EC8094 SATELLITE COMMUNICATION

QUESTION BANK
PART-A
Q.
Questions
No.
1. Explain what is meant by noise factor.
Noise factor is defined as an alternative way of representing amplifier noise. In defining
the noise factor of an amplifier, the source is taken to be at room temperature denoted
by To which is usually taken as 290k,hence the output noise from the amplifier is
N0,out = F GKT0
Where G is available power gain of the amplifier and F is its noise factor.
2. What is meant by polarization interleaving?
Overlap occurs between channels, but these are alternatively polarized left hand circular
and right hand circular to reduce interference to acceptable levels. This is referred to as
polarization interleaving.
3. Write down the link power budget equation.
The losses for the link have been identified, the power at the receiver, which is the power
output of the link, may be calculated simply as [EIRP] [LOSSES] [GR], where the last
quantity is the receiver antenna gain. The major source of loss in any ground-satellite link
is the free-space spreading loss [FSL], the basic link-power budget equation taking into
account this loss only. However, the other losses also must be taken into account, and
these are simply added to [FSL].
The losses for clear-sky conditions are
[LOSSES] = [FSL] +[RFL] +[AML]+[AA] -[PL] equation for the received power is then
[PR] = [EIRP] X [GR] -[LOSSES]
Where
[PR] - the received power, dBW
[EIRP] - equivalent isotropic radiated power, dBW [FSL] free-space spreading loss, dB
[RFL] - receiver feeder loss, dB
[AML] -antenna misalignment loss, dB
[AA] - atmospheric absorption loss, dB [PL] polarization mismatchloss, dB
4. How does a CDMA receiver function for the purpose of synchronization maintenance
and reliable data reconstruction?
A system and method for communicating information signals is by using spread spectrum
communication techniques. PN sequences are constructed that provide orthogonality
between the users so that mutual interference will be reduced, allowing higher capacity
and better link performance. With orthogonal PN codes, the cross-correlation is zero
over a predetermined time interval, resulting in no interference between the
orthogonal codes, provided only that the code time frames are time aligned with each
other.
5. Explain the need of a reference burst in a TMDA system.
The reference burst is used to provide timing references to various stations accessing the
TDMA transponder. It does not carry any traffic information and consists of a primary and
secondary reference burst for redundancy.
 PART-B

Q.
Questions
No.
6. a) From the calculation of system noise temperature prove that C/N ratio is directly
proportional to G/T ratio.

b) Explain what is meant by saturation flux density. The power received by a 1.8 m
parabolic antenna at 14 GHz is 250 pW. Calculate the power flux density a) in W/m 2
and b) in dB W/m2 at the antenna.
7. a) With test set-up explain the procedure of EIRP and antenna gain measurement.
Antenna gain is usually defined as the ratio of the power produced by the antenna from a
far-field source on the antenna's beam axis to the power produced by a hypothetical lossless
isotropic antenna, which is equally sensitive to signals from all directions.
 Two direct methods of measuring the Rx gain can be used; integration of the Rx sidelobe
pattern or by determination of the 3dB and 10dB beamwidths. The use of pattern integration
will produce the more accurate results but would require the AUT to have a tracking system.
In both cases the test configurations for measuring Rx gain are identical, and are illustrated
in Figure In order to measure the Rx gain using pattern integration the AUT measures the
elevation and azimuth narrowband (±5° corrected) sidelobe patterns. The AUT then
calculates the directive gain of the antenna through integration of the sidelobe patterns.

The Rx gain is then determined by reducing the directive gain by the antenna
inefficiencies. In order to measure the Rx gain using the beamwidth method, the AUT
measures the corrected azimuth and elevation 3dB/10dB beamwidths.From these results the
Rx gain of the antenna can be directly calculated using the formula below.

Where,
G is the effective antenna gain (dBi)
Az3 is the corrected azimuth 3dB beamwidth (°) El3 is the elevation 3dB beamwidth (°)
Az10 is the corrected azimuth 10dB beamwidth (°) El10 is the elevation 10dB beamwidth (°)
FLoss is the insertion loss of the feed (dB)
RLoss is the reduction in antenna gain due to reflector inaccuracies, and is given by:
RLoss =4.922998677(Sdev f )2 dB
where: Sdev is the standard deviation of the actual reflector surface (inches)
b) What do you mean by the term frequency reuse? Discuss the technique relating to
satellite communication in detail with example.

Polarization isolation refers that carriers, which may be on the same frequency but with
opposite senses of polarization, can be isolated from one another by receiving antennas
matched to the incoming polarization. With linear polarization, vertically and horizontally
polarized carriers can be separated in this way, and with circular polarization, left-hand
circular and right-hand circular polarizations can be separated. Because the carriers with
opposite senses of polarization may overlap in frequency, this technique is referred to as
frequency reuse.
In radio resource management for wireless and cellular network, channel allocation schemes
are required to allocate bandwidth and communication channels to base stations, access
points and terminal equipment.
The objective is to achieve maximum system spectral efficiency in bit/s/Hz/site by means of
frequency reuse, but still assure a certain grade of service by avoiding co-channel
interference and adjacent channel interference among nearby cells or networks that share the
bandwidth. There are two types of strategies that are followed:-
 Fixed: FCA, fixed channel allocation: Manually assigned by the network operator
 Dynamic:
 DCA, dynamic channel allocation,
 DFS, dynamic frequency selection
 Spread spectrum

8. a) List and explain the steps of link power budget analysis for downlink.
The downlink of a satellite circuit is the one in which the satellite is transmitting the signal
and the earth station is receiving it. Equation can be applied to the downlink, but subscript D
will be used to denote specifically that the downlink is being considered.

In the above equation, the values to be used are the satellite EIRP, the earth- station receiver
feeder losses, and the earth-station receiver G/T. The free space and other losses are
calculated for the downlink frequency. The resulting carrier-to-noise density ratio appears at
the detector of the earth station receiver.

b) How the performance of the system affects due to system noise? Derive the
expression for system noise at the receiving earth station.

(a) Configure the spectrum analyzer as follows:

Centre Frequency: Adjust for beacon or test signal frequency (to be advised). Use marker to
peak and marker to centre functions.
 Frequency Span: 100 KHz
 Resolution Bandwidth: 1 KHz
 Video Bandwidth: 10 Hz (or sufficiently small to limit noise variance)
 Scale: 5 dB/div
 Sweep Time: Automatic
 Attenuator Adjust to ensure linear operation. Adjust to provide the "Noise floor delta"
described in steps 7 and 8.
(b) To insure the best measurement accuracy during the following steps, adjust the
spectrum analyzers amplitude (reference level) so that the measured signal, carrier or noise,
is approximately one division below the top line of the spectrum analyzers display.
(c) Record the frequency and frequency offset of the test signal from the nominal
frequency: For example, assume the nominal test frequency is 11750 MHz but the spectrum
analyzer shows the peak at 11749 MHz The frequency offset in this case is -1 MHz
(d) Change the spectrum analyzer centre frequency as specified by SES WORLD SKIES
so that the measurement is performed in a transponder guard band so that only system noise
power of the earth station and no satellite signals are received. Set the spectrum analyzer
frequency as follows:
Centre Frequency = Noise slot frequency provided by the PMOC
(e) Disconnect the input cable to the spectrum analyser and confirm that the noise floor
drops by at least 15 dB but no more than 25dB. This confirms that the spectrum analyser’s
noise contribution has an insignificant effect on the measurement. An input attenuation value
allowing a "Noise floor Delta” in excess of 25 dB may cause overloading of the spectrum
analyser input. (i) Reconnect the input cable to the spectrum analyser.
(j) Activate the display line on the spectrum analyser.
 (k) Carefully adjust the display line to the noise level shown on the spectrum analyser.
Record the display line level.
(l) Adjust the spectrum analyser centre frequency to the test carrier frequency recorded
in step (e).
(m) Carefully adjust the display line to the peak level of the test carrier on the spectrum
analyser. Record the display line level.
(n) Determine the difference in reference levels between steps (l) and (j) which is the
(C+N)/N.
(o) Change the (C+N)/N to C/N by the following conversion:
This step is not necessary if the (C+N)/N ratio is more than 20 dB because the resulting

correction is less than 0.1 dB.

(p) Calculate the carrier to noise power density ratio (C/No) using:

The 2.5 dB figure corrects the noise power value measured by the log converters in the
spectrum analyser to a true RMS power level, and the SAcorr factor takes into account the
actual resolution filter bandwidth.
(q) Calculate the G/T using the following:

where,
EIRPSC – Downlink EIRP measured by the PMOC (dBW) Acorr – Aspect correction
supplied by the PMOC (dB)
FSL – Free Space Loss to the AUT supplied by the PMOC (dB) La – Atmospheric
attenuation supplied by the PMOC (dB)
(r) Repeat the measurement several times to check consistency of the result.

9. a) Write the design aspects and explain the technical features of TDMA frame
structure.
• TDMA systems divide the channel time into frames. Each frame is further partitioned
into time slots. In each slot only one user is allowed to either transmit or receive.
• Unlike FDMA, only digital data and digital modulation must be used.
• Each user occupies a cyclically repeating time slot, so a channel may be thought of as
a particular time slot of every frame, where N time slots comprise a frame.
 Figure: TDMA Channels
Features:
• Multiple channels per carrier or RF channels.
• Burst transmission since channels are used on a timesharing basis. Transmitter can be
turned off during idle periods.
• Narrow or wide bandwidth – depends on factors such as modulation scheme, number
of voice channels per carrier channel.
• High ISI – Higher transmission symbol rate, hence resulting in high ISI. Adaptive
equalizer required.

Figure:TDMA Channels time slot

• A guard time between the two time slots must be allowed in order to avoid
interference, especially in the uplink direction.

• Efficient power utilization: FDMA systems require a 3- to 6-dB power back off in
order to compensate for inter-modulation effects.
• Efficient handoff: TDMA systems can take advantage of the fact that the transmitter
is switched off during idle time slots to improve the handoff procedure. An enhanced link
control, such as that provided by mobile assisted handoff (MAHO) can be carried out by a
subscriber by listening to neighboring base station during the idle slot of the TDMA frame.
• Efficiency of TDMA: It is a measure of the percentage of bits per frame which
contain transmitted data. The transmitted data include source and channel coding bits.

Where bOH includes all overhead bits such as preamble, guard bits, etc.

b) Express FDMA in detail and also enumerate the interference in detail.

 In FDMA, each user is allocated a unique frequency band or channel.
During the period of the call, no other user can share the same frequency band.

Figure : FDMA Channels

 All channels in a cell are available to all the mobiles. Channel

 assignment is carried out on a first-come first- served basis.
 The number of channels, given a frequency spectrum BT, depends on
the modulation technique (hence Bw or Bc) and the guard bands between the
channels 2Bguard.
 These guard bands allow for imperfect filters and oscillators and can
be used to minimize adjacent channel interference.
 FDMA is usually implemented in narrowband systems.

Figure: FDMA/TDD
Nonlinear effects in FDMA:
 In a FDMA system, many channels share the same antenna at the
BS. The power amplifiers or the power combiners, when operated at or near
saturation are nonlinear.
 The nonlinear ties generate inter-modulation frequencies.
 Undesirable harmonics generated outside the mobile radio band
cause interference to adjacent services.
 Undesirable harmonics present inside the band cause interference to
other users in the mobile system.

PART-C

Q.
Questions
No.
10. a) Explain the impact of rain on link performance. Consider the governing equation for
uplink and downlink rain fade margin and elaborate in detail.

In the C band and, more especially, the Ku band, rainfall is the most significant cause of
signal fading. Rainfall results in attenuation of radio waves by scattering and by absorption of
energy from the wave. Rain attenuation increases with increasing frequency and is worse in
the Ku band compared with the C band. This produces a depolarization of the wave; in effect,
the wave becomes elliptically polarized. This is true for both linear and circular polarizations,
and the effect seems to be much worse for circular polarization. The C/N0 ratio for the
downlink alone, not counting the PNU contribution, is PR/PND, and the combined C/N0
ratio at the ground receiver is
 The reason for this reciprocal of the sum of the reciprocals method is that a
single signal power is being transferred through the system, while the various
noise powers, which are present are additive. Similar reasoning applies to the
carrier-to-noise ratio, C/N.

Fig 2.17 (a) Combined uplink and downlink (b) power flow diagram

b)i) A certain 6/4 GHz satellite uplink has earth station EIRP is 80 dBW; Earth station
satellite distance is 35780 Km; attenuation due to atmospheric factors is 2 dB; satellite
antennas aperture efficiency is 0.8 ; satellite antennas aperture area is 0.5m 2 ; satellite
receiver effective noise temperature is 190K; satellite receivers bandwidth is 20MHZ.
Determine the link margin for satisfactory quality of service if the threshold value of
received carrier to noise ratio is 25 dB.
ii) A geostationary satellite transmits 5W of power with an antenna having a gain of
28dB. The downlink is operated at 4GHz and the receive antenna is a dish with
diameter of 3.6m. Compute the EIRP transmitted and the power received by the
receiving antenna. Assume the receiver antenna efficiency to be 0.7 and all the other
losses to be 2dB.
(i)
(ii)