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Unformatted text preview: early, this length may be modified as needed to suit different hardware characteristics or channel bit error rates (such as, for example, a noise-free channel used during ground testing). When bit synchronization has been achieved, the decoder leaves the INACTIVE state and enters the SEARCH state. BECAUSE THE PROBABILITY OF ACHIEVING BIT SYNCHRONIZATION IS PRIMARILY HARDWARE-DEPENDENT, THIS ANALYSIS WILL ASSUME THAT BIT SYNCHRONIZATION HAS ALREADY BEEN ACHIEVED. D-3.2 CLTU Start Sequence Factor Once the on-board decoder is in the SEARCH state, it begins looking for the required CLTU Start Sequence. The Start Sequence provides two functions: 1) to resolve the ambiguity between a "one" and a "zero" if needed (e.g., when NRZ-L symbol representation is used), and 2) to delimit the beginning of the CLTU. The Start Sequence is a fixed-pattern marker 16 bits long. It follows immediately after the acquisition sequence and before the first codeblock of the CLTU. As a consequence of recognizing the CLTU Start, the start of the first codeblock is then also delimited, since it immediately follows the start sequence. In decoding serially transmitted block codes, it is usually necessary to know the point at which the block starts and its length. The codeblock length which has been chosen for the mission must be known a priori, and must remain constant. Two operating strategies for recognizing the start sequence are presented in Reference [4]: In the first strategy (denoted by subscript A), the decoder requires that the entire 16 bits be received without error. In the second (denoted by subscript B), the decoder allows one of the 16 bits to be in error and will still declare the start of a CLTU. For the first strategy (that is, all 16 bits must be correct to declare a CLTU start) the probability of rejecting (missing) a CLTU start is found by simply taking the probability that one or more bit errors may fall on any of the 16 bits of the start sequence: 1 For example, the probability of the TDRSS transponder achieving bit lock in response to this acquisition sequence is equal or greater than 0.9999. Issue 6 Page D-6 January 1987 CCSDS REPORT CONCERNING TELECOMMAND: SUMMARY OF CONCEPT AND SERVICE <Prob. of rejecting a START, no errors allowed> = PsA = 1 - (1 - p)16 [EQ. A] where p is the channel bit error rate. For the second strategy (in which one bit error is allowed) it is necessary for TWO or more bit errors to appear in the start sequence before it will be rejected. In this case, <Prob. of rejecting a START, 1 error allowed> = PsB = 1 - [(1 - p)16 + 16p(1-p)15] [EQ. B] These values are tabulated in Table D-1. Table D-1: Probability of Rejection of CLTU Start Sequence (for case where no errors are allowed (PsA ) and one error allowed (PsB )) ________________________________________________________________ Channel Bit Error Rate Case 10-4 10-5 10-6 ________________________________________________________________ PsA 1.60 x 10-3 1.60 x 10-4 160 x...
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This document was uploaded on 03/06/2014.

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