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From Telegraph Double DottingTo

Microwave QPRS(Quadrature Partial Response Signalling)

George de WitteVE3AYB

QCWA Sep 20, 2011

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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

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First Route Valentia Harbour,Ireland to Trinity Bay NL

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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

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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

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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 CableStripped

Shore endCross-section

Deep SeaCross-section

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Nyquist Theory 1928

X(f)=Gt(f)*C(f)*Gr(f)

X(f)=T )X(f)=0 ) -1/2T<f<1/2T

sin πt/Tx(t)= ------------- πt/T

F=1/T is called Nyquist frequency

0

BPF BW=Fnyquist

LPF BW=0.5Fnyquist

Gt(f) C(f) Gr(f)Sampler &Threshold Detector

+Xn Xn

Input Data

Output Data

Transmit Filter

ChannelResponse

ReceiveFilter

X(f)

Noise

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More Nyquist Theory

( T 0<f<(1-α)/2TX(f)= ( (T/2[1-sin(πT(f-1/2T)/α)] (1-α)/2T <f<(1+α)/2T

sin πt/T cos απt/Tx(t)= ------------- ---------------- πt/T 1-4α2t2/T2

1/2T 1/T

- 6 dB point

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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

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Duobinary Concept• Doubles the speed• First mentioned by A Lender in 1963 paper. • Also called Partial Response• Easiest implementation is a simple digital filter

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

3 Level 3 Level Encoder OutEncoder Out

+

1 symbol delay T

Nyquist Filter

Σ+

+BinaryTo

Bipolar

1011 1V,-1V 2V,0,-2V

Data In

Xn-1

Xn

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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

Duobinary Decoder

ΣSlicerAnd

Sampler

1011

+

-

1 symbol delay T

Nyquist Filter

Σ

1 symbol delay T

+

+BinaryTo

Bipolar

1011

Bipolar to

Binary

1V,-1V 2V,0,-2V 1V,-1V

Data In Data Out

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Some More Duobinary Theory

Encoder is a Digital Filter

H(f)=1+e-j2πfT

Euler’s Formula : H(f)=cos (πfT)

H(f)=cos (πfT)

1/2Tf

1

0.5

1/3T

1 symbol delay T

Σ+

+

Xn-1

Xn

-2T -T 0 T 2T 3T

- 6 dB point

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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

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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

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Gulstad Relay

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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

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TD-2 1958

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TD-2 equipment

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Typical TD-2 Microwave Tower

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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

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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

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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 HierarchyDS-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)

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What is optimum Modulation Method?

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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

I-axis

Q-axis

TWTTrmtFilter

RcvrFilter

4QAMQPRS

μW

4QAM

Baseband

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FCC Mask

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QPRS Eye Pattern

I-axis

Q-axis

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Transmitter Implementation

SerialTo

Parallel

70 MHzOscillator

2 LevelAM

Modulator

2 LevelAM

Modulator

0/90 degPhase

Splitter

0 deg

90 deg

Σ

Data

90 Mb/s

Data

45 Mb/s2L 70MHz I-signal

2L 70MHz Q-signal

8 GHzOscillator

TWTAmp

Up-converter

½QPRSBPF

4QAM

70 MHz

4QAM 8 GHz

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Receiver Implementation

8 GHzOscillator

DownConverter

70 MHzAGC Amp

70 MHzCarrier

Recovery

2 LevelAM

Demod

2 LevelAM

Demod

0/90 degPhase

Splitter

0 deg

90 deg

½QPRSLPF

½QPRSLPF

ΣData

Detector

1 Symbol delay

+

-

ParallelTo

Serial

ΣData

Detector

1 Symbol delay

-

+

3 Level I-signal

3 Level Q-signal

45 Mb/s I data

45Mb/s Q Data

90 Mb/s

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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

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THANK YOU