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                                    Satellite Communications Tutorial
                                                       J P Silver


                   1 ABSTRACT
This tutorials discusses the key areas of satellite                       2.1 ATMOSPHERIC ABSORPTION
communications, discussing the various elements of a              Figure 1 & Figure 2 indicates the average atmospheric
satellite communications system eg antennas, path                 absorption as a function of frequency at different alti-
loss etc. The communication system elements can                   tudes above sea-level and the effects of rain and fog.
then be connected together and analysed to deter-                 Note that the figures cover different frequency ranges.
mine a link budget.
                                                                  Note 1. The first graph shows resonant absorption
                                                                  peaks due to different molecules in the atmosphere at
  2 FREQUENCIES FOR MICROWAVE                                     particular frequencies. Usually these frequencies are
    SATELLITE COMMUNICATIONS                                      avoided for communications applications, though in
The frequencies used for microwave satellite communi-             special cases they may be deliberately used so that the
      cations are determined by                                   signal will not propagate beyond a certain range - eg
(i) the absorption of the atmosphere as a function of             covert military signals, or mobile communications where
      frequency                                                   the limited frequency range available means that the
(ii) the antenna size needed to produce a beam with the           same frequency must be re-used many times in different
      required angular spread                                     communication                                     cells.
(iii) international agreements/regulations

   Figure 1 Average atmospheric absorption of millimeter waves. A: Sea level ; T = 20˚C; P = 760mm; PH2O = 7.5g/m3. B : 4
   km; T = 0˚C; PH2O = 1g/m3 .
                                                                                                              2 of 12

                                                             - see below for the allocation from 4990 to 7075MHz.
                                                                                    Allocation to Services
                                                                      Region 1                Region 2             Region 3
                                                             4990 – 5000 FIXED
                                                                          MOBILE except aeronautical mobile
                                                                          RADIO ASTRONOMY
                                                                          Space Research (passive)
                                                             5350 – 5255 RADIOLOCATION
                                                                          Space Research
                                                                          713 798
                                                             5650 – 5725 RADIOLOCATION
                                                                          Space Research (deep space)
                                                                          664 801 803 804 805
                                                             5725 – 5850
                                                             FIXED SATELLITE
                                                             801 803 805
                                                             806 807 808
                                                             5850 – 5925                 5850 – 5925          5850 – 5925
                                                             FIXED                       FIXED                FIXED
                                                             FIXED-SATELLITE             FIXED-               FIXED-
                                                             (Earth-to-space)            SATELLITE            SATELLITE
                                                             MOBILE                      (Earth-to-space)     (Earth-to-space)
   Figure 2 Atmospheric absorption of millimeter
                                                                                         MOBILE               MOBILE
   waves due to fog and rain.                                806                         Amateur              Radiolocation
Note 2 The second graph covers a much broader fre-                                       Radiolocation        806
quency range, from microwave to optical and beyond. It       5850 – 5925        FIXED
shows that although rain and fog increase the attenuation                       FIXED-SATELLITE
of microwave signals the attenuation is still considerably                     (Earth-to-space)
less at the lower microwave frequencies (up to 15GHz,                           MOBILE
                                                                                791 809
for example) than at optical frequencies, so that micro-
wave frequencies will maintain communication links           Note:
and remote sensing observations under conditions where       • Region 1: Europe, Africa, N Asia; Region 2: N &
optical links will fail.                                         S America; Region 3: rest of Asia
                                                             • Upper case entries eg FIXED indicate a definite
                     2.1.1 Antenna size                          allocation for the service in the frequency band.
The basic (approximate) relationship between wave-               Lower case entries show services that may be al-
length and antenna size is θ (radians) ≈ λ D where               lowed.
                                                             • Numbers - such as 795 - refer to regulations which
θ is the angular breadth of the main beam between the            apply to the frequency band.
3dB points and D is the maximum dimension across the
antenna aperture. An aperture of 10 wavelengths will
give a beamwidth of about 6°. At low frequencies the               2.3 ORBITING AND GEOSTATIONARY
wavelength is large, implying a large antenna. As the                         SATELLITES
frequency increases the antenna size reduces for a given
beamwidth but the attenuation of the atmosphere in-
                                                                               2.3.1 Orbiting satellites
creases. A compromise must be made. Note that at-
mospheric attenuation is not a problem for satellite-to-
                                                             • lower orbits - cheaper to launch. Eg remote sensing
satellite links, so these may involve mm-wave frequen-
                                                             satellites at about 800km altitude (about 1/8 earth ra-
cies and very small antennas.
                                                             • not available all the time for communication links
      2.2 INTERNATIONAL REGULATIONS                          • ideal for collecting data - eg remote sensing - trans-
The use of different frequency bands for different appli-         mitting data back periodically to fixed earth sta-
cations has been agreed through various international             tions. Earth coverage obtained by rotation of earth
agencies                                                          beneath satellite.
                                                                                                                      3 of 12

•   receive antennas must track satellite                             •    Data:
•   lower coverage than geostationary
                                                                      radius of orbit:     42 000km (about 7 times earth radius)
                                                                      altitude:            36 000km
            2.3.2 Geostationary satellites
                                                                      orbital period:      24hours
•   occupy fixed position with respect to earth above
    the equator - no tracking required
•   3 satellites provide coverage for most of earth's sur-
    face (not polar regions)
                                                        3 LINK BUDGET

                                                                                          PT .G T    Isotropic power ie
                                            2 x 36,000 x 1000
                                                              = 0.24 Sec                  4π .R 2    If TX transmits as a
                                                 3 x 10 8                                            sphere.

             GT                                                                                       Rx
      Tx                                                      Effective area = Aeff


                                                                                                       PT .G T
     Received Power                                                                                    4π .R 2

            PT GT Aeff                                                The link attenuation α in dB is given by
    PR =
                                                                                                    ⎛ 4πR ⎞ ⎛⎜ 1

                                                                                 ⎛    ⎞                               ⎞
    Aeff is the receive antenna effective area                         α = 10 log⎜ PT ⎟ = 10 log
                                                                                 ⎝ PR ⎠
                                                                                                    ⎜ λ ⎟ ⎜⎝ G G      ⎟
                                                                                                    ⎝ ⎠

                                             4 π Aeff
    General antenna relationship: G    =
                                               λ2                              ⎛ 4πR ⎞
                   ⎛ λ ⎞
                             2                                         = 20 log⎜     ⎟ − GT [dB ] − G R [dB ]
                                                                               ⎝ λ ⎠
    PR = P G T G R ⎜
                   ⎜     ⎟
                   ⎝ 4πR ⎠
                                                                      The first term is called the free space loss - due to the
    GR is the Rx antenna gain                                         spreading of the radiation, not absorption.

 PT GT is the Effective 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 - EIRP[dBW] = 10log10 ( PT GT ) .
                                                                                                                     4 of 12

                                                                                   − 116[dBW] = PT [dBW]+ 30 + 40 − 203− 5
             3.1 DBW (DECIBEL WATTS)
                                                                                   ∴ PT dBW = 22dBW = 159W
Link budget calculations are often carried out using
powers measured in dBW. The power is measured rela-
                                                                                   and   EIRP = 22 dBW + 30dB = 52 dBW
tive to a 1 watt reference power.

          Power in dBW = 10 log
                                         Power in Watts                         3.3 ANTENNA BEAMWIDTH AND GAIN
                                            1 Watt                        The satellite antenna beamwidth must correspond to the
                                                                          area of the earth to be illuminated. This determines the
                                          ⎛ 4πR ⎞                         gain of the antenna. The earth station antenna must be
PR [dBW ] = EIRP[dBW ] + G R [dB] − 20 log⎜     ⎟                         able to select a particular geostationary satellite - the
                                          ⎝ λ ⎠
                                                                          satellite spacing in the crowded parts of the geostation-
                                                                          ary orbit is about 2°, though there may also be frequency
Corrections must be added to PR for additional losses                     discrimination between neighbouring satellites. The
due to                                                                    following approximate results for a circular aperture
                                                                          antenna may be used to estimate suitable antenna sizes
1.    antenna efficiency - power is lost in the antenna                   and gains.
      feed structure, also in connections to the receiver
2.    atmospheric absorption due to water and oxygen
                                                                                          ( )
      molecules                                                                    G =η
3.    polarisation mismatches of Tx and Rx antennas                                      λ
4.    antenna misalignments - ie boresights of Tx and
                                                                          η is the antenna efficiency, typically 0.6 to 0.7, D is the
      Rx antennas not aligned
                                                                          antenna diameter

An additional loss factor L is introduced to the link                              θ 3dB = 70 λ D
budget equation to take account of these losses. The
equations become
                                                                          the 3dB beamwidth in degrees of the antenna.

                            ( )λ     2
          P R = PT GT G R
                              4πR        L
                                                                                  3.4 SYSTEM NOISE TEMPERATURE

                                                     ( )
                                                                          For satisfactory operation a communication link must
PR [dBW ] = PT [dBW ] + GT [dB] + G R [dB]− 20 log            − L[dB]     have:
                                                                          1.    a large enough signal for the receiver sensitivity,
Typically L is about 5dB.                                                       and
                                                                          2.    a high enough S/N ratio or BER at the receiver
         3.2 LINK BUDGET CALCULATION                                            output for good quality communication
                                                                                eg for TV reception international regulations re-
Calculate the power that must be transmitted from a geo-                        quire a S/N ratio ≥ 47dB
stationary satellite to give a power of -116dBW (2.5 ×
10-21 W) at a receiver on the earth. Assume f=10GHz,                      Information is conveyed by modulating a high frequency
                                                                          carrier with a message signal. The basic quality of a link
G R = 40dB , GT = 30dB and additional losses of                           is expressed in terms of its carrier to noise ratio C/N
5dB.                                                                      where C is the power for the unmodulated carrier and N
                                                                          is the noise power, both measured at the receiver input.
R = altitude = 36000km                                                    The signal to noise ratio for an information signal - ie a

                                                     ( )
                                                                          modulated carrier - depends upon both the C/N ratio for
PR [dBW ] = PT [dBW]+ GT [dB]+ G R [dB]− 20 log                 − L[dB]   the link and the type of modulation used - ie AM, FM,
                                                          λ               FSK, PSK etc.
                                                                                                                    5 of 12

The noise power associated with the link is specified by             ture (which must be in degrees K, ie absolute tempera-
the system noise temperature Ts. This is made up from                ture) using the general relationship
three contributions:
                                                                               available noise power = kTB
   1. antenna noise       TA                                                   where k is Boltzmann's constant and B
   2. antenna - receiver connection - a cable or
   waveguide TC                                                                is the bandwidth. k = 1.38 × 10-23 J K-1
   3. receiver noise      TR       this may include RF,
                                                                     A useful figure to remember is that at 290K the available
   mixer and IF stage contributions                                  noise power density is -174dBm/Hz
In each case the noise power in watts (this is the avail-
able noise power) is calculated from the noise tempera-
                                      3.5 ANTENNA NOISE TEMPERATURE TA


             Antenna Noise Power
                  NA = kTA.B

                                                                                           Other RF sources eg
                                                PR                                             sources etc

                                                                         Ground wave

            Radiation into the                                          Earth surface
           Back lobes from the
            surface reflections

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

                                                                       3.5.1 Antenna pointing to the sky (ground station
Referring Figure 3, the antenna noise is due to energy,
which is fed to the antenna by unwanted radiation
                                                                     In this case the output noise power from the antenna has
sources, such as stars and galaxies and other communi-
                                                                     two components which are represented by the sky tem-
cation signals. (The latter are not strictly noise signals
                                                                     perature, Tsky , and the earth temperature Tearth
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/N ratio and so they are included              Tsky is due to noise originating in the atmosphere. It
here.) Also, the atmosphere itself behaves as a resistive            varies with frequency and the elevation angle E of the
medium, which supplies noise power to the antenna.                   antenna. The sky temperature is higher for E=0° (an-
The output noise power from the antenna N A = kTA B                  tenna pointing to the horizon) because of the longer path
will depend on the positions and temperatures and emis-              of the radiation through the atmosphere. Elevation an-
sivities of the noise sources and the gain and polar radia-          gles of less than 10° are usually avoided. The two dia-
tion pattern of the antenna.                                         grams Figure 4 and Figure 5 show Tsky for different
                                                                     frequency ranges.
                                                                                                       6 of 12

                                                                         Radiation into the
                                                                        Back lobes from the
                                                                         surface reflections

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

                                                            For a small (≈ 0.5m) antenna          Tearth ≈ 100K
   Figure 4 Antenna sky noise temperature as a func-        If an antenna points towards the Sun the noise effective
   tion of frequency and antenna angle.
                                                            temperature is about 10 000K. This situation should be

                                                                     3.5.2 Antenna pointing to the earth
                                                            Usually 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.

                                                              3.6 ANTENNA-TO-RECEIVER CONNECTING
                                                            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 loss L
Figure 5 Sky noise for clear air and 7.5 g/m3 of water
vapour concentration (φ is the elevation angle)

For E ≥ 10° and f ≤ 15GHz Tsky ≤ 40K.                                                                         RX
Tearth arises from radiation which feeds into the antenna        IL = L (eg 2dB)     gain = 1/L
via the back lobes of the antenna radiation pattern.
                                                            Noise figure F = L ∴ effective noise temperature

                                                            Te =T0 (L−1) and Gain G = 1/L

                                                            Tc = To (F-1) = 290(L-1) Where, To = 290K
                                                                                                                                                            7 of 12

                 3.7 RECEIVER NOISE                                                                                       Tm     Tif
                                                                                                 TR = Trf +                   +
Receiver noise includes contributions from thermal                                                                        Grf   Grf Gm
noise, shot noise and possibly flicker noise. These may
arise in the input RF section of the receiver, the mixers                                        Figure 6 shows typical equivalent noise temperatures
used for frequency translation and the IF stages. A                                              and figures for various devices, which may be used in
schematic diagram of a simple receiver and its equiva-                                           microwave receivers.
lent noise circuit is shown below. The total receiver
noise figure TR can be calculated from the individual                                            2000

contributions from the usual formula for cascaded cir-                                           1500                                                                       8
                                                                                                 1000                 Mixer

TR =To (FR −1) FR is the receiver noise figure                                                    700                                                                       5
                                                                                                  500          Tunnel diode Amplifier                                       4

                                                              Equivalent noise temperature (K)
In the schematic receiver shown in Figure 7.                                                      300                                           Amplifier                   3

                                                                                                                                                                                    Noise figure (dB)
                                                                                                  200           Bipolar                                                     2
                            Tm     Tif                                                            150
           TR = Trf +           +
                            Grf   Grf Gm                                                          100
                                                                                                   70                                                                   1.0
                                                                                                   50            Uncooled
Note: This formula follows from the corresponding for-                                                           Parametric amplifier
mula for the noise figure Ftotal for cascaded stages,                                              20                                                                   0.25
                F − 1   F − 1
         = F1 + 2     + 3     + ... with
Ftotal                                                                                             10                                Cooled parametric amplifier
                 G1     G1G2
                                                                                                   7                                                                        0.125
each noise figure replaced by its equivalent effective
noise temperature using T e=To (F −1) .
                                                                                                         0.2    0.4 0.6      1   2      4   6     10    20 40      60 100

                                                                                                                                 Frequency (GHz)

                                                                                                       Figure 6 Typical equivalent noise temperature and
LNA        (low noise amplifier)                                                                       noise figures of various devices

           Trf = 50K       Grf = 23dB [Grf = 200]
           Tm = 500K        Gm = 0dB    [ Gm = 1]

IF stage

TIF = 1000K           GIF = 30dB       [ GIF = 1000]

                500    1000
∴ TR = 50 +         +         = 50 + 2. 5 + 5 = 57.5 K
                200   200 × 1

Usually, the mixer has conversion loss eg
suppose Gm = − 10dB ∴ Gm = + 0.1

                500     1000
∴ TR = 50 +         +            = 50 + 2. 5 + 50 = 102. 5K
                200   200 × 0 .1
                                                                                                                         8 of 12

                                             L.N.A                                                                   I.F
  TA                                                                 Mixer                   I.F Filter

  Antenna            cable                                 Gain = Gm
                                       Gain = Grf          Noise = Tm
                                       Noise = Trf
                                                                                                          Gain = Gif
                                                                                                          Noise = Tif


                                                                  Noise equivalent circuit

                                                                Tm                       Tif
                                                Gain Grf                   Gm                     Gain Gif

                                          ___                              RX
                                          ___              TR

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

            3.8 SYSTEM TEMPERATURE
If we consider the system temperature for a combination                          TS = (TA +TC ) L+ TR
of the antenna and the receiver with a receiver tempera-                (ie at receiver input use noise temperature x gain)
ture of 102.5K:

  Antenna + Receiver                                                                   TA     ⎛ L−1 ⎞
                                                                                   =      +290⎜     ⎟+TR
                                                                                       L      ⎝ L ⎠
                TA           TR                                         Using the figures above,

                                                                                         50       ⎛ 1.58−1 ⎞
         Therefore, TS = TA + TR                                                 TS =        + 290⎜        ⎟ +102.5= 240.6K
                                                                                        1.58      ⎝ 1.58 ⎠
TS = TA + TR = 50 + 102.5 = 152. 5K
                                                                        ie. adding cable with 2dB IL increases TS from 152.5
If we now add a cable with IL 2dB [⇒ IL = 1.58] be-                     to 240.6K. This illustrates the very significant effect
tween the antenna and the receiver:                                     attenuation at the input has on noise. For this reaon the
                                                                        LNA is often connected directly to the receive antenna.
TC = 290 F − 1 = 290 L − 1

Then, the system temperature at the receiver input
                                                                                                                                           9 of 12


          GT                                                                                                         Satellite
                                            Rx                                                                     Transponder
 Tx                                                                                                                  Gain = G
                                                          GR                                 Cu

                PT                                                                          uplink
                                     PR                                                                                                      Ld = Dielectric Loss

                             R                        ˜ C = Carrier power

                                                                     ⎛ λ ⎞ 1
                                                                                                                                                         Cd at receiver
                                                   = (PT + G T ).GR .⎜       ⎟ .
                                                                     ⎝ 4π .R ⎠ L

                                       ⎛ λ ⎞ 1

From before:           PR =(PT GT )G R ⎜     ⎟                                                                                      Power at earth station/Power at
                                       ⎝ 4πR ⎠ L                                                                                    satellite down link output

If system temperature is TS (includes antenna noise TA ,
                                                                                      Figure 8 Schematic of the RF uplink and downl link
cable and receiver noise)                                                             signal path

Noise power (single link) at receiver input is
                                                                                   received down-link carrier power
 N = kTS B                                       Free space loss
                                                                                   Cd = Cu G Ld
                                            ⎛ GR ⎞ ⎛ λ ⎞ 1 1

      (link ) = PR = PT GT                  ⎜
                                            ⎜     ⎟ ⎜
                                                  ⎟       ⎟
                                                                                   total received down-link power
    N           kTs B                       ⎝ T s ⎠ ⎝ 4πR ⎠ L kB
                                                                                   N = N u G Ld + N d
                                 (Tx)            Bandwidth                         Here N u is the uplink noise at the transponder (satel-
⎛ GR   ⎞
⎜T     ⎟ is the receiver figure of merit.
                                                                                   lite). N d is the noise added to the down link.
⎝ S    ⎠
                                                                                             N   N G Ld + N d  N                                      Nd
Usually the down link is the most critical due to the lim-                         Hence,       = u           = u                               +
ited power which is available on board the satellite ( PT )                                  Cd     Cu G Ld    Cu                                     Cd
and the antenna gain GT (limited by its size). Hence,
                                                                                   and so
the most critical receiver is the earth station

           eg Intelstat ground station                                                       (C      N )total =
⎛ GR   ⎞                                                                                                          (C N ) uplink
                                                                                                                                  +(C N )downlink

⎜T     ⎟=40.7 dBK −1
       ⎟                  at 4 GHz
⎝ S    ⎠
                                                                                   Because of the limited power available on the satellite
The analysis above applies to a single link - ie up-link or                        for the downlink the C/N ratio for this link is usually
down-link, but information transmitted via satellite in-                           lower than that for the uplink, and this is the main de-
volves both links. With reference to Figure 8 the total                            termining factor for the overall C/N ratio.
C/N ratio for the two links can be found as follows:
                                                                                   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
                                                                                                                                                                                    10 of 12

(C N )total =                                    1                                                               TV signals and/or computer signals. They are transmit-
                (C N )uplink+(C N )
                      -1           -1
                                          +(C I )
                                                             +(C I )
                                                                          +(C N )
                                                                                                                 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-
      Calculations using the above relationships apply to clear                                                  bined for transmission by multiplexing. The converse
      air propagation conditions, but allowance has to be                                                        process of demultiplexing is carried out at the receiver
      made for additional attenuation and noise which may be
      introduced on each link due to rainfall or other possible                                                  The multiplexing techniques used are
      meteorological conditions. The margin that must be
      allowed depends upon the required reliability (eg link                                                     i)   Frequency Division Multiplexing (FDM) - each
      maintained for 99.99% of time, averaged over one year)                                                     message is placed in a different frequency range   by
      and the range of climatic conditions which are predicted                                                   modulating a different carrier frequency. All the mes-
      along the link. The margins also vary with frequency                                                       sages are combined for     transmission.
      and the angle of elevation. Typical margin values are
      2dB (C band) and 8dB (Ku band).                                                                            Each satellite link will have a certain bandwidth. The
                                                                                                                 bandwidth may be divided into sub-bands with different
        4 MODULATION AND MULTIPLEXING                                                                            sub-bands assigned to each earth station. The figure
                                                                                                                 below shows a set of satellite transponders for (a) a C
                 TECHNIQUES                                                                                      band and (b) a Ku band system.
      Each earth station will, in general, be transmitting and                                                   The C band transponder uses a single down converter
      receiving many messages simultaneously to and from a                                                       (D/C) and signal processing at 4GHz, whereas the Ku
      satellite. The messages may be 'phone calls, ratio and
      band system uses D/C to 1GHz for signal processing                                                         be fed together to the HPA (high power amplifier) for
      followed by up-conversion (U/C) for the down-link.                                                         amplification.
      Each sub-band will contain many messages, which will
                                                                                            Frequency DEMUX

                            6GHz                                                                                                                               4GHz
                                                                                                                                               Frequency MUX




                                      C-Band Transponder
                                      Figure A


                                                                                                                Multiple Transponders

                                                                                                                     1GHz         11GHz

                                                                                     Frequency DEMUX

                           14GHz                                                                                                                                                  11GHz
                                                                                                                                                                  Frequency MUX



                                                                                                                            U/C’s     H.P.A
                                   KU-Band Transponder
                                   Figure B

                                                                                                              Equilizer      Multiple Transponders

           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.
                                                                                                           11 of 12

In the C band 6/4GHz transponder (Figure 9 A):                                            Non-linear      Distorted fo,
                                                                                          saturation      2fo, 3fo etc
•   the uplink is at the higher frequency, so D λ is
    greater for the (common) receive/transmit antenna –
    it will have a higher gain
•   the input filter is a fairly wideband band-
    pass‘roofing’ 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.                                                              Vin

•   frequency demultiplexing – divides input signal into
                                                                                             Pure sinewave
    sub-bands to reduce non-linear distortion during
    amplification. Each sub-frequency band is proc-                                  fo
    essed by a single transponder.
•   equalisers – correct for phase differences between          Figure 10 The diagram shows the non-linear (in the
    the different frequency components of a signal              saturation region) Vout vs vin curve for an ampli-
    which are introduced by filters, de-multiplexers etc        fier. If a sine-wave is applied to the input the non-
•   HPAs – high power amplifiers – to increase power            linearity will distort the amplified output sinewave
    levels before re-transmission on the down-link.             as shown.
    Non-linear performance in the HPAs can intoduce          Intermodulation can be reduced using back-off, as
    harmonics, intermodulation distortion etc                shown in Figure 11 Figure 11. The input signal signal
•   band-pass filters at various points remove out-of-       power is reduced to move below the non-linear segment
    band products from the HPAs etc and reduce the           of the characteristic. The amount of back-off can be
    background noise, but they cannot remove in-band         expressed in terms of either the input signal back-off or
    products – eg 3rd order intermodulation (IM) prod-       the output signal backoff. A disadvantage of using
    ucts                                                     back-off is that it reduces the efficiency of the amplifier
                                                             because the RF output from the amplifier is reduced
The Ku (14/11GHz) system (Figure 9 – B) has many of          whilst it is still consuming the same DC power.
the same elements, but the down-link frequency
                                                                                                       Saturation - IMD
(11GHz) is too high for the elements in each trans-
ponder, so the input is mixed down from 14GHz to                                 PSAT
1GHz for de-multiplexing and equalisation, then mixed
up to 11GHz for power amplification, frequency MUX             Output
and re-transmission.

Because each transponder will be processing a very                Pout
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
                                                                                     Pin                Back off
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       Figure 11 shows how distortion can be reduced by backing
                                                             off the input signal from the saturation region to the linear
harmonics of the input signal, which are introduced by
the amplifier. The non-linearity may also be represented
in terms of the amplifier power transfer characteristic,     The amount of back-off needed to avoid intermodulation
which also shows the saturation and saturation power of      increases with the number of messsages (ie modulated
the amplifier.                                               carriers) in the signal which is applied to the trans-
                                                                                                         12 of 12

ponder. One solution is to increase the number of trans-       tions may arrive simultaneously at the satellite antenna
ponders on board the satellite so that each need only          from which they are fed to the transponder which will
handle a restricted bandwidth and number of carriers.          process the signals in several ways - eg amplification,
This, of course, increases the satellite mass, so a suitable   error detection and correction, filtering and frequency
compromise must be reached between the number of               changing - before feeding the signals back to the satellite
transponders and the intermodulation.                          antenna for the down link. The uplink and the down
                                                               link operate at different frequencies to avoid direct cou-
Back-off modifies the formula for the down-link C/N            pling of signals from the transmit to the receive channels
ratio by making : PT = Pos − BOo                               eg 6/4GHz (C band), 14/11GHz Ku band). The higher
Where, Pos is the output power of the HPA at satura-           frequency is used for the up-link because the satellite
                                                               antenna has limited size and a higher noise temperature
tion and BOo is the output backoff power . Pos is
                                                               (usually 290K). The gain is higher at the upper fre-
normally known for a given amplifier, then BOo is              quency for a fixed antenna size.
adjusted dynamically according to the strength of the
input signal.                                                  Similarly, the signals transmitted from a satellite will
                                                               usually be received by all the earth stations. Most of the
Solid state amplifiers are superior to TWT amplifiers in       messages received will not be needed by a specific earth
their linearity. Considerable attention has been devoted       station - they must be filtered out during de-
to techniques for linearising HPAs to improve their effi-      multiplexing. In a typical analogue system a trans-
ciency. This involves extending the linear part of the         ponder may have a bandwidth of 36MHz, but this will
power amplifier characteristic.                                be subdivided into 12 sub-bands, each with a bandwidth
                                                               of 3MHz. When an earth station receives messages
ii) Time Division Multiplexing (TDM) - each message is         from its vicinity via the PSTN network it sorts them out
transmitted at a different time. TDM is usually used           into their destination earth stations. All the messages for
with digitally coded messages. Whereas with FDM each           a particular earth station are combined to one sub-band
message is transmitted continuously using a restricted         for the uplink. They are all processed by the satellite
bandwidth, with TDM each message is only transmitted           transponder and transmitted to the earth stations, but
for a small fraction of the available time, but during that    each earth station will only process its own sub-band.
time it uses all the available bandwidth.
                                                               As noted earlier, multiplexing and modulation are sepa-
Clearly, a system must be established to regulate the          rate processes and so various combinations of the differ-
timeslots for each message. This scheduling will itself        ent techniques available for each can be used. Accord-
require the communication of earth stations via the satel-     ing to Glover and Grant, the predominant multiplex-
lite which imposes a network management overhead on            ing/modulation/multiple access technique in current use
the available bandwidth/transmission time. An appro-           for PSTN satellite telephony is FDM/FM/FDMA, but
priate balance must be struck between the complexity of        this leads to large intermodulation products. Increas-
the 'housekeeping' of the communication system and the         ingly, digital modulation (PCM) is replacing analogue
useful communication capacity.                                 techniques, leading to TDM/PSK/TDMA.

An advantage of TDM is that intermodulation distortion         With the systems described so far the communication
can be avoided, because only one message is being am-          capacity between different earth stations is essentially
plified at any one time.                                       'designed in' when the bandwidths assigned to each sta-
                                                               tion are fixed, and changes cannot easily be made even
iii) Code Division Multiplex (CDM) - each message              if demand changes. Capacity can be increased, and
includes a unique code which means that TDMA can be            made more flexible, by
used with different signals being transmitted simultane-
ously - the code allows the elements of the different          i)    using multiple spot beams that can be steered as
messages to be grouped correctly. CDM uses a very              required to different points on the earth's surface, and
wide bandwidth and so this technique is sometimes also
known as a spread spectrum technique.                          ii) by using a switching matrix on board the satellite
                                                               to co-ordinate the message transmission with the beam
              4.2 MULTIPLE ACCESS
Multiple access refers to the fact that many earth stations
share the same satellite. Signals from several earth sta-

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