DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION

PAGE MEHRAN UNIVERSITY OF ENGINEERING AND TECHNOLOGY, JAMSHORO

DEPARTMENT OF TELECOMMUNICATION ENGINEERING

SATELLITE COMMUNICATION (1st Term, Final Year)

Lab Experiment # 1/8

Name: ___________________________________________________Roll No: _____________

Score: _____________________ Signature of lab tutor: ____________ Date: _______________

INTRODUCTION TO TV SATELLITE PERFORMANCE OBJECTIVEUpon successful completion students will be able to describe the main aspects related to the TV satellite.DISCUSSION Up-link and Downlink Satellite for TV transmission broadcasting receive the signal from the earth station across a radio connection called up-link and broadcast this signal back toward Earth with the downlink, as shown in figure 1.1. The broadcasting to the earth is carried out with a Frequency modulation of a carrier in band C (from 3.4 GHz to 4.2 GHz) or in band Ku (from 10.95 GHZ to 12.75 GHz). In the last few years, QPSK digital modulation has become the satellite transmission standard and this requires the use of the Ku band.

Figure 1.1: Uplink and DownlinkFootprint

The satellite re-transmits the signal to ground, covering a reception area called footprint (covering map). This area varies according to the power of the satellite, the kind of antenna mounted on board, the antenna pointing direction. An example of footprint (Astra satellite) is shown in figure 1.2. A signal is stronger in the center of the pointing area and weaker toward the outside. Lines can be plotted joining point in which the signal has the same intensity; such lines defines the area covered by the satellite and give indications on the dimensions of the dish to be used in the earth reception. Moving from the center of the footprint toward the outside, the antenna sizes must be increased if the quality of the received signal (i.e. the signal-to-noise ration) is to be kept the same.

Figure 1.2: FootprintEquivalent Isotropically Radiated Power (EIRP)

The power received at ground, in the different places inside the covering area, is only a fraction of the one transmitted by the satellite. The satellites are characterized by the power they radiate in the beam axis, defined by the variable called EIRP. This corresponds to the power (expressed in dBW) the transmitted should radiate if using an isotropic antenna. The EIRP value determines the diameter of the parable of the received necessary to obtain a fixed value of the signal-to-noise ratio, and so the reception quality level.

Geostationary orbit and Satellite positionAll TV satellite occupies an orbit around the ground located at about 36,000-km height on the vertical line of the equator as shown in figure 1.3. At that level the speed to keep in orbit coincides to the earth rotation speed, and the satellite is fixed in respect to a point in the earth. The orbit, too, it called Geostationary. The orbit position of each satellite is expressed in longitudinal degree in respect to the meridian of Greenwich (figure 1. 4) and represents a main datum for the proper orientation of the receiver parabolic antenna.

Figure 1.3: Geostationary Orbit

Figure 1.4: Satellite Distance and Position

Distance of the satellite

The exact distance D between satellite and receiving station, used for the calculation of the path attenuation, is measured with the following formula (figure 1. 4).D= R2+ (R+H)2 2R(R+H) Cos

With: R= 6370 km (average earth radius)

H= 35800 km (average satellite-to-earth distance)

Cos = Cos (latRx). Cos (logRx-longSat)

latRx = receiving station latitude

longRx = receiving station longitude

longSat = Satellite longitude

Frequency sub-division

Nowadays the satellites for TV broadcasting use two frequency bands:

1. C band:Up-link: from about 5.7GHz to about 6.5GHz Down-link: from 3.4GHz to 4.2GHz

2. Ku band:Up-link: from about 14GHz to about 14.5GHz

Down-link: from 10.95GHz to about 12.75GHz, with the following sub-division:

10.95 11.70GHz FSS band (Fixed Satellite Services) for TV communication

11.70 12.50GHz DBS band (Direct Broadcasting Satellite) for direct diffusion

12.50 12.75GHz SMS band (Satellite Mobile Services)

PolarizationIn order to have more channels on the same band, the same satellite can transmit using different polarity formats: linear (horizontal or vertical) or circular (RHCP right hand Circular Polarization or LHCP left Hand Circular Polarization) (figure 1. 5). In the plant receiver from earth, you can select the polarization you want with a polarizer, set before the converter. The most recent DBS satellites tend to use the circular polarization; the other satellites with linear polarization must sometimes be skew adjusted in the reception equipment.

Figure 1.5: Linear and Circular PolarizationBroadcasting and coding standards

At the moment signals are broadcasted with PAL, NTSC, SECAM standards, but the MAC standard (Multiplexed Analog Components) is being used more and more.

The MAC includes more standards (C-MAC, D-MAC, D2-MAC), and defines a digital broadcasting system enabling a superior clearness of the digital superior to the 50% in respect to PAL ones and an audio with quality equal to one of a compact-disc.In case of Pal, Secam or NTSC transmission, the signal broadcasted by the satellite consists of a carrier which is frequency modulated by the video+audio composite signal (figure 1. 6a), where the audio information is shifted over the video band with the frequency modulation of a subcarrier

First the modulation of the audio signal (two signal in case of stereo transmission) crosses pre-emphasis circuit, which purpose is to improve the transmission quality.

In the stereophonic audio transmission there are the following main methods:

Multiples system: the left and right audio channels are combined together in a way similar to the one used for FM stereo radio broadcasting, and transmitted using a single under- carrier. Matrix system (Warner Amex): the sum and the difference of the right and left channels are transmitted across two different under-carriers, usually ranging between 5 and 8.5 MHz (depending on the satellite broadcasting system) (figure 1. 6b).

Discreet system (Wegener Panda): the right and left channels are transmitted separately on two different under-carriers.

Sound-in-Sync system: the audio signal is transmitted, in PCM digital format, during the line and field returns.

Figure 4: Examples of Transmission Standards (a) PAL mono (b) NTSC stereoIn the last few years, a new transmission standard is being used which employs the QPSK digital modulation. This new standard has to respect the frequency channeling used by the existent satellites to co-exist with the last analog method. The QPSK modulation employed by the standard DVB or more exactly, the DVB-S (Digital Video Broadcasting-Satellite) method uses the simplest digital modulation diagram i.e. the carrier is modulated directly from a bit-stream or a bit flow.

To increase the efficiency of the transmission, more complex digital modulations are being used. Starting from the simple QAM (Quadrature Amplitude Modulation) as shown in figure 1.7, you can reach the more complex QPSK ones.

Figure 1.7: QPSK Modulation and Demodulation Block Diagram

In the QAM, there are two input signals (I and Q) modulating two carriers shifted of 90o coming from the same local oscillator. In this way, we have a signal in time with the phase shifts that are typically represented with a constellation, as shown in figure 1.8. In this case, which is the simplest one, we have only four states. This is the simplest QPSK modulation. Each state or carrier carries 2-bit information that is called a Symbol.

Figure 1.8: QPSK Signal ConstellationTo increase the spectral efficiency, the tendency is to increase the number of bits per symbol, because the symbol-rate, determining the used bandwidth, is equal to the bit-rate. In this example the QPSK is 2, as it is equal to the speed expressed in symbol/second divided by the number of bits constituting a symbol. The speed limit is caused by the effect of noise, causing as error in the information reception. Increasing the number of states and consequently, the symbol rate, it is necessary to have a higher S/N or C/N ratio. Actually, the existent S/N ratio is low, the modulation with four layers is the maximum usable limit.

REVIEW QUESTIONS

Q. # 1: Which frequency bands are used for TV signal broadcasting via satellite?

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Q. # 2: On which orbit is the TV satellite allocated, and how is their position expressed?

Answer: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Q. # 3: Which graph supplies the information about the area of the earth surface reached by signal emitted by the satellite, and power of the signal received on the earth.

Answer: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Q. # 4: Which parameter defining the power received on the earth? Describe the meaning.

Answer: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Q. # 5: Which techniques are used to transmit more TV channels inside the same frequency band?

Answer: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Q. #6: What is the benefit of using QPSK in satellite transmission? And what is the maximum usable limit for the modulation?

Answer: ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

FINAL CHECK LIST Submit your answers to questions before the next laboratory

Radio Communications Lab