For Sir Ahmed Sikander Page 0

F-11

Iqbal Uddin Khan

MS - Telecommunication

F-11

Satellite Communication Assignments

SATELLITE COMMUNICATION ASSIGNMENTS F-11

1 | P a g e Iqbal Uddin Khan

Assignment – One

Q1 ) Define Up Converter and Down Converter with the help of example.

Answer

Up Converter

The Up converter accepts the modulated IF carrier and convert it to the frequency of Up Link of satellite,

which is relatively high frequency as compared to the modulated IF.

Down Converter

The Down Converter receives the modulated RF carrier and converts it to IF for the processing section of

transponder. IF are relatively low frequency as compared to RF carriers.

Elaboration by Example

KU – Band Transponder

The Ku (14/11GHz) system, satellite receives the signal of 14 GHz from ground satiation(s) and by

passing through Band pass filter and Low Noise Amplifier it goes through Down Converter (DC). The C

band transponder uses a single Down Converter (DC) and signal processing at 1GHz. The need of down

conversion to 1 GHz is necessary to reshape or regain the signal according to their channel. The process

of De-multiplication and equalization of signals can be done efficiently, on low frequency and

independently on each channel as compare to high frequency.

After each channel is done with the necessary processing, the signal is fed into Up Converter to set their

frequency to 11GHz so that can be transmitted again to earth station. Before transmission each

channel’s strength is amplified by feeding it in to High power amplifiers, mostly SSPA (Solid state Power

Amplifiers). After amplification through necessary band pass the Up Converted signals are multiplex

again, for transmitting further to destination earth station(s).

SATELLITE COMMUNICATION ASSIGNMENTS F-11

2 | P a g e Iqbal Uddin Khan

Q2 ) Why the satellite downlink frequency is less than the uplink frequency, explain.

Answer

From the Earth Station we have to penetrate the atmosphere to reach the satellite station and battle

against the gravitational pull. So very high power is required to transmit the signal from ground to space

and that much power is available on ground. So the Up Link frequency is higher.

a) In contrast, Propagation of signal is easier from space to earth station as it is supported by

gravitational pull so low power is required as satellites are dependent on solar power, so use of

power is very critical and secondly to generate high frequency is a power requiring with more

complex circuitry so to avoid all comparatively low frequency is used in Down Link.

b) Now as the frequency is increased the amount of interference caused by the atmosphere or any

other obstacles are reduced. As higher frequency signal has more signal energy in it, so it can

penetrate the atmosphere more easily.

c) One more thing is that the Satellite is a equipment that needs to be light weight, as less pay

load to transport the less in pay, so it cannot have high power amplifiers on board so by default

the down link frequency is lower than uplink frequency.

Q3 ) What is the difference between BER and C/N?

Answer

BER (Bit Error Rate) is the rate at which errors occur in a transmission system. This can be directly

translated into the number of errors that occur in a string of a stated number of bits. Furthermore, BER

can also be said the Probability of Error. Mathematically can be represented as:

BER = Number of Errors / Number of bits transmitted

BER is different to every other modulation scheme, as ratio of noise is different for all. In general BER

can be determined by three variables:

1. Erf – error function.

2. Eb – energy in one bit.

3. NO – noise power spectral density.

Here we can see that BER is the function of Eb / NO. For Digital signal Eb / NO is used.

C/N is said to be the carrier power of the whole usable band width.

In other words the Difference between BER and C/N is that BER is error rate of bandwidth and C/N is the

power of bandwidth.

SATELLITE COMMUNICATION ASSIGNMENTS F-11

3 | P a g e Iqbal Uddin Khan

Q4 ) Draw the block Diagram of Satellite Earth Station and explain each Block.

Answer

Below are multiple Block Diagrams of Satellite Ground/Earth stations

Diagram from Course material

A VSAT Earth terminal

SATELLITE COMMUNICATION ASSIGNMENTS F-11

4 | P a g e Iqbal Uddin Khan

C – Band Earth station Receiver section

C – Band Earth station Transmitter section

The typical satellite Ground/Earth station’s components are common to all type of satellite platform, so I

use VSAT setup to demonstrate the working of individual block of earth station.

In VSAT earth terminal two blocks are shown:

I. Indoor Unit

II. Outdoor Unit

SATELLITE COMMUNICATION ASSIGNMENTS F-11

5 | P a g e Iqbal Uddin Khan

I. Indoor Unit

The indoor unit is said to be the base-band equipment, used by user or any other attached mechanism.

II. Outdoor Unit

The outdoor unit is consisting of the blocks that are relevant to the ask question so details are below:

A. Diplexer

B. Band-Pass filters

C. LNA

D. SSPA

E. Down Converter

F. Up Converter

Explanation of Blocks:

A. Diplexer

The Diplexer is generally a passive filter, used to differentiate the paths for the transmitter and

receiver according to the frequency they use. It is basically made of a Low-pass and High-pass filter.

Block Diagram of Diplexer

A satellite Diplexer

SATELLITE COMMUNICATION ASSIGNMENTS F-11

6 | P a g e Iqbal Uddin Khan

B. Band-Pass filters

A band-pass filter is a device that passes frequencies within a certain range and attenuates frequencies

outside that range.

The graph showing the magnitude transfer function versus frequency for a band-pass filter.

C. LNA

In the satellite communication in Earth station, the weak RF downlink signal is amplified in a special low-

noise amplifier (LNA). The LNAs are constructed using low-noise devices such as a tunnel-diode for

amplification aka Tunnel Diode amplifier.

D. SSPA

At the final stage of the earth station’s transmitter includes a power amplifier known as Solid-State

Power Amplifier (SSPA).They are constructed by the use of Semiconductor devices. Solid-state power

amplifiers provide output powers of 10-30W with efficiencies of 20-35% with gains of approximately 50

dB.

E. Down Converter

A down converter translates a carrier frequency from a high frequency to a low frequency. In satellite

communications a down converter is used in the earth station to take the downlink frequency and

translate it in Intermediate frequency for further process.

F. Up Converter

An up converter translates a carrier frequency from an intermediate frequency to a high frequency

inside earth station or an up converter is used to take the baseband signal and translate it in frequency

according to the uplink frequency.

SATELLITE COMMUNICATION ASSIGNMENTS F-11

7 | P a g e Iqbal Uddin Khan

Assignment – Two

The Derivation of Received Power Equation (Pr).

To begin the derivation of the Friis Equation, consider two antennas in free space (no obstructions

nearby) separated by a distance R:

Transmit (Tx) and Receive (Rx) Antennas separated by R.

Assume that Watts of total power are delivered to the transmit antenna. For the moment, assume

that the transmit antenna is Omni-directional, lossless, and that the receive antenna is in the far field of

the transmit antenna. Then the power density p (in Watts per square meter) of the plane wave incident

on the receive antenna a distance R from the transmit antenna is given by:

If the transmit antenna has an antenna gain in the direction of the receive antenna given by , then

the power density equation above becomes:

The gain term factors in the directionality and losses of a real antenna. Assume now that the receive

antenna has an effective aperture given by . Then the power received by this antenna ( ) is

given by:

SATELLITE COMMUNICATION ASSIGNMENTS F-11

8 | P a g e Iqbal Uddin Khan

Since the effective aperture for any antenna can also be expressed as:

The resulting received power can be written as:

This is known as the Friis Transmission Formula. It relates the free space path loss, antenna gains and

wavelength to the received and transmits powers. This is one of the fundamental equations in antenna

theory, and should be remembered.

Reference for Power receive