satellite link design - wordpress.com · in satellite communications, three factors influence...

SATELLITE LINK DESIGN

Dr. Marwah Ahmed Networks and

Communication

Department

1

Outlines

26-May-16 Networks and Communication Department

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Introduction

Basic Transmission Theory

System Noise Temperature and G/T Ratio

Design of Downlinks

Satellite Communication Link Budget

Introduction


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In Satellite Communications, three factors influence system

design: frequency band, atmospheric propagation effect, and

multiple access technique.

Table 1. Frequency bands allocated for the satellite.

Satellite Communications, 2/E by

Timothy Pratt, Charles Bostian, &

Jeremy Allnutt

Copyright © 2003 John Wiley & Sons.

Inc. All rights reserved.

Figure 10.25 (p. 422)

One-way propagation delay for the three

orbits: LEO, MEO, and GEO. The one-way

delay figures shown above have been

calculated assuming the radio signal

propagates at the speed of light in a

vacuum, i.e., 3 X 108 m/s. That is, no

account has been taken of any delay due

to the refractive index of the atmosphere

not being unity. Also, no account has been

taken of any processing delay imposed on

the signal from any source coding, channel

coding, modulation, or access scheme

used.

Introduction: Types of satellites


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From the figure below, explain, how do satellites

perform communications? And define the two major

elements of satellite communications system!

Class Work!


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Networks and Communication Department



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The calculation of the power received by an earth

station from a satellite transmitter is fundamental to

the understanding of satellite communications.

Consider a transmitting source, in free space,

radiating a total power 𝑃𝑡 watts uniformly in all

directions as shown in Figure 4.2.



Jeremy Allnutt



Figure 4.2 (p. 101)

Flux density produced by

an isotropic source.


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At a distance 𝑅 meters from the hypothetical

isotropic source transmitting RF power 𝑃𝑡 watts, flux

density crossing the surface of a sphere with radius

𝑅 is given by:

𝐹 = 𝑃𝑡

4𝜋𝑅2 𝑊 𝑚2



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All real antennas are directional and radiate more

power in some directions than in other. Any real

antenna has a gain 𝐺 𝜃 , as sketched below.



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For a transmitter with output 𝑃𝑡 watts driving a

lossless antenna with gain 𝐺𝑡, the flux density in the

direction of the antenna boresight at distance 𝑅

meter is:

𝐹 = 𝑃𝑡𝐺𝑡4𝜋𝑅2

The product 𝑃𝑡𝐺𝑡 is often called the effective

isotopically radiated power or EIRP




Jeremy Allnutt



Figure 4.3 (p. 102)


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If we had an ideal receiving antenna with an aperture area of 𝐴 𝑚2, as shown in Figure 4.3, we will collect power 𝑃𝑟 watts given by:

𝑃𝑟 = 𝐹 × 𝐴 watts

A practical antenna with a physical aperture area of 𝐴𝑟 𝑚

2 will not deliver the power transmitted, and some is absorbed by lossy components. This reduction in efficiency is described by using an effective aperture 𝐴𝑒 where:

𝐴𝑒 = 𝜂𝐴 𝐴𝑟

And 𝜂𝐴 is the aperture efficiency of the antenna.



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The power received by a real antenna is

𝑃𝑟 = 𝑃𝑡 𝐺𝑡 𝐺𝑟4𝜋𝑅 𝜆 2

This expression is known as the link equation, and it is

essential in the calculation of power received in any

radio link.

The term 4𝜋𝑅 𝜆 2 is known as the path loss, 𝐿𝑝.

Therefore, we can write the 𝑃𝑟 as:

𝑝𝑜𝑤𝑒𝑟 𝑟𝑒𝑐𝑖𝑒𝑣𝑒𝑑 = 𝐸𝐼𝑅𝑃 ×𝑅𝑒𝑐𝑒𝑖𝑣𝑖𝑛𝑔 𝑎𝑛𝑡𝑒𝑛𝑛𝑎 𝑔𝑎𝑖𝑛

𝑃𝑎𝑡ℎ 𝑙𝑜𝑠𝑠 watts



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In Communication systems, decibel quantities are commonly

used to simplify calculations:

𝑃𝑟 = 𝐸𝐼𝑅𝑃 + 𝐺𝑟 − 𝐿𝑝 dBW

In practice, we will need to take account of more complex

situation in which we have losses in the atmosphere due to

attenuation by oxygen, water vapor, and rain, losses in the

antennas at each end of the link.

𝑃𝑟 = 𝐸𝐼𝑅𝑃 + 𝐺𝑟 − 𝐿𝑝 − 𝐿𝑎 − 𝐿𝑡𝑎 − 𝐿𝑟𝑎 dBW

where 𝐿𝑎 is the attenuation in the atmosphere, and 𝐿𝑡𝑎 , 𝐿𝑟𝑎 are

the losses associated with the transmitting and receiving,

respectively.




Jeremy Allnutt



Figure 4.4 (p. 104)

A satellite link.

System Noise Temperature and G/T

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Is a useful concept in communications receivers, since it provides a way to determining how much thermal noise is generated by active and passive devices in the receiving system.

The noise power is given by:

𝑃𝑛 = 𝑘𝑇𝑠 𝐵𝑛

where 𝑘 denotes Boltzman’s constant = 1.39× 10−23 J/K = -228.6 dB W/K/Hz, 𝑇𝑠 is the system noise temperature of source in kelvin degrees, and 𝐵𝑛 represents the noise bandwidth in which the noise power is measured, in Hz

Noise Temperature

Research Work!


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Calculate the total noise power at the output of the

IF amplifier of the receiver in the below figure.

G/T Ratio for Earth Stations


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The link equation can be rewritten in terms of 𝐶𝑁 at the

earth station:

𝐶

𝑁=

𝑃𝑡𝐺𝑡𝐺𝑟𝑘𝑇𝑠 𝐵𝑛

𝜆

4𝜋𝑅

2

= 𝑃𝑡𝐺𝑡𝑘 𝐵𝑛

𝜆

4𝜋𝑅

2𝐺𝑟𝑇𝑠

Thus, 𝐶 𝑁 ∝ 𝐺𝑟 𝑇𝑠 , and can be used to specify the quality of a

receiving earth station or a satellite receiving system.

Design of Downlink

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Design of Downlinks

The design of any satellite communication is based on meeting a minimum C/N ratio for a specified percentage of time.

Any satellite link can be designed with very large antenna to achieve high C/N ratios under all conditions, but cost will be high.

The art of good system design is to reach the best compromise of system parameters that meet the specification at the lowest cost.

For example if a satellite link is designed with sufficient margin to overcome a 20 dB rain fade rather than a 3 dB fade, earth station antennas with seven times the diameter are required.

Design of Downlinks

All satellite communication links are affected by rain

attenuation.

In the 6/4 GHz band the effect of rain on the link is small

In the 14/11GHz (Ku) band and even more in the 30/20 GHz

(Ka) band, rain attenuation becomes important.

Rain attenuation is very variable phenomenon, both with time

and place.

Link Budget

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Link Budgets

C/N is simplified by the used of link budget

A link budget is a tabular method for evaluating the received

power and noise power in a radio link.

Link budget must be calculated for an individual transponder,

and must be repeated for each of the individual links.

Link budgets are usually calculated for the worst case, the one

in which the link will have the lowest C/N ratio.

Link Budgets

Factors which contribute to a worst case scenario include: an

earth station located at the edge of the satellite coverage

zone where the received signal is typically 3 dB lower than the

center of the zone.

This is because the satellite antenna pattern, maximum path

length from the satellite to the earth station, low elevation

angle at the earth station giving the highest atmospheric path

attenuation in clear air and maximum rain attenuation on the

link causing loss of received signal power and increase in

receiving system noise temperature.

Link Budgets

The calculation of carrier to noise ratio in a satellite link is

based on the two equations for received signal power and

received noise power

Pr = EIRP + Gr - Lp - La – Lr – Lt dBw

A receiving terminal with a system noise temperature TsK and

a noise bandwidth Bn Hz has a noise power Pn

Pn = KTsB

Q & A

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