Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Short Uplink Codes:
Acquisition and Termination Alternatives
Kenneth Andrews
Jet Propulsion Laboratory
California Institute of Technology
© 2015 California Institute of Technology
Government sponsorship acknowledged.
CCSDS Spring Meetings
Pasadena March 23-27, 2015
1
Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Problem
•  New LDPC codes will out-perform the BCH (63,56) code used in the existing
telecommand (TC) standard by about 8 dB.
•  To take advantage of this increased performance, the current acquisition and
termination techniques must be reconsidered.
•  Both must work with a symbol SNR of about
Es/N0=0 dB.
A maximum of one TC Transfer
•  The “TC Coding and Synchronization”
Frame is deleted (rejected) for
Green Book gives a performance requirement:
every 103 frames transmitted.
CCSDS 230.1-G-2, Section 9.2
Acquisition sequence
•  The existing TC standard relies on finding a 16-bit synchronization marker.
•  This is not sufficiently reliable when the SNR is about Es/N0=0 dB. [1,2]
Termination sequence
•  The existing TC standard relies on finding an undecodable BCH codeword.
•  This doesn’t make sense in the context of LDPC codes.
[1] Pfletschinger, Navarro, Closas, and Bertinelli, “Start Sequence Detection”, CCSDS presentation, Fall 2014.
[2] Andrews, “Short LDPC Codes in the Emergency Uplink Setting”, CCSDS presentation, Fall 2014.
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Acquisition sequence alternatives
1.  Use a 64-symbol acquisition sequence
•  16- and 32-symbol sequences are insufficient; 64-symbols are sufficient. [1,2]
•  Disadvantage: 64 symbols is a lot of overhead on a short (e.g. 128 symbol)
codeword. FACT: For critical commands, NASA spacecraft typically use
commands that are one codeword long (56 information bits). These
commands use the “TC Synchronization and Channel Coding” standard, but
not the “TC Data Link Protocol” standard.
•  Potential disadvantage: Sequence detection is threshold test, where the
threshold is SNR-dependent. This requires knowledge of the SNR.
[1] Pfletschinger, Navarro, Closas, and Bertinelli, “Start Sequence Detection”, CCSDS presentation, Fall 2014.
[2] Andrews, “Short LDPC Codes in the Emergency Uplink Setting”, CCSDS presentation, Fall 2014.
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Acquisition sequence alternatives (continued)
2.  Use a 16-bit acquisition sequence, and verify it with tentative LDPC decoding
of the following data.
•  Advantage: The overhead is small, which is valuable with short-blocklength codes
when sending critical commands.
•  Disadvantage: Decoding is required. However, this effort is wasted only if the 16-bit
pattern is close but incorrect.
•  Disadvantage: Acquisition cannot be determined until decoding is complete. However,
decoding these codes is simple, and data rates are typically low.
Reproduced from Pfletschinger,
Navarro, Closas, and Bertinelli,
“Start Sequence Detection”,
CCSDS presentation, Fall 2014.
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Jet Propulsion Laboratory
Acquisition and Termination Sequences
California Institute of Technology
Acquisition sequence alternatives (continued)
3.  Compromise. Use a 32-bit acquisition sequence, and verify it with tentative
LDPC decoding of the following data.
•  Careful “correlating” can produce FER ≈ 10-3, and about 3 decoding iterations would
confirm acquisition.
LDPC (512,256), (256,128), and (128,64) codes
20
18
16
Average iterations
14
12
10
8
6
4
2
0
1
2
3
4
Eb/N0 (dB)
5
6
7
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Acquisition sequence alternatives (summarized)
1.  Use a 64-symbol acquisition sequence.
2.  Use a 16-bit acquisition sequence, and verify it with tentative
LDPC decoding of the following data.
3.  Compromise. Use a 32-bit acquisition sequence, and verify it with
tentative LDPC decoding of the following data.
Question: Is alternative 3 a compromise worth making, or does it
capture the worst of both alternatives?
Better ideas? Opinions?
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Tail sequence alternatives
1.  Use the existing 64-symbol undecodable BCH codeword
• 
• 
• 
While its BCH property has no significance, it is a 64-symbol marker with heritage.
Pattern: hexadecimal C5 C5 C5 C5 C5 C5 C5 79
Potential disadvantage: When derandomized, it might be possible to confuse this
with the start of a CLTU (SCID=2FC, VCID=26, length=39F)
2.  Use a 128-symbol “undecodable” sequence that does not resemble a
codeword of the (128,64) code.
• 
• 
• 
Similarly, would one use a 512-symbol non-codeword as a tail sequence for the
(512,256) code?
Disadvantage: These are longer tail sequences than necessary for reliable
detection.
Disadvantage: Squeezing a non-codeword into a code is poor practice. Effectively,
it represents an additional signaling point in GF(2n), but without the usual distance
properties and reliability. Some decoder might decode it.
GF(2k) GF(2n) encoding The codewords have consistent distance proper7es An undecodable tail sequence is weak, and it weakens the nearby codewords. 7
Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Tail sequence alternatives (continued)
3.  Use no tail sequence. Rely on the upper protocol
layers to determine when a message is complete.
• 
• 
• 
Disadvantage: If there is more than one Transfer Frame
in a CLTU, then the Frame Sequence Number is not
sufficient.
Potential disadvantage: This may require cross-layer
interaction at the receiver.
No advantage: The transmitter is on the ground, so
there is no significant cost to transmitting a tail
sequence. The flight receiver can use it or not, as it
chooses.
[1] Figures from “TC Space Data Link Protocol” Blue Book, CCSDS 232.0-B-2, Sept. 2010
AI_14_11 Provide proposal for CLTU termina7on based on TF length ﬁeld value 8
Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Tail sequence alternatives (continued)
4.  Encode a distinctive message, such as the all-ones vector.
• 
• 
• 
• 
• 
• 
This would not look like any message allowed by the “TC Space Data Link
Protocol” standard.
Pattern: hexadecimal 00 C6 61 A5 97 ... (systematic encoder, randomized)
Advantage: It is as reliable as the data transfer itself is.
Potential advantage: It could be found before decoding with a correlator, or
afterwards by matching the message. However, the frame-length field is also
available after decoding.
Potential disadvantage: The invalid S/C ID, etc. could confuse ground or flight
systems.
Disadvantage: As with option 2, this is a long tail sequence.
[1] Figures from “TC Space Data Link Protocol” Blue Book, CCSDS 232.0-B-2, Sept. 2010
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Tail sequence alternatives (continued)
5.  Use the length-64 sequence generated by randomizing the all-ones
vector.
• 
• 
• 
Pattern: hexadecimal 00 C6 61 A5 97 16 F9 0A
No message allowed by the “TC Space Data Link Protocol” standard would
begin with a similar pattern.
As a 64-bit tail sequence, rather than a full-length codeword, it cannot be
confused for a codeword.
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Acquisition and Termination Sequences
Jet Propulsion Laboratory
California Institute of Technology
Tail sequence alternatives (summarized)
1.  Use the existing 64-symbol undecodable BCH codeword.
2.  Use a 128-symbol “undecodable” sequence that does not resemble
a codeword of the (128,64) code.
3.  Use no tail sequence. Rely on the upper protocol layers to
determine when a message is complete.
4.  Encode a distinctive message, such as all-ones.
5.  Use the length-64 sequence generated by randomizing the all-ones
vector
Suggestion: Choose alternative 5, since it appears to avoid the
(minor) drawbacks of the others.
Other ideas? Opinions?
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