chap 8 earthsegment

7/30/2019 Chap 8 Earthsegment

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IntroductionReceive-only home TV Systems

Master Antenna TV System

Community Antenna TV System

Transmit-Receive Earth Stations

Homework Problems


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Introduction

The earth segment of a satellite communications

system consists of the transmit and receive earthstations.

The simplest of those are the home TV receive-only

(TVRO) system

The most complex are the terminal stations usedfor international communication networks.

Also, included in the earth segment are:

Earth station on ships at sea Commercial and military land stations

Commercial and military aeronautical mobile

stations


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Receive-Only Home TV Systems

Direct Broadcast Satellite (DBS) service: Some variation in thefrequency bands and different geographical regions. In Americas,

the downlink band is 12.2 to 12.7 GHz. (500mhz)

C-band larger receiving antenna (about 3-m diameter) for network

relay to commercial TV outlets. It differs from Ku-band receive-only

systems lies in the frequency of operation of the outdoor unit, andEIRP of Ku-band is higher.

INTELSAT-702 77dBW EIRP 80dBW 30,240,000 74dBW EIRP 77dBW 24,192,000 71dBW EIRP 74dBW 18,144,000 68dBW EIRP 71dBW 12,096,000 EIRP 68dBW 6,048,000


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Receive-Only Home TV Systems Polarization interleaving: Frequency reuse by

using the alternately polarized LHC/RHC and/orvertical/horizontal to reduce interference to

acceptable levels. Interleaving

The signal fed to the indoor unit is normally a

wideband signal covering the range 950-1450MHz

The 70MHz Intermediate frequency (IF) carrier

must be demodulated and the base bandinformation used to generate a vestigial single

sideband (VSSB) signal which is fed into one of

the VHF/UHF channel of a standard TV set.


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There are many types of communication, however, such astelevision, video, facsimile and high-speed data signals that have a

large bandwidth incorporating significant low-frequency content.

Single sideband (SSB) systems have a poor low-frequency

response so vestigial-sideband (VSB) modulation is used, which is

derived by filtering double-sideband (DSB) so that one sideband iscompletely passed and just a trace, or vestige, of the other sideband

is included.

cos(A+B) = cosAcosB - sinAsinB

cos(A-B) = cosAcosB + sinAsinB

cosAcosB = (cos(A+B) + cos(A-B) )/2 Let A = wt & B = wct

cos wt cos wct = (cos(wc + w) t+ cos(wc - w)t )/2

There are two side band frequencies: wc+w and wc-w


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SSB versus AM: (1) Since the carrier is not transmitted,there is a reduction by 50% of the transmitted power (-

3dBm). --In AM @100% modulation: 1/2 of the power is

comprised of the carrier; with the remaining (1/2) power

in both sidebands. (2) Because in SSB, only one

sideband is transmitted, there is a further reduction by

50% in transmitted power (-3dBm (+) -3dBm = -6dBm).

(3) Finally, because only one sideband is received, the

receiver's needed bandwidth is reduced by one half--thus

effectively reducing the required power by the transmitteranother 50% (-3dBm (+) -3dBm (+) -3dBm = -9dBm).


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Receive-Only Home TV Systems(continued)

A vestigial sideband (VSB) in radio communication is asideband that has been only partly cut off or suppressed. VSB

transmission is similar to single-sideband (SSB) transmission,

in which one of the sidebands is completely removed. In VSB

transmission, however, the second sideband is not completely

removed, but is filtered to remove all but the desired range offrequencies.

Television broadcasts (regardless of NTSC, PAL, or SECAM

analog video format) use this method if the video is

transmitted in AM, due to the enormous bandwidth used. It

may also be used in digital transmission, such as the ATSC-standardized 8-VSB.

The bandwidth of vestigial-sideband systems is therefore only

slightly larger than that of SSB systems.


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Receive-Only Home TV Systems(continued)

Advanced TelevisionSystems Committee

(ATSC)Digital VideoBroadcasting-Terrestrial(DVB-T)

Integrated Services

Digital Broadcasting-


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Master Antenna TV Systems

A master antenna TV (MATV) systems is used

to provide reception of DBS TV/FM channels to

a small group of users, for example, to the

tenants in an apartment building. It consists of a single outdoor unit feeding a

number of indoor units.

The advantage is that only one outdoor unit is

required, but separate LNA/Cs and feedercables are required .


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Community Antenna TV System

The community antenna TV (CATV)

system employs a single outdoor unit, withseparate feeds available, like the MATV.

Instead of having a separate receiver for

each user, all the carrier are demodulatedin a common receiver-filter system.

In a remote areas where a cable

distribution system may not be installed,the signal can be rebroadcast from a low-

power VHF TV transmitter.


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Transmit-Receive Earth System

A transmit-only station is required in relaying TV signals

to the remote TV receive-only stations. Transmit-receive stations provide both functions and are

required for telecommunications traffic.

A number of different classes of earth stations are

available, depending on the service requirements. Trafficcan be broadly classified as heavy route, medium, and

thin route.

In a thin-route circuit, a transponder channel (36 MHz,

C-band) may be occupied by a number of single carriers,each associated with its own voice circuit. The mode of

operation is known as single carrier per channel (SCPC).


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Transmit-Receive Earth System(continued)

A medium-route circuit also provide multiple access,either on the basis of frequency-division multiple access

(FDMA) or time-division multiple access (TDMA),

multiplexed base band signals being carried in either

case. In 6/4-GHz heavy-route system, each satellite channel

(bandwidth 36 MHz) is capable of carrying over 960 one-

way voice circuits simultaneously or a single-color

analog TV signal with associated audio (in somesystems two analog TV signals can be accommodated).


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The largest satellite station in

the world, Goonhilly Earth

Station is located on the

Goonhilly Downs near Helston

on the Lizard peninsula.

The station has over 60 dishes

on the site enabling it totransmit to every corner of the

globe via space, and through

undersea fibre optic cables.

Goonhilly simultaneouslyhandles millions of

international phone calls,

emails, and TV broadcasts.

chap 8 earthsegment

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