO RECOGNIZE THE START SEQUENCE OF THE NEXT CLTU. The decoder enters SEARCH state when a CODEBLOCK REJECTION is encountered (in this case caused by the tail sequence.) However, it is possible for the decoder to "miss" a tail sequence (i.e., the intended function, CODEBLOCK REJECTION, does not occur). Such a condition could happen if bit errors on the channel suitably change the tail sequence to make it an apparently valid codeblock, or (in SEC mode) make it appear (wrongly) to be correctable. This requires the received tail sequence to have at least 2 introduced bit changes in the TED mode, or 1 introduced bit change in the SEC mode. Upon missing a tail sequence, the decoder remains in DECODE state after the first CLTU's tai...
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This document was uploaded on 03/06/2014.

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