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

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Presentation on theme: "Proposal for a FEC-Coded AO-40 Telemetry Link 2002 AMSAT Annual Meeting Phil Karn, KA9Q"— Presentation transcript:

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 – 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 E b /N 0 : energy per bit / noise spectral density – E b in joules; N 0 in watts/Hz = joules – dimensionless, usually expressed in dB – aka "digital S/N ratio" or "per bit SNR" E s /N 0 : Energy per symbol / noise spectral density – Without FEC, E b /N 0 = E s /N 0 – With FEC, E s /N 0 = E b /N log 10 (code rate) – E s /N 0 <= E b /N 0

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 – 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 E b /N 0 ~= 2.5 dB – vs ~10 dB for uncoded BPSK on AWGN

13 FEC Performance

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

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 E s /N 0 = 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 – E b /N 0 = E s /N 0 = 10 dB FEC, differential PSK demod, measured – E b /N 0 = 6dB; E s /N 0 = 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 – E b /N 0 = 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 E s /N 0 ratios of strong, low rate FEC codes

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