1. Proposal for a FEC-Coded AO-40 Telemetry Link
2002 AMSAT Annual Meeting
Phil Karn, KA9Q

2. The AO-40 Telemetry Format
Same as Phase 3-A (1980)
400bps BPSK, suppressed carrier
Manchester coding
no FEC
4 byte sync byte data + 2 byte CRC + idle
Requires strong, steady signals
Highly susceptible to fading
One bad bit destroys whole frame

3. Uncoded BPSK Performance

4. Why FEC? Substantially improved link margins Allows one or more of:
especially dramatic against fading
Allows one or more of:
Reduced spacecraft power ($$)
Reduced ground G/T (smaller receive antennas)
Improved link margin during off-axis operation
Higher data rates
but not on AO-40 (limited by hardware)
Now well within capability of average PC

5. Terminology Forward Error Correction (FEC): Bit: data bit from user
Adding redundant info to enable receiver correction of transmission errors without retransmission
Bit: data bit from user
Symbol: data bit from encoder output
modems handle symbols, not bits
Code Rate: bit rate / symbol rate
e.g., rate ½ = 2 channel symbols per user data bit

6. Eb/N0 : energy per bit / noise spectral density
Ebin joules; N0 in watts/Hz = joules
dimensionless, usually expressed in dB
aka "digital S/N ratio" or "per bit SNR"
Es/N0 : Energy per symbol / noise spectral density
Without FEC, Eb/N0 = Es/N0
With FEC, Es/N0 = Eb/N0 + 10log10(code rate)
Es/N0 <= Eb/N0

7. AWGN: Additive White Gaussian Noise
Classic model for satellite or deep-space channel
NO fading!

8. AO-40 Hardware Constraints
400 bps BPSK, suppressed carrier
Manchester encoding
no benefit or penalty
Differential encoding
turns out to be useful
IHU limitations on memory, CPU
not a problem with chosen scheme

9. FEC Design Requirements
Obey AO-40 hardware constraints
Assume Pentium-class PC with soundcard for demodulation and decoding
no need to preserve hardware BPSK demods
Keep frame transmission time reasonably short
reduce payload instead to accommodate overhead
Design primarily for fade resistance
Good AWGN performance desirable, but secondary

10. My Design Choices 256 data bytes/frame
vs present 512 bytes/frame
Frame transmission time: sec
Concatenated RS-convolutional code
Overall code rate: 0.4; reasonably optimal
user data rate = 0.4 * 400 = 160 bps
Scrambling for reliable symbol timing recovery
Extra layer of interleaving
also interleaves sync vector

11. Concatenated Coding Two layered FEC codes
Reed-Solomon code + convolutional code
byte interleaver between codes
First flown on Voyager (1977); standard practice ever since
Now being slowly replaced with Turbo coding
but turbo codes are still patented

12. Proposed Codes (160,128) Reed-Solomon code (rate 0.8)
Shortened from CCSDS standard (255,223) code
128 8-bit data symbols bit parity per block
Corrects up to 32/2 = 16 symbol errors/block
Rate ½ constraint length 7 convolutional code
CCSDS standard, very widely used
Viterbi decoded
Steep threshold at Eb/N0 ~= 2.5 dB
vs ~10 dB for uncoded BPSK on AWGN

13. FEC Performance

14. Encoder Block Diagram my CCSDS standard addition 256 data bytes 65-bit
sync
vector
pad
3 bits
tail
6 bits
256 data bytes
2:1 byte
Interleaver
Scrambler
Convolutional
encoder
r=1/2 k=7
(160,128)
Reed-Solomon
Encoder
65x80 bit
block interleaver
5200 channel symbols
(2560+6)*2 = 5132 bits
8x2x160 = 2560 bits my
addition
CCSDS standard

15. Coherent BPSK Demodulation
Costas or Squaring loop required on suppressed carrier signal
traditionally used on Phase 3
Optimum performance on AWGN
Bad choice on fading channel
may spread outside loop bandwidth
sudden carrier phase jumps lose lock

16. Noncoherent BPSK Demodulation
Use each symbol as phase reference for next
Requires differential encoding at transmitter
Phase 3 already does this in hardware
Easy to implement in both SW and HW
"Instant" lockup
Excellent fade performance
Theshold effect, much like FM
small (~0.5 dB) penalty at Es/N0 = 10 dB
So why are most Phase 3 demods coherent??

17. Prototype Encoder: ~1kB code + ~2kB RAM Decoder libraries:
fits easily into IHU
Decoder libraries:
Viterbi decoder in C/MMX/SSE/SSE2
~14 Mb/s on 1.8 GHz P4
Reed-Solomon codec in C
General purpose DSP (filtering, etc)
Prototype demod/decoder in C
< 1% of 1GHz PIII when locked

18. AWGN Performance Uncoded BPSK demod, ideal
Eb/N0 = Es/N0 = 10 dB
FEC, differential PSK demod, measured
Eb/N0 = 6dB; Es/N0 = 2 dB
3 dB worse than coherent PSK
Link margin still 8dB better

19. Fading Performance
Tested configuration: 3.3 Hz sinusoidal envelope, 2 nulls/cycle
Eb/N0 = 8 dB (2 dB worse than AWGN)
Actual performance depends on fade envelope
slow fading worse than fast fading
short fades more tolerable than long fades
fade depth irrelevant

20. Status Linux prototype developed and working
all software open source GPL
Decoder should be easily ported
to AO40RCV, etc
Encoder in IPS needed
IPS-like code in C written
Restructure IPS pseudo-DMA subsystem
eliminate inter-frame padding
desirable, not absolutely necessary

21. Planned Improvements Equalizer for AO-40 transmit filter
~1 dB ISI loss with current matched filter
Implement coherent demodulator
Use noncoherent first, switch to coherent if necessary
Improve performance on weak nonfading signals

22. Thoughts on Future Links
Not constrained by existing AO-40 hardware
FEC is now a no-brainer
should be mandatory on all future AMSAT links!
Adapt design to specific requirements
uplinks and downlinks may use different modulation & coding
encoding easier than decoding

23. Future Modulation Choices
BPSK still ideal for low speed links
QPSK for high rate links (rate >> freq uncertainty)
Noncoherent demod for fading links
but threshold effect limits coding gain
Add residual carrier on low speed links
find with FFT, track with simple PLL
Manchester keeps data away from carrier
avoid squaring losses of Costas and squaring loops
essential for low Es/N0 ratios of strong, low rate FEC codes