Successful Use of a High Rate LDPC Code with - CWE
Download
Report
Transcript Successful Use of a High Rate LDPC Code with - CWE
Spring Technical Meeting, Portsmouth, VA 05/03-07/2010
Spring Technical Meeting, Bordeaux, Fr
04/15-18/2013
First Flight:
Successful Use of a High Rate LDPC Code
With High Data Rate in a Restricted Band
H. Garon, V. Sank, W. Fong
NASA/GSFC
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
01 of 11
Statement of the Problem
LDCM Characteristic:
Sun-synchronous polar orbit (16 day
repeat cycle at nominally 705 km)
with a finite number of ground stations
Imaging mission with high data
rate of 384 Mbps in 375 MHz X band.
Requires low coding overhead.
CFDP with 1 G Byte file size but using
CFDP class where entire file is retransmitted if there is an error.
EESS X band
Abuts DSN X band
Restriction:
Imposed
Requirement:
Limited contact
time
Sensors generate
data at high data
rate
Limited time/capacity to
repeat transmissions and
large file size drives
BER < 10-12
Limited onboard
storage (4 Tbits)
Bandwidth limited
to 375 MHz
Power Spectral Flux
Density restriction at
Deep Space Network
antennas
Band pass filtering
aboard S/C to achieve
PSFD at DSN
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
02 of 11
Selection of Codec
• Unlike DSN, expect excess signal margin operationally
• S/C science data rate, and limited storage capacity forces
high RF data rate.
• High Data Rate in restricted bandwidth channel, forces
high rate code (384 Mbps => 440 Msps)
• Large file and low retransmission, requires BER < 10-12
• High data rate restricts Complexity of encoder
– while decoder important, it is on the ground
• Absence of BER floor
LDPC 7/8 chosen even though there was no flight heritage.
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
03 of 11
Will it work?
(Establishing link viability prior to flight)
• Questions …
– Flight experience for LDPC 7/8? Receiver/Decoder availability?
– Assuming LDPC 7/8 is used, what is the impact on truncating the
transmission spectrum to less than 375/440 in order to meet spectral
emission requirements?
– Similarly, impact of radiated spectrum asymmetrical about carrier?
– The spectrum truncation is accompanied by significant phase
distortion. Can we properly compensate for that distortion on the
ground?
– Can we achieve a BER < 10-12 ?
Absolute requirement to build both end-to-end
simulation and hardware emulation in order to
gain confidence in flight application.
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
04 of 11
End-to-End Simulation Using Mathworks
Matlab/Simulink
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
05 of 11
End-to-End Emulation Using Test Bench
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
06 of 11
Key to hardware emulation
Custom bandpass filter designed using simulation to replicate distortion
introduced aboard spacecraft
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
07 of 11
Emulation Spectrum Compared To Flight
Actual on orbit
LDCM spectrum
Original emulation
of LDCM spectrum
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
08 of 11
Mission Events
• 2/11/13 Launch
• 2/14/13 X-band SSOH files have been successfully received at both Gilmore Creek
and Sioux Falls ground stations.
•
The Svalbard ground station corrected a configuration setting so they should be able to receive X-band files on future contacts. There was a
problem with getting S-Band SSOH back from Svalbard . The X-Band mission data from Svalbard uses a different link and is unaffected.
•
•
•
•
3/17/13
3/21/13
3/25/13
3/27/13
OLI & TIRS activation
Released its first X band images of Earth
X-band downlink with International Collaborators
LDCM hit a 400 scenes/day milestone
All with the rate 7/8 LDPC code.
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
09 of 11
Conclusion:
• Demonstrated in flight
A viable 440 Msps link with BER < 10-12 and implementation loss less
than 2 dB (including ground station loss) despite an asymmetrical
spectrum truncation to 375 MHz,
• Commercial off-the-shelf availability of
Flight encoder chips qualified to Level 1 (GEO environment at 1 Gbps)
along with
Commercial hardware using sliced data frame, now proven.
Multiple vendors offering FPGA cores and high data rate receivers with
LDPC decoder (Zodiac, RT Logic, Avtec, …) (both sliced and not sliced)
Successful cooperation within CCSDS to standardize the codec.
Now we can say that not only does it look good, but the rate 7/8 LDPC code
works well in practice.
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
10 of 11
The Real Conclusion, an Error Free Image
http://www.nasa.gov/images/content/736042main_onion-skin-still-detail.jpg
“Successful Use of a High Rate LDPC…”
SLS-C&S 16 April 2013
11 of 11