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8/11/2019 Fundamentals Satellite Communication Part 4
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Satellite Communications: Part 4
Signal Distortions & Errors
and their Relation to Communication
Channel Specifications
Howard Hausman
April 1, 2010
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Satellite Communications: Part 4
Signal Distortions & Errorsand their Relation to Communication
Channel Specifications
Communications Problem Signals Formats & Distortions
Signal Errors
Phase Noise Group Delay Distortion
Amplitude Distortion
Combined Signal Distortions Adjacent Channel Interference
Time Domain Effects
Summary -2
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Process
Transmit an idea
Receiver the signal Receive the idea
Each step is a point of error
Added medium or device will increase the likelihood of errors
Specifications are designed to bring the error to acceptable level-
4
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Vectors Modulation
Digital Communications hasalmost universally replacedAnalog Communications
Analog required higherS/N than digital
Digital TransmissionEfficiency
16 QAM
5
Maximized usingamplitude and phaseinformation - Vector
Vector location defines a
symbol A Symbol is a collection of
Bits (1s & 0s) -
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Error Vectors
Vector Errors (EV) distort
the original signal EVM, (Error Vector
Measurements) common
ErrorVector
ResultantReceivedSignal
distortion
Can cause the resultantvector to point to the
wrong symbol -
6
TransmittedSignal
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Decoded Vectors Vectors are decoded into bits in the time domain
7
- ne ym olocation defines 6 bits1 0 1 101
Bit rate is 6 times symbol rate
(64QAM)
Base band is 6 times the IFbandwidth -
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Time Domain Measurements Time domain symbols are superimposed on each other
Time domain errors are identified using Eye Diagrams -
8
Eye Diagram
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1. Dont interfere with yourneighbor Frequency Domain
frequency
power
2. Recover the correct symbol Vector Measurements 3. Recovery the bit information
Time Domain -
Equipment Specifications
Three Areas of Concern
9
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Signals Formats & Distortions
Signal Areas of Concern
Large signal
Deterministic &Random effects(other than thermalnoise)
10
Average
PowerOperatingRegion Peak
Power
Distortion
SmallSignal -
Thermalnoise
Linear operating region must allow
for the signal peaks (AM) -
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Constant Amplitude (CW) Transmission Formats
Binary Phase-Shift KeyingBPSK (2-QAM)Used for low speed
communications
QPSK & 8PSK are usedfor higher speedcommunications
Q
IBPSK
1
0
0001 Q
12
11 10
I
8PSK
0101
I
Q
001
000
100101111
110
010
Note: Vector phase is the onlyinformation needed to recover data -
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Quadrature Amplitude Modulation
(QAM)
64 QAM
Signal amplitude and phase must beresolved
Used for much higher speed communication
where bandwidth is severely limited -
Q16 QAM
256 QAM13
I0000
1111 1110 0101 0111
0100 011011001101
1010
1011
1000
1001 0010 0011
0001
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Decision Regions - System Diagram
14
Transmit
Vector is on apoint
Receiver Vector
is in a decisionregion -
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CW Decision Regions
BPSK
Threshold90
QPSK
Threshold45
AcceptableRegion
AcceptableRegion 22.5
15
Vector Phase Only
Phase Thresholds
BPSK: 90
QPSK: 45
8PSK: 22.5
-
Decisionregion
8PSK
0101
I
Q001
000
100101111
110
010
22.5
Threshold22.5
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Lines between the
constellation points are the
threshold levels
Signals residing in the
square are assume to
QAM Decision
Region
16
reside at the discretevector location.
Note vector outside the
square - Wrong Code
Codes are set such thatall surrounding codes have
a 1 bit error -
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QAM GeometricEffects
Maximum angle error
is dependent on SymbolLocation
Outer Symbols Toleratethe least angle error
17
Allowable ErrorWindow is smaller forMore Complex
Modulations -
Modulation Error2QAM 90.04QAM 45.0
16QAM 16.932AM 10.964QAM 7.7128QAM 5.1
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16APSK & 16 QAM
I
Q
00001010
1011
1000
1001 0010 0011
000116 QAM
16 APSK Smaller peak to averageratio than 16QAM
16APSK more immune to PhaseNoise than 16QAM ~
18
16APSK
1111 1110 0101 0111
0100 011011001101
16QAM 16.916APSK 22.5
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Random Errors
Highly uncertain
Characterized by a probability
Signal Errors
19
s r u on
Characterized by their standard deviation
Errors are statistical
Function of the number of standard
deviations to the threshold (Multiples of )Thermal Noise Low Noise AmplifierPhase Noise-Local Oscillators - -
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Standard Deviation & RMS Noise
pdf is area underGuassian curve from -to a1
P(a11)=1-
.841=.159
0.8
0.9
1Probability Density Function
1
Probabilitydensity
is Average (Mean)=standard deviation: Relates tothe function spreading
P(V+1)=.682
P(>|1|) = .318
P(>|2|) = .046
P(>|3|) = 2.7x10-3
P(>|4|) = 6.3x10-5
P(>|5|) = 5.7x10-7 -
20
4 3.5 3 2.5 2 1.5 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 40
0.1
0.2
0.3
0.4
0.5
0.6
0.7
6 10 4
pdfi
p i
3.53.5 Vi
a1
a2function(pdf)
Guassian
probabilityCurve
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Standard Deviation & RMS Noise=1 RMS
Noise is the ideal signal
point Error Probability =
-a +a
number of from to a (>0) ExampleP(a=|4|) Bit Error
= 6.3x10-5 -
21
Noise VectorRotates
Signal Vector
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Deterministic Errors
Deterministic know everything withcomplete certainty
Examples: Filter ripple Causes Side Band amplitude
errors
May change with frequency &
UndistortedSignal
22
Temperature
Characteristics are completelyknown
Knowing the signal spectrumtransmitted
Possible to correct the distortion
DistortedSignal due togain ripple
Gainripple
-
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Deterministic Effects: Predictable & Correctable
AM/AM Distortion-Power Amplifier,
ADC QuantizationAM/PM Distortion-Power AmplifierGroup Delay Distortion-Filters
-
Examples of Deterministic Errors
23
Supply, 3rd Order Interference At set-up & periodically thereafter
Learning codes are sent
Distortion is compensated theimprove BER -
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Deterministic effects add directly: A + B = C Probabilistic (Noise) effects add RMS: SQRT(A2 + B2) = C
A, B, & C are standard deviations
Large number of deterministic effects add as noiseGaussian Theorem -
Random & Deterministic Effects
24-
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Random Noise in a Boundary
Bit Error: Received VectorFalls Outside Boundary
Signal Vector (Red) Random Noise (Yellow) Rotates around signal
Threshold
1
3
25
Gaussian AmplitudeDistribution
BER is related to the numberof s to the threshold - Noise Vector
Rotates
SignalVector
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Random Noise + Deterministic Errors
in a Boundary Bit Error: ReceivedVector Falls OutsideBoundary Signal Vector (Red) Random Noise (Yellow) Rotates around
Noise VectorRotates &
Adds withDeterministicvector
Threshold
12
26
signal vector (360) Deterministic vector(Green) adds an error tothe signal vector
BER is the number of sto the threshold Number of s wentfrom 3 to 2
SignalVector
Noise
Amplitude
0
p(n)
Gaussiandistribution isoffset -
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Long Term Frequency Stability
Time frame: Typically hours to years
Temperature variations are long term
Data: F Fo Parts Per Million PPM
Phase Noise
Oscillator Stability
Short Term Frequency Stability Residual FM F Large: Change in frequency F is
much greater than the rate of frequency change, fm
(F/fm = >> 1) Allen Variance - F small: Rate of change : t >1
Second
Phase Noise: F small: Rate of Change: t < 0.1 sec.-
-27
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Phase Noise - Short Term Stability
Measures oscillator Stability over short periods of time
Typically 0.1 Second to 0.1 microsecond
Noise varies the oscillator phase/frequency
Not amplitude relatedNoise level increases close to the carrier
Typical offset frequencies of interest: 10Hz to 10MHz
ffff
Howard Hausman August 2009 28
Noise further from the carrier is usually masked by AMthermal noise
Phase Noise cannot be eliminated or affected by filtering
Phase & Frequency are related:
Frequency is the change in phase with respect to time
/ t d/dt as t 0 -
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Phase (Frequency) Noise
Specified and measured as a spectral density function in a 1 Hzbandwidth
dBc/Hz at a given offset from the carrier
Level in dBc = 20 Log (/2) where is in radians
Modulation index () of noise in a 1 Hz bandwidth
Measurement at Frequency offset from the carrier is the time intervalof phase variation
29
Measurement bandwidth orResolution Bandwidth is the dwell
time of the measurement1Hz resolution bandwidth is a 1second measurement time -
Resolution
BW
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Total RMS Phase NoiseEach 1 Hz bandwidth
(dBc/Hz) is the result of
narrow band modulation (
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Thermal Noise: Random in all
directions
-a
Phase Noise vs Thermal Noise
31
Phase Noise: Random on theAngular Axis
Independent of Signal
Power Errors occur on Both Symbol
Boundaries -
+a
ErrorError
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Phase Noise & Error Probability
Gaussian Function = Average angle
Standard Deviation
RMS = 1 (Standard
Deviation)
Probability of Error (BER) is related
32
to the number of s to the boundary
s are in degrees RMS ProbabilityDensityFunction
P(>|1|) = .318
P(>|2|) = .046
P(>|3|) = 2.7x10-3
P(>|4|) = 6.3x10-5
P(>|5|) = 5.7x10-7 -
22 5
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System PhaseNoise
Constant AmplitudeModulation (e.g. 8PSK) Phase Noise threshold
0101
I
Q
001
000
100101111
110
010 22.5
22.5
Threshold22.5
8PSK
33
is constant (22.5 ) QAM ModulationAllowable Phase Noiseis a function of Bit
Position Figure shows allowablephase error for 64QAM -
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RMS Phase Noise Integration Limits
Sum ONLY over Applicable
Frequencies
Typically 1/50 Symbol Rate
to 1 Symbol Rate ( f1 to f2 )
Ex: For 5Msymbols/sec
T ical inte rated BW
34
100kHz to 5 MHzIntegrate in segments
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Intelsat Phase Noise Specification
-60
-50
-40
-30
Bc/Hz
Phase Spec / IESS-308/309dBc/Hz, Single Side Band
35
Dont make symbol rate too low
Phase Noise close to the carrier is higher
See why low data rate modulators use BPSK -
-90
-80
-70
d
10 100 1K 10K 100K 1MOffset Freq.: Hz
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Phase errors reduces the
number of standard deviations to
threshold Maximum Angular error MAX
Random + Deterministic Phase
Distortion
36
Distortion Error 3= DistortionRMS =1.0
MAX = 5
MAX = 2 [P(>|2|) = .046 ]
Should be 5 [P(>|5|) = 5.7x10-7]-
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Phase Noise Allocation BudgetTotal Phase noise budget is the RMS sum of all thecomponentsOscillators have the highest phase noise
37
Power Amplifier phase errors are caused when signal peaks -
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Quadrature of
the initial vectors
are effected Fixed Offset of
Vectors
Group Delay Distortion
38
roup
DelayError
Group Delay
Distortion is
deterministic
Distortion is a
function offrequency -
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Amplitude Distortion3rd OrderIntercept PointSignal increases
Amplitudecompresses:
AM/AMDistortionPhase Power at 1 dB
Com ression
39
Gain
Compression
c anges:
AM/PMDistortion
Create Two Tone
Intermodulation(IMD) distortion3rd and 5th
Order Products
SaturatedPower
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AM/AM Mechanism(Non-Random Effect)
Gain vs.Amplitudeis Non-Linear
ClippingReducesAmplitude
40
Gain Compressionresults in AM/AMDistortion
Amplitudevariation is Non-Linear -
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AM/PM Mechanism
(Non-Random Effect)
Offset CreatesAM/PM (Phase
changes withamplitude)
Clipping AmplitudeChanges Zero
Crossing
/ ,
2.5/
41
AM/PM occursbefore AM/AM
AM/PM Distortion is morepronounced at the outer symbolsPeak to Average ratio has apronounced effect on phasedistortion -
T T I t d
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Two Tone Intermods
22
6
Typical Spec
42
o 1st, 2nd , & 3rd Harmonics MixTogether Forming IMDo Level of Compression DeterminesHarmonics Amplitudes & IMD Tones
IMD is a RotatingSpurious at the end of the
signal vector -
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Combined Signal Distortions
Thermal Noise
+ Phase Noise
+ GroupDelay
Thermal Noise
Phase Noise
AM/PM
43
AM/AM
Group Delay
Intermodulation
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Symbol Error Probability
Each Symbol has adifferent probability of Error(Pi )
44
Assume all symbols areequally likely Calculate Expected SymbolError Probability
is the Random (RMS)variation is the deterministicoffset -
Adj t Ch l I t f
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Spectral Re-Growth
Modulated Spectrum ispre filtered to providedless than -40dBc of sideband interference
Adjacent Channel Interference
Non-linearities increasethe side lobe level
Typical maximumallowable spectral re-
growth is 30dBc -
45
Spurious Signal
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Spurious Signal
Spurious signals are discrete non-signal relatedinterference
Individual spurious signals occur from multiple sources
Add non-coherently Typical Specification is 60dBc for the entire
ransmitter chain
In band interference is controlled: -20 dBcinterference effects C/N < 0.04dB
-60dBc protects small carriers
Carrier power is a function of Bandwidth -
46
O t f B d N i P O t t
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Out of Band Noise Power Output
Transmitters have high C/N
Noise Figure is usually not an issue
Output noise power can interfere with adjacentcarriers
Maximum output noise is given in dBm/Hz
Noise Power output = Noise Figure (dB) + Gain fromsignal generator (primary oscillator) to final output(dB) -174dBm/Hz (thermal noise)
Low Noise Figure and High Gain High output noise
power -
47
Time Domain Effects
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Time Domain Effects
RecoveredData
Eye Diagrams
A means of assessing Received signal quality
48
Fold Data 1s & 0s Overlap
Establish an Area ofKnown Good Data in Time
and Voltage Larger the EYE lesserrors -
E Di S ifi ti
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Eye Diagram Specifications
Noise +Distortion
Noise
Inter-Symbol
Interference
Recovered Pulse must avoid the RED area RED area is an error in the amplitude or time
49
Optimum ThresholdVoltage
Optimum Sampling Time -
Margin
Zero
CrossingJitter
ccep a e
samplingarea
D t i th Ti & F D i
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Data in the Time & Frequency Domain
Ideal ReceivedData
20Msymbols/SecNRZ
IF Frequency
50
Spectrum o BPSK NRZ
data alternate 1s &0sCarrier (RED) issuppressed
Only Odd Harmonics-
Fund3rd 5th
UpperSide Band
LowerSide Band -
Freq
Typical Gain Specification
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Typical Gain Specification :
1.0 80
2.5
, + 0.04 /, 24 + 0.25
+ /1.0
51
GainVariationIncludesRipple
Gain: dBFrequency80 MHz
Slope
Amplitude Distortion
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p ude s o o(Gain Ripple Specification)
Ideal ReceivedSignal 1 dB peak to peak
Amplitude Rippleon 3rd Harmonic
52
Added Ringing 15% -
3rd 1dB
Typical Group Delay Specification
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Typical Group Delay Specification
, 10.95 12.75
80
0.01 /
Group Delay is usually parabolic
Edges rise with the skirts of the filter
53
0.005 2
0.5 /
LinearSymmetry ofthe Parabola
ParabolicDepth ofthe Parabola
Ripple-
Delay Distortion
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Delay Distortion
Modulated Signal through a 70 MHz Filter Side bands should notchange in amplitude or phase(delay)
Delay curve & effect onsidebandsSymmetryUpper & Lower
Typical Group Delay distortion in a70MHz filter
ec/Div
5dB/Div
Amplitude
Lowerrd
Upper 3rd
5nSec
54
sidebands
Depth of ParabolaSide band harmonics
Ripple-All sidebandseffected
Upper 3
rd
Harmonic isdelayed 5 nSec Lower 3rd Harmonic isdelayed 10 nsec -
6
n
70MHz
88MHz52MHz
6MHz/Div
Group
Delay
Fund3rd 5th
Upper Side BandLower Side Band
Freq
10nSec
Effect of Delay / Phase Distortion on
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Effect of Delay / Phase Distortion on
20Msymbol/Sec Data
Added Ringing 30%
55
2nsec Delay of 3rd
Harmonic onRinging 30%Delay distortion can be more critical than AmplitudeDistortion -
20Msymbols/Sec Data
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20Msymbols/Sec Data
Amplitude & Phase Distortion
2nsec delayat 3rd harmonic+ 1dB Ripple on
Added Ringing 37%
56
e
HarmonicIncreasedpulse ripple to
37% Judge how theEYE is closing -
Thermal Noise Noise Figure
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Thermal Noise Noise FigureA Signal Level Related Function
Random effect in the time domain
Thermal noise is a concern at lower signal levels
Systems should have at least a 30dB Input signal
57
to internal noise ratio Typical effect on the system
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Clock Jitter
Delay
Signal withnoise
58
Jitter
Clock Jitter is the uncertainty related tothe start of the dataCaused by Zero crossing uncertainty onthe recovered signal Thermal noise & Phase noisecontribute to clock jitter
Zero CrossingUncertainty -
Inter-pulse interference
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Ideal Filter ( = 0) has
a poor pulse response(SinX/X)Filter shaping lowers
FrequencyResponse
= 0.0 = 0.5 = 1.0
p
Nyquist Filtering: Raised CosineFilter Response
59
nter-sym o nter erence
Characterized by (Frequency Response)Typically = 0.35)Trade-Off is Frequencyselectivity vs PulseResponse
Time DomainResponse
= 0.0
= 0.5 = 1.0
Ripple inadjacentpulses isInter-pulse
Interference -
PulseWidth
Typical Data
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Typical Data
Eye Diagrams
e
Pulse Distortion
ZeroCrossing
Jitter
60
Time (Tb)
Amplitud
NoiseMargin
bT
IntersymbolInterference
lOptimumSampling isin thecenter of
the Eye -
Summary
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u a y
Satellite 2010 Convention in Washington The good news for satellite is that it goes where
fiber and Wi-Fi dont go. Its the most versatile
communications technology available, and there isnot a substitute.
Quality of received signal relates to:
Modulator
Transmitter
Transmission medium
Receiver
Demodulator
Each segment requires has separate requirements &individual concerns
61