Proposal for a FEC-Coded AO-40 Telemetry Link

1. 2002 AMSAT Annual Meeting Phil Karn, KA9Q karn@ka9q.net http://www.ka9q.net Proposal for aFEC-Coded AO-40 Telemetry Link

2. The AO-40 Telemetry Format • Same as Phase 3-A (1980) • 400bps BPSK, suppressed carrier • Manchester coding • no FEC • 4 byte sync + 512 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? • Substantiallyimproved link margins • 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): • 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: 13.00 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 + 32 8-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 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 • 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