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From Telegraph Double Dotting To Microwave QPRS (Quadrature Partial Response Signalling)

From Telegraph Double Dotting To Microwave QPRS (Quadrature Partial Response Signalling). George de Witte VE3AYB QCWA Sep 20, 2011. Background 19 th Century Telegraphy. 1837 Wheatstone 5 needle, 6 wire system Euston-Camden Town 1852 6000 miles of 6 wire Telegraph in UK

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From Telegraph Double Dotting To Microwave QPRS (Quadrature Partial Response Signalling)

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  1. From Telegraph Double Dotting To Microwave QPRS (Quadrature Partial Response Signalling) George de Witte VE3AYB QCWA Sep 20, 2011

  2. Background 19th Century Telegraphy • 1837 Wheatstone 5 needle, 6 wire system Euston-Camden Town • 1852 6000 miles of 6 wire Telegraph in UK • 1850 Dover-Calais Single Wire Cable based on Gutta-Perra insulator • Soon after many other short distance submarine cables in Europe • 1858 First Transatlantic Cable from Ireland to NewFoundland • It worked for 3 days, it took hours to send a few words • Unknowns : cable strength, armour, dispersion, HV breakdown • Cable laying and Cable recovery • 1866 Second Transatlantic Cable • Recent submarine survey found “plateau” in North Atlantic • Stronger Cable 1 inch, but weighed 9000 tonnes/ 2300 miles • Was more successful and inspired many more cables

  3. First Route Valentia Harbour,Ireland to Trinity Bay NL

  4. The Economic Reality • The investments were high >1M$ in 1865 $$’s • A cable laying ship had 2000 men on board (and livestock to feed them) • A skilled Telegraph Operator was paid as much as a Bank Manager • Operating Speed was 8 WPM • Telegram Tariff for 20 words incl address was $150 • Despite all that 21 transatlantic cables by 1928 The incentive to improve speed was phenomenal

  5. Key Theoretical Contribution by Lord Kelvin • The common belief was that higher battery voltage was required • The first 1858 cable was likely destroyed by High Voltage • In 1855 he contended that the cable speed was proportional to the square root of the length • He defined what we today call pulse dispersion R R R R C C C

  6. Cable Laying Ship The Great Eastern Cable ship measured ship-shore continuity during laying

  7. Shore end Cross-section Shore end of Cable Stripped Deep Sea Cross-section

  8. Nyquist Theory 1928 Channel Response Transmit Filter Receive Filter Output Data Input Data Noise Sampler & Threshold Detector C(f) Gt(f) Gr(f) + Xn Xn X(f) BPF BW=Fnyquist LPF BW=0.5Fnyquist X(f)=Gt(f)*C(f)*Gr(f) X(f)=T ) X(f)=0 ) -1/2T<f<1/2T sin πt/T x(t)= ------------- πt/T F=1/T is called Nyquist frequency 0

  9. More Nyquist Theory ( T 0<f<(1-α)/2T X(f)= ( (T/2[1-sin(πT(f-1/2T)/α)] (1-α)/2T <f<(1+α)/2T sin πt/T cos απt/T x(t)= ------------- ---------------- πt/T 1-4α2t2/T2 - 6 dB point 1/2T 1/T

  10. Eye Pattern 30% Raised Cosine • In a perfect Raised Cosine Channel the Intersymbol Interference is theoretically zero • Cable Attenuation is well known to be proportional to sqrt(f*L) • The sqrt(f) response approximates a raised cosine response with large α • Therefore the max cable speed is at the 6 dB down cable attenuation ie the Nyquist frequency

  11. Duobinary Concept • Doubles the speed • First mentioned by A Lender in 1963 paper. • Also called Partial Response • Easiest implementation is a simple digital filter 1011 1V,-1V 2V,0,-2V Xn Binary To Bipolar + + 3 Level Encoder Out Nyquist Filter Σ Data In + 1 symbol delay T Xn-1 Data In 1 0 1 1 1 0 0 0 1 0 1 0 Bipolar Data In 1 1 -1 1 1 1 -1 -1 -1 1 -1 1 -1 + Xn + Xn-1 (3 Level) 2 0 0 2 2 0 -2 -2 0 0 0 0

  12. Duobinary Decoder 1011 1V,-1V 1011 2V,0,-2V 1V,-1V Binary To Bipolar + Slicer And Sampler Bipolar to Binary + Nyquist Filter Σ Σ - + 1 symbol delay T 1 symbol delay T Data Out Data In Data In 1 0 1 1 1 0 0 0 1 0 1 0 Bipolar Data In 1 1 -1 1 1 1 -1 -1 -1 1 -1 1 -1 Xn + Xn-1 (3 Level) 2 0 0 2 2 0 -2 -2 0 0 0 0 - Bipolar Data Out 1 -1 1 1 1 -1 -1 -1 1 -1 1 -1 Data Out 1 0 1 1 1 0 0 0 1 0 1 0

  13. Some More Duobinary Theory + Encoder is a Digital Filter H(f)=1+e-j2πfT Euler’s Formula : H(f)=cos (πfT) Σ Xn + Xn-1 1 symbol delay T - 6 dB point 1 H(f)=cos (πfT) 0.5 1/2T 1/3T f -2T -T 0 T 2T 3T

  14. Eye Pattern Duobinary • Duobinary can also be achieved with a cosine shaped Low Pass Filter • With a perfectly truncated cosine frequency response the Intersymbol Interference is zero. • The 6 dB down frequency is 0.6 * raised cosine case • For the same cable attenuation, the signalling speed for duobinary is almost double the binary raised cosine case

  15. Double -dotting • In 1898 Gulstad published paper : “Vibrating Cable Relay” • In hindsight invented Duobinary long before Lender in 1963 • Up until then cables were run at the “natural” speed (ie Nyquist Frequency) • He proposed doubling the speed, so that a 10101 dotting pattern resulted in almost zero Voltage output on the cable • As decoder on the Receive side he used a Polarized Relay with 2 extra windings • Each extra winding was connected with its own battery and RC network • The battery voltage and RC values were carefully adjusted so that one winding cancelled the second half of a detected + pulse and the second extra winding did the same for a –pulse • Gulstad called it “Vibrating Cable Relay” because in the absence of a cable signal, it vibrates at the “natural” frequency

  16. Gulstad Relay

  17. Long Haul Microwave • In US Bell Labs centre of expertise compliments DoD • Radar technology during WWII advanced microwave technology • Post war economic boom demanded intercity TV and telephone • First microwave was TD-2 • All vacuum tube, key part 416C planar microwave tube • Operated in 4 GHz band, 240 Voice channels or 1 TV • 1950 First Route NYC- Chicago • 1958 First Canadian coast to coast route

  18. TD-2 1958

  19. TD-2 equipment

  20. Typical TD-2 Microwave Tower

  21. Microwave status mid 70’s • Equipment all solid state except TWT amplifier (10 watt) • Telephone companies “owned” 4 GHz common carrier band • 12 RF duplex channels 20 MHz each • Separate Transmit/Receive antenna’s • Easy to recognize Bell Type A towers with 2+2 horn reflector antenna's • Capacity 1 TV or 1200 VF channels • Modulation FDM-FM (FDM=SSB-SC with 4 KHz spacing) • Everybody else (CN-CP, Western Union, MCI) “owned” 6 GHz band • 8 RF duplex channels 30 MHz each • Duplex 10-12 ft parabolic antenna’s • Capacity 1 TV or 1800 VF Channels

  22. Digital World was coming in 1976 • Bell Northern Research was working on DMS digital CO switch • The “toll” interface was digital • Technology existed for intra-city digital transmission • T1 carrier @ 1.544 Mb/s • Hence a need developed for intercity digital microwave

  23. Requirements for a Digital Microwave • Overbuild on existing 4 GHz network • CRTC made 8 GHz band available in Canada • Channel plan was 40 MHz • VF capacity similar to existing analog FDM-FM ie 1200+ • Digital Hierarchy DS-1 1.544 Mb/s 24VF • DS-2 6.312 Mb/s 96VF • DS-3 44.736 Mb/s 672 VF • By default capacity was set at 2 DS-3 (1344VF) or 91 Mb/s • Modulation efficiency requirement 2.25 bit/sec • FCC/CRTC defined a spectral emission mask (Spectrum Management)

  24. What is optimum Modulation Method?

  25. Choice of Modulation Method • Constraint; cost of TWT amplifier • Amplitude and phase nonlinearity of TWT • Modulation candidates • 8PSK very difficult to implement, but constant envelope • 16QAM 4 level PAM on I and Q axis  AM vs TWT • QPRS 2 level QAM on I and Q axis => TWT => filter => QPRS • Filter can be split between Transmitter and Receiver • Transmit Filter standard Tchebyshew and meets FCC Q-axis 4QAM QPRS I-axis 4QAM Rcvr Filter Trmt Filter TWT Baseband μW

  26. FCC Mask

  27. QPRS Eye Pattern Q-axis I-axis

  28. Transmitter Implementation Data 90 Mb/s Data 45 Mb/s 2L 70MHz I-signal 2 Level AM Modulator 4QAM 8 GHz 0 deg 4QAM 70 MHz Σ Up- converter 0/90 deg Phase Splitter Serial To Parallel TWT Amp ½QPRS BPF 90 deg 2 Level AM Modulator 2L 70MHz Q-signal 8 GHz Oscillator 70 MHz Oscillator

  29. Receiver Implementation 3 Level I-signal 45 Mb/s I data + Σ ½QPRS LPF 2 Level AM Demod Data Detector 90 Mb/s - 0 deg 1 Symbol delay Down Converter 70 MHz AGC Amp 0/90 deg Phase Splitter Parallel To Serial 3 Level Q-signal 90 deg + Σ 2 Level AM Demod ½QPRS LPF Data Detector - 1 Symbol delay 8 GHz Oscillator 70 MHz Carrier Recovery 45Mb/s Q Data

  30. Summary • Field Trial held 1976 Avonmore –Kemptville • Impact of Propagation much worse than expected • Added Space diversity with Phase-Adaptive Combiner at 70 MHz • Added Automatic Amplitude Slope Equalizer • Commercial Introduction 1978 Toronto-Winnipeg • Eventually Extended coast-to-coast • The Worlds First successful Long Haul Digital Microwave

  31. THANK YOU

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