First Flight: Successful Use of a High Rate LDPC Code With High Data Rate in a Restricted Band

First Flight: Successful Use of a High Rate LDPC Code

With High Data Rate in a Restricted Band

H. Garon, V. Sank, W. Fong H. Garon, V. Sank, W. Fong

NASA/GSFC NASA/GSFC

Spring Technical Meeting, Portsmouth, VA 05/03-07/2010Spring Technical Meeting, Bordeaux, Fr 04/15-18/2013

“Successful Use of a High Rate LDPC…”SLS-C&S 16 April 2013 01 of 11

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 usingCFDP class where entire file is re-transmitted if there is an error.

EESS X band

Abuts DSN X band


LDCM Characteristic: Restriction: Imposed Requirement:

Statement of the Problem

Limited contact time

Sensors generate data at high data rate

Limited onboard storage (4 Tbits)

Bandwidth limited to 375 MHz

Power Spectral Flux Density restriction at Deep Space Network antennas

Limited time/capacity to repeat transmissions and large file size drives BER < 10-12

Band pass filtering aboard S/C to achieve PSFD at DSN

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.


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.


End-to-End Simulation Using Mathworks Matlab/Simulink


End-to-End Emulation Using Test Bench


Key to hardware emulationCustom bandpass filter designed using simulation to replicate distortion

introduced aboard spacecraft


Emulation Spectrum Compared To Flight

Original emulation of LDCM spectrum

Actual on orbit LDCM spectrum


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 OLI & TIRS activation• 3/21/13 Released its first X band images of Earth• 3/25/13 X-band downlink with International Collaborators • 3/27/13 LDCM hit a 400 scenes/day milestone

All with the rate 7/8 LDPC code.


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.


http://www.nasa.gov/images/content/736042main_onion-skin-still-detail.jpg

The Real Conclusion, an Error Free Image


http://www.nasa.gov/images/content/736042main_onion-skin-still-detail.jpg

First Flight: Successful Use of a High Rate LDPC Code With High Data Rate in a Restricted Band

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