Transcript Sep 20, 2011
From Telegraph Double Dotting To Microwave QPRS (Quadrature Partial Response Signalling)
George de Witte VE3AYB QCWA Sep 20, 2011 1
Background 19 th 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 2
First Route Valentia Harbour,Ireland to Trinity Bay NL
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The Economic Reality
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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
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Key Theoretical Contribution by Lord Kelvin
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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 C R C R C R
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Cable Laying Ship The Great Eastern
Cable ship measured ship-shore continuity during laying
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Shore end of Cable Stripped Shore end Cross-section Deep Sea Cross-section
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Input Data Xn
Nyquist Theory 1928
Transmit Filter Gt(f) Channel Response C(f) Noise + Receive Filter Gr(f) Sampler & Threshold Detector Output Data Xn X(f) BPF BW=F nyquist LPF BW=0.5F
nyquist
X( f)=Gt(f)*C(f)*Gr(f) X(f)=T ) X(f)=0 ) -1/2T 0 8 ( T X(f)= ( (T/2[1-sin( πT(f-1/2T)/α)] sin πt/T cos απt/T x(t)= ------------- --------------- πt/T 1-4α 2 t 2 /T 2 0 - 6 dB point 1/2T 1/T 9 • 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 10 • • • • Doubles the speed First mentioned by A Lender in 1963 paper. Also called Partial Response Easiest implementation is a simple digital filter Data In 1011 Binary To Bipolar 1V,-1V X n + + Σ 2V,0,-2V Nyquist Filter + 1 symbol delay T X n-1 3 Level Encoder Out Data In 1 0 1 1 1 0 0 0 1 0 1 0 Bipolar Data In Xn + Xn-1 (3 Level) 1 1 2 + -1 1 0 0 1 1 2 2 -1 -1 -1 1 0 -2 -2 0 -1 1 0 0 -1 0 11 1011 Binary To Bipolar 1V,-1V Data In 1 symbol delay T + + Σ 2V,0,-2V Nyquist Filter + - Σ Slicer And Sampler 1V,-1V Bipolar to Binary 1011 1 symbol delay T Data Out Data In Bipolar Data In Xn + Xn-1 (3 Level) Bipolar Data Out Data Out 1 0 1 1 1 0 0 0 1 0 1 0 1 1 -1 1 1 1 -1 -1 -1 1 -1 1 -1 2 1 0 0 -1 1 2 2 1 1 0 -2 -2 0 -1 -1 -1 1 1 0 1 1 1 0 0 0 0 -1 1 0 -1 0 1 0 1 0 12 X n 1 symbol delay T + + Σ X n-1 Encoder is a Digital Filter H(f)=1+e -j2πfT Euler’s Formula : H(f)=cos (πfT) 1 0.5 - 6 dB point H(f)=cos (πfT) 1/3T f 1/2T -2T -T 0 T 2T 3T 13 • 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 14 • 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 15 16 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 17 18 19 20 • 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 21 • 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 22 • 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) 23 24 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 • QPRS 4 level PAM on I and Q axis AM vs TWT 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 4QAM I-axis TWT Trmt Filter μW Rcvr Filter Baseband QPRS 25 Mask 26 Q-axis I-axis 27 Data 90 Mb/s Data 45 Mb/s Serial To Parallel 2 Level AM Modulator 0 deg 0 / 90 deg Phase Splitter 2L 70MHz I-signal Σ 4QAM 70 MHz Up converter 4QAM 8 GHz TWT Amp 90 deg ½ QPRS BPF 2 Level AM Modulator 2L 70MHz Q-signal 70 MHz Oscillator 8 GHz Oscillator 28 Receiver Implementation Down Converter 70 MHz AGC Amp 8 GHz Oscillator 2 Level AM Demod 0 deg 0/90 deg Phase Splitter 90 deg 2 Level AM Demod ½ 3 Level I-signal + QPRS LPF Σ 3 Level Q-signal ½ QPRS LPF + Σ 70 MHz Carrier Recovery Data Detector 45 Mb/s I data 1 Symbol delay Parallel To Serial 90 Mb/s Data Detector 1 Symbol delay 45Mb/s Q Data 29 • 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 30 31More Nyquist Theory
Eye Pattern 30% Raised Cosine
Duobinary Concept
Duobinary Decoder
Some More Duobinary Theory
Eye Pattern Duobinary
Double -dotting
Gulstad Relay
Long Haul Microwave
TD-2 1958
TD-2 equipment
Typical TD-2 Microwave Tower
Microwave status mid 70’s
Digital World was coming in 1976
Requirements for a Digital Microwave
What is optimum Modulation Method?
FCC
QPRS Eye Pattern
Transmitter Implementation
Summary
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