Satellite Communication - Satellite Technology

• Satellite technology is used for communication.
• Voice and video calling , internet, fax, television and radio channels are the
services.
• Long distances spanning and inoperable for other forms of communication
is possible.
• Comprises of transponder, antenna,comm payload, switching systems,
command and control system.

1. SATELLITE
COMMUNICATION

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2. Satellite Communication
•Satellite technology is used for communication.
•Voice and video calling , internet, fax, television and radio channels are the
services.
•Long distances spanning and inoperable for other forms of communication
is possible.
•Comprises of transponder, antenna,comm payload, switching systems,
command and control system.

satellite
Tx earth station Rx earth station
UP link Down link
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3. Principle Of Operation
•Txm of signal from earth station to satellite through channel : Up link.
•Repeater : a ckt to increases the strength of rxd signal and then transmits
it. It works as a transponder.
•Txm from satellite to earth station : Down link.
•Uplink frequency : frequency at which the earth station is communicating
with satellite.
•Transponder receives the signal from 1
st
earth station and converts it to
another frequency and sends it down to the 2
nd
earth station.
•Downlink frequency : frequency at which the satellite communicates with
earth station.
•Earth station send the infm to satellite in the form of high powered, high
frequency (GHz range) signals.
•The satellite receives and retransmit the signals back to the earth where
they are rxd by other earth stations in the coverage area of the satellite.
•Satellite’s footprint is the area which receives a signal of useful strength
from the satellite.


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4. Features
•Satellites used in communication are in geostationary orbit. Some of them
are placed in highly elliptical orbits.
•Provides global availability. Large distances can be covered easily.
•Superior reliability.
•Superior performance ( uniformity, speed).
•High scalability.
•Deployment cost is high.
•Less vulnerable ( used in defense dept.)
•Provide weather information.
•Helpful during disasters as the services rarely fail.
•High amount of data txm is possible.

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5. Advantages / Disadvantages
Adv
•Area of coverage is more.
•Each and every corner can be
covered.
•Txm cost is independent of
coverage area.
•More bandwidth
•More broadcasting possibilities.
Disadv
•Launching of satellites into orbits
is a costly process.
•Propagation delay is more.
•Repairing activities are difficult.
•Free space loss is more.
•There can be congestion of
frequencies.
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6. Applications
•Radio broadcasting and voice communications.
•TV broadcasting (DTH).
•Internet applications ( GPS, internet surfing).
•Military applications and navigations.
•Remote sensing applications.
•Weather forecasting.

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8. Multiple Access Techniques
•To accommodate a number of users, many traffic
channels need to be made available.
•In principle, there are three basic ways to have many
channels within an allocated bandwidth:
–Frequency Division Multiple Access (FDMA)
–Time Division Multiple Access (TDMA)
–Code Division Multiple Access (CDMA)
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9. Concepts and Models of FDMA, TDMA and
CDMA
•In one BS radio service range, may be many MSs are located.
•MS must distinguish which signal meant for itself among
many signals being transmitted by other users.
•BS should be able to recognize the signal sent by a particular
user.
•In cellular system, MS not only can distinguish a signal from a
serving BS but also can discriminate the signal from adjacent
BS. – Multiple access techniques important in mobile cellular
system!
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10. Concepts and Models of FDMA, TDMA and
CDMA (cont.)
•A Radio Signal can be presented as a function of
frequency, time and code.
s (f,t,c) = s(f,t)c(t)
Where
s(f,t) – a function of frequency and time
c(t) – a function of code
•When c(t) = 1 then
s (f,t,c) = s(f,t)
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11. Concepts and Models of FDMA, TDMA and
CDMA (cont.)
•System employs different carrier frequency – FDMA system.
•System uses distinct time – TDMA system.
•System uses different code – CDMA system.
•In wireless communications, it is necessary to utilize limited
frequency bands at the same time, allowing multiple
users(MSs) to share radio channel simultaneously.
•To provide simultaneous two-way communication (duplex
communication) :
–Frequency division duplexing (FDD)
–Time Division Duplexing (TDD)
•FDMA uses FDD, TDMA & CDMA uses TDD @FDD
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12. FDMA
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13. FDMA
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14. FDMA
Advantages:
•Using well established
technology.
•No need for network timing.
•No restriction regarding the
type of baseband or the
type of modulation.
Disadvantages:
•Inter-modulation noise in
the transponder leads to
interference with other links
– satellite capacity
reduction.
•Lack of flexibility in channel
allocation.
• Requires up-link power
control to maintain quality.
•Weak carrier tend to be
suppressed.
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15. TDMA
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16. TDMA
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17. TDMA
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18. CDMA
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19. CDMA
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20. CDMA
•CDMA is a system based on spread-spectrum
technology.
•Spread-spectrum – transmission technique wherein
data occupy a larger bandwidth than necessary.
•There are two basic types of implementation
methodologies:
–Direct Sequence (DS)
–Frequency Hoping (FH)


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21. TD-CDMA
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22. SDMA
(SPACE DIVISION MULTIPLE ACCESS)
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25. Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA

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27. Optical Link
•A fiber optic link or fiber channel is a part of an optical fiber
communications system which provides a data connection between two
points.
•It consists of a data tx, a txm fiber and a rx.
•For very long txm distance, extremely high data rates (Gb/s or Tb/s) can be
achieved.
•The tx converts the electronic input signal into modulated light beam.

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28. Point-to-Point Links
Key system requirements needed to analyze optical fiber links:
1. The desired (or possible) transmission distance
2. The data rate or channel bandwidth
3. The desired bit-error rate (BER)
(a) Core size
(b) Core index profile
(c) BW or dispersion
(d) Attenuation
(e) NA or MFD
MMF or SMF LED or laser pin or APD
(a) Emission wavelength
(b) Spectral line width
(c) Output power
(d) Effective radiating area
(e) Emission pattern
(a) Responsivity
(b) Operating λ
(c) Speed
(d) Sensitivity

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•The light beam pulses are then fed into a fiber – optic
cable where they are transmitted over long distances.
•At the receiving end, a light sensitive device known as
a photocell or light detector is used to detect the light
pulses.
•This photocell or photo detector converts the light
pulses into an electrical signal.
•The electrical pulses are amplified and reshaped back
into digital form.

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Countd.
•Both the light sources at the sending end and the light
detectors on the receiving end must be capable of operating
at the same data rate.
•The circuitry that drives the light source and the circuitry that
amplifies and processes the detected light must both have
suitable high-frequency response.
•The fiber itself must not distort the high-speed light pulses
used in the data transmission.
•They are fed to a decoder, such as a Digital – to – Analog
converter (D/A), where the original voice or video is
recovered.

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Countd.
•In very long transmission systems, repeater units must be used
along the way.
•Since the light is greatly attenuated when it travels over long
distances, at some point it may be too weak to be received
reliably.
•To overcome this problem, special relay stations are used to pick
up light beam, convert it back into electrical pulses that are
amplified and then retransmit the pulses on another beam.
•Several stages of repeaters may be needed over very long
distances.
•But despite the attenuation problem, the loss is less than the loss
that occurs with the electric cables.

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71. Photo Detectors
•Optical receivers convert optical signal (light)
to electrical signal (current/voltage)
•There are several photodetector types:
–Photodiodes, Phototransistors, Photon multipliers,
Photo-resistors, vaccum photodiodes, pyroelectric
detectors, SC photodiodes etc.
•Made up of Si and Ge compounds like GaAs,
InGaAs etc…

72. Requirements
•Compatible physical dimensions (small size)
•Low sensitivity (high responsivity) at the
desired wavelength and low responsivity
elsewhere  wavelength selectivity
•Low noise and high gain
•Fast response time  high bandwidth
•Insensitive to temperature variations
•Long operating life and low cost

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74. pin energy-band diagram
Cut off wavelength depends on the
band gap energy μm
)(
24.1
eVEE
hc
gg
c 
Cut off wavelength:

75. Thermal detectors
•Measurable response of a thermal detector is a rise in temperature.
•Absorbers are used to absorb the incoming radiation and convert it
to heat.
•When light falls on the device, it raises its temperature, which, in
turn, changes the electrical properties of the device material, like its
electrical conductivity.
• Examples of thermal detectors are thermopile (which is a series of
thermocouples), pyroelectric detector etc.
•Main virtue of thermal detectors is their relatively flat responsivity
over a wide wavelength region
•Disadvantages are noisiness and slow responsivity compared to
quantum detectors
•It does not require cooling.

76. Photon detectors
•Photon detectors work on the principle of conversion of
photons to electrons or quantum effect or photon effect.
•Unlike the thermal detectors, such detectors are based
on the rate of absorption of photons rather than on the
rate of energy absorption.
•However, a device may absorb photons only if the energy
of incident photons is above a certain minimum
threshold.
•Photon detectors, in terms of the technology, could be
based on
– Vacuum tubes - e.g. photomultipliers
–Semiconductors - e.g. photodiodes
•For optical fiber applications, semiconductor devices are
preferred because of their small size, good responsivity
and high speed.
•Cryogenic cooling methods are required.

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77. Photodiodes
•Photodiodes meet most the requirements,
hence widely used as photo detectors.
•Positive-Intrinsic-Negative (pin) photodiode

•Avalanche Photo Diode (APD)

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79. Physical Principles of Photodiodes
•As a photon flux Φ penetrates into a semiconductor, it will be
absorbed as it progresses through the material.
•If α
s(λ) is the photon absorption coefficient at a wavelength λ,
the power level at a distance x into the material is
Absorbed photons trigger
photocurrent I
p in the
external circuitry

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81. pin energy-band diagram
Cut off wavelength depends on the
band gap energy μm
)(
24.1
eVEE
hc
gg
c 
Cut off wavelength:

82. Quantum Efficiency
•The quantum efficiency η is the number of the
electron–hole carrier pairs generated per incident–
absorbed photon of energy hν and is given by
I
p is the photocurrent generated by a steady-state
optical power P
in incident on the photodetector.

83. Avalanche Photodiode (APD)
•APD has an internal gain M, which is obtained by
having a high electric field that energizes photo-
generated electrons.
•These electrons ionize bound electrons in the
valence band upon colliding with them which is
known as impact ionization
•The newly generated electrons and holes are also
accelerated by the high electric field and gain
energy to cause further impact ionization
•This phenomena is the avalanche effect

84. Responsivity ()
Quantum Efficiency () = number of e-h pairs
generated / number of incident photons


0
/
/
p
Iq
Ph


 0
p
I q
Ph


  
mA/mW APD PIN
M  
M = 1 for PIN diodes

85. Responsivity c
g
hc
E

When λ<< λ
c absorption is low
When λ > λ
c; no absorption

86. Photodetector Noise
•In fiber optic communication systems, the photodiode is
generally required to detect very weak optical signals.
•Detection of weak optical signals requires that the
photodetector and its amplification circuitry be optimized to
maintain a given signal-to-noise ratio.
•The power signal-to-noise ratio S/N (also designated by SNR)
at the output of an optical receiver is defined by
SNR Can NOT be improved by amplification

87. Quantum (Shot Noise) )(2
22
MFBMqIi
pQ

F(M): APD Noise Figure F(M) ~= M
x
(0 ≤ x ≤ 1)
I
p: Mean Detected Current
B = Bandwidth
q: Charge of an electron
Quantum noise arises due optical power fluctuation
because light is made up of discrete number of photons

88. Dark/Leakage Current Noise )(2
22
MFBMqIi
DDB
 BqIi
LDS
2
2

Bulk Dark Current Noise
Surface Leakage
Current Noise
I
D: Dark Current
I
L: Leakage Current
There will be some (dark and leakage ) current without any
incident light. This current generates two types of noise
(not multiplied by M)

89. Thermal Noise
The photodetector load resistor R
L contributes to
thermal (Johnson) noise current LBT
RTBKi /4
2

K
B: Boltzmann’s constant = 1.38054 X 10
(-23)
J/K
T is the absolute Temperature

90. Signal to Noise Ratio 22
2
2 ( ) ( ) 2 4 /
p
p D L B L
iM
SNR
q I I M F M B qI B k TB R

  
Detected current = AC (i
p) + DC (I
p)
Signal Power = <i
p
2
>M
2

91. Coherent detectors
•Used for coherent detection
•By tracking an optical txr by an optical rxr, we
can extract phase and frequency information
carried by transmitted signal.
91
Balanced
photo
detector
DSP
Optical local
oscillator
Optical
signal
Optical
coherent
mixer

92. Coherent detectors
•Coherent mixers also called heterodyne mixers
•Local oscillator used is tunnable laser.
•It tunes its frequency to intradyne with
received signal frequency through optical
coherent mixer.
•Intradyne means frequency difference
between local oscillator and received optical
carrier is small.
•But it does not have to be zero.
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93. Digital filters
•An optical filter is a device that selectively transmits light of
different wavelengths, usually implemented as a glass plane or plastic
device in the optical path, which are either dyed in the bulk or
have interference coatings.
•The optical properties of filters are completely described by
their frequency response, which specifies how the magnitude and phase
of each frequency component of an incoming signal is modified by the
filter.
•Filters mostly belong to one of two categories. The simplest, physically, is
the absorptive filter; then there are interference or dichroic filters.
•Optical filters are commonly used in photography (where some special
effect filters are occasionally used as well as absorptive filters), in
many optical instruments, and to colour stage lighting.
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94. Types
•Band pass filters - Light of a certain wavelengths can pass through these
filters
•Long pass filters - Long wavelengths of light can pass through these filters
•Short pass filters - Short wavelengths of light can pass through these
filters
•Neutral density filters - These filters have almost constant attenuation
inside the visible spectrum
•Contrast enhancement filters - Specially developed filters for display
applications that deliver clarity (green displays) and true color rendition in
full color displays
•Multiband filter - Light of several bands of wavelengths can pass through
this filter
•Photo filters - UVW-365, Yellow 490, Grey filter 25 & 50, Orange 565, Red
600 & 625, Yellow-Orange 550

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95. Advantages/ Disadvantages
Advantages
•Accurate signal values within the
available resolution.
•High reproducibility.
•Processing flexibility.
•Stable system operation.

Disadvantages
•Large size and physical
complexity.
•Difficulty of processing high-
frequency signal in real time.
•Significant arithmetic errors.
•High power consumption.
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