Satellite Link Design Fundamentals. Link budget is actually the sum of all the losses between: Transmitter - Satellite & back down to a Receiver. These.

Satellite Link DesignFundamentals

Link budget is actually the sum of all the losses between: Transmitter - Satellite & back down to a Receiver. These losses are reduced by any gain you have at the transmitter, satellite or receiver. So in order to see if your signal is still going to be big enough to use after it has been sent to a receiver via satellite, the gains and losses are effectively added together and the result will be the net gain or loss. A loss means your signal has got smaller, and a gain means it has got bigger. The four factors related to satellite system design:

– The weight of satellite – The choice frequency band – Atmospheric propagation effects– Multiple access technique

The major frequency bands are 6/4 GHz, 14/11 GHz and 30/20 GHz (Uplink/Downlink)

At geostationary orbit there is already satellites using both 6/4 and 14/11 GHz every 2 (minimum space to avoid interference from uplink earth stations) -> Additional satellites higher BW

Low earth orbit (LEO) & medium earth orbit (MEO) satellite systems are closer and produces stronger signals but earth terminals need omni directional antennasLow earth orbit (LEO) & medium earth orbit (MEO) satellite systems are closer and produces stronger signals but earth terminals need omni directional antennas

The design of any satellite communication is based on – Meeting of minimum C/N ratio for a specific percentage of time – Carrying the maximum revenue earning traffic at minimum cost

TDRSSGroundStation

TDRSSGroundStation

NASA andCustomerGround

Operations

NASA andCustomerGround

Operations

Space-Space Link

Fwd: 2.025-2.120 GHz (S-band)2.1064 GHz (MA)13.775 GHz (Ku-band)22.55-23.55 GHz (Ka-band)

Fwd: 2.025-2.120 GHz (S-band)2.1064 GHz (MA)13.775 GHz (Ku-band)22.55-23.55 GHz (Ka-band)

Space-Space Link

Fwd: 2.025-2.120 GHz (S-band)2.1064 GHz (MA)13.775 GHz (Ku-band)22.55-23.55 GHz (Ka-band)

Fwd: 2.025-2.120 GHz (S-band)2.1064 GHz (MA)13.775 GHz (Ku-band)22.55-23.55 GHz (Ka-band)

Space-Ground Link

Fwd: 14.6-15.225 GHz

Rtn: 13.4-14.05 GHz

Space-Ground Link

Fwd: 14.6-15.225 GHz

Rtn: 13.4-14.05 GHz

1 of 2 Single Access (SA) Antennas S & Ku-Band for F1-F7 S, Ku, & Ka-Band for F8-F10Field of View (Primary): ±22° E-W, ±28.0° N-S Extended FOV (HIJ only): ±76.8° E-W*, ±30.5° N-S**

S-Band Phased – Array forMultiple-Access (MA) Service

1 Fwd, 5 Rtn Links for F1-F7***

1 Fwd, 5 Rtn Links for F8-F10Field of View (Primary): ±13° conical

RTNLink

FWDLink

CustomerSpacecraft

* - 76.8° outboard** - 24°E-W (inboard)*** - Demand Access Service allows large expansion on the number of non-coherent return link services available through F1 – F7

Primary site atWhite Sands, NM- STGT- WSGTU

Additional site atGuam to supportTDRS at 85E- GRGT

Primary site atWhite Sands, NM- STGT- WSGTU

Additional site atGuam to supportTDRS at 85E- GRGT

Transponder

Earth station (site A) Earth station(site B)

IRRADIUM

downlink

uplink

downlink

uplink

A comprehensive look will be taken at the important parameters that govern the design of a satellite

communication link. The significance of each one of these parameters will be discussed vis-a-vis the overall link

performance in terms of both quantity and quality of services provided by the link, without of course losing sight

of the system complexity of both the Earth station and the space segment and the associated costs involved therein

Transmission Equation:The transmission equation relates the received power level at the destination, which could be the Earth station or the satellite in the case of a satellite communication link, to the transmitted RF power, the operating frequency and the transmitter--receiver distance.

For an isotropic antenna in free space conditions, the power supplied to the antenna, PT, is uniformly

distributed on the surface of a sphere of which the antenna is the center

Power Flux density:

The power flux-density is the power radiated by the antenna in a given direction at a sufficiently large

distance, d, per unit of surface area is:

The power flux-density radiated in a given direction by antenna having a gain, GT, in that direction is:

The equivalent isotropically radiated power (EIRP) = PT GT

The power received by an antenna with area AR is:

The gain of any antenna, for example GR, is:

The expression for the received power is modified to

The term (4 ∕ )𝜋𝑑𝜆 2 represents the free space path loss 𝐿P. The above expression is also known as the Friis transmission equation. The received power can be expressed in decibels as

The above equation can be modified to include other losses, if any, such as losses due to atmospheric

attenuation, antenna losses, etc. For example, if 𝐿A, 𝐿TX and 𝐿RX are the losses due to atmospheric

attenuation, transmitting antenna and receiving antenna respectively, then the above equation can be

rewritten as

Application:

A geostationary satellite at a distance of 36 000km from the surface of the Earth radiates a power of 10

watts in the desired direction through an antenna having a gain of 20 dB. What would be the power

density at a receiving site on the surface of the Earth and also the power received by an antenna having

an effective aperture of 10m2?

Satellite Link Parameters

Noise considerations

Propagation considerations

Choice of operating frequency

Interference-related problems

Overall design of a complete satellite communications system involves many complex trade-offs to obtain a cost-effective solutionsFactors which dominate are

Downlink EIRP, G/T and SFD of SatelliteEarth Station AntennaFrequencyInterference

General Architecture

HPA / Transceiver LNA / LNB

G/T & SFD

EIRP down

UplinkDownlink

Uplink Path Loss

Rain Attenuation

Downlink Path Loss

Rain Attenuation

EIRP Up G/T ESGt

Pt

Transmit Earth Station Antenna Gain Power of Amplifier

Uplink Path Loss Rain Attenuation

Satellite G/T EIRP (Equivalent Isotropic

Radiated Power) SFD (Saturated Flux

Density) Amplifier Characteristic

Downlink Path Loss Rain Attenuation

Receiving Earth Station Antenna Gain LNA /LNB Noise Temperature Other Equipment

Signal Power Calculation

Antenna Gain

G = ( * d / ) 2 [dBi]

Where, = C / f , C = Speed of lightf = frequency of interest = efficiency of antenna (%),d = diameter of antenna (m)

Antenna Beam width

3dB = 70 * C / df [degrees]Where,C= 3x108 m/s (Velocity of Light)

EIRPIs the effective radiated power from the transmitting side and is the

product of the antenna gain and the transmitting power, expressed as

EIRP = Gt + Pt –Lf [dB]Where,

Lf is the Feed LossesSignal Power (Pr)

Pr = EIRP – Path Loss + Gr (sat) [dB]Where,Path Loss = (4D / ) 2

D is the Slant Range (m)

Thermal NoiseIs the noise of a system generated by the randommovement of electronics, expressed as

Noise Power = KTBWhere,

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

Effective Temperature: Te = T1 + (T2/G1)

Where,T1= Temperature of LNAT2= Temperature of D/CG1= Gain of LNA

Noise TemperatureTs = Tant / Lf+(1-1/Lf)Tf

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

Noise Calculation

Choice of operating frequency:The choice of frequency band from those allocated by the International Telecommunications Union

(ITU) for satellite communication services such as the fixed satellite service (FSS), the broadcast

satellite service (BSS) and the mobile satellite service (MSS) is mostly governed by factors like

propagation considerations, coexistence with other services, interference-related issues, technology

status, economic considerations. The most common used frequency bands areHigher frequency bands offer higher bandwidths but suffer from the disadvantage of severe rain-induced attenuation, particularly above 10 GHz. Also, above 10 GHz, rain can have the effect of reducing isolation between orthogonally polarized signals in a frequency re-use system. It may be mentioned here that for frequencies less than 10 GHz and elevation angles greater than 5◦, atmospheric attenuation is more or less insignificant.

Common Frequency Allocations

• L band 0.950 - 1.450 GHz Note: GPS at 1.57542 GHz

• C band 3.7 - 4.2 GHz (Downlink) 5.925 - 6.425 GHz (Uplink)

• Ku band 11.7 - 12.2 GHz (Downlink) 14 - 14.5 GHz (Uplink)

• Ka band 18.3 - 18.8, 19.7 - 20.2 GHz

(Downlink) 30 GHz (Uplink)

• V band 40 - 75 GHz 60 GHz allocated for unlicensed

(WiFi) use 70, 80, and 90 GHz for other

wireless

Propagation ConsiderationsAttenuation is defined as the difference between the power that would have been received under ideal conditions and the actual power received at a given time.

Where,A(t) is the attenuation at any given time t𝑃𝑟𝑖𝑑𝑒𝑎𝑙( ) is the received power under ideal conditions at time t𝑡𝑃𝑟𝑎𝑐𝑡𝑢𝑎𝑙( ) is the actual received power at time t𝑡

Free-space Lossit implies remoteness from all material objects or forms of matter that could influence propagation of electromagnetic waves. That means the Power received by an earth station antenna will be equal to 𝑃t∕(4 2) 𝜋𝑅 where 𝑃t is the transmitted power and 𝑅 is the distance of the receiving antenna from the transmitter. In the case of uplink, the Earth station antenna becomes the transmitter and the satellite transponder is the receiver. It is the opposite in the case of downlink.

The free-space path loss component can be computed from

where FS is the free space loss and = operating wavelength. Also, = ∕ , where𝐿 𝜆 𝜆 𝑐 𝑓 𝑐 = velocity of electromagnetic waves in free space 𝑓 = operating frequency

If is taken in km/s and in MHz, then the free-space path loss can also be computed from𝑐 𝑓

Gaseous Absorptionthere are specific frequency bands where the absorption is maximum, near total. The first absorption band is caused due to the resonance phenomenon in water vapor and occurs at

There are two transmission windows in which absorption is either insignificant or has a local minimum. The first window is in the frequency range of 500 MHz to 10 GHz and the second is around 30 GHz. This explains the wide use of the 6/4 GHz band. The increasing interest in the 30/20 GHz band is due to the second window, which shows a local minimum around 30 GHz. Losses at the 14/11 GHz satellite band are within acceptable limits with values of about 0.8 dB for 5◦ elevation and 0.2 dB for 15◦ elevation.

Attenuation due to RainLosses due to rain increases with an increase in frequency and reduction in the elevation angle

Attenuation of electromagnetic waves due to rain (Arain) extended over a path length of 𝐿 can be computed from

where 𝛼 = specific attenuation of rain in dB/km. Specific attenuation again depends upon various factors like rain drop size, drop size distributionIn practice, rain attenuation is estimated from = 𝛼 𝑎𝑅𝑏 where 𝑎 and 𝑏 are frequency and temperature-dependent constants and 𝑅 is the surface rain rate at the location of interest.

e

Ionosphere-related Effects: Traveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.

• The disturbances of major concern are:

• Scintillation: Scintillations are variations in the amplitude, phase,

polarisation, or angle of arrival of radio waves, caused by

irregularities in the ionosphere which change over time. The main

effect of scintillations is fading of the signal.

• Polarisation rotation.

Polarisation: is the property of electromagnetic waves that describes the direction of the transverse electric field. Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other it is necessary to adopt a convention to determine the polarisation of the signal. Conventionally, the magnetic field is ignored and the plane of the electric field is used.

• Linear Polarisation (horizontal or vertical):• the two orthogonal components of the

electric field are in phase;• The direction of the line in the plane

depends on the relative amplitudes of the two components.

• Circular Polarisation:• The two components are exactly 90º out

of phase and have exactly the same amplitude.

• Elliptical Polarisation:• All other cases.

Linear Polarisation Circular Polarisation Elliptical Polarisation

Types of Polarisation

Depolarisation:

Since raindrops are not perfectly spherical, as a polarised wave crosses a raindrop, one

component of the wave will encounter less water than the other component.

There will be a difference in the attenuation and phase shift experienced by each of the

electric field components, resulting in the depolarisation of the wave.

𝐴PR is the attenuation due to polarization rotation in dB

Polarisation vector relative to the major and minor axes of a raindrop.

Cross-Polarisation DiscriminationDepolarisation can cause interference where orthogonal polarisation is used to provide isolation between signals, as in the case of frequency reuse.The most widely used measure to quantify the effects of polarisation interference is called Cross-Polarisation Discrimination (XPD)The mismatch also produces a cross-polarized component, which reduces the crosspolarization discrimination (XPD), given by

To counter depolarising effects circular polarising is sometimes used.Alternatively, if linear polarisation is to be used, polarisation tracking equipment may be installed at the antenna.

Illustration of the various propagation loss mechanisms on a typical earth-space path

Refractive effects

(tropospheric

scintillation) cause

signal loss.

The absorptive effects of the

atmospheric constituents

cause an increase in sky noise

to be observed by the

receiver

The ionosphere can cause the electric vector of signals passing through it to rotate away from their original polarization direction, hence causing signal depolarization. the sun (a very “hot”

microwave and millimeter

wave source of incoherent

energy), an increased noise

contribution results which

may cause the C/N to drop

below the demodulator

threshold.

The ionosphere has its principal impact on signals at

frequencies well below 10 GHz while the other

effects noted in the figure above become increasingly

strong as the frequency of the signal goes above 10

GHz

Signal TransmissionLink-Power Budget Formula• Link-power budget calculations take into account all the gains and

losses from the transmitter, through the medium to the receiver in a telecommunication system. Also taken into the account are the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna.

• The decibel equation for the received power is:• [PR] = [EIRP] + [GR] - [LOSSES]Where:

• [PR] = received power in dBW• [EIRP] = equivalent isotropic radiated power in dBW• [GR] = receiver antenna gain in dB• [LOSSES] = total link loss in dB

• dBW = 10 log10(P/(1 W)), where P is an arbitrary power in watts, is a unit for the measurement of the strength of a signal relative to one watt.

Interference between the Satellite and Terrestrial Links

Types of Interference (1/2)Cross-Pol Interference – Accidental / very common

• Generally caused by: incompatible modulation types transmitted in the opposite polarization field to digital services on the cross-pol; poorly aligned antennas in bursting networks; and/or lack of training/experience of the uplink operators.

• Becoming more prevalent as installation margins are squeezed.• Mitigation: monitoring, detection and geolocation tools, carrierID, training.

Adjacent Satellite Interference – Accidental / common

• Generally caused by: operator error, or poor inter-system coordination. Transmitting antenna is poorly pointed.

• Caused by lack of installation expertise but becoming more prevalent as two degree spacing between satellites in the geostationary arc becomes more common.

• Mitigation: monitoring, detection and geolocation tools, carrierID, coordination between satellite operators.

Adjacent Carrier Interference – Accidental / minimum occurrence

• Generally caused by: operator error, or equipment failure (unlocked equipment).

• Relatively infrequent• Mitigation: monitoring, detection and geolocation tools, carrierID.

! X

Y

!

!

Adjacent satellitesignal

Unauthorised Access – Accidental & Deliberate• Term given to a signal which is not resident as cross-pol or adjacent

satellite or carrier.

Accidental: very common• Generally caused by: equipment failure, human error, improper

commissioning, and terrestrial interference.• Interference from proliferation of terrestrial (e.g. microwave) systems.• Mitigation: monitoring, detection and geolocation tools, carrierID,

training. Unfortunately terrestrial systems often have priority and so becomes dead capacity.

Deliberate: relatively rare• Generally caused by: unauthorised “borrowing” of bandwidth for test

purposes (e.g. at commissioning), piracy, and hostile attempts to deny service.

• Becoming more prevalent though geopolitical motivation.• Mitigation: monitoring, detection and geolocation tools. While hostile

jamming is generally easy to locate, it is almost impossible to remove without political intervention, which can prove difficult.

!

Ways to Detect Interference

Passive• Wait for end customer complaints or local authority report• Compare spectrum plot of the transponder with the nominal frequency

plan• Check for unauthorized carriers, spurious

Active • Continually scan signals and transponders of interest, generate alarms

for out-of-tolerance conditions• Analog Spectrum Analyzer• Digital Spectrum Analyzer

• Pro-active; problem can be cleared before it is noticed by the customer

Detection ToolsAnalogue• Legacy Spectrum Analyser

Digital• DSP based Spectrum Analyser

A DSP based monitoring system allows for advanced signal analysis and demodulation.It also allow to perform carrier under carrier investigation.

In Case of Interference between Two Satellites

ReceivingStation

Transmission from satellite B on its downlink, in addition to being received by its intended Earth station shown by a solid line again, also finds its way to the receiving antenna of the undesired Earth station through the side lobe shown by the dotted line. Quite obviously, this would happen if the off-axis angle of the radiation pattern of the Earth station antenna is equal to or more than the angular separation 𝜃 between the adjacent satellites

PrimarySatelliteSecondary

Satellite

Source

𝜃 is the angular separation between two satellitesas viewed by the Earth stations

𝛽 is the angular separation between the satellites asviewed from the centre of the Earth; i.e. 𝛽 is simply the difference in longitudinal positionsof the two satellites

𝜃 and 𝛽 are interrelated by the following expression:

where𝑑A = slant range of satellite A𝑑B = slant range of satellite B 𝑟 = geostationary orbit radius

For a known value of 𝜃, the worst case acceptable value of the off-axis angle of the antenna’s radiation pattern can be computed

The desired carrier power 𝐶D for the downlink channel in dBW can be expressed as

whereEIRP= desired EIRP (in dBW, or decibels relative to a power level of 1 W)𝐿D = downlink path loss for the beam from the desired satellite (in dB)

𝐺 = Earth station antenna gain in the direction of the desired satellite (in dB)

The interfering carrier power for the downlink channel ( D) in dBW is given by𝐼

whereEIRP′ = interfering EIRP (in dBW)𝐿D′ = downlink path loss for the beam from interfering satellite (in dB)

′ 𝐺 = Earth station antenna gain in the direction of the interfering satellite (in dB)The expression for (C/I) in the case of downlink can then be written as

where (C/I)D is the C/I for the downlink channel in dB.If the path losses are considered as identical, then

the term ( 𝐺 − 𝐺′) is the receive Earth station antenna discriminationThis gives

A similar calculation can be made for the uplink interference, where a satellite may receivean unwanted signal from an interfering Earth station. In the case of uplink, the expression forC∕I can be written as

where(C/I)U = C/I for the uplink channel in dBEIRP = EIRP of the desired Earth station in dBWEIRP′ = EIRP of the interfering Earth station in the direction of the satellite in dBW

𝐺 = gain of the satellite receiving antenna in the direction of the desired Earth station indB

′ 𝐺 = gain of the satellite receiving antenna in the direction of the interfering Earthstation in dBEIRP′ is further equal towhereEIRP = EIRP of the interfering Earth station in dBW∗

𝐺𝐼 = on-axis transmit antenna gain of the interfering Earth station in dB 𝜃 = viewing angle of the satellite from the desired and interfering Earth Stations

The overall carrier-to-interference ratio ( ∕ ) for adjacent satellite interference is given by𝐶𝐼

where the subscripts U and D imply uplink and downlink respectively. Where the interference is noise like, it is possible to combine the effects of noise and interference. The combined carrier-to-noise ratio (C∕NI) is given by

Applications: Page 362Refer to Figure 7.19. The EIRP values of Earth stations A and B are 80 dBW and 75 dBW respectively. The transmit antenna gains in the two cases are 50 dB each. If the gain of the receiving antenna of the satellite uplinked from Earth station A is 20 dB in the direction of Earth station A and 15 dB in the direction of Earth station B, determine the carrier-to interference ratio at the satellite due to interference caused by Earth station B. Assume that the viewing angle of the satellite from the two Earth stations is 4◦.

A B

sat

DesiredInterferene

Link BudgetThe link budget is a way of analyzing and predicting the performance of a microwave communication link for given values of vital link parameters that contribute to either signal gain or signal loss

A typical satellite consists of a number of repeaters (transponders), each of which provides a large-capacity communication channel. Each transponder has a receiver tuned to a frequency range that has been allocated for uplink communication signals from Earth to the satellite. Following the receiver, each transponder consists of a frequency shifter to lower the received signals to a downlink frequency, a filter tuned to the frequency of the transponder and a power amplifier to transmit signals back to Earth The communication capacity of a satellite is determined by the number of transponder channels and the volume of communication that can be transmitted on each channel. Although this varies from one type of satellite to another, the most commonly used satellite in 1995 had 24 transponders. Each can carry a colour TV signal (or 6 digitally compressed TV signals) or at least 1200 telephone voice signals in one direction. Each new generation of satellites tends to have increased communication capability

Satellites – Satellite Subsystems

Attitude and Orbit Control System

–Rocket motors to move satellite back to the correct orbit

–Keep antennas point toward to earth

Telemetry, tracking, command and monitoring

–Telemetry system monitor satellite health, tracking system is located

at the earth station and provides information about elevation & azimuth angles of the satellite

Power system

–Electrical power from solar cells

Communication subsystem

–Major component of communications satellites, one or more

antennas & a set of receivers and transmitters (transponders)

•The linear or bent pipe transponders; amplifiers the received signal & retransmits it a different, usually lower frequency

•Base-band processing transporters; used with digital signals, converts the received signal to base-band, process it, & then retransmits a digital signal

Satellite communication system & interfacing with terrestrial entities

What Are Satellite Payloads?

• The payloads on communications satellites are effectively just repeaters. They receive the signals that are transmitted to them and then retransmit them at a different frequency back to earth

• Modern satellites do more than this. They receive the signals and then demodulate them to access the data, the data can then be processed before being modulated and retransmitted. The data can be stored for later retransmission or modulated using a different method, even at a different data rate

A wireless repeater

1. Transmitter power T𝑃2. Power loss in the waveguide connecting the transmitter output to the antenna input T𝐿3. Transmitting antenna gain T𝐺4. Free-space path loss P𝐿

5. Attenuation due to rain, clouds, fog, etc., 𝐴6. Receive antenna gain R𝐺7. Power loss in the waveguide connecting the receive antenna output to the receiver input R𝐿8. Received signal power R𝑃

The power balance equation describing the link budget in this case would be given by

It may be mentioned here that all the power levels in the above expression are in dBW and the gain, attenuation and loss terms are in dB.With reference to a satellite link, such an equation can be written for both the uplink as well as the downlinkThe uplink and downlink power balance equations can be written as