Sattelite Communications

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Satellite Communications Tutorial

1ABSTRACTThis tutorials discusses the key areas of satellite

communications, discussing the various elements of a

satellite communications system eg antennas, path

loss etc. The communication system elements can

then be connected together and analysed to deter-

mine a link budget.

2FREQUENCIES FOR MICROWAVESATELLITE COMMUNICATIONS

The frequencies used for microwave satellite communi-

cations are determined by

(i) the absorption of the atmosphere as a function of

frequency

(ii) the antenna size needed to produce a beam with the

required angular spread

(iii) international agreements/regulations

2.1ATMOSPHERIC ABSORPTIONFigure 1 & Figure 2 indicates the average atmospheric

absorption as a function of frequency at different alti-

tudes above sea-level and the effects of rain and fog.

Note that the figures cover different frequency ranges.

Note 1. The first graph shows resonant absorption

peaks due to different molecules in the atmosphere atparticular frequencies. Usually these frequencies are

avoided for communications applications, though in

special cases they may be deliberately used so that the

signal will not propagate beyond a certain range - eg

covert military signals, or mobile communications where

the limited frequency range available means that the

same frequency must be re-used many times in different

communication cells.

Figure 1 Average atmospheric absorption of millimeter waves. A: Sea level ; T = 20C; P = 760mm; PH2O = 7.5g/m3. B : 4

km; T = 0C; PH2O = 1g/m3.

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Figure 2 Atmospheric absorption of millimeter

waves due to fog and rain.

Note 2 The second graph covers a much broader fre-

quency range, from microwave to optical and beyond. It

shows that although rain and fog increase the attenuation

of microwave signals the attenuation is still considerablyless at the lower microwave frequencies (up to 15GHz,

for example) than at optical frequencies, so that micro-

wave frequencies will maintain communication links

and remote sensing observations under conditions where

optical links will fail.

2.1.1Antenna sizeThe basic (approximate) relationship between wave-

length and antenna size is D(radians) where is the angular breadth of the main beam between the3dB points andD is the maximum dimension across the

antenna aperture. An aperture of 10 wavelengths willgive a beamwidth of about 6. At low frequencies the

wavelength is large, implying a large antenna. As the

frequency increases the antenna size reduces for a given

beamwidth but the attenuation of the atmosphere in-

creases. A compromise must be made. Note that at-

mospheric attenuation is not a problem for satellite-to-

satellite links, so these may involve mm-wave frequen-

cies and very small antennas.

2.2INTERNATIONAL REGULATIONSThe use of different frequency bands for different appli-

cations has been agreed through various internationalagencies

- see below for the allocation from 4990 to 7075MHz.

Allocation to Services

Region 1 Region 2 Region 34990 5000 FIXED

MOBILE except aeronautical mobile

RADIO ASTRONOMY

Space Research (passive)

795

5350 5255 RADIOLOCATION

Space Research

713 798

5650 5725 RADIOLOCATION

Amateur

Space Research (deep space)

664 801 803 804 805

5725 5850

FIXED SATELLITE

(Earth-to-space)

RADIOLOCATIONAmateur

801 803 805

806 807 808

5850 5925

FIXED

FIXED-SATELLITE

(Earth-to-space)

MOBILE

806

5850 5925

FIXED

FIXED-

SATELLITE

(Earth-to-space)

MOBILE

Amateur

Radiolocation

806

5850 5925

FIXED

FIXED-

SATELLITE

(Earth-to-space)

MOBILE

Radiolocation

806

5850 5925 FIXED

FIXED-SATELLITE

(Earth-to-space)

MOBILE

791 809

Note:

Region 1: Europe, Africa, N Asia; Region 2: N &S America; Region 3: rest of Asia

Upper case entries eg FIXED indicate a definiteallocation for the service in the frequency band.

Lower case entries show services that may be al-

lowed.

Numbers - such as 795 - refer to regulations whichapply to the frequency band.

2.3ORBITING AND GEOSTATIONARYSATELLITES

2.3.1Orbiting satellites

lower orbits - cheaper to launch. Eg remote sensingsatellites at about 800km altitude (about 1/8 earth ra-

dius).

not available all the time for communication links ideal for collecting data - eg remote sensing - trans-

mitting data back periodically to fixed earth sta-

tions. Earth coverage obtained by rotation of earthbeneath satellite.




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receive antennas must track satellite lower coverage than geostationary

2.3.2Geostationary satellites

occupy fixed position with respect to earth abovethe equator - no tracking required

3 satellites provide coverage for most of earth's sur-face (not polar regions)

Data:

radius of orbit: 42 000km (about 7 times earth radius)

altitude: 36 000kmorbital period: 24hours

3LINK BUDGET

TxGT

PT

R

Rx

GR

PR

Effective area = Aeff

2

TT

.R4

.GP

Isotropic power ieIf TX transmits as asphere.

2

TT

.R4

.GP

Sec0.24

10x3

1000x36,000x28

=

Received Power

PP G A

RR

T T eff =

4 2

effA is the receive antenna effective area

General antenna relationship: GAeff

=4

2

2

4

= RGGPP RTTR

GR is the Rx antenna gain

P GT Tis theEffective Isotropic Radiated Power (EIRP).

It gives a measure of the power flux. For each satellite

contours of constant EIRP can be plotted on the earth's

surface. A minimum value of EIRP is required for each

type of receiver (eg DBS). Usually the EIRP is given in

units of dBW - ( )TTGP1010logEIRP[dBW] = .

The link attenuation in dB is given by

=

=

GG

R

P

P

RTR

T 14

log10log10

2

[ ] [ ]dBGdBGR

RT

=

4log20

The first term is called the free space loss - due to the

spreading of the radiation, not absorption.




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3.1DBW (DECIBEL WATTS)

Link budget calculations are often carried out using

powers measured in dBW. The power is measured rela-

tive to a 1 watt reference power.

Power in dBW 10 logPower in Watts

1 Watt=

[ ] [ ] [ ]

+=

RGEIRPP RR

4log20dBdBWdBW

Corrections must be added toRP for additional losses

due to

1. antenna efficiency - power is lost in the antenna

feed structure, also in connections to the receiver

2. atmospheric absorption due to water and oxygen

molecules

3. polarisation mismatches of Tx and Rx antennas

4. antenna misalignments - ie boresights of Tx and

Rx antennas not aligned

An additional loss factorL is introduced to the link

budget equation to take account of these losses. The

equations become

( )LR

GGPP RTTR1

4

2

=

[ ] [ ] [ ] [ ] [ ]dB4log20dBdBdBWdBW LR

GGPP RTTR ++=

TypicallyL is about 5dB.

3.2LINK BUDGET CALCULATION

Calculate the power that must be transmitted from a geo-

stationary satellite to give a power of -116dBW (2.5

10-21 W) at a receiver on the earth. Assume f=10GHz,

dB40=RG , dB30=TG and additional losses of5dB.

R = altitude = 36000km

[ ] [ ] [ ] [ ] [ ]dB4log20dBdBdBWdBW LR

GGPP RTTR ++=

[ ] [ ] 52034030dBWdBW116 ++=

TP

=TP dBW dBW = 159W22

and EIRP = 22 dBW + 30dB = 52 dBW

3.3ANTENNA BEAMWIDTH AND GAINThe satellite antenna beamwidth must correspond to the

area of the earth to be illuminated. This determines the

gain of the antenna. The earth station antenna must be

able to select a particular geostationary satellite - the

satellite spacing in the crowded parts of the geostation-

ary orbit is about 2, though there may also be frequencydiscrimination between neighbouring satellites. The

following approximate results for a circular apertureantenna may be used to estimate suitable antenna sizes

and gains.

( )

DG

2

=

is the antenna efficiency, typically 0.6 to 0.7, D is the

antenna diameter

3 70dB = D

the 3dB beamwidth in degrees of the antenna.

3.4SYSTEM NOISE TEMPERATUREFor satisfactory operation a communication link must

have:

1. a large enough signal for the receiver sensitivity,

and

2. a high enough S/N ratio or BER at the receiver

output for good quality communication

eg for TV reception international regulations re-

quire a S/N ratio 47dB

Information is conveyed by modulating a high frequency

carrier with a message signal. The basic quality of a link

is expressed in terms of its carrier to noise ratio C/N

where C is the power for the unmodulatedcarrier and N

is the noise power, both measured at the receiver input.

The signal to noise ratio for an information signal - ie a

modulated carrier - depends upon both the C/N ratio for

the linkand the type of modulation used - ie AM, FM,

FSK, PSK etc.




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The noise powerassociated with the link is specified by

the system noise temperature Ts. This is made up from

three contributions:

1. antenna noise TA

2. antenna - receiver connection - a cable or

waveguide TC

3. receiver noise TR this may include RF,

mixer and IF stage contributions

In each case the noise power in watts (this is the avail-

able noise power) is calculated from the noise tempera-

ture (which must be in degrees K, ie absolute tempera-

ture) using the general relationship

available noise power = kTBwhere kis Boltzmann's constant andB

is the bandwidth. k= 1.38 10-23 J K-1

A useful figure to remember is that at 290K the available

noise power density is -174dBm/Hz

3.5ANTENNA NOISE TEMPERATURE TA

PR

Radiation into theBack lobes from thesurface reflections

Ground wave

Other RF sources egsatellites,galactic

sources etc

satellite

Antenna Noise PowerNA = kTA.B

Earth surface

Figure 3 Antenna noise temperature as a result of other noise sources including galactic and other satellites.

Referring Figure 3, the antenna noise is due to energy,

which is fed to the antenna by unwanted radiation

sources, such as stars and galaxies and other communi-

cation signals. (The latter are not strictly noise signals

in that they will not be random, but their effect on the

communication link will be the same as for noise - ie

they will worsen the S/Nratio and so they are included

here.) Also, the atmosphere itself behaves as a resistive

medium, which supplies noise power to the antenna.

The output noise power from the antenna N kT BA A=

will depend on the positions and temperatures and emis-

sivities of the noise sources and the gain and polar radia-

tion pattern of the antenna.

3.5.1Antenna pointing to the sky (ground stationantenna)

In this case the output noise power from the antenna has

two components which are represented by the sky tem-

perature, Tsky , and the earth temperature Tearth

Tsky is due to noise originating in the atmosphere. It

varies with frequency and the elevation angleEof the

antenna. The sky temperature is higher for E=0 (an-tenna pointing to the horizon) because of the longer path

of the radiation through the atmosphere. Elevation an-

gles of less than 10 are usually avoided. The two dia-grams Figure 4 and Figure 5 show Tsky for different

frequency ranges.




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Figure 4 Antenna sky noise temperature as a func-

tion of frequency and antenna angle.

Figure 5 Sky noise for clear air and 7.5 g/m3 of water

vapour concentration ( is the elevation angle)

For E 10 and f 15GHz Tsky

40K.

Tearth arises from radiation which feeds into the antennavia the back lobes of the antenna radiation pattern.

Radiation into theBack lobes from thesurface reflections

For a large (5m) Cassegraine antenna Tearth 10K

For a small ( 0.5m) antenna Tearth 100K

If an antenna points towards the Sun the noise effective

temperature is about 10 000K. This situation should be

avoided.

3.5.2Antenna pointing to the earthUsually the beamwidth is less than or equal to the angle

subtended by the earth, so that the earth fills the beam.

Then the noise temperatutre of the antenna is about

290K, the physical temperature of the earth.

3.6ANTENNA-TO-RECEIVER CONNECTINGCABLE

Although it is a passive element the cable or waveguide

that connects the antenna to the receiver has a noise tem-

perature TC which, contributes to the system noise tem-

perature. A passive component with an insertion lossL

has

RXIL = L (eg 2dB) gain = 1/L

Noise figure F=L effective noise temperature

( )10 = LTTe and Gain G = 1/L

Tc = To (F-1) = 290(L-1) Where, To = 290K




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3.7RECEIVER NOISE

Receiver noise includes contributions from thermal

noise, shot noise and possibly flicker noise. These mayarise in the input RF section of the receiver, the mixers

used for frequency translation and the IF stages. A

schematic diagram of a simple receiver and its equiva-

lent noise circuit is shown below. The total receiver

noise figureRT can be calculated from the individual

contributions from the usual formula for cascaded cir-

cuits.

( )1= RoR FTT FR is the receiver noise figure

In the schematic receiver shown in Figure 7.

mrf

if

rf

m

rfRGG

T

G

TTT ++=

Note: This formula follows from the corresponding for-

mula for the noise figure Ftotal for cascaded stages,

...11

21

3

1

21total +

+

+=

GG

F

G

FFF with

each noise figure replaced by its equivalent effective

noise temperature using ( )1= FTT oe

.

Example

LNA (low noise amplifier)

T G Grf rf rf = =50 23K dB [ = 200]

MixerT G Gm m m= = =500 0K dB [ 1]

IF stage

T G GIF IF IF

= = =1000 30K dB [ 1000]

= + +

= + + =TR 50500

200

1000

200 150 2 5 5 57 5. . K

Usually, the mixer has conversion loss eg

suppose dBG Gm m= = +10 0 1.

= + +

= + + =T KR 50500

200

1000

200 0 150 2 5 50 102 5

.. .

mrf

if

rf

m

rfRGG

T

G

TTT ++=

Figure 6 shows typical equivalent noise temperatures

and figures for various devices, which may be used in

microwave receivers.

15

20

30

50

70

100

150

200

300

500

700

1000

1500

2000

0.2

10

7

1.5

0.125

1.0

0.25

2

3

4

5

6

7

8

0.60.4 21 64 2010 10040 60

Frequency (GHz)

Equivalentnoisetemperatu

re(K)

Noisefigure(dB)

Mixer

Tunnel diode Amplifier

Cooled parametric amplifier

FETAmplifier

UncooledParametric amplifier

BipolarTransistoramplifier

Figure 6 Typical equivalent noise temperature and

noise figures of various devices




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LO

Receiver

Tc

cable

TA

Antenna

L.N.A

Mixer I.F Filter

I.FAmplifier

Gain = GifNoise = Tif

Gain = GmNoise = TmGain = Grf

Noise = Trf

Noise equivalent circuit

Trf

Gain Grf GmTm

Gain GifTif

RXTR

_________

Figure 7 System setup including the antenna, antenna cable feed and receiver. The gains and noise temperatures are de-

fined throughout the system.

3.8SYSTEM TEMPERATUREIf we consider the system temperature for a combination

of the antenna and the receiver with a receiver tempera-

ture of 102.5K:

RX

Antenna + Receiver

TA TR

Therefore, TS = TA + TR

T T TS A R= + = + =50 102 5 152 5. . K

If we now add a cable with IL 2dB [ IL = 1.58] be-tween the antenna and the receiver:

T F LC = = 290 1 290 1

Then, the system temperature at the receiver input

( ) RCAS TLTTT ++= (ie at receiver input use noise temperature x gain)

RA T

L

L

L

T+

+=

1290

Using the figures above,

K6.2405102581

1581290

581

50=+

+= .

.

.

.ST

ie. adding cable with 2dB IL increases TS from 152.5

to 240.6K. This illustrates the very significant effect

attenuation at the input has on noise. For this reaon the

LNA is often connected directly to the receive antenna.




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3.9C/N RATIO AT RECEIVER OUTPUT

Tx

GT

PT

R

RxGR

PR

L

1.

R.4.).GGP(

2

RTT

+=

C = Carrier power

From before: ( )LR

GGPP RTTR1

4

2

=

If system temperature is TS (includes antenna noise TA ,

cable and receiver noise)

Noise power (single link) at receiver input is

N kT BS=

( )kBLRT

GGP

BkT

P

N

C

s

RTT

s

R 11

4link

2

==

merit.offigurereceivertheis

S

R

T

G

Usually the down linkis the most critical due to the lim-

ited power which is available on board the satellite ( PT

)

and the antenna gain GT (limited by its size). Hence,

the most critical receiver is the earth station

eg Intelstat ground station

GHz4at740 1=

dBK

T

G

S

R .

The analysis above applies to a single link - ie up-link or

down-link, but information transmitted via satellite in-

volves both links. With reference to Figure 8 the total

C/N ratio for the two links can be found as follows:

Satellite

TransponderGain = G

Ld = Dielectric Loss

uplink

Nv

Cd at receiver

Power at earth station/Power at

satellite down link output

Cu

Figure 8 Schematic of the RF uplink and downl link

signal path

received down-link carrier power

C C G Ld u d=

total received down-link power

N N G L N u d d= +

HereuN is the uplink noise at the transponder (satel-

lite). dN is the noise added to the down link.

Hence,N

C

N G L N

C G L

N

C

N

Cd

u d d

u d

u

u

d

d

=+

= +

and so

( ) ( ) ( ) 1-downlink1-uplinktotal

1

NCNCNC

+=

Because of the limited power available on the satellite

for the downlink the C/N ratio for this link is usually

lower than that for the uplink, and this is the main de-

termining factor for the overall C/N ratio.

The total C/N ratio is also reduced by interference on

each link, and intermodulation distortion in the trans-

ponder, so a more complete expression is

EIRP

(Tx)

Free space loss

Bandwidth




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( )( ) ( ) ( ) ( ) ( ) 1

intermods

1-

downlink

1-

uplink

1-

downlink

1-

uplink

total

1++++

=NCICICNCNC

NC

Calculations using the above relationships apply to clear

air propagation conditions, but allowance has to be

made for additional attenuation and noise which may be

introduced on each link due to rainfall or other possible

meteorological conditions. The margin that must be

allowed depends upon the required reliability (eg link

maintained for 99.99% of time, averaged over one year)

and the range of climatic conditions which are predicted

along the link. The margins also vary with frequency

and the angle of elevation. Typical margin values are

2dB (C band) and 8dB (Ku band).

4MODULATION AND MULTIPLEXINGTECHNIQUES

Each earth station will, in general, be transmitting and

receiving many messages simultaneously to and from a

satellite. The messages may be 'phone calls, ratio and

TV signals and/or computer signals. They are transmit-

ted by modulating a carrier signal in some way - AM,

FM, PM (analogue), or ASK, FSK, PSK etc (digital). In

a multicarrier system the different messages are com-bined for transmission by multiplexing. The converse

process ofdemultiplexing is carried out at the receiver

The multiplexing techniques used are

i) Frequency Division Multiplexing (FDM) - each

message is placed in a different frequency range by

modulating a different carrier frequency. All the mes-

sages are combined for transmission.

Each satellite link will have a certain bandwidth. The

bandwidth may be divided into sub-bands with different

sub-bands assigned to each earth station. The figure

below shows a set of satellite transponders for (a) a C

band and (b) a Ku band system.

The C band transponder uses a single down converter

(D/C) and signal processing at 4GHz, whereas the Ku

band system uses D/C to 1GHz for signal processing

followed by up-conversion (U/C) for the down-link.

Each sub-band will contain many messages, which will

be fed together to the HPA (high power amplifier) for

amplification.

6GHz

DC

L.N.A

Multiple Transponders

FrequencyDEMUX

Equilizer

H.P.A

FrequencyMUX

4GHz

14GHz

DC

L.N.A

Multiple Transponders

FrequencyDEMUX

Equilizer

H.P.A

FrequencyMUX

11GHz

1GHz

U/Cs

1GHz 11GHz

C-Band TransponderFigure A

KU-Band TransponderFigure B

Figure 9 Schematic of two satellite transponders. The top one is a C-Band system and the one on the bottom is a Ku-

Band system. HPA = High power amplifier; DC = Downconverter; U/C = Upconverter.




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In the C band 6/4GHz transponder (Figure 9A):

the uplink is at the higher frequency, so D is

greater for the (common) receive/transmit antenna

it will have a higher gain

the input filter is a fairly wideband band-passroofing filter to allow all the uplink frequen-

cies in, but eliminating out-of-band noise

LNA low noise amplifier D/C down converter to 4GHz (the down-link fre-

quency) for signal processing error correction,

amplification, signal channelling etc.

frequency demultiplexing divides input signal intosub-bands to reduce non-linear distortion during

amplification. Each sub-frequency band is proc-essed by a single transponder.

equalisers correct for phase differences betweenthe different frequency components of a signal

which are introduced by filters, de-multiplexers etc

HPAs high power amplifiers to increase powerlevels before re-transmission on the down-link.

Non-linear performance in the HPAs can intoduce

harmonics, intermodulation distortion etc

band-pass filters at various points remove out-of-band products from the HPAs etc and reduce the

background noise, but they cannot remove in-band

products eg 3

rd

order intermodulation (IM) prod-ucts

The Ku (14/11GHz) system (Figure 9 B) has many of

the same elements, but the down-link frequency

(11GHz) is too high for the elements in each trans-

ponder, so the input is mixed down from 14GHz to

1GHz for de-multiplexing and equalisation, then mixed

up to 11GHz for power amplification, frequency MUX

and re-transmission.

4.1NON-LINEAR BEHAVIOUR IN HPASBecause each transponder will be processing a very

large number of messages simultaneously any non-

linearity in the transponder amplifier will lead to inter-

modulation which causes interference between the mes-

sage signals by transferring modulations from one fre-

quency range to another. The diagram Figure 10 shows

a non-linear amplifier voltage transfer characteristic and

the way in which it leads to signal distortion. The dis-

tortion is normally represented in terms of additional

harmonics of the input signal, which are introduced by

the amplifier. The non-linearity may also be represented

in terms of the amplifier power transfercharacteristic,

which also shows the saturation and saturation powerof

the amplifier.

Vout

Pure sinewave

Vin

Non-linearsaturation

Distorted fo,2fo, 3fo etc

fo

Figure 10 The diagram shows the non-linear (in the

saturation region) Vout vs vin curve for an ampli-

fier. If a sine-wave is applied to the input the non-

linearity will distort the amplified output sinewave

as shown.

Intermodulation can be reduced using back-off, as

shown in Figure 11 Figure 11. The input signal signal

power is reduced to move below the non-linear segment

of the characteristic. The amount of back-off can be

expressed in terms of either the input signal back-offor

the output signal backoff. A disadvantage of usingback-off is that it reduces the efficiency of the amplifier

because the RF output from the amplifier is reduced

whilst it is still consuming the same DC power.

Pout

Back offPin

Saturation - IMD

PSAT

Outputpower

backoff

Figure 11 shows how distortion can be reduced by backing

off the input signal from the saturation region to the linear

region.

The amount of back-off needed to avoid intermodulation

increases with the number of messsages (ie modulated

carriers) in the signal which is applied to the trans-




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ponder. One solution is to increase the number of trans-

ponders on board the satellite so that each need only

handle a restricted bandwidth and number of carriers.

This, of course, increases the satellite mass, so a suitablecompromise must be reached between the number of

transponders and the intermodulation.

Back-off modifies the formula for the down-link C/N

ratio by making : P P BOT os o= Where, Pos is the output power of the HPA at satura-

tion and BOo is the output backoff power . Pos is

normally known for a given amplifier, then oBO is

adjusted dynamically according to the strength of the

input signal.

Solid state amplifiers are superior to TWT amplifiers in

their linearity. Considerable attention has been devoted

to techniques for linearising HPAs to improve their effi-

ciency. This involves extending the linear part of the

power amplifier characteristic.

ii) Time Division Multiplexing (TDM) - each message is

transmitted at a different time. TDM is usually used

with digitally coded messages. Whereas with FDM each

message is transmitted continuously using a restricted

bandwidth, with TDM each message is only transmitted

for a small fraction of the available time, but during that

time it uses all the available bandwidth.

Clearly, a system must be established to regulate the

timeslots for each message. This scheduling will itself

require the communication of earth stations via the satel-

lite which imposes a network management overhead on

the available bandwidth/transmission time. An appro-

priate balance must be struck between the complexity of

the 'housekeeping' of the communication system and the

useful communication capacity.

An advantage of TDM is that intermodulation distortion

can be avoided, because only one message is being am-

plified at any one time.

iii) Code Division Multiplex (CDM) - each message

includes a unique code which means that TDMA can be

used with different signals being transmitted simultane-

ously - the code allows the elements of the different

messages to be grouped correctly. CDM uses a very

wide bandwidth and so this technique is sometimes also

known as a spread spectrum technique.

4.2MULTIPLE ACCESSMultiple accessrefers to the fact that many earth stations

share the same satellite. Signals from several earth sta-

tions may arrive simultaneously at the satellite antenna

from which they are fed to the transponder which will

process the signals in several ways - eg amplification,

error detection and correction, filtering and frequencychanging - before feeding the signals back to the satellite

antenna for the down link. The uplink and the down

link operate at different frequencies to avoid direct cou-

pling of signals from the transmit to the receive channels

eg 6/4GHz (C band), 14/11GHz Ku band). The higher

frequency is used for the up-link because the satellite

antenna has limited size and a higher noise temperature

(usually 290K). The gain is higher at the upper fre-

quency for a fixed antenna size.

Similarly, the signals transmitted from a satellite will

usually be received by all the earth stations. Most of the

messages received will not be needed by a specific earth

station - they must be filtered out during de-

multiplexing. In a typical analogue system a trans-

ponder may have a bandwidth of 36MHz, but this will

be subdivided into 12 sub-bands, each with a bandwidth

of 3MHz. When an earth station receives messages

from its vicinity via the PSTN network it sorts them out

into their destination earth stations. All the messages for

a particular earth station are combined to one sub-band

for the uplink. They are all processed by the satellite

transponder and transmitted to the earth stations, but

each earth station will only process its own sub-band.

As noted earlier, multiplexing and modulation are sepa-

rate processes and so various combinations of the differ-

ent techniques available for each can be used. Accord-

ing to Glover and Grant, the predominant multiplex-

ing/modulation/multiple access technique in current use

for PSTN satellite telephony is FDM/FM/FDMA, but

this leads to large intermodulation products. Increas-

ingly, digital modulation (PCM) is replacing analogue

techniques, leading to TDM/PSK/TDMA.

With the systems described so far the communication

capacity between different earth stations is essentially

'designed in' when the bandwidths assigned to each sta-tion are fixed, and changes cannot easily be made even

if demand changes. Capacity can be increased, and

made more flexible, by

i) using multiple spot beams that can be steered as

required to different points on the earth's surface, and

ii) by using a switching matrix on board the satellite

to co-ordinate the message transmission with the beam

direction.


Documents

Sattelite Communications