msc4-linkdesign-16spet03-5

EE5404 link design 1

September 16, 2003

Satellite CommunicationsSatellite Communications

Chen, Chen, ZhiZhi NingNing

e-mail: [email protected]: http://www1.i2r.a-star.edu.sg/~chenzn


7 Link Design7 Link Design

7.1 Introduction

7.2 RF Link Design• carrier-to-noise ratio C/N• uplink• downlink• overall system C/N• intermodulation noise • interference*


7.1 Introduction: 7.1 Introduction: general descriptiongeneral description

satellite system desired destination

signal quality Carrier-to-Noise ratio, C /N

proper link parametersSo far, we have discussed the important issues related to the RF link of a satellite communication systems. To deliver the messages with the acceptable quality to the desired destination, we should conduct the link design using carrier-to-noise ratio. The C/N will be determined by properly choosing various link parameters, which affect the RF link design.Ea

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7.1 Introduction: 7.1 Introduction: general descriptiongeneral description

uplin

k

downlink

In satellite systems, the RF link is the source-to-destination path. It includes the uplink—from a transmitting earth station to a satellite, the satellite path within the satellite, and the downlink—from the satellite to a receiving earth station. Through the RF link, the message is delivered from the earth station A to the earth station B. The satellite in the sky acts a relay and greatly extends the communication coverage.

Satellite path

Satellite

Earth station BEarth station A


7.1 Introduction: 7.1 Introduction: general description: earth stationgeneral description: earth station

As mentioned above, the link design involves many factors such as the earth station related factors. They include •Geographical location rain attenuation, satellite look angle, satellite EIRP, path-loss

•Transmit antenna gain & power earth station EIRP

•Receive antenna gain & system noise temperature G/T and sensitivity of the receiver at an earth station

•Intermodulation noise the carrier-to-noise ratio•Equipment characteristics (eg. de-modulator implementation budget, cross-polar

discrimination, filter characteristics) the additional link budget

G/T: the ratio of the receive antenna gain to the system noise temperature


7.1 Introduction: 7.1 Introduction: general description:satellitegeneral description:satellite

Also many factors are satellite related, such as•Satellite location: coverage region and earth station look angle•Satellite transmit antenna gain & radiation pattern: satellite EIRP of a transmitter at the satellite and coverage region•Satellite receive antenna gain & radiation pattern: G/T of a receiver at the satellite and coverage region•Transmitted power : EIRP of a transmitter at the satellite •Transponder gain and noise characteristics: EIRP of a transmitter & G/T of a receiver at the satellite. •Intermodulation noise: carrier-to-noise power at a receiver of the earth station


7.1 Introduction: 7.1 Introduction: general description: radio channelgeneral description: radio channel

The radio channel related factors include

•Operating radio frequency: path loss & link budget

•Modulation/coding characteristics: carrier-to-noise ratio

•Propagation characteristics: link margin & scheme of modulation/coding

•Channel: total inter-system noise

Earth station relatedSatellite relatedRadio channel related

link design of

overall system


7.1 Introduction: 7.1 Introduction: general description: RF linkgeneral description: RF link

HPA up-converter modulator

LNAdown-

converterde-

modulatorHPAup-

convertermodulator

LNAdown-converter

de-modulator

RF LINK

C/N

C/Ndown-

converterdown-

converterbasebandbase

band

RF LINK

A typical satellite communication system, including all the basic elements, where the connection is built up via a satellite in space. The RF link covers all the channels between the modulator of the transmitter at an earth station to the demodulator of the receiver in the destination earth station. The target of RF link design is to estimate the C/N at the input of a receiver.


7.1 Introduction: 7.1 Introduction: frequency allocation

In addition, the frequency allocations for satellite services are mainly dependant on RF link losses, the state of technology, and economy. The studies on atmosphere effects and antenna noise have shown that the frequencies used for satellite systems should be allocated in the range of 1-12GHz. At lower operating frequencies, the devices may be larger. The equipment will be larger and heavy. This results in the higher cost especially for satellites.

frequency allocation

FSS- C band: ~6 GHz uplink , ~4 GHz downlink - INTELSAT- X band: ~ 8 GHz uplink , ~7 GHz downlink- government- Ku band: ~ 14 GHz uplink , ~12 GHz downlink- EUTELSAT, Telecom I & II -INTELSATVSAT – Very Small Aperture Terminals (C and Ku band)MSS-L band ~ 1.6 GHz uplink, ~1.5 GHz downlink for the mobiles -Ku band for Network Control Centre and Hubs.DBS- Ku band: ~12 GHz downlink Frequencyuplink > Frequencydownlink


7.1 Introduction: 7.1 Introduction: frequency allocationfrequency allocation

Within a specific band such as C-band, the higher frequency is always allocated for uplink and the lower ones for downlink.

In the uplink, the RF signals will be transmitted by the antennas at the earth stations and received by antennas at satellites. Use of higher operating frequencies allows the physically smaller receive antennas at the satellites.

In the downlink, the RF signals will be transmitted by the antennas at the satellites and received by the antennas at the earth stations. The physical size of the antenna at the satellite is unchanged but the gain will decrease due to the lower operating frequencies compared to the antenna in transmitting mode (electrically smaller size). The higher gain of the larger receive antennas can compensate for this reduction in the gain of the satellite antenna.


7.2 RF link Design: 7.2 RF link Design: carriercarrier--toto--noise ratio (noise ratio (C/NC/N))

To measure the performance of the RF link, we define an important parameter — carrier-to-noise ratio (C/N). We use the C/N to conduct the RF link design. The carrier-to-noise ratio is the ratio of carrier power to noise power at any point of the link.

N

c

PPNC =

(7.1)dBNc PPNC −=

Particularly, the carrier-to-noise ratio at the input of the de-modulator (receiver) is the most important in a system design because the goal of the RF link design is to achieve the desired carrier-to-noise ratio at the input of the receiver at the destination earth station.



In general, we don’t consider the bandwidth of the receiver to generalise the discussion. So, the carrier-to-noise spectral density (or carrier-to-noise density) is often used in link budget.

HzdBNNNco BNCBPPNC +=+−= (7.2)

at output: Pc, PNat input: Pc, PN

LNAG1, Te1

antenna Tantreceiver

FcableL:1 load

The carrier power is the received signal power by the antenna and then transmitted to the receiver via a network including a transmission system (cable or waveguide), LNAs and other lossy networks (transmission system). So, the C/N can be defined at the input or output of the receiver.



The carrier is the power received by a receiver. As discussed in Chapter 5, the receiver power should include the EIRP at the transmitter and all the losses occurring in the RF channel.

C=Pc=EIRP-LOSSES dB

On the other hand, the noise power at the input of the receiver include all the noise generated within the RF channel. They have been discussed in Chapter 6.

N=Pn=κTsBn dB or No=Pn=κTs dBHz

So, the C/N at the receiver can be expressed as, C/N=EIRP+Gr-κTsBn-LOSSES dB

or C/No=EIRP+Gr-κTs-LOSSES dBHz or C/No= EIRP+Gr/Ts-κ -LOSSES dBHz (7.3)


So, we can use (7.3) to evaluate the C/N and assess the performance of a satellite system. C/N= EIRP+Gr-κTsBN -LOSSES dB (7.3)

LNAGa, Ta

Input C, N

antennaGr, Tant

receiverF

cableL:1

RF signalsEIRP

receiving system

load

7.2 RF link Design: 7.2 RF link Design: C/NC/N--exampleexample

transmit system receive system RF channelEIRP=Gt+Pt dB Gt: gain of the transmit antenna; Pt: the transmitted power

Gr= Gr: gain of the receive antenna

κTsBN=κ+Ts+BN dB; κ=1.38×10-23 J/K; Boltzmann’s constantTS: equivalent system noise temperature; BN: operating bandwidth

LOSSES=PL+RFL+AML+AA+PML dB: all the losses in RF link

2em4λ

π Ae rad


7.2 RF link Design: 7.2 RF link Design: C/NC/N--example 1example 1

Example:A satellite link operating at 12GHz has the receiver of G/T=19dB and receiver feeder loss is 1 dB. The free-space loss is 206dB. The atmospheric absorption loss is 2dB, and the antenna pointing loss is 1dB. EIRP is 48 dB. Depolarization loss may be zero. Calculate C/No.

Solution:The total link loss is the sum of all the losses.LOSSES=PL+RFL+AML+AA=206+1+1+2=210 dBκ=-228.6 dBC/No= EIRP+G/T-κ-LOSSES=48+19+228.6-210=85.6 dBHz #

transmitterEIRP

RF channellosses

receive antennaGr

receiver networkthermal noise Ts

C/No= EIRP+Gr/Ts -κ-LOSSES dBHz (7.3)


7.2 RF link Design: 7.2 RF link Design: C/NC/N--example 2example 2

C/N= EIRP+G /T-κ-LOSSES-Bn dB

Example: In a link budget operating at 12GHz with a 10-MHz bandwidth, the free-space loss is 208dB, the antenna pointing loss is 1dB, and the atmospheric absorption loss is 2.5dB. The satellite receiver G/T ratio is 19dBK-1 and feeder losses are 1.2dB. The earth station ERIP is 49dB. Calculate C/N.

Solution:ERIP=49 dB; G/T=19 dB; κ=-228.6 dBKLOSSES=PL+RFL+AML+AA+PML=208+1.2+1+2.5=212.7 dBC/No= 49+19 +228.6-212.7= 83.9 dBHzBn=10log107=70 dBHzC/N= C/No – Bn= 83.9–70= – 13.9 dB


Satellite path

Satellite

uplin

k

downlink7.2 RF link Design: 7.2 RF link Design: uplinkuplink

As mentioned, the radio link of satellite communication systems consists of uplink, satellite path and downlink. So, the link design includes the C/N calculation for each link. First, we discuss the C/N in the uplink where the earth station transmits the signals andthe satellite receives them. We use eq. (7.4) for the calculation. Earth stations

C/NoU= EIRPU +G /TU -κ-LOSSESU dBHz (7.4)

[PL]+[RFL]+[AML]+[AA ]+[PML]

channel

Of the parameters, the EIRP is for the earth station, the G/T and receiver feeder losses are for the satellite receiver.

The path loss occurs in free-space and the atmospheric attenuation in radio channel. All the losses are frequency-dependent and can be calculated at the uplink frequency.

The antenna misalignment loss and polarization mismatch loss are related to both transmit antenna at the earth station and the receive antenna at the satellite.

receiversatellite

earthstation

free-space receiversatellite

transmit antenna at earth station & receive antenna at satellite

all frequency-dependent!!!


C/NoU= EIRPU -LOSSESU dBHz

7.2 RF link Design: 7.2 RF link Design: uplink=uplink+satellite pathuplink=uplink+satellite path

feednetwork

widebandreceivers

feednetworkcoupler transmitter

C/NoU= EIRPU +G /TU -κ-LOSSESU dBHz

Uplink=uplink +satellite pathAs described above, the uplink is the link between the transmit antenna at the earth station and the receive antenna at the satellite. So, the losses exclude the losses after the receive antenna.Usually, the EIRP at the transmit antenna is determined by the amplifier at the satellite. So, we consider the satellite path before the amplifier as part of the uplink. So, the losses should include the receiver feed losses after receive antenna.

Sometimes, we even consider all the satellite path as the part of the uplink.


7.2 RF link Design: 7.2 RF link Design: uplink:transponderuplink:transponder

In the uplink design, we are going to calculate the power flux density illuminating at the receive antenna not EIRP at the transmitter. The power flux density is determined by the requirement from an amplifier in the transponder.Transponder: The transponder is the series of interconnected units which forms a single communication channel between the receive and transmit antennas at a communication satellite.The figure shows a transponder consists of some basic elements, such as input filter, wideband receiver, 3-dB coupler, demultiplexer, attenuator, TWTA and multiplexer. The TWTA, travelling-wave tube amplifier is a power amplifier in a satellite transponder.

receive antenna

inputfilter

widebandreceiver

3-dBcoupler demultiplexer multiplexer

transmit antennaattenuator TWTA

Transponder


7.2 RF link Design: 7.2 RF link Design: uplink:transponder: TWTAuplink:transponder: TWTA

TWTA: travelling-wave tube amplifier TWTAs are widely used in transponders to provide the final output power required to the transmit antennas at the satellite.TWTA can provide the same amplification over a very wide bandwidth (a wideband amplifier). However, its power transfer characteristics may be nonlinear when its input power level is higher than certain value as shown. So, the input power levels to a TWTA must be carefully controlled to minimize signal distortion due to its non-linearity. The point of the maximum output power Pout, max is a saturation point, where the corresponding input power is called saturation power Ps.The region between a thermal noise limit point and a 1-dB compression point is a linear region. At the compression point, the output power level will drop 1dB below the extrapolated straight line. So, due to the power transfer characteristics of the TWTA, we should control the input power levels to avoid signal distortion.

Pout saturationpoint

1dB

Pout, max

thermal noise limit

Ps PinLinear region


7.2 RF link Design: 7.2 RF link Design: uplink:saturation flux densityuplink:saturation flux density

In RF link design, we must take saturation of a TWTA it into account. To describe the effect of the saturation power on the link design, we define the power flux density required at the receive antenna to produce the saturation of the TWTA called saturation flux density, ψ. The saturation flux density ψ is a specified quantity in link budget calculations. After obtaining it, we can calculate the required EIRP at the earth station.

Assuming that the minimum saturation flux density of the TWTA is ψm. It is the power flux density at a receive antenna when the transmission is in free-space. We do not consider any possible losses between the transmit antenna at an earth station and the receive antenna at the satellite. So, we can calculate the required EIRP at the earth station. The R is the distance between the earth station and the satellite.

22m m/W

4EIRPΨ

Rπ= (7.5)



2m 41log10EIRPRπ

+=Ψ dBW/m2

2

2

2

41log10

4log10

4log10PL

R

R

ππλ

λπ

+=

−=−

πλ4

log102

=oA fA log2045.21o −−=

πλ4

log10PLEIRP2

m −−=Ψ

We also rewrite it in dB form. Consider the definition of the path loss in free-space, we can rewrite it.

Further we define the last term as the effective area of an isotropic antenna Ao (with G=1 or 0dB) and re-arrange the expression. The Ao is only frequency-dependent.

This formula can be used to calculate the required minimum EIRP transmitted by the earth station under the saturation flux density ψm appearing at the receive antenna at the satellite.

dB

dBW/m2

or f in GHzdBm2

EIRP=Ψm+PL+Ao dBW



Based on previous discussions, we should consider other link losses, such as AA, AML, PML. Therefore, the EIRP should includes the compensation for the losses.

AMLPMLAAPLEIRP oSS +++++Ψ= A dBW (7.6)

The ERIPS at the earth station is the minimum value for the saturation flux density at a satellite. This formula is used to evaluate the minimum EIRP which the earth station must provide to produce a given saturation flux density at a satellite receive antenna. In other words, to produce a given saturation flux density at a satellite receive antenna, the earth station must provide the minimum ERIP calculated in (7.6).


7.2 RF link Design: 7.2 RF link Design: uplink:example 3uplink:example 3

ProblemAn uplink operates at 14 GHz, and the flux density required to saturate the transponder is –120 dB W/m2. The free-space loss is 207 dB and the other propagation losses amount to 2 dB. Calculate the earth-station EIRPS required for the saturation.

SolutionEIRPSU=ΨS+Ao+PL+LOSSES

At 14GHz, Ao= –21.45-20log f = –(21.45+20log14)= –44.37dBm2

The losses in the propagation path amount to 207+2=209 dB. So, EIRPSU= –120 –44.37+209=44.63 dBW


7.2 RF link Design: 7.2 RF link Design: uplink:uplink:CC//NN at a receiverat a receiver

EIRPSU=ΨS+Ao+PL+AA+AML+PML dBW (7.6)

C/No= EIRP+Gr/Ts -κ-LOSSES dBHz (7.3)

Then, we can calculate the carrier-to-noise density ratio at the input of the receiver at the satellite. Please note that the losses in (7.3) include the RFL. Substituting (7.6) for EIRP in (7.3) gives

C/No=ΨS+Ao+PL+AA+AML+PML+Gr/Ts-κ-LOSSES= ΨS+Ao +Gr/Ts-κ-RFL dBHz (7.7)

The (7.7) shows that we can calculate the carrier-to-noise density ratio at the input of the receiver at the satellite after determining the saturation power flux density. This calculation is independent of the link from an earth station to the receive antenna at a satellite because the saturation power flux density is defined at the receive antenna at the satellite.

The term ΨS+Ao is required by the TWTA at a receiver but determines the transmitted EIRP at an earth station with losses together. The term Ts-κ-RFL is determined by the receiving system.


7.2 RF link Design: 7.2 RF link Design: uplink:input uplink:input backoffbackoff

As mentioned above, the transfer characteristics of the TWTA is not always linear. When the input power level is higher than certain value, the transfer characteristics will be nonlinear. For multiple carrier operations, this non-linearity results in intermodulation (noise!). So, the TWTA must operate at really linear region to suppress the effect of intermodulation and TWTA must back off its operating point from the saturation point (in nonlinearportion for single carrier!) to a linear portion of the transfer characteristics. So the difference between the saturation EIRP and the required EIRP is termed input backoff (BOin).

Pout

Pout max0dB

1-dB compression

point

backoffoperation

pointbackoff

saturationpoint

Ps0dB

Pin EIRPU=EIRPSU-BOinLinear region

•EIRPU : saturation EIRP for multiple-carrier•EIRPSU : saturation EIRP for single carrier•BOin :input backoff


7.2 RF link Design: 7.2 RF link Design: uplink:input uplink:input backoff

The truth is that we should design the proper EIRP. Too low EIRP can’t meet the saturation power flux density to waste the TWTA capability. However, too high EIRP will cause the waste of the EIRP for single-carrier or the intermodulation for multi-carrier.

We can modify the uplink budget again due to the input backoff. The carrier-to-noise density ratio (7.7) at the input of the receiver can be calculated in (7.8). The input backoff is taken into account.

backoff

C/NoU= ΨS+Ao +Gr/Ts-κ-RFL-BOin dBHz (7.8)


7.2 RF link Design: 7.2 RF link Design: uplink:input uplink:input backoffbackoff:example:example

Problem:An uplink at 14GHz requires a saturation power flux density of 90dB dB W/m2 and an input backoff of 11dB. The satellite G/T is -7dBK-1, and receiver feeder losses amount to 0.5dB. Calculate the carrier-to-noise density ratio.

Solution:The carrier-to-noise density ratio at the input of satellite: (without path loss and other fade losses)C/NoU= ψs +Ao -BOin +G /TU -κ-RFL dBHz Ao=-44.37dB, Saturation power flux density ψs = -90dB, input backoff BOin=11dB, satellite G/T= -7dBK-1, -κ=228.6dB, RFL=0.5dB.

C/NoU= -90-44.37-11-7+228.6-0.5=75.73 dBHz


7.2 RF link Design: 7.2 RF link Design: uplink:HPA+TFLuplink:HPA+TFL

In the uplink design, we often design the EIRP at the transmit antenna. To supply the transmitted power required by a system, a high-power amplifier HPA must be used to supply the required output EIRP at the earth station. If an HPA has the power gain of Gt and the input power PHPA, the EIRP can be obtained

EIRP=PHPA+GT dBW

Another important loss TEL, transmit feeder losses should be considered when we design the link between the input of the HPA and the antenna at the earth station. This loss is similar to the receive feeder losses and occurs in the transmitter at the earth station. For a typical earth station, the transmit feeder losses may include waveguide, filter and coupler losses between the output of the HPA and the transmit antenna. So, the EIRP at the transmit antenna can be calculated.

EIRP=PHPA+GT -TFL dBWPHPA =EIRP-GT +TFL dBW


7.2 RF link Design: 7.2 RF link Design: uplink:HPA+TFLuplink:HPA+TFL

In an earth station!

TFL

waveguidefiltercouplerHigh-power

amplifier

Transmitantenna

ERIPPHPA PHPA+GT PHPA+GT-TFL


7.2 RF link Design: 7.2 RF link Design: uplink:HPAuplink:HPA→→TWTATWTA

PHPA=EIRP-GT+TFL

Transmitterat earth station transponder

EIRP=PHPA+GT-TFL

PL+LOSSES(AA+PML+AML)

EIRPSU=ΨS+PL+Ao+LOSSES -BOin

ΨS

for multi-carrierfor single carrier

for single carrier

EIRPSU=ΨS+Ao+RFL-BOin

for multi-carrier

This figure shows the EIRP at different points of an uplink. We can calculate the required input power of an HPA to supply the required EIRP at the transmit antenna at an earth station. Based on the saturation power flux density required by a TWTA, we can calculate the required EIRP at the transmit antenna at an earth station or at the receive antenna at the satellite for single and multiple carrier.


7.2 RF link Design: 7.2 RF link Design: uplink:HPAuplink:HPA→→TWTATWTA

for single carrier

C/NoU= ΨS+Ao +Gr/Ts-κ-BOin dBHz (7.8)

transponder

for multi-carrier

C/NoU= ΨS+Ao +Gr/Ts-κ-RFL-BOin dBHz (7.8)

for single carrierfor multi-carrier

Then, we can calculate the carrier-to-noise density ratio at different points based on the calculated ERIP.


Satellite path

Earth stations

Satellite

uplin

k

downlink7.2 RF link Design: 7.2 RF link Design: downlinkdownlink

Similar to the analysis of uplink+satellite path, downlink can be evaluated based on the C/N calculation. We also use eq. (7.4) for the calculation.

C/NoD= EIRPD +G /TD -κ-LOSSESD dBHz (7.4)

[PL]+[RFL]+[AML]+[AA ]+[PML]

channel

receiverearth station

satellite Of the parameters, the EIRP is for the transmit antenna at a satellite, the G/T and feeder losses are for the receiver at an earth station.

The path loss occurs in free-space and the atmospheric attenuation in radio channel. All the losses are frequency-dependent and can be calculated at the uplink frequency.

The antenna misalignment loss and polarization mismatch loss are related to both receive antenna at the earth station and the transmit antenna in the satellite.

receiverearth station

free-space

receive antenna at earth station & transmit antenna at satellite

all frequency-dependent!!!


7.2 RF link Design: 7.2 RF link Design: downlink: example 4downlink: example 4

ProblemA satellite signal occupies the bandwidth of 40MHz, and it must provide a C/N at the destination earth station of 25dB. Given that the total transmission losses are 200dB and the destination earth station G/T ratio is 31 dB/K. Find the satellite EIRP required.

Solution: EIRPD =C/N-G /TD +κ+LOSSESD +B [J=W/Hz, κT=No (J=W/Hz)]B=10log(4×107) =76 dBHz; C/N=25dB; -G /TD= -31dBK-1; κ= -228.6dBJK-1; LOSSESD=200dB.The required satellite EIRP = 25-31-228.6+200+76=41.4 dBW


7.2 RF link Design: 7.2 RF link Design: downlink:output downlink:output backoffbackoff

Pout saturationpoint

5 dB

backoffoperation

point

inputbackoff

BOin

outputbackoffBOout

The satellite EIRP depends on the saturation output power of the TWTA for the single carrier operation. For the multi-carrier operation, the input backoff is necessary to keep the TWTA operate in its linear region for the depression of the intermodulation. The input backoff results in the difference between the saturation output power and the output power at the operation point in the linear portion. The difference is termed output backoff. The output backoff is not linearly related to the input backoff. Often, the relationship between the input and output backoff is

BOout=BOin-5dB,

where linear portion has a 1:1 change in dB. So, the EIRP is EIRPD=EIRPSD-BOoutPinLinear portion

•EIRPD : output EIRP for multiple-carrier•EIRPSD : saturation output EIRP for single carrier•BOout : output backoff


7.2 RF link Design: 7.2 RF link Design: downlink:at earth stationdownlink:at earth station

The C/No at a receive antenna of an earth station can be calculated.

C/NoD= EIRPSD –PL +G /TD –κ – LOSSESD – BOout dBHz (7.9)

Where the EIRPSD is the EIRP produced by the transmit antenna at the satellite for single carrier operation. G /TD is for the receive antenna of the earth station. LOSSESD includes all the path losses such as PL, AA, AML, PML. BOout is the output backoff for the multi-carrier operation, which should be allowed.

If considering the receive feeder loss, RFL occurring between the receive antenna and the receiver at the receiving earth station, the C/No can be rewritten as

C/NoD= EIRPSD –PL +G /TD –κ – LOSSESD –RFL – BOout dBHz (7.9)


7.2 RF link Design: 7.2 RF link Design: downlink:example 5downlink:example 5

Problem:For a multi-carrier system, the specified parameters for a downlink are satellite saturation EIRP of 25dBW; output back of 6dB; path loss of 196dB; allowance for other losses of 1.6dB; and earth station G/T is 41dBK-1. Find the carrier-to-noise density ratio at an earth station.

Solution:The carrier-to-noise density ratio at the earth stationC/NoD= EIRPSD-BOout +G /TD-κ-LOSSESD –RFL dBHzEIRPSD=25dB, PL=-196dB, output backoff: BOout=6dB, G/T=41dB, -κ=228.6dB, RFL=0dB, other losses:LOSSESD=1.6dB.

C/NoD=91 dBHz


7.2 RF link Design: 7.2 RF link Design: downlinkdownlink

Receiver of earth station

single carrier

EIRPD=EIRPSD-BOout

Transmitterat a satellite

C/NoD= EIRPSD –PL +G /TD –κ – LOSSESD – BOout

multi-carriersingle carrier

C/NoD= EIRPSD –PL +G /TD –κ – LOSSESD –RFL – BOout

multi-carrier

This figure shows the EIRP at the different point of the downlink. We can calculate the output EIRP at the transmit antenna at the satellite for both single and multiple carrier operations.


7.2 RF link Design: 7.2 RF link Design: overall link=uplink+downlinkoverall link=uplink+downlinkNoU NoD

The complete system link consists of the uplink and downlink. On the uplink, noise density NoUis applied at the satellite receiver input with the receive carrier CU at the same point. So the carrier-to-noise density ratio on the uplink is CU/NoU not in dB.

The received carrier power for the downlink is C. It is G times of the input carrier power at the satellite. G is the system power gain from the satellite input to the earth station input, including the satellite transponder and transmit antenna gain, the downlink losses, and the earth station receive antenna gain and receiver feed losses.

The noise density also appears at the receiving earth station input which is also multiplied by G, and in addition, the earth station introduces its own noise density including all the noise caused on the downlink, denoted by NoD. So the noise density at the end of the overall link is GNoU+ NoD.

+CU

GC = G CUNo= GNoU+ NoD

+


G C = G CUNo= GNoU+ NoD

No/C =(G NoU+ NoD )/C

=(G NoU )/ C + NoD /C

=(G NoU )/ (G C U) + NoD /C

= NoU / CU + NoD /C

=(No/C)U +(No/C)D (7.10)

where (No /C)D=NoD/CD =NoD/C and No/C is

noise density to carrier ratio for overall system.

After determining the carrier-to-noise ratios for both uplink and downlink, we can evaluate the carrier-to-noise ratio.

For convenient, we describe the link with the noise density-to-carrier ratio instead of the carrier-to-noise ratio, not in dB.

So, we can obtain the noise density-to-carrier ratio, namely the total noise density over the total noise density at the input of the receiver of the receiving earth station for the overall link.

It is clear that the overall noise density-to-carrier ratio is simply the sum of the relevant noise-to-carrier ratio of both uplink and downlink.

Then, we can obtain the carrier-to-noise ratio by combining the carrier-to-noise ratios of the uplink and downlink.

7.2 RF link Design: 7.2 RF link Design: overall link=uplink+downlinkoverall link=uplink+downlinkNoDNoU

CU+ +

Power flow diagram in Uplink +Downlink


7.2 RF link Design: 7.2 RF link Design: overall link:example 6overall link:example 6

Problem:For a satellite system the individual link carrier-to-noise spectral density ratios are: uplink 100 dBHz; downlink 87 dBHz. Find the combined carrier-to-noise ratio.

Solution:No/C=(No/C)U +(No/C)D (7.10)No/C=(No/C)U +(No/C)D=10-10+10-8.7=2.095×10-9 Hz-1

C /No=10log[1/(No/C)]=-10log(2.095 ×10-9 )=86.79 dBHz

The combined C/No approaches to the lower one when one of the ratios for uplink and downlink is much less than the other.


7.2 RF link Design: 7.2 RF link Design: intermodulation intermodulation noise

In the previous discussion on the noise, we just discussed thermal and antenna noise. In practice, we should consider the intermodulation noise. The intermodulation occurs where multiple carriers pass through any device with non-linear characteristics. In satellite communication systems, it most commonly occurs in the TWTA in the satellite as discussed above. Both amplitude and phase nonlinearities cause the intermodulationproducts.

The noise due to Intermodulation products in the transponder is defined as another C/N ratio, which should be considered in the C/N for overall system. The carrier-to-intermodulation-noise ratio is usually found experimentally, or in some cases it may be determined by simulation. Once this ratio is known, it can be combined with the carrier-to-thermal-noise ratio discussed above.

noise

No/C=(No/C)U +(No/C)D+ (No/C)IM (7.11)


7.2 RF link Design: 7.2 RF link Design: intermodulation intermodulation noisenoise

f1

TWTAamplitude & phase

non-linearities

Intermodulation noise

f2

fn

Carrier-to-Intermodulation noise ratio

experiments computation

(No/C)IM

∆f

3rd order intermodulation products from f1 and f2

f1 f2(2f1-f2) (2f2-f1)

Figure shows the third-order intermodulation products fall on neighbouring carrier frequency. They result in the interference there.


7.2 RF link Design: 7.2 RF link Design: intermodulation intermodulation noise: example 7noise: example 7

Problem:For a satellite system the carrier-to-noise ratios are: uplink 23dB; downlink 20dB, intermodulation 24dB. Find the overall carrier-to-noise ratio.

Solution:No/C=(No/C)U +(No/C)D+ (No/C)IM (7.11)The overall noise-to-carrier ratio:N/C=10-2.4+10-2.3+10-2=0.0019So, the overall carrier-to-noise ratio in dBC/N=10log(N/C)-1= -10log(0.0019)= -27.2 dB


7.2 RF link Design: 7.2 RF link Design: interference*interference*

earthStation 1

earthStation 2

F

C1

E

C2

A1

A2

B2

B1

Satellite 1 Satellite 2 In addition, there are many ways to cause the interference between different sources in the same or different networks. as shown in FigureA1 Terrestrial station transmissions, causing interference to reception by earth stations. A2 earth station transmissions, causing interference to reception by terrestrial stations. B1 Space station transmissions of one system, causing interference to reception by earth stations of another system.B2 Earth station transmissions of one system, causing interference to reception by space stations of another system.C1 Space station transmissions, causing interference to reception by terrestrial stations.C2 Terrestrial station transmissions, causing interference to reception by space stations.E Space station transmissions of one system, causing interference to reception by space stations of another system.F Earth station transmissions of one system, causing interference to reception by earth stations of another system.

terrestrialstation


7.2 RF link Design: 7.2 RF link Design: interference*interference*

If we consider the interference into the link budget, the link budget equation for the overall system can be modified as the new one.

The interference can be caused in both uplink and downlink.

Intermodulation Noise Downlink-interference

OverallSystemLink No/C=(No/C)U +(No/C)D+ (No/C)IM +(No/C)IU +(No/C)ID

Downlink Uplink-interferenceUplinkOR

C/No=[(No/C)U +(No/C)D+ (No/C)IM +(No/C)IU +(No/C)ID]-1


ConclusionConclusion

•EIRP: Equivalent Isotropic Radiation Power

•Transmission Losses: Path Loss; Feeder Losses; Antenna Misalignment Losses; Atmospheric Losses

•Link Power Budget Equation

•System Noise: Antenna noise; Amplifier Noise Temperature; Noise Factor/Figure; Lossy Network Noise Temperature, System Noise Temperature, Intermodulation noise, Interference noise

•Carrier-to-Noise (density) Ratio

•System Link Design: Uplink & Downlink


Problem for Noise

The receiver noise figure is 12dB, the cable loss is 5dB, the LNA gain is 50dB, and its noise temperature 150K. The antenna noise temperature is 35K. Calculate the noise referred to the input.

inputantenna

Tant

receiverF

LNAG1, Te1

cableL:1


Solution

n=3TS =Tant + Te1(cable) + Te2(LNA)/ G1+ Te3(receiver)/ (G1 G2)

Te1 =To(L -1)

Te2 =150KG1 =1/L

Te3 =To(F-1) TS =Tant + Te1 + To(L -1) /G1+ To(F -1) L/ G1

TS =Tant + Te1 + Te2/ G1+ Te3/ (G1 G2)+…+ Ten/ (G1 G2... Gn-1)

The receiver noise figure F=12dB=15.85. The cable loss L=5dB=3.16.The LNA gain G1= 50dB=105, noise temperature Te1=150K. The antenna noise temperature Tant =35K. To =290 K

TS =Tant + Te1 + To(L -1) /G1+ To(F -1) L/ G1so TS =35 + (3.16-1) ×290+ 3.16×150+290× (15.85 -1) × 3.16/ 105=1136 K


Problem for Link Design

Problem:

A multiple carrier satellite system operates in the 6/4GHz band with the following

characteristics.

Uplink: saturation flux density of -67.5 dBW/m2; input backoff

of 11dB; satellite G/T of -11.6dBK-1;

Downlink: satellite saturation EIRP of

26.6dBW; output backoff of 6dB; path loss of 196.7dB; earth station G/T of

40.7 dBK-1. The other losses are ignore.

Calculate the carrier-to-noise density ratios for uplink, downlink,

and combined value.


Solution for uplink:

C/NoU= ψs +PL+Ao -BOin +G /TU -κ-LOSSESU dBHz all in dB.ψs =-67.5; saturation flux densityPL=0; at the satellite receive antennaAo]= -37; at 6GHz for uplinkBOin =11; input backoffG /TU = -11.6; satellite-κ=228.6LOSSESU =0; ignoredC/NoU=-67.5+0-37-11-11.6+228.6-0=101.5 dBHz


Solution for downlink:

C/NoD= EIRPSD +G /TD -BOout -κ-LOSSESD dBHz all in dBEIRPSD = 26.6; satelliteBOout =6; output backoffG /TD =40.7; earth station-κ=228.6LOSSESD =PL=196.7; in downlinkC/NoD=26.6+40.7-6+228.6-196.7=93.2 dBHz


Solution for combined uplink & downlink:

No/C=(No/C)U +(No/C)D

(No/C)U =10-10.15

(No/C)D =10-9.32

No/C=5.49×10-10 Hz-1

C/No=-10log (No/C)=10log(5.49×1010)=92.6 dBHz

The combined C/No approaches to the lower one when one of the ratios for uplink and downlink is much less than the other.

If there are other losses for example, AML, AA, RFL, or (No/C)IM, how to calculate the problem mentioned above???


Thank you &Thank you &Good Luck !!!Good Luck !!!

msc4-linkdesign-16spet03-5

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