rfm_pdd day 5-b

7/31/2019 RFM_PDD Day 5-b

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Okey Ugweje, PhD Page 1

Day 5 (b)

Link Budget Analysis


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y g j


RF/Microwave Systems

Microwave LinkLink Budget Definitions/ModelingSystem Gain and LossLink Budget CalculationsPlanning a Point to Point SystemUnderstanding Digital MicrowaveSystem Testing

Interpreting Microwave Alarms

Microwave Link Budget & SystemEvaluation


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Microwave Links

In many terrestrial microwave systems (e.g., for long distancecommunication) Relay, Repeater or regenerators are used

Basic Rules

Repeaters are used for larger coverage and some repeater process the signal before retransmissionCoverage depends on frequency, power, environment, etc.,

3 0 m i l e s

3 0 m ile s 30 miles3 0 m i l e s


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Microwave Links

The rule for the repeater systems is to use alternatepolarization between the Tx and Rx at alternating repeatersDifferent frequencies are used for transmitting and receiving

V H V H V H V H V H V H V H V H

Half Band Transmission Half Band ReceptionFrequency 1

Frequency 2

Microwave tower Microwave tower

Frequency #1

Frequency #2 RxRx

TxTx


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Link Budget Definitions


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Link Budget Definitions - 1

Focuses on the FRONT ENDs and the CHANNEL

Format MultiplexChannelEncoderSource

Encoder Spread

Format DemultiplexChannelDecoderSource

Decoder Despread

Bits orSymbol

To otherdestinations

From othersources

Digitalinput

Digitaloutput

Sourcebits

Sourcebits

Channelbits

Carrier & symbolsynchronization

Channelbits

$

mi

mi MultipleAccess

Waveforms

MultipleAccess

PerformanceMeasure

$

Pe

Modulate

Demodulate&

Detect

Tx

Rx


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RF Propagation Principles

Propagation Models and link Budgets

Link Budget Is the calculation which allows us to dimensions theemitted power and antenna gain to the ratio of signal rates, used modulation and the noisereception.

It uses the precedent formulas in logarithmic form,

which gives the following table can be put ingraphic form (hypsogramme of link figure a) and b)


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RF Propagation Principles

Propagation Models and link Budgets


9/76 Okey Ugweje, PhD Page 9

RF Propagation PrinciplesPropagation Models and link Budgets

Link Budget:Transmitter power : P t (dBW) = 10 log P t (W)Losses in transmission guides: t (dB)Transmission antenna gain: G t (dB) = 10 log G tEffective Isotropic Radiation Power EIRP (dBW)Transmission attenuation : - t (dB) =-20log (4 d/)

Atmospheric attenuation : a

(dB) = - .dReceiver antenna gain : +g r (dB) = 10 log G tLosses in reception guides : - r (dB)Received power : P r (dBW) = 10 log P r (W)



Link Budget Definitions - 2Link Budget (LB) analysis is concerned with determining thesystem operating points and performance

It comprises of the entire communication system frominformation source to information sink

LB consist of the calculation and tabulation of Useful signal power Interfering noise power available at the receiver

It is a balance sheet of gains and losses within thecommunication resourcesSince most parameters of a communication link are statistical,link budget is simply an estimate of the operating pointsPurposes of a link budget are:

To determine the actual system operating points To establish that the BER the system requirement or margin



Link Budget Definitions - 3LB is the basic tool for providing the system engineer withoverall system requirements

It shows the overall system design and performanceFrom LB one can know whether the system will meet its requirement comfortably, marginally or not at all LB is often used as a score sheet in considering

System tradeoff Configuration changesSubsystem nuances

Subsystem interdependenceLB can help predict equipment weight, size, power requirement, technical risk and cost



Simplified Satellite Link Model

In a satellite channel, there are different types of lossesLosses or degradation are caused by either signal loss,noise or both

All sources of degradation must be accounted for to get theoperating points

A complete model of a satellite channel with all the differenttypes of losses are shown below

Modulate

m t ( )Transmit

FilterSatellite(TWTA)

DetectReceive

Filter+ +S t ( ) X t ( ) Y t ( )

Z t ( ) R t ( )

n t d

( )n t u

( ) AWGNAWGN

Interferer 1

Interfere N Interferer N

Interferer 1

DownlinkUplink

Transponder



Detail Satellite System Model - 1

Modem

Transmitter

LO PhaseNoise

ISI

Limiter

Band-limiting

Efficiency

AM/PM

IM Products

Modulation

Pointing

Efficiency

ISI

Feeder loss

Receiver

Radomenoise

Polarization Atmospheric Space Pointing

Radomenoise

Adjacent Channel Interference Co-channelInterference

IM Noise

AntennaGalactic, star,

terrestrialnoise

Receiver

Modem

LO PhaseNoise

Band-limiting

synchronization

Imple-mentation

Antenna

Information SourceTransmitting

Terminal Information Sink

ReceivingTerminal

10 11 12 13 10

9

87

6

5 3

1 2

4

13

2120

2

18

8

17

16

9

19



Detail Satellite System Model - 21. Bandlimiting Noise

Signal loss causes by BPF limiting of the energy in a signal2. Intersymbol Interference (ISI)

ISI occurs due to overlapping of signal pulses as a result of improper filtering in the Tx, Channel and Rx

Channel induced distortion which spreads/disperses pulsesMultipath effects (echo)

3. Local Oscillator (LO) Phase LossWhen LO is used in signal mixing (e.g.,coherent receivers),

phase fluctuations or jitter adds phase noise to the system4. AM-to-PM Conversion

Phase noise that occurs in nonlinear devices such as thetraveling-wave-tube (TWT)

Amplitude fluctuation produce phase variations and noise



Detail Satellite System Model - 35. Limiter Loss or Enhancement6. Multiple Carriers Intermodulation (IM) products

A multiplicative noise introduced by the interaction of different carriers

7. Modulation Loss

8. Antenna Efficiency Antennas are transducers that convert electronic signals toEM and vice versaThe ability of an antenna to do this conversion is known as

its efficiency9. Radome Loss and Noise

A radome is a protective cover used with some antennasfor shielding against weather effects

Since it is in the path of the signal, it can cause signal loss



Detail Satellite System Model - 410.Pointing Loss

Loss due to the misalignment of the either the Tx or Rxantenna

11.Polarization LossLoss due to polarization mismatch between the Tx/Rxantennas

12.Atmospheric Loss and NoiseLoss due to weather or atmospheric condictions

13.Space LossLoss as a function of distance of radio wave propagation

14.Adjacent Channel InterferenceLoss due to other frequencies spilling over to adjacentbands

15.Co-channel Interference

Loss cause by two signals using the same frequency



Detail Satellite System Model - 516.Intermodulation (IM) Noise

Multiple carrier signals interacting in a nonlinear device17.Galactic or Cosmic, Star, and Terrestrial Noise

Loss caused by celestial bodies18.Feeder Line Loss

Loss between the receiving antenna and the receiver frontend

19.Receiver NoiseLoss caused by thermal noise within the receiver

20.Implementation NoiseLoss caused by the difference in theory and practice

21.Imperfect Synchronization ReferenceLoss due to error in synchronization and timing




Link Budget Calculations(System Gain and Loss)



Link Budget Parameters

Modulator Transmitter Receiver Demodulator

Transmit Antenna

Receive Antenna

OutputInput

C N

P N

EIRP R

GkT

EIRP RGT k

EIRP L GT k

R

Req

Req

s R

eq

0 0

2

2

2

4 4

1

4

1

1

FH IK

C N

EIRP LGT

k dB

dB s dB R

eq dB

dB0NMQP NMQPin dB

Free SpaceLoss

Transmitter

Parameters ReceiverParameters

Nature

Sometimes the noise figure F is used instead of the equivalent noise temperature T eq

E

N CT N

C N R

avg s

s0 0 0

1



Parameter & Variable Definitionsd = Tx-Rx separationf = Frequency

= WavelengthD = Antenna Diameter

= Antenna EfficiencyP r = Receive power

P t = Transmit power N o = Thermal noise power C = Carrier power G t = Transmit gain

G r = Receive gainLs = Free space lossLaf = Antenna Feeder LossLtf = Transmitter Feed LossLat = Atmospheric Loss

Lrf = Rain Fade LossLp = Pointing Loss

T s = System noise temperatureT a = Antenna noise temperatureT = Feeder ambient temperatureT e = Effective receiver temperatureF = Noise figure

= Flux densityEIRP = Effective Isotropic Radiated Power G/T = Gain-to-Temperature ratio (Rx sensitivity)C/N o = Carrier-to-noise ratioE b /N o = Energy per bit to noise ratioR b or R s = Channel Bit or Symbol RateA = Area of antennaAe = Effective area of antennak = Boltzmanns constantW = Noise bandwidth



Link Budget Calculations - 11. Free Space Loss:

Recall that free space propagation model is used to predictthe signal strength when the Tx and Rx have direct LOSRx is located at some distance d from the Tx

received power is a function of distance, d The power density at some distance, d is

Received power P r , (also known as Friis free spaceequation) is

where Aer is the effective area of the receiving antenna, PL fs =(4 d/ )2 is the free space path loss (a geometric dilution factor)

22( ) / 4

t P p d watts md

2( )4

t er r er

P AP p d A

d

24 /

r fs

EIRP EIRPP d

PLd



Link Budget Calculations - 2The received power, P r , can be written in many other forms

2( ) 4t t er

r

PG A

P d d

2 2

t et er

r

P A A

P d d

24

t et r r

P A GP d

d

2

24

t t r r

PG GP d

d

Antenna effective area Ae and physical area Ap is related by

The gain of the antenna is given by

Antenna effective area for an isotropic antenna (G = 1) is

2

4e A

G

2 / 4e A

2ewhere

c A and

f

total extrated power

incident power flux densitye p A A



Link Budget Calculations - 3Since P r (d) is a function of distance squared, it diminishes asthe distance increases at the rate of 20 dB/decadeThe total attenuation factor, , over the RF link is given by

The dB value of is sometimes called Path Loss (PL)

Path Loss is the signal attenuation in dB between the Tx andthe Rx

2 24 1 ( )t r t r et er

P d d LP G G A A

2

10 2 210log 10log 10 (4 )t t r

r

P G GPL

P d

i k d C l l i 4



2. Thermal NoiseNoise due to fluctuations of electric current or motion of electrons in a conductor It is modeled as AWGNMean Square Value

wherev n = noise voltage

k = Boltzmanns constant, 1.38x10-23

J/K = -228.6dBW/KHzR = resistance, ohmsW = noise bandwidth, Hz

T = absolute temperature, Kelvin

Link Budget Calculations - 4

2

4o

nv kT WR

Li k B d C l l i 5



Available noise power, N , into a matched load is given by

N is independent of frequency (hence the term white) It also does not depend on magnitude of the resistance

However, it depends on ambient temperature of the RxThis leads to the concept of effective noise temperature for noise sources

The max single-sided power spectral density, N 0 , (noise

power in 1-Hz bandwidth) is

This representation is valid for most communication systems

It is usually referred to as non-quantum thermal noise

Link Budget Calculations - 5

2

4nv

N kTW watts R

0 / N

N kT watts HzW



However, in the higher millimeter wave and infrared region,the exact quantum formula is used

Note: at very high freq, thermal noise vanishes leaving onlythe quantum noise (2 nd term)

3. Noise Figure (F)F relates the SNR at input of the network to the SNR atoutput of the networkIt measures the SNR degradation caused by the networkIt is a parameter that expresses the noisiness of a two portnetwork or device, compared to a reference noise source

/

1o

Q hf

kT

hf N hf watts Hz

e



whereS i = signal power at the amplifier input port

N i = noise power at the amplifier input portN ai = amplifier noise referred to the input portG = amplifier gain

F is not an absolute measure of noise but a measure of how

much more noise in the system compared to the referenceThe noise figure of any device is a measure of how muchnoisier the device is than the reference

1

i

ii ai

S N i aiin ai

GS i iout G N N

SNR N N N F

SNR N N



4. Noise Temperature The computation of RF link performance is usually carried out interms of an equivalent system Noise TemperatureIt is a measure of Rx system performance comprising the linkthermal noise, radio noise from atmosphere and outer space,and device noise

It is the temperature used in the terminal figure of merit (G/T s)

Each stage of the receiver is characterized by an effective or equivalent noise temperature, T R

Gain, GBandwidth, W

Noise Figure, FNoise Temperature, T R

TaReceiver Front End

OutputNout

TL

Ts

01 1 290

o RT F T F K


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Individual stages in the system are characterized by a noisefigure, F and a noise temperature, T

So, we talk about composite noise temperature, T comp , and composite noise figure, F comp

G1, F 1, T 1

Ta

OutputNout

TL

TsG 2, F 2, T 2 Gn, F n, T n...

321

1 1 2 1 2 1

ncomp

n

T T T T T

G G G G G G

321

1 1 2 1 2 1

1 11 ncomp

n

F F F F F

G G G G G G


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From these equations, it is evident that the first stage isimportantThe first stage is most susceptible to added noiseIt should have as low a noise figure, F 1, and noisetemperature, T 1, as possibleIt should also have as high gain, G

1, as possible

In most cases, we can model the system as a 2-stagenetwork

Hence the composite noise figure becomes 1compF L L F LF

Feed Line

OutputNout

L = power loss factor

LG,F, T

G = 1/L


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The composite temperature becomes

whereTR = effective noise temperatureT0 = reference noise temperature

Although noise figure is a measure relative to a reference,the noise temperature has no such constraint

5. System Temperature

0 01 1comp RT L L F T T LT

Feed LineNout

TLTR

T A

1 290 1 290

1 290

s A comp A L R

o o A

o A

T T T T T LT

T L K L F K

T LF K


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6. Bit Energy-to-Noise Power ratio, E b /No

In communication system, we required a particular E b /N o toachieve a specified BER

Let M = the margin or safety factor (E b/No)reqd = required value of E b/No (E b/No)r = received or actual value of E b/No

The margin, M , is the difference between required E b /N o and the actual or received E

b /N

o

0 0 0

br E P S R N N N

( ) ( )

0 0

b bdB dB

r reqd

E E M

N N


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It can also be stated as

Hence in dB

Also

0 0 0

b br

r reqd

E E P R M R N N N

0

r

b

GT

E s o N reqd

EIRP M RkL L

0( ) ( ) ( )( )( / ) ( / ) ( ) ( )

b E dB dBW dBr dBi N reqd

dB bit s dBW Hz dB dBs o

M EIRP G

R kT L L


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7. Total Noise Power:The total noise power in satcom is determine not only bythe uplink and downlink noise power, but by factors such as

Intermodulation in the satellite transponders (C/N o)IM Adjacent channel interference (C/N o) AC

Co-channel Interference (C/N o)CCInterference from other sources (C/N o)OTThus the overall noise power can be written as

where

C N

C N

C N

C N o o o ototal uplink downlink noiseH K H K H K H K

C N

C N

C N

C N

C N o o o o onoise IM AC CC OT H K H K H K H K H K


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You have seen that central to Link Budget computation isNOISE

Noise generated by active electron devices (e.g.,intermodulation)Thermal noise inherent in the motion of electronsNoise received from outer space

We have already seen that noise is affected by thetemperature of the systemUsually, we compute the signal-to-noise ratio (SNR)

However, since we are concerned with modulated signal, weoften compute the carrier-to-noise ratio (CNR)

f k


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Summary of Pertinent Link Equations - 1

Wavelength:

Affective Antenna Area:

Antenna Gain:

EIRP:

Lf = total loss of feeder and

duplexer

2

4G

Ae

c f

2

2410 log

2 D

G

t t

f

t t f

PG EIRP

L

P G LdBW dB dB

Flux Density:

Flux Density Received:

when = the the saturation flux

density at satellite can be determinedFree Space Loss:

2

2

2 2 /

earth station4

4

4

fs

dBW s dBdB m

EIRPd

EIRP

PL

EIRP L

24Pd

24 fs

d PL

f k


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Received Power:

Carrier Power:

Thermal Noise Power:

System Noise Temp.:

Carrier-to-noise ratio:

C/N depends onrequired information raterequired SNR or BERmodulation scheme and associatedbandwidth

Summary of Pertinent Link Equations - 2

N kTW o

2

22 4 44t t t

r e

P PG Gt C G Ad d

P G Gt t r Pr PL fs

1

1

s an sf

sf e sf nr

T T L T

T L T L F

G

21

4 /

1

ot t r r

o s

r

fs s

PGP GC N kW N T d

EIRP GPL kW T

0

r

sdB

dB dB

dB dB fs dB

C G EIRP

N T

L W k

M h d f Li k B d A l i


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Analysis Approach1. Define all probable and applicable parameters2. Before computation:

Identify the radio terminals characteristicsi.e., transmitters, receivers, antennas

Specify radio terminals operating characteristicsacceptable loss, efficiency, required power, etc.

Specify communication systems requirement (requiredBER, margin, etc.)The characteristics of the propagation medium

Specify interference and noise properties3. Compute the link budget using assumed or given parameters4. Develop a table for the link budget

Note that link budget are typically in decibels

Method of Link Budget Analysis


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A Case Study

Link Budget Calculation

LEO Satellite Link


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LEO Satellite Link

100~300 Miles

LEO Satellite

Forward Link Reverse Link

Uplink Downlink

User

TerrestrialNetwork

User

TerrestrialNetwork

Part 6: Communication Link Analysis

Overall design of a complete satellite communications systeminvolves many complex trade-offs to obtain a cost effectivesolutions

S l Li k B d A l i


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Sample Link Budget Analysis

Satellite-to-Earth Station

For this link, we have made the following assumptionsGeosynchronous satellite has one spot beam is usedEarth station is covered by the satellite beam, 3 dB contour Band of operation is L-band

Line of sight propagation with no multipath

GEO

User

TerrestrialNetwork

User

TerrestrialNetwork

EIRP up

Ls, Lo

G/T

Part 6: Communication Link Analysis

G/Tdown

HPATransmitter

LNA/LNB

Downlink Path lossRain AttenuationUplink Path lossRain attenuation

P t f I t t 1


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Factors which dominate areDownlink EIRP, G/T and saturated flux density (SFD) of SatelliteEarth Station AntennaFrequency

Interference

Parameters of Interest - 1

Transmit Earth Station Antenna GainPower of Amplifier

UplinkPath LossRain Attenuation

Parameters of Interest 2


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SatelliteG/TEIRP (Equivalent Isotropic Radiated Power)SFD (Saturated Flux Density)

Amplifier Characteristic

DownlinkPath LossRain Attenuation

Parameters of Interest - 2

Receiving Earth Station Antenna GainLNA /LNB Noise TemperatureOther Equipment

Signal Power Calculation


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Antenna Gain

where, = c/f ,

c = Speed of light (c= 3x108

m/s (Velocity of Light))f = frequency of interest = efficiency of antenna (%),

Signal Power Calculation

Antenna Beam width

2

dBid

G

3 70 degreesdBc

df


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EIRPIs the effective radiated power from the transmitting sideand is the product of the antenna gain and the transmittingpower, expressed asEIRP = G t + P t Lf [dB]

where,Lf is the Feed Losses

Signal Power (Pr)P r = EIRP Path Loss + G r (sat) [dB]

where,

and 2D is the Slant Range (m)

4Path Loss dB

D

Noise Calculation


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Noise Calculation

Ts

AntennaT A

LNA

LO

Mixer

TMX GMX

IF AMP

TLNA, G LNA

TIF, G IF

Down Converter

Te

Receiver

T1

Ts

LF

TF Feeder

Antenna

Te

Ta

G/T

Thermal Noise


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Thermal NoiseThermal Noise is the noise of a system generated by therandom movement of electronics, expressed as

where,K= (-228.6 dBJ/K)T= Equivalent Noise Temperature (K)B= Noise Bandwidth of a receiver

Noise Power, No = KTB

Effective Temperature


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Effective TemperatureEffective Temperature

where,T1 = Temperature of LNAT2 = Temperature of D/CG 1 = Gain of LNA

21

1e

T T T G

Noise Temperature


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Noise TemperatureNoise Temperature

where,Tant = Temperature of antennaLf = Feed LossesTf = Feed Temperature

(1 1/ )ant

S f f f

T T L L T



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Being a first stage in the receiving chain, LNA is the major factor for the System Temperature CalculationLowering the noise figure of LNA, lowers the systemtemperature

Antenna temperature depends on the elevation angle from theearth station to satellite

Sys s eT T T

G/T


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G/TG/T (Gain to System Noise Temperature)

This is the Figure of merit of any receiving systemIt is the ratio of gain of the system and system noisetemperature

-10 log( ) [ / ]G

G Tsys dB K T

Link Analysis


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Link AnalysisC/N Uplink

Path Loss Noise BW [ ]e uU sat C G

EIRP K dB N T

C/N Downlink

Path Loss Noise BW [ ]sat d d e

C G EIRP K dB N T

C/N Total

1 1 1 1 1 1

[ ]T U d IM adj xp

C C C C C C dB

N N N I I I

Energy per bit per Noise Power Density


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Energy per bit per Noise Power Density

Eb/No (Energy per bit per Noise Power Density)Is the performance criterion for any desire BERIt is the measure at the input to the receiver Is used as the basic measure of how strong the signal isDirectly related to the amount of power transmitted from theuplink station

Eb /No = (C/N) T + Noise BW Information Rate

Bit Error Rate (BER)


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Bit Error Rate (BER)Why is it used?

To represent the amount of errors occurring in atransmissionTo express the link quality

What is it?BER is an equipment characteristicBER is directly related to E b/NoBER improves as the E b/No gets larger

Probability of error

0

1exp

2b

e

E P

N

Carrier Parameters


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Carrier ParametersSolution - Carrier Performance:

Eb/No ThresholdBit Error Rate (BER)Rain Attenuation

Performance:Typical E b/No values for different FEC

E b/No for FEC 1/2 Eb/No for FEC 3/4 E b/No for FEC 7/8 BER

6.5 (dB) 8.0 (dB) 9.1 (dB) 10 -6

7.1 (dB) 8.7 (dB) 9.7 (dB) 10 -7

7.6 (dB) 9.2 (dB) 10.4 (dB) 10 -8

9.9 (dB) 11.0 (dB) 12.1 (dB) 10 -10

Performance: Application specificDigital voice links:

BER threshold 10 -3

Data links:

BER threshold: 10-4



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Effective TemperaturePerformance - Rain Attenuation:

AvailabilityRain MarginsTypically 99.60 % for Ku-BandTypically 99.96 % for C-Band

Performance - Additional Margins: Adjacent Satellite Interference (ASI)Interference Margins

Link Budget Exercises


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Link Budget ExercisesLink Budget

http://www.wirelessconnections.net/calcs/BudgetCalc.asp

http://my.athenet.net/~multiplx/cgi-bin/wireless.main.cgi

General conversionshttp://www.spectrummicrowave.com/conversions.asp http://www.spectrummicrowave.com/vswr.asp

Examples
http://www.spectrummicrowave.com/conversions.asphttp://www.spectrummicrowave.com/vswr.asphttp://www.spectrummicrowave.com/vswr.asphttp://www.spectrummicrowave.com/conversions.asp


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ExamplesExample 1: Wireless Link to Dial-Up Modem

As an example, consider a datalink intended to provide awireless link between a laptopcomputer and a dial-up modemin a home environment asshown in Figure

In order to support a throughput of 28.8 kbps, the link should bedesigned for about 40 kbps. The additional data rate is needed

to accommodate framing, overhead, checksums which may berequired for the wireless link.

WIRELESS LINK TO MODEM

Examples 1: Requirements


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Examples 1: RequirementsRequired data rate = 40 kbps

(28.8 kbps plus framing, overhead and checksum)Range = 5 metersDesired BER =10 -6

Probability of Bit Error for CommonModulation Methods

Example 1: FCC and ETSI Regulations


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Example 1: FCC and ETSI Regulations For unlicensed systems not employing spread spectrumtechniques, RF power is limited to -1.25 dBm, or about 0.75mW

For details on RF power limitations, refer to FCC Regulations15.247 and 15.249If spreading is employed, RF power can be increased to 1W(U.S. operations)

For Europe, ETSI regulations (ETSI 300, 328) limit RF power for spread spectrum radios to 20 dBm, or 100mWIn Nigeria, you may have to refer to NCC regulationsTherefore, spreading is attractive because it allows for transmission of up to 1000 times more RF power

Example 1: So Should Spread Spectrum


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Example 1: So, Should Spread Spectrum Techniques Be Used In This Case?

Spread spectrum offers some interference rejection properties,but it also entails higher complexitySpread spectrum systems are fairly robust in the presence of multipath

Direct Sequence Spread Spectrum (DSSS) systems willreject reflected signals which are significantly delayedrelative to the direct path or strongest signal

Therefore, the application should first be evaluated todetermine if it can be reliably serviced by a low power, non-spread spectrum radioIf not, then spread spectrum high power radios should beconsidered

Example 1: Frequency Selection


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Example 1: Frequency SelectionThere are several bands available for unlicensed operation(see Table)

Recall that in the Multipath environment, the higher thefrequency, the higher the propagation loss.Hence, a lower frequency is better in terms of propagation loss

It is generally less expensive to build radios at lower

frequencies

World Wide Unlicensed Frequency Allocation RF Power Limits

Example 1: Frequency Selection


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Example 1: Frequency SelectionOther considerations include available bandwidth andregulatory limitations

The available bands are 900 MHz, 2.4 GHz, and 5.725 GHz.The easy choice is 900 MHz, but this band is getting crowdedwith things like cordless phonesFor such a short link, 900 MHz is still a good choice

Example 1: Modulation Technique


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p qMost radio chip set uses Phase Shift Keying (PSK)modulation, but some of the motivating factors behindthis choice are not applicable in this instance

A simpler method is Frequency Shift Keying (FSK)With FSK, two separate frequencies are chosen, onefrequency representing a logical zero, the other representing logical one. Data is transmitted byswitching between the two frequencies

So a good choice of modulation would therefore beFSK

Example 1: Modulation Technique


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p qThe separation of two frequencies relative to the bit rate iscalled modulation index (h)

h = frequency separation / bit rate = f / R A modulation index of 1 (h = 1) is a good choice for a lowcost application, unless there are restrictions on bandwidth

When h = 1, the frequencies are said to be orthogonal.This form of modulation is called Orthogonal FSK (OFSK)Choosing h = 1 results in a simple but fairly robust receiver design

In this case, the frequencies would be separated by40 kHz

Why?

Example 1: System Bandwidth & Noise Floor


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p y

Generally, modulation technique dictates the required systembandwidth (or visa versa, depending on design constraints)

For FSK modulation and h = 1, the bandwidth is typically about2 times the data rate (see Table), or 80kHz.

We can now compute the noise power as follows:N = kTB

= 1.38 x 10 -23 J/K x 290K x 80,000 s -1

= 2.4 x 10 -13 mW

= -126dBm

Typical Bandwidth for Various Digital Modulation Schemes

Example 1: System Bandwidth & Noise Floor


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p y

This figure represents a theoretical noise floor for an idealreceiver

A real receiver noise floor will always be higher, due to noiseand losses in the receiver itself

Noise Figure (NF) is a measure of the amount of noise addedby the receiver itself

A typical number for a low cost receiver would be about 15dB

This number must be added to the thermal noise to determinethe receiver noise floor:

Receiver Noise Floor (RNF)

RNF = -126dBm + 15dB

= -111dBm

Example 1: Receiver Sensitivity


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Example 1: Receiver SensitivityThe first step in performing the link budget is determining therequired signal strength at the receiver input

This is referred to as receiver sensitivity (Prx)This is a function of the Modulation Technique and the desiredBERFor this case, the modulation technique is OFSK and requiredBER = 10 -6

For 10 -6 BER, we have:

Eb/N

o= 14.2 dB = 26.3

SNR = (E b/No) * (R/B T)= 26.3 * (40 kbps/ 80 kHz)= 11 dB

Example 1: Receiver Sensitivity


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Example 1: Receiver SensitivityPrx = Receiver Noise Floor + SNR

= -111 dB + 11 dB= - 100 dBm

Example 1: Fade Margin


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a p e : ade a gIn an indoor environment, multipath is almost always presentand tends to be dynamic (constantly varying)

Severe fading due to multipath can result in a signal reductionof more than 30dBIt is therefore essential to provide adequate link margin toovercome this loss when designing a wireless system

Failure to do so will adversely affect reliability.

The amount of extra RF power radiated to overcome thisphenomenon is referred to as fade margin

The exact amount of fade margin required depends on thedesired reliability of the link, but a good rule-of-thumb is 20dBto 30dB

Assume 30dB for this design

Example 1: Link Calculation


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pPropagation loss (Part Loss) can be computed as:

Note that lambda is the free space wavelength at the carrier frequency

= c/f

= 3 x 108

ms-1

/900MHz= 0.33 meters

PL fs = 20 x log 10 (4*pi*d/ ) = 20 x log 10 (4*pi*5 meters/0.33 meters)= 46 dB

Example 1: Link Calculation


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pFinally, some assumption must be made about transmitand receive antenna gain valuesFor a simple dipole antenna, an assumption of 0 dB gainis reasonableThis number will be taken for the gain of both the transmitantenna gain (G

tx) and receive antenna gain (G

rx).

Now, the required transmitter power (Ptx) can becomputed:

P tx = P rx G tx G rx + PL fs + Fade Margin= - 100dBm 0dB 0dB + 46dB + 30dB= - 24dBm

Example 1: Conclusions


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pThis exercise shows that the wireless modem link can bereliably served by an OFSK radio operating at 900MHz using

as little as -24dBm transmit power. FCC regulations permitting

Exercise


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Universal Serial Bus (USB) is rapidly replacing the serial porton personal computers. USB provides high speed flexibleinterconnectivity between a PC and its peripherals. Despite itsflexibility, USB has a range limitation of 5 meters.

USB has two modes of signaling. The full speed signaling rateis 12Mbps, while the low rate is 1.5Mbps. The low speed rate isdesigned to support devices such as mice and keyboards.However, a radio capable of providing 1.5Mbps throughput

could be used in a wireless hub application, though it could notsupport the full hi-speed rate of 12Mbps. A wireless hub could support bulk transfers, and possiblyisochronous applications such as wireless audio if ratebuffering were available at the transmit side of the link.

Link Budget for Wireless USB

Exercise


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Design the Link budget for a wireless digital link capable of 1.5Mbps throughput at up to 100 feet indoors. A somewhathigher data rate will be required in order to accommodateframing, overhead, and checksum for the wireless link.

Typically, throughput is about 70% to 75% of peak data rate.Therefore, the required data rate for the wireless link is roughly2Mbps. The requirement are as follows:


Exercise


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The requirement are as follows:


Data Rate = 2Mbps(1.408Mbps + framing,

overhead, checksum)Range = 30 meters indoors(100 feet)Desired BER =10 -6

rfm_pdd day 5-b

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