From Telegraph Double Dotting
To
Microwave QPRS
(Quadrature Partial Response Signalling)
George de Witte
VE3AYB
QCWA Sep 20, 2011
1
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
2
First Route Valentia Harbour,Ireland to Trinity Bay NL
3
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
4
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
C
R
C
R
C
5
Cable Laying Ship The Great Eastern
Cable ship measured ship-shore continuity during laying
6
Shore end
Cross-section
Shore end of Cable
Stripped
Deep Sea
Cross-section
7
Nyquist Theory 1928
Input
Data
Xn
Transmit
Filter
Channel
Response
Gt(f)
C(f)
Noise
+
Receive
Filter
Output
Data
Sampler &
Threshold
Detector
Gr(f)
Xn
X(f)
BPF BW=Fnyquist
X(f)=Gt(f)*C(f)*Gr(f)
LPF BW=0.5Fnyquist
X(f)=T )
X(f)=0 ) -1/2T<f<1/2T
0
sin πt/T
x(t)= ------------πt/T
F=1/T is called Nyquist
frequency
8
More Nyquist Theory
(T
X(f)= (
(T/2[1-sin(πT(f-1/2T)/α)]
0<f<(1-α)/2T
(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
9
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
10
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
Data In
1V,-1V
Binary
To
Bipolar
2V,0,-2V
Xn
+
Data In
Xn + Xn-1 (3 Level)
+
3 Level
Encoder Out
+
1 symbol
delay T
Bipolar Data In
Nyquist
Filter
Σ
1
1
1
1
1
0
-1 1
+
2 0 0
1
1
-1 -1 -1 1
-1 1
-1
2
2
0
0
0
1
0
Xn-1
0
0
1
-2 -2 0
0
1
0
0
11
Duobinary Decoder
1011
1V,-1V
2V,0,-2V
Binary
To
Bipolar
Data In
+
+
Nyquist
Filter
Σ
Bipolar
to
Binary
Slicer
And
Sampler
Σ
-
+
1 symbol
delay T
1 symbol
delay T
Data In
0
1
1
1
0
1
-1 1
1
1
-1 -1 -1 1
-1 1
-1
0
0
2
2
0
0
0
Bipolar Data Out
2
1
-1 1
1
1
-1 -1 -1 1
-1 1
-1
Data Out
1
0
1
1
0
0
0
Xn + Xn-1 (3 Level)
1
1
0
0
1
-2 -2 0
0
0
1
0
1
Data Out
1
Bipolar Data In
1011
1V,-1V
0
1
0
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Some More Duobinary Theory
+
Encoder is a Digital Filter
Σ
Xn
+
Xn-1
1 symbol
delay T
H(f)=1+e-j2πfT
Euler’s Formula : H(f)=cos (πfT)
- 6 dB point
1
0.5
H(f)=cos (πfT)
1/3T 1/2T
f
-2T
-T
0
T
2T
3T
13
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
14
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
15
Gulstad Relay
16
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
18
TD-2 equipment
19
Typical TD-2 Microwave Tower
20
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
21
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
22
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)
23
What is optimum Modulation Method?
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
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
4QAM
QPRS
I-axis
TWT
Trmt
Filter
μW
Rcvr
Filter
Baseband
25
FCC Mask
26
QPRS Eye Pattern
Q-axis
I-axis
27
Transmitter Implementation
Data
Data
90 Mb/s
45 Mb/s
2L 70MHz I-signal
2 Level
AM
Modulator
0 deg
Serial
To
Parallel
4QAM 8 GHz
4QAM
0/90 deg
Phase
Splitter
Σ
70 MHz
Upconverter
TWT
Amp
½QPRS
BPF
90 deg
2 Level
AM
Modulator
2L 70MHz Q-signal
70 MHz
Oscillator
8 GHz
Oscillator
28
Receiver Implementation
3 Level I-signal
2 Level
AM
Demod
½QPRS
+
-
0 deg
Down
Converter
70 MHz
AGC Amp
Σ
LPF
90
Mb/s
Parallel
To
Serial
3 Level Q-signal
90 deg
½QPRS
+
Σ
LPF
8 GHz
Oscillator
Data
Detector
1 Symbol
delay
0/90 deg
Phase
Splitter
2 Level
AM
Demod
45 Mb/s I data
70 MHz
Carrier
Recovery
Data
Detector
1 Symbol
delay
45Mb/s Q Data
29
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
30
THANK YOU
31