00_Satellite Communication Intro

8/12/2019 00_Satellite Communication Intro

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Customer Educational Services

Satellite Networks Ltd.Satellite Networks Ltd.Satellite Communications - 1Proprietary and Confidential

This document contains information proprietary to Gilat Satellite Networks Ltd. and may not be

reproduced in whole or in part without the express written consent of Gilat Satellite Networks Ltd. The

disclosure by Gilat Satellite Networks Ltd. of information contained herein does not constitute any

license or authorization to use or disclose the information, ideas or concepts presented. The contents of

this document are subject to change without prior notice.


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The smaller Earth Stations are also known as Remote Terminal in the VSAT terminology.

VSAT (Very Small Aperture Terminal)


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C-Band is power restricted by the ITU to avoid interference with terrestrial microwave systems


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Ku means "K-under" referring that is the band below the K-band (in the original German, "Kurz-

unten)


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Satellite Networks Ltd.Satellite Networks Ltd.Satellite Communications - #Proprietary and Confidential

In Ku-band, noticeable signal degradation could occur with heavy rain fall.


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Intermediate Frequency is any middle step frequency no matter which band in particular.

At the Hub side the RFT (Radio Frequency Transceiver) is in charge of up-converting and down-

converting the IF (intermediate frequency to the Satellite frequency (C, Ku or KA band)

The BUC (Block Up-Converter) up-converts the IF to The satellite frequency.

The LNB (Low Noice Block) down-converts from the Satellites frequency to the IF.

At the Remote Terminal side:

The Remote Terminal is composed of two sections:

The IDU (In Door Unit), that is the actual VSAT.

The ODU (Out Door Unit), that is the Antenna, BUC and LNB

IF are used as an intermediate step for several reasons:

Circuits and cables performs oddly at high frequencies.

The RFT, BUC, and LNB vendors use oscillators instead of having to tune all the circuitry to

specific frequencies.

Inexpensive and more selective filters are possible when using narrow bandwidths.


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Dish antenna - directional antenna consisting of a parabolic reflector for microwave or radio

frequency radiation

A block up converter (BUC) is used in the transmission (uplink) of satellite signals. It converts a band

(or "block") of frequencies from a lower frequency to a higher frequency and amplifies the signal.

BUCs are generally used in conjunction with LNBs (Low Noise Blocks). The BUC, being an up-

converting device, makes up the "transmit" side of the system, while the LNB is the down-converting


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device and makes up the "receive" side.


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Feed - A device mounted at the focal point of antenna that gathers signals reflected from the dish.

The Block Up-convertor is assembled with the LNB in association with an OMT, Orthogonal Mode

Transducer to the Feed-horn that faces the reflector parabolic dish.

An orthomode transducer is a microwave duct component of the class of microwave circulators. It is

commonly referred to as an OMT, and commonly referred as a polarization duplexer.

Wherever there are two polarizations of radio signals (Horizontal and Vertical), the transmitted and

received radio signal to and from the antenna are said to be orthogonal. This means that the

modulation planes of the two radio signal waves are at 90 degrees angles to each other. The OMT

device is used to separate two equal frequency signals, of high and low signal power. Protectiveseparation is essential as the transmitter unit would seriously damage the very sensitive low (V)

micro-voltage, front-end receiver amplifier unit at the antenna.


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Any object that moves around the earth has an orbit.

The orbit is defined by 3 factors.

The first is the shape of the orbit, which can be circular or elliptical.

The second is the altitude of the orbit. The altitude is constant for a circular orbit but changes

constantly for an elliptical orbit.

The third factor is the angle the orbit makes with the equator. An orbit that brings the satellite over

the poles or close to it has a large angle. An orbit that makes the satellite stay close to the

equator has a small angle.


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LEO Advantages:

The LEO Satellites are small, easy to launch and less expensive if you compare to MEO and

GEO Satellites.

Short round trip signal delay.

Small path loss.

LEO Disadvantages:

A large number of Satellites (Constellations) are needed to have uninterrupted continuity on a

larger coverage area.

Shorter life, 5-8 years.


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MEO Advantages:

Less Satellites are needed than LEO constellations.

Larger coverage areas than LEO Satellites.

Moderate launch cost.

Small round trip delays.

MEO Disadvantages:

Need constellation of Satellite to have uninterrupted connectivity


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GEO Advantages:

Keeping track of a GEO Satellite is relatively easy as the Satellite remains almost stationary in

respect to earth.

The coverage area is very large because the height of the satellite orbit.

Continuous communication is possible by one satellite only.

Small energy storage is required as high solar radiation exist most of the time.

Average 14-17 Year Lifespan

GEO Disadvantages:

Because of the altitude there is higher propagation delay.

Grater path loss and attenuation related to the longer travel distance of the signal.


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Satellite launching is more difficult and more expensive.


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Footprint The area of the Earths surface from which an Earth Station can transmit to or receive

from a particular satellite.


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There is a trade-off between the size of the geographic area in which signals can be transmitted or

received and the amount of power that can be used to send or receive the signal. Therefore,

modern satellites support a variety of beam types to allow the satellite to focus its power at

different levels to particular locations. Use of beams other than global also allows satellites to

employ frequency reuse to increase capacity.


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In the Diagram: The vertical axis is represented here in general as a percentage of maximum but

would ordinarily be a measurement of voltage, current, etc. Note the smooth variation that starts atzero, rises to a positive peak, falls back through zero to a negative peak, and then rises again to

where it started. The whole process then repeats. Each repeat is referred to as a cycle. The time it

takes to complete one cycle is called the Period and is denoted with the symbol T (T for Time). The

reciprocal of the Period is the Frequency, f. Thus, f = 1/T. The frequency indicates how many cycles

exist in one second. To honor one of the 19th century researchers in the field, instead of calling the

unit cycles per second, we use Hertz, named after Heinrich Hertz and abbreviated Hz. In the

example above, f = 1/10 ms, or 100 Hz (100 cycles in one second).

The amplitude (vertical) of the wave can be expressed as a peak quantity, which would be the

change from the center zero line up to the most positive value. Amplitude may also be expressed as

peak-to-peak, the distance from the most negative to the most positive. Note that for a sine wave

this will always be twice the peak value, although for other sorts of waves which may be


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asymmetrical, that may not be the case.


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Since these sine waves do not go through their maximum and minimum points at the same instant of

time, a phase difference exists between the two waves. For the two waves the phase difference is90.


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In digital modulation, an analog carrier wave is modulated by a digital bit stream.

Digital modulation methods can be considered as digital-to-analog conversion, and the

corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier


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signal are chosen from a finite number of alternative symbols.


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The waveforms used to represent a 0 and a 1 bit differ by exactly half a carrier period.

In Gaussian minimum shift keying, input bits with rectangular (+1, -1) representation are converted

to Gaussian (bell-shaped) pulses by a Gaussian filter before further smoothing by a frequency


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modulator.


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BPSK is the simplest form of PSK. It uses two phases which are separated by 180and so can also

be termed 2-PSK.

This modulation is the most robust of all the PSKs since it takes serious distortion to make the

demodulator reach an incorrect decision. It is, however, only able to modulate at 1 bit/symbol and so

is unsuitable for high data-rate applications when bandwidth is limited.


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With four phases, QPSK can encode two bits per symbol, twice the rate of BPSK. Analysis shows

that this may be used either to double the data rate compared to a BPSK system while maintainingthe bandwidth of the signal or to maintain the data-rate.


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Forward error correction (FEC) requires that the decoder should also be capable of correcting a

certain number of errors, i.e. it should be capable of locating the positions where the errors occurred.


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The task of the interleaver is to scramble bits in a pseudo-random manner.

The first turbo code, based on convolutional encoding, was introduced in 1993. Since then, several


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schemes have been proposed and the term turbo codes has been generalized to cover block

codes as well as convolutional codes. Simply put, a turbo code is formed from the parallel

concatenation of two codes separated by an interleaver.


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00_Satellite Communication Intro

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Transcript of 00_Satellite Communication Intro