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chapter 5


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									      CHAPTER 5
Fiber Optic Communication
   Fiber optic system : a communication system where
    the information is put on a light beam and transmitted
    through a guided cable.

   Light frequencies used in fiber optic systems are
    between 4.3x1014 and 7.5x1014 Hz.

   Thus the higher the carrier frequency, the wider the
    bandwidth, and consequently, the greater the
    information-carrying capacity.

   Block diagram of a fiber optic communication link, is
    illustrated in Figure 5.1.
                Input            Driving           Light      Light
                signal           circuit         Electrical Electrical    Coupler

                                   VIC             Light source

                                                                         cable      Repeater

                                                 Light detector

                                 IVC                          Light      Coupler
                  signal                         Electrical

                           VIC - Voltage-to-Current converter
                           IVC - Current-to-Voltage converter

Figure 5.1 Basic elements of a fiber optic communication system
The main elements in an optical
 fiber communication link are:-

1.   Driving circuitry:
     Driving circuitry serves as an electrical interface between the
     input circuitry and the light source. It converts voltage signal
     to current signal (voltage to current converter) to drive the
     light source.

2.   Light Source:
     Light source can be either a light-emitting diode (LED) or
     Laser (Light Amplification by Stimulated Emission of
     Radiation). Light source is to convert electrical energy to
     optical energy, where the amount of light emitted is
     proportional to the amount of drive current. Another words,
     the light intensity depends on the amplitude variations of the
     input signal.

3.   Light Source-to-fiber coupler:
     This is an interface to couple the light emitted by the source
     into the optical fiber cable.
4.       Fiber Optics Cable:
         It is a long thin strand of glass or plastic fiber used to
         transfer signal in a form of light from a point to another
         point. Most fiber cables have a circular cross section with a
         diameter of only a fraction of an inch (Its size is almost the
         same as the size of human hair).

         The characteristics of light transmission through a glass fiber
         depend on many factors, for examples:-

           The composition of the fiber.
           The amount & type of light introduced into the fiber.
           The diameter and length of the fiber.
The fiber optic consists of three parts:-

    The core, where the light is passing through.
    The cladding, which surrounds the core with a lower
     refractive index. It is to ensure that the light waves
     remain within the core, to protect the fiber core from
     scratches and to strengthen the fiber core.
    Protective jacket/coating. This is the outer coating, made
     of specially formulated plastic coating that provides a first
     level shock and resistance to damage and moisture, for
     the fiber. Typically it is a clear protective coating or a
     material made of stranded steel or a special yarn known
     as Kevlar. Kevlar is strong and preferred over steel as it is
     an insulator. Kevlar may forms a protective sleeve or
     jacket over the cladding.

     Fiber optic cable are also available in a flat ribbon form,
     which are easier to use and more space-efficient,
     especially for multiple fibers.
5.   Fiber-to-detector coupler:
     This is an interface between fiber and light-detector to
     couple as much light as possible from the fiber cable into
     the light detector.

6.   Light detector:
     The commonly used light detector or photo sensor is
     either a PIN (p-type-intrinsic-n-type) diode or an APD
     (avalanche photodiode). Both the APD and PIN diode
     convert light energy to electrical energy or current.
     Consequently, a current-to-voltage converter (IVC) is
     required, which transforms variations of detector current
     to output signal voltage variations.

           Plastic    Kevlar    Protective   Cladding
           Outer      Sleeve                            Core

             Figure 5.2 Typical layers in a fiber optic cable
Types of fiber optic cable

   There are three commonly types of fiber optic cable:

1. Single mode fiber:
   The single mode or mono-mode has a very small core
    diameter, typical core sizes are 2 μm to 15 μm diameter.

   So, only a single path that light may take as it propagates
    down the cable.
   All light rays follow approximately the same path down the
    cable and take approximately the same amount of time to
    travel the length of the cable.
   Here, modal dispersion is zero.
   Modal dispersion is where the pulse at the end of the cable
    is lower in amplitude due to the attenuation of the light in
    the cable and increase in duration due to the different
    arrival times of light rays.
Single mode fiber
2. Multimode step index fiber:

   The centre core is larger of about from 50 μm to
    100 μm diameter.

   It has a large light-to-fiber aperture and allows
    more light to enter the cable.

   The light rays are propagated down the cable in
    a zig-zag pattern, continuously reflecting off the
    interface boundary, resulting many paths.

   As a result, all light rays do not follow the same
    path, with different propagation time.

   Modal dispersion exists.
Multimode step index fiber
3. Multimode graded index fiber:

   The centre core is about from 50 μm to 100 μm diameter.

   Multimode graded-index fiber is characterized by a central core
    that has a refractive index that is non-uniform; it is maximum at
    the centre and decreases gradually toward the side of the cable.

   Light is propagated down the fiber by refraction, which results in
    a continuous bending of the light rays, at many different angles.

   As they propagate down the fiber, the light rays that travel in
    the outermost area travel a greater distance, but with a higher
    velocity, than the rays traveling near the centre (because
    velocity is inversely proportional to the refractive index).

   Finally, all the light rays arrive at the end point at almost the
    same time, resulting less modal dispersion.
Multimode graded index fiber
Advantages fiber optic cables
 over conventional electrical

1. Wider bandwidth: Fiber optic cables have
  higher information-carry capability.

2 Lower loss/attenuation: With fiber-optic cables,
  there is less signal attenuation over long
  distances. The loss is low as 0.2 dB/km, and
  the repeater spacing’s are longer, thus
  reducing both system cost and complexity.

3. Light weight: Glass or plastic cables are much
   lighter than copper cables and offer benefits in
   those areas where low weight is critical.
        Advantages – cnt’d

4. Small size: Practically, fiber-optic cables are
   much smaller in diameter than electrical
   cable. Therefore, more can be contained in a
   smaller space.

5. Strength: Fiber-optic cables are stronger
   than electrical cable and can support more
   weight. They are manufactured with very
   high tensile strengths, can be bent or twisted
   without damage. So, they are superior in
   terms of storage, transportation, handlings
   and installation than corresponding copper
6. Security: Fiber-optic cables cannot be “tapped” as
   easily as electrical cables, and they do not radiate
   signals that can be picked up for eavesdropping
   purposes. There is less need for complex and
   expensive encryption techniques.

7. Interference immunity: Fiber-optic cables do not
   radiate signals as some electrical cables and cause
   interference to other cables. They are also
   immune to pickup of interference from other
   sources. As they are dielectric waveguides, they
   are free from electromagnetic interference (EMI),
   radio frequency interference (RFI) or switching
   transients giving electromagnetic pulses (EMP).
   They have no optical interference between them
   and no crosstalk.

8. Greater safety: Fiber -optic cables do not carry
   electricity. Therefore, there is no shock hazard, no
   arching, no sparks and no short circuits. They are
   also insulators so are not susceptible to lightning
   strikes as electrical cables.
9. System reliability and easy maintenance: The
   lifetime is about 20-30 years, it yields good
   reliability, reduces maintenance time, manpower
   and maintenance costs.

10. Potential low cost as the total costs are
  continuing to decline.

11.   No problems of corrosions.
Attenuation/losses in fiber optics

Cable attenuation
   Cable attenuation of a fiber optic cable is expressed in
    dB per unit length, dB/km.

   The total cable attenuation depends on the cable

   For a fiber optic cable of a length, L (in km), the cable
    attenuation is,

    [10 log10 (Po/Pi)]/L , in dB/km.

where Po is the output power and Pi is the input power.
The possible causes of cable attenuation
1.   Scattering losses:
     i.e. the radiation of propagated light due to the microscopic
     imperfection of the fiber. When light rays that are
     propagating strike these impurities, they are diffracted.
     Diffraction causes the light to disperse or spread out in
     many directions, which represent a loss in light power.
     Scattering also refers to the light loss because of light
     waves entering at the wrong incident angle and being lost
     in the cladding due to refraction. This is called Rayleigh
     scattering loss.

2.   Absorption losses:
     Absorption refers to how the light waves are actually
     “soaked-up” in the fiber core due to the impurity of the
     glass or plastic or due to any imperfections.
Bending losses: occur because the light rays on the outside of
  a sharp bend cannot travel fast enough to keep up with the
  other rays and are lost.
   Table 5.1 shows losses in a typical in a typical multimode fiber
    as a function of wavelength with Rayleigh and absorption
    components shown for comparison, showing three windows.

   Window is a short range of wavelength where the attenuation
    is at minimum.

              Table 5.1 Loss spectrum of typical optical fiber
Optical signal attenuation:

    Optical signal attenuation is expressed in dB,

         Attenuation (dB) = 10 log10 (Po/Pi)

where Po is the output power and Pi is the input power.

    The possible causes of cable attenuation are:

1.   Splicing loss:
     Splicing is a permanent jointing of two fibers.

2.   Coupling losses:
     These represent a large source of loss in commercial fiber
     optic system. They occur at either; light source-to-fiber
     connections, fiber-to-fiber, or, fiber-to-photo detector
     connections, mainly due to misalignments.
Applications of Fiber-Optic Cable:
     Long-haul, backbone public and private networks
     Local loop networks
     Fiber backbone networks (LAN connectivity)
     High-resolution image and digital video
     TV studio-to- transmitter interconnection, elimination
      microwave radio link.
     Closed-circuit TV systems used in building for security.
     Secure communications systems at military bases.
     Computer networks, wide area and local area.
     Shipboard communications.
     Aircraft communications and controls.
     Interconnection of measuring and monitoring
      instruments in plants and laboratories.
     Data acquisition and control signal communications in
      industrial process control systems.
     Public Switching Telephone
          Networks (PSTN)
The public telephone network accommodates two types of

1.    Private-line circuits or dedicated circuits – the customers
      lease the equipments, transmission media/facilities and
      services from the telephone companies or service
      providers, on a permanent basis, eg large banks.

2.    Public subscribers – the customers share the equipments
      and facilities that are available to all the public
      subscribers to the network, includes transmission facilities
      and telephone switches. Since subscribers to the public
      network are interconnected only temporarily through
      switches, the network is called Public Switched Telephone
      Network (PSTN).
PSTN’s primary characteristics

          They include:

    i.     Analog access, 300 to 3,400 kHz
    ii.    Circuits-switched duplex connection
    iii.   Switched bandwidth 64 kbps, or 0.3 – 3.4 kHz for
           analog exchanges
    iv.    Immobility or very limited mobility
    v.     Many functions in common with another bearer
PSTN Teleservices
The terminals that can be connected to PSTN are as shown in

                  Telephone                 Cordless Telephone



          Figure 5.3 Terminals that can be connected to PSTN
   Fixed telephone:
    For fixed telephone, the generated information (voice) is

   Fax service:
    For fax service, the generated information is digital, but
    has a built-in modem providing an analog signal. It is a
    distributive service involves unidirectional communication
    to one recipient or broadcast to many recipients.

   Data communication:
    For data communication, computer generates digital
    signal and connected to PSTN through a modem, i.e. data
    communication via modem.

   Video, multimedia and teleworking:
    The limited bandwidth of the PSTN is the bottleneck for
    the video and multimedia services into PSTN, eg video
    telephony service. The most tcommonly used teleworking
    is a telephone, a fax and a computer with a modem – all
    are connected to the PSTN.
    Cordless telephone:
     Cordless telephone has an analog to digital
     converter ADC in the terminal.

Since the PSTN interface is analog, fax and data traffic
must be converted to analog signals in the frequency
band 0.3 to 3.4 kHz with the help of built-in modem or
stand-alone modem. The modem adapts the signal to
the PSTN by converting information from digital form to
analog form and vice versa.
Integrated Services Digital
     Network (ISDN)

        ISDN is to support a wide range of voice
    (telephone) and non-voice (digital data) applications
    in the same network using a limited number of
    standardized facilities includes both switched and non-
    switched connections. Services introduced into an
    ISDN should be compatible with 64-kbps switched
    digital connections, which is the basic building block of
    ISDN. ISDN provides service features, maintenance
    and network management function.
                                    Primary Rate
 Basic Rate Interface

      Network       2B+D              30B+D
      Terminal             N-ISDN                  PBX

Figure 5.4 Basic ISDN concepts
Services in ISDN

   Telephony
   Telefax (FAX)
   Videoconferencing and video-telephony
   High quality sound
   Data communication including internet access
   Combinations of teleservices
              Mobile and Cellular
Introduction to Cellular Telephone

   The key principle of Cellular Telephone or Cellular Radio is
    determined by sub-dividing a relatively large geographic
    market area (called coverage zone) into small sections
    (called cells).

   It uses the concept of frequency re-use to increase the
    capacity of a mobile telephone channel

   Cellular telephone system allows a large number of users to
    share the limited number of common usage of radio
Fundamental Concept of Cellular
   Each coverage zone is further sub-divided into hexagonal-
    shaped cells that fit together to form a honey-comb
    pattern, as shown in Figure 5.5. Each geographical area is
    allocated a fixed number of cellular voice channels.

                      6            2
                      5            3

                 Figure 5.5 Hexagonal honey-comb patterns
The concept of a cellular are:
1.   A cellular system allows frequency reused in the same area
     by splitting the entire region into many smaller cells.

2.   Each cell has a base station.

3.   Because of low power of base station, the same frequency
     can be used for another cell, which is not adjacent (i.e.
     adjacent cells have different frequency groups).

4.   Each mobile user is initially assigned a frequency and
     communicates with the closest base stations.

5.   When the mobile user crosses to some other cell, it is given a
     new base station (RBS), i.e. the cellular phone changes from
     one channel to another as it crosses cell boundaries, even
     while the conversation is in progress.

6.   The RBSs are linked together so that a conversation can pass
     from one cell to another. This will link the users in any two
     cells regardless of their channel assignments.
Physical size of a cell varies, depends on
 user density and calling pattern:

1. Large Cell - Macrocells:
  Radius: Between 1.8 km - 28 km
  Base station transmit power:1 W - 6 W

2. Small Cells - Microcells:
  Radius:Less than 450 m
  Base station transmit power:0.1 W - 1 W
  Applications:High density areas eg large cities and inside
  Limitations :Low effective working areas, reflections and
   signal delays

3. Very small cells - Picocells:
  To provide reliable communication indoors, well-shielded
   areas or ares with high level of interference, eg underground
  Advantage: Can use the same frequency as regular cells in
   the same area.
   Figure 5.6 illustrates the base station location in a
    hexagonal-shaped cells.

       Centre-excited cell         Edge-excited cell   Corner-excited cell
       (omni-directional           (sectored             (sectored
       antenna)                    Directional          Directional antenna)

                           Figure 5.6: Locations of base stations
Frequency Reuse

   Frequency reuse is the process in which the same set of
    frequencies or channels can be allocated to more than one
    cell, provided that the cells are separated by sufficient
   Each cellular base station is allocated a group of channel
    frequencies that are different from those of neighboring cells,
    and base station antennas are chosen to achieve a desired
    coverage pattern within a cell.

The main objectives of frequency re-use are:
(i) To keep the transmitted power from each base station to a

(ii) To keep the position of the antenna of the base station just
    high enough to provide for the area coverage of the
    respective cells.
Concept of frequency reuse:
     A coverage
     area                G       Cluster 1
                     F       B
                     E       C
                 G       D
        F            B       G
                 A       F        B
       E             C       A
                 D       E        C
     Cluster 3                   Cluster 2
    Figure 5.7 Cellular frequency reuse concept
   Cluster is a group of cells.
   From Figure 5.7 above:
   The coverage area consists of 3 clusters.
   Each cluster has 7 cells
   Each cell is assigned the same number of full-duplex
    telephone channels
   Cell with the same letter use the same set of channel
Total number of channels in a cluster:

       F =G N           (channels/cluster)

       where G = Number of channels in a cell
             N = Number of cells in a cluster

                (Typical values:         N = 3, 7, 12)

When a cluster is duplicated m times within a given service area, the
total number of full-duplex channel in a service area becomes,

       C= mGN          (channels/service area)
        = mF

       where C = Number of full-duplex channels in an area
             m = number of clusters in a service area
                  (clusters/service area)
             G = Number of full-duplex channels in a cell
             N = Number of cells in a cluster
             i.e C  m
Cellular System Topology
   Radio Network is defined by a set of radio-frequency
    transceivers (base stations) located within each of the cells.
    Figure 5.8 illustrates a simplified cellular telephone system
    that includes the basic components: Mobile unit, Base station
    and Mobile Telephone Switching Office (MTSO).




                                        MTSO                         RBS

                               PSTN                           PSTN
                                       Trunk Circuit
       Wired Telephone
                                                                           Wired Telephone

        Figure 5.8 Simplified cellular telephone system topology
Mobile Unit :

    Mobile Unit communicates directly with the base stations
     (over dedicated data links - metallic & non-metallic) and the
     base station communicates directly with a MTSO (over free-

Base Station :

1.   Serves as central control for all users within a cell.

2.   Consists of a low-power radio transceiver, power amplifiers
     and cell-site controller

3.   Provides an interface between mobile telephone sets and
     the MTSO.
Mobile Telephone Switching Office (MTSO)

1. Controls channel assignments, call processing, call setup
   and call termination, includes signaling, switching,
   supervision, and allocating radio-frequency channels.
2. Provides a centralized administration and maintenance
   point for the entire network
3. Interfaces with the public telephone network over wire line
   voice trunks for the conventional wired telephones and
   interfaces with data links.
4. Most MTSO are connected to the SS7 signaling network,
   which allows cellular telephone to operate outside their
   service area.
         Satellite Communication

Introduction to Satellite
 Communication System

   A satellite system consists of:

(i)a transponder (a radio repeater in the sky),
(ii) a ground-based station to control its operation
(iii) a user network of earth stations that provide the facilities for
    transmission and reception of communication traffics through
    the satellite system.
   Satellites received a signal from the ground station, do the
    amplification and frequency translation, and broadcast it to
    earth stations that are able to receive transmissions.
   A satellite begins at a single earth station, passes through
    the satellite, and ends at one or more earth stations.
   Satellite communication systems utilize microwave terminals
    both on satellites and in earth stations for high reliable and
    high-capacity system.
   Figure 5.9 below shows the basic elements of a
    communications satellite.
                              Satellite in geosynchronous orbit


      Uplink                                                                Uplink
                   Downlink                              Downlink

           Earth station                                    Earth station
Satellite transponder
    Satellite transponder acts like a repeater, consists of a
     receiver and a transmitter. The main functions of a satellite
     transponder are:
(i) to pick up the transmitted signal from the transmitter on
     the earth,
(ii) to amplify the signal,
(iii ) to translate the carrier frequency to another frequency
(iv) to retransmit the amplified signal to the receiver on the
 Figure 5.10 below shows a block diagram of a satellite

                Band pass      Low-noise          Mixer        BPF     Low-power
                filter (BPF)   amplifier (LNA)                         amplifier


                                                 Frequency converter
         From                                                              To earth
         earth station                                                      station
   The BPF limits the total noise.
   The LNA amplifies the received signal and fed it to the frequency
    converter which converts the high-band uplink frequency to the low-
    band downlink frequency.
   This is an RF-to-RF repeater.
   Each RF satellite channel requires a separate transponder.

Satellite system links
1. Up link: It is the path of the satellite signal from the earth
   transmitter to the receiver of the satellite. The frequency signal
   being transmitted from the earth station to the satellite is called the
   uplink frequency. For example: the uplink frequency for a C-band is
   6 GHz.

2. Down link: It is the path of the satellite signal from the satellite
   transmitter to the receiver on the earth. The retransmitted signal
   from the satellite to the receiving stations is called the down-link.
   For example: the downlink frequency for a C-band is 4 GHz.

Note: The uplink and downlink use different carrier frequencies to
avoid interference, and the frequency conversion is done in the
Earth stations
   Figure 5.11 shows an earth station transmitter.

   From Figure 5.11, the intermediate frequency (IF)
    modulator converts the input baseband signals to an FM,
    a PSK, or a QAM modulated intermediate frequency.

   The up- converter (mixer and bandpass filter) converts
    the IF to an appropriate RF carrier frequency.

   The high power amplifier (HPA) provides adequate input
    sensitivity and output power to propagate the signal to
    the satellite transponder.
                                                          To satellite

         Baseband in   Modulator                      High Power
Data                                 Mixer     BPF   High power
          FDM or       (FM, PSK                       Amplifier
Video     PCM/TDM      or QAM)                       (HPA)



      Figure 5.11 An earth station transmitter
   Figure 5.12 shows an earth station receiver.
   From Figure 5.12, LNA which is a highly sensitive and low-
    noise device amplifies the received signal.
   The RF-to-IF down-converter is a mixer and bandpass filter
    combination, which converts the received RF signal to an
    intermediate frequency (IF).


                Low noise                         Demodulator   Baseband out   Tel
                               Mixer       BPF
                 Amplifier                        (FM, PSK       (FDM or       Data
                 (LNA)                            or QAM)        PCM/TDM)      video

                             Figure 5.12     An earth station receiver
             sat1                         sat2

        Uplink/Downlink                     Uplink/Downlink

  station1           Earth

                    Intersatellite link
                     Figure 5.13 Intersatellite
Frequency allocations

   Table 5.1 shows the frequency bands for the
    satellite frequency range.

   The most common carrier frequencies used for
    satellite communications are the 6/4 GHz (C-band)
    and 14/12 GHz (Ku-band) bands, especially for
    voice and data telecommunications.

   At these bands, the signal attenuation is minimum.

   The first number is the uplink frequency, and the
    second number is the downlink frequency, that is 6
    GHz is the uplink frequency and 4 GHz is the
    downlink frequency for a C-band.
Table 5.1 Satellite frequency bands
Satellite Orbit

   Three basic types of orbits are:

1. Polar Orbit: It is a north-south orbit, normally used for navigation,
   weather satellites, meteorological and land resource satellite
   systems. It is not used for telecommunication purposes.

2. Elliptically inclined orbit: It is used for Russian domestic systems,
   with inclination of 63 degrees and a 12-hour orbit period, but visible
   for 8 hours only. So, 3 satellites are needed for continuous coverage.

3. Circular Equatorial Orbit: It is called geosynchronous orbit.
   This satellite in a geosynchronous orbit is at a height of about
   35,800 km, has 24-hour orbit period, and its angular speed is equal
   to the rotational speed of the earth. So, it appears stationary or
   motionless over a fixed point on the earth’s surface. The satellite is
   visible from 1/3 of the earth’s surface, so 3 satellites are needed for
   full coverage of the earth.

   The above basic orbits are as shown in Figure 5.14.
Figure 5.14 Three satellite orbits
Advantages of geosynchronous

1. The satellite remains almost stationary relative to the
   earth station, so, the computer-controlled tracking of the
   satellite is minimized.

2. A geosynchronous satellite is permanently in view, so,
   there are no breaks in transmission. There is no need to
   switch from one satellite to another.

3. Due to high altitude, it can cover a large area on earth
   (about 1/3 of the earth), and a large number of earth
   stations may intercommunicate. Three satellites can give
   global coverage except in the polar regions.

4. The effects of Doppler shift are negligible. Doppler shift is
   the change in the apparent frequency of the radiation to
   and from the satellite caused by the motion of the satellite
   to and from the earth station.
Disadvantages of geosynchronous
1. Latitudes greater than 81.250 North and South are not

2. Due to the high latitudes of geosynchronous satellite, the
   received signal power, which is inversely proportional to
   the square of the distance, is very weak, and the signal
   propagation delay is about 300 msec.

System performance

   Figure 5.15 shows a simplified block diagram of a satellite
    system which is showing the various gains and losses that
    may affect the system performance.
   When evaluating the performance, the uplink and downlink
    parameters are considered separately, and then the overall
    performance is determined by combining them in the
    appropriate manner.
                Pcap    G/Te(s)

                    Lp(up)              Lp(down)

               Prad                             Pcap
              Gtx                    Grx(e)
 Pt                                                  G/Te(e)

      Lfb                                      Lfb

Transmitter                             Receiver
 Figure 5.15 A simplified satellite links
Satellite System Parameters

1. Effective Isotropic Radiated Power (EIRP):

 EIRP is defined as an equivalent transmit power.

       EIRP = Pin x   Gtx

 Where,     Pin   =    antenna input power

            Gtx   =    transmit antenna gain


 EIRP (dBW) = 10 log10 Pin (dBW) +      Gtx (dB)
In respect to the transmitter output:

      Pin = Pt         - Lbo - Lfb
    (dBW) (dB)      (dB) (dB)

Where,    Pt =       saturated output power
      Lbo    =       back-off loss
      Lfb    =       feeder & branching loss


  EIRP =  Pt -        Lbo -     Lfb + Gtx
  (dBW)  (dBW)         (dB)        (dB)        (dB)
Example 1:
A satellite earth station transmitter operates with an
   antenna input
power of 10 kW, a back-off loss of 3 dB and a total feeder
branching loss of 4 dB. The transmit antenna gain is 40 dB.
Determine the station EIRP.


EIRP = Pt    -       Lbo     -     Lfb      +    Gtx
(dBW)  (dBW)         (dB)           (dB)         (dB)

       = 10 log10 (10x103) - 3 - 4 +40
       = 40 -3 - 4 + 40
       = 73 dBW
Example 2
A satellite transponder has a gain of 50 dB. Its receiving and
transmitting antenna have equal gain of 20 dB. If the
receiving antenna receives a signal power of 10 uW from the
earth transmitter, determine the signal power at the output
of the satellite transmitting antenna.


                                 Satellite Transponder
                                 Gain = Gs = 50 dB
                Receiving                                Transmitting
                antenna                                  antenna
                Gr=20 dB                                 Gt=20 dB

                     Pin=10 uW                           Po= ?

            Uplink                                                 Downlink
Received power,   Pin   =     10 uW
                        =     10 log10 10 x 10-6 W
                                      1 x 10-3 W
                        =     -20 dBm

             or   Pin   =     10 log10 10 x 10-6 W
                                       1 W
                        =     -50 dBW

  Total gain GT   =     Gr + Gs + Gt
                  =     20 + 50 + 20
                  =     90 dB

  But,   Po =     Pin    +    GT
            =     (-50) dBW   +        90 dB
            =     40 dBW
            =     104 W or 10 kW
Applications of satellite communications

Some of the applications of satellite communications are:

  (i) Digital audio broadcasting
  (ii) Television broadcasting
  (iii) Serving remote areas
  (iv) Weather satellites
  (v) Remote monitoring and control
  (vi) Vehicle tracking
  (vii) Mobile communications
  (viii)Maritime and air navigation
  (ix) Video teleconferencing
  (x) Defence communications.
Comparisons between satellite system and
  terrestrial microwave system

 Advantages of a satellite system include:
 1.   It can access to wide geographical area.
 2.   Wide bandwidth
 3.   High reliability
 4.   Distance insensitive cost.
 5.   Independent of terrestrial infrastructure
 6.   Rapid installation, and low cost per added site
 7.   Uniform service characteristics
 8.   Single provider

 Disadvantages of a satellite system include:
 1.   High initial cost
 2.   It has propagation delay
 3.   Regulatory licensing requirement and limited orbital
       parking slots.

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