Fundamentals Satellite Communication Part 4

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

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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

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Fundamentals Satellite Communication Part 4