1 / 23

Proposal for a FEC-Coded AO-40 Telemetry Link

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

Télécharger la présentation

Proposal for a FEC-Coded AO-40 Telemetry Link

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related