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Satellite CommunicationsSystems: Systems, Techniques and Technologies, 5th edition. Gerard Maral, Michel Bousquet John Wiley & Sons, 2009.

Chapter 5Uplink, Downlink and Overall Link Performance; Intersatellite Links

Chapter 5: Uplink, Downlink and Overall Link Performance; Intersatellite Links

Our goal: Tools to evaluate

link budget Link performance

from origin to destination station

Overview: configuration of a link antenna parameters radiated/received power noise power spectral density individual link performance influence of the

atmosphere/mitigation overall link performance with

transparent/regenerative satellite

multibeam antenna coverage intersatellite link performance

Chapter 5: roadmap (1/4) 5.1 Configuration of a link5.2 Antenna parameters

gain, radiation pattern and angular beamwidth, polarisation

5.3 Radiated power effective isotropic radiated power (EIRP), power flux

density5.4 Received signal power

Power captured by the receiving antenna and free space loss

Example 1: Uplink received power Example 2: Downlink received power Additional losses

Chapter 5: roadmap (2/4) 5.5 Noise power spectral density at the receiver input

The origins of noise, noise characterisation, noise temperature of an antenna

System noise temperature5.6 Individual link performance

Carrier power to noise power spectral density ratio at receiver input

Clear sky uplink/downlink performance5.7 Influence of the atmosphere

Impairments caused by rain, other impairments, link impairments—relative importance

Link performance under rain conditions

Chapter 5: roadmap (3/4)5.8 Mitigation of atmospheric impairments

Depolarisation mitigation, attenuation mitigation, site diversity, adaptivity, cost-availability trade-off

5.9 Overall link performance with transparent satellite Characteristics of the satellite channel Expression for (C/N0)T Overall link performance for a transparent satellite without

interference or intermodulation5.10 Overall link performance with regenerative

satellite Linear satellite channel without interference Non-linear satellite channel without interference Non-linear satellite channel with interference

Chapter 5: roadmap (4/4)5.11 Link performance with multibeam antenna coverage vs monobeam coverage

Advantages of multibeam coverage Disadvantages of multibeam coverage

5.12 Intersatellite link performance Frequency bands Radio-frequency links Optical links

Configuration of a Link uplinks: from the earth

stations to the satellites downlinks: from the

satellites to the earth stations radio frequency

modulated carriers intersatellite links:

between the satellites

Configuration of a Link Quality of service (QoS) for the

connection between the end users baseband signal-to-noise ratio (S/N) –

analogue communication bit error rate (BER) – digital

communication QoS depends on the individual link

performance C/N0 (Hertz)

C: the received carrier power N0: the noise power spectral

density

Configuration of a Link

Transmitter (Tx) GT transmit antenna gain in the direction of the

receiver PT power radiated by the transmitter in the

direction of the receiver EIRP (Effective Isotropic Radiated Power)

EIRP = PTGT (W)


Receiver (Rx) GR receive antenna gain in the direction of the

transmitter C power of the modulated carrier at the receiver

input T system noise temperature (all sources of noise

in the link contribute to it)• Conditions the noise power spectral density N0


Receiver (Rx) C/N0 the link performance can be calculated at

the receiver input G/T (Figure of merit) receiver performance

measure G overall receiver gain

Path loss (L)

Chapter 5: roadmap (1/4) 5.1 Configuration of a link5.2 Antenna parameters

gain, radiation pattern and angular beamwidth, polarisation

5.3 Radiated power effective isotropic radiated power (EIRP), power flux

density5.4 Received signal power

Power captured by the receiving antenna and free space loss

Example 1: Uplink received power Example 2: Downlink received power Additional losses

Gain The gain of an antenna is the ratio of the power

radiated (or received) per unit solid angle by the antenna in a given direction to the power radiated (or received) per unit solid angle by an isotropic antenna fed with the same power

Gmax the gain is maximum in the direction of maximum radiation (the electromagnetic axis of the antenna, also called the boresight)

Gmax = (4π/λ2)Aeff

λ = c/f c speed of light, 3 × 108 m/s f frequency of the electromagnetic wave Aeff effective aperture area of the antenna

In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is a dimensionless unit of measurement called a steradian (symbol: sr).

Source: Wikipedia

Gain Antenna with a circular aperture or reflector of

diameter D and geometric surface A = πD2/4Aeff = ηA = η(πD2/4)

η efficiency of the antennaGmax = (4π/λ2)Aeff

= (4π/λ2) η(πD2/4) = η(πD/λ)2

= η(πDf/c)2 Expressed in dBi (the gain relative to an isotropic

antenna), the actual maximum antenna gain is: Gmax , dBi = 10 log[η(πD/λ)2] = 10

log[η(πDf/c)2]

Gain The efficiency η of the antenna is the product of

several factors which take account of the illumination law, spill-over loss, surface impairments, ohmic and impedance mismatch losses, and so on:

η = ηi × ηs × ηf × ηz ........... Illumination efficiency ηi

Uniform illumination (ηi = 1) → high secondary lobes Attenuate the illumination at the reflector boundaries

(aperture edge taper) Cassegrain antenna

• Best compromise: illumination attenuation at the boundaries of 10 to 12 dB

• ηi of the order of 91%

Gain Spill-over efficiency ηs

Ratio of the energy intercepted by the reflector to the total energy radiated by the primary source

Large view angle → high spill-over efficiency If illumination level at the boundaries becomes less

with large values of view angle then illumination efficiency collapses

A compromise leads to a spill-over efficiency of the order of 80%

Surface finish efficiency ηf Effect of surface roughness on the gain of the antenna Actual parabolic profile differs from the theoretical one A compromise must be found between the effect on

the antenna characteristics and the cost of fabrication

Gain The effect on the on-axis gain is of the form:

ηf = ∆ G = exp[-B(4πε/λ)2] εthe root mean square (rms) surface error, i.e. the

deviation between the actual and theoretical profiles measured perpendicularly to the concave face

B a factor, less than or equal to 1, whose value depends on the radius of curvature of the reflector

The other losses, including ohmic and impedance mismatch losses, are of less importance

Overall efficiency η The product of the individual efficiencies, is typically

between 55% and 75%

Gain Gmax vs. D for different frequencies at η = 0.6. A 1 m

antenna at 12 GHz has a gain of 40 dBi Dividing the frequency

by 2 (f = 6 GHz) reduces the gain by 6 dB, so Gmax = 34 dBi

Keeping frequency constant (f = 12 GHz) and increasing the size of the antenna by a factor of 2 (D = 2m) increases the gain by 6 dB (Gmax = 46 dBi)

Radiation pattern & angular beamwidth

Radiation pattern variations of gain with direction A circular aperture or reflector antenna this pattern

has rotational symmetry The main lobe contains the direction of maximum

radiation Side lobes should be kept to a minimum

Polar coordinates

Cartesian coordinates


Polar coordinates

Cartesian coordinates

Angular beamwidth angle defined by the directions corresponding to a given gain fallout with respect to the maximum value

3 dB beamwidth (θ3 dB) angle between the directions in which the gain falls to half its maximum value


3 dB beamwidth (θ3 dB) Related to the ratio λ/D by a coefficient whose

value depends on the chosen illumination law Uniform illumination the coefficient has a value

of 58.5° Non-uniform illumination attenuation at the

reflector boundaries, θ3 dB increases and the value of the coefficient depends on the particular characteristics of the law. The value commonly used is 70° which leads to the following expression:

θ3 dB = 70(λ/D) = 70[c/(fD)] (degrees)


3 dB beamwidth (θ3 dB) In a direction θ with respect to the boresight, the

value of gain is given by:G(θ)dBi = Gmax,dBi – 12(θ/θ3 dB)2 (dBi)

This expression is valid only for sufficiently small angles (θ between 0 and θ3 dB/2)

θ3 dB = 70[c/(fD)] Df/c = 70/θ3 dB

Gmax = η(πDf/c)2 = η(70π/θ3 dB )2 For η = 0.6

Gmax = 29000/(θ3 dB )2


Gmax vs. θ3 dB for three values of efficiency


3 dB beamwidth (θ3 dB) For η = 0.6

Gmax = 29000/(θ3 dB )2 10log Gmax = 10log[29000/(θ3 dB )2] Gmax , dBi = 44.6 – 20logθ3 dB (dBi)

20logθ3 dB = 44.6 – Gmax , dBi logθ3 dB = 2.23 – Gmax , dBi /20

logθ3 dB = log102.23 – log10 Gmax , dBi /20

logθ3 dB = log170 – log10 Gmax , dBi /20

θ3 dB = 170/[10 Gmax , dBi /20] (degrees)


3 dB beamwidth (θ3 dB)

G(θ)dBi = Gmax,dBi – 12(θ/θ3 dB)2 (dBi)Differentiating with respect to θ

dG(θ)/dθ = – 24θ/(θ3 dB)2

Or∆G = [– 24θ/(θ3 dB)2]∆θ

∆G gain fallout in dB at angle θ degrees from the boresight, for a depointing angle ∆θ degrees about the θ direction

The gain fallout is maximum at the edge of 3 dB beamwidth (θ = ½θ3 dB) ∆G = – 12 ∆θ/θ3 dB

Polarisation Wave radiated by an antenna →Two components

Electric field and magnetic field They are orthogonal and perpendicular to the direction of

propagation of the wave They vary at the frequency of the wave

By convention, the polarisation of the wave is defined by the direction of the electric field

Polarisation In general, the direction of the electric field is not

fixed; i.e., during one period, the projection of the extremity of the vector representing the electric field onto a plane perpendicular to the direction of propagation of the wave describes an ellipse; the polarisation is said to be elliptical

Parameters characterising polarisation: direction of rotation (with respect to the

direction of propagation): right-hand (clockwise) or left-hand (counter-clockwise)

axial ratio (AR): AR = Emax/Emin, that is the ratio of the major and minor axes of the ellipse. When the ellipse is a circle (axial ratio = 1 = 0 dB), the polarisation is said to be circular. When the ellipse reduces to one axis (infinite axial ratio: the electric field maintains a fixed direction), the polarisation is said to be linear;

inclination Τof the ellipse

Polarisation

Two waves are in orthogonal polarisation if their electric fields describe identical ellipses in opposite directions Two orthogonal circular polarisations described as right-

hand circular and left-hand circular (the direction of rotation is for an observer looking in the direction of propagation)

Two orthogonal linear polarisations described as horizontal and vertical (relative to a local reference)

Frequency re-use by orthogonal polarisation Two polarised antennas must be provided at each end One antenna which operates with the two specified

polarisations mutual interference due imperfections of the antennas/

depolarisation of the waves by the transmission medium

Polarisation

Polarisation

Two orthogonal linear polarisations a, b the amplitudes, assumed to be equal, of the electric

field of the two waves transmitted simultaneously ac, bc the amplitudes received with the same polarisation ax, bx the amplitudes received with orthogonal

polarisations

Polarisation

Some definitions: Cross-polarisation isolation: XPI = aC/bX or bC/aX

XPI (dB) = 20 log(aC/bX) or 20 log(bC/aX) (dB) Cross-polarisation discrimination (when a single

polarisation is transmitted): XPD = aC/aX XPD (dB) = 20 log(aC/aX) (dB)

In practice, XPI and XPD are comparable and are often included in the term ‘isolation’.

For a quasi-circular polarisation characterised by its value of axial ratio AR, the cross-polarisation discrimination is given by: XPD = 20 log[(AR + 1)/(AR - 1)] (dB)

Conversely, the axial ratio AR can be expressed as a function of XPD by:AR = (10XPD/20 + 1)/(10XPD/20 - 1)

The antenna is thus characterised for a given polarisation by a radiation pattern for nominal polarisation (copolar) and a radiation pattern for orthogonal polarisation (cross-polar)

Cross-polarisation discrimination is generally maximum on the antenna axis and degrades for directions other than that of maximum gain

Polarisation

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