This preview shows page 1. Sign up to view the full content.
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 noisefree 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 HARDWAREDEPENDENT, THIS ANALYSIS WILL ASSUME THAT BIT
SYNCHRONIZATION HAS ALREADY BEEN ACHIEVED. D3.2 CLTU Start Sequence Factor Once the onboard 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 NRZL symbol representation is used),
and 2) to delimit the beginning of the CLTU.
The Start Sequence is a fixedpattern 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 D6 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(1p)15] [EQ. B] These values are tabulated in Table D1.
Table D1: 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
104
105
106
________________________________________________________________
PsA 1.60 x 103 1.60 x 104 160 x...
View
Full
Document
This document was uploaded on 03/06/2014.
 Spring '14

Click to edit the document details