Sat & mob commn lizy

08.804 Satellite & MobileCommunication (T)
KERALA UNIVERSITY B-TECH 8th SEMESTER
B-
lizytvm@yahoo.com Lizy Abraham
+919495123331 Assistant Professor
` Department of ECE
LBS Institute of Technology for Women
(A Govt. of Kerala Undertaking)
Poojappura
Trivandrum -695012
Kerala, India

SYLLABUS
• 08.804 SATELLITE & MOBILE COMMUNICATION (T) L-T-P : 3-1-0 Credits: 4

• Module I Communication Satellite- Orbits & launching methods-Kepler‘s law-Inclined Orbits-
Geostationary orbits, Effect of Orbital Inclination, Azimuth and Elevation, Coverage Angle and Slant
Range, Eclipse, Satellite Placement. Space segment subsystems & description, Earth Station-
Antenna, High Power Amplifiers, Up converter, Down converters, Monitoring and Control. Satellite
link- Basic Link and Interference analysis, Rain Induced Attenuation and Cross Polarization
Interference-Link Design.Mobile Satellite Networks.
• Module II Cellular concept:-hand off strategies, Interference and system capacity-: Cell splitting,
Sectoring, Repeaters, Microcells. Link budget based on path loss models. Propagation
models(outdoor):- Longely-Rice Model, Okumura Model. Mobile Propagation:- Fading and doppler
shift, impulse response model of multipath channel, parameters of multipath channel. Fading effect
due to multipath time delay spread and doppler shift. Statistical models for multipath flat fading:-
Clarks model, Two-ray Rayleigh Model. Multiple Access- TDMA overlaid on FDMA,SDMA, FHMA.
GSM:- Architecture, Radio subsystem, Channel types, Frame Structure. Introduction to Ultra
Wideband Communication System.
• Module III Direct sequence modulation, spreading codes, the advantage of CDMA for wireless,
code synchronization, channel estimation, power control- the near-far problem, FEC coding and
CDMA, multiuser detection, CDMA in cellular environment. Space diversity on receiver techniques,
multiple input multiple output antenna systems, MIMO capacity for channel known at the receiver
-ergodic capacity, space division multiple access and smart antennas.

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SYLLABUS - TEXTBOOKS
• Text books:
• 1. Dennis Roody, Satellite communication,2/e, McGraw Hill.
• 2. Theodore S. Rappaport: Wireless communication principles and practice,2/e, Pearson Education
• 3. Simon Haykin, Michael Mohar, Modern wireless communication, Pearson Education,2008
• References:
• 1. Tri. T. Ha, Digital satellite communication,2/e, Mcgraw Hill.
• 2. M. Ghavami, L. D. michael, k Rohino, Ultra-wide band signals in communication engineering,
Wiley Inc.
• 3. William stallings: Wireless communication and networks, Pearson Education, 2006
• 4. William C Y Lee: Mobile cellular Telecommunications,2/e, McGraw Hill.
• 5. MadhavendarRichharia: Mobile satellite communications: principles and trends, Pearson
Education,2004.
• Question Paper
• The question paper shall consist of two parts. Part I is to cover the entire syllabus, and carries 40
marks. This shall contain 10 compulsory questions of 4 marks each. Part II is to cover 3 modules,
and carries 60 marks. There shall be 3 questions from each module (10 marks each) out of which 2
are to be answered.
• (Minimum 40% Problem, derivation and Proof)

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Kepler’s 3rd Law: Law of Harmonics

• The squares of the periods of two planets’
orbits are proportional to each other as the
cubes of their semi-major axes:
T12/T22 = a13/a23
• Orbits with the same semi-major
axis will have the same period.

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Equinox
• Earth’s axis of rotation is not perpendicular to that of sun’s
equatorial plane and instead is tilted at an angle of about 23
degrees.
• The day that the Earth's North Pole is tilted closest to the sun is
called the summer solstice. This is the longest day (most daylight
hours) of the year
• The winter solstice, or the shortest day of the year, happens when
the Earth's North Pole is tilted farthest from the Sun.
• In between, there are two times when the tilt of the Earth is zero,
meaning that the tilt is neither away from the Sun nor toward the
Sun. These are the vernal equinox — the first day of spring — and
the autumnal equinox – the first day of fall.
• Equinox means "equal." During these times, the hours of daylight
and night are equal. Both are 12 hours long.

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• The right ascension of the ascending node is
the angle measured eastward from the Vernal
Equinox to the ascending node.
• The Vernal Equinox is the Sun's apparent
ascending node (marking the beginning of the
Northern hemisphere's spring.

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Solar Eclipses for Geo-stationary
Satellites
• Between 28 February and 11 April, and between 2
September and 14 October, roughly 21 days either side
equinoxes, satellites in geostationary orbits will pass
through the shadow of the earth once every day.
• While in the earth’s shadow the satellite gains no
power from its all important solar cells. So, either a
satellite is forced to shut down, or if 24-hour operation
is necessary, to switch over to batteries.
• Earth caused eclipses can continue around equinox
with the satellite being in the shadow for up to 70
minutes each day.

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• A solar day is the length of time between two
successive passes of the sun across the same spot
in the sky. That time period is, on average,
24:00:00, hours, or one mean solar day.

•
A sidereal day is the length of time between two
successive passes of the fixed stars across the sky.
That time period is 23:56:04, or one sidereal day.

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azimuth and elevation
• azimuth and elevation - an angular coordinate
system for locating positions in the sky.
• Azimuth is measured clockwise from true
north to the point on the horizon directly
below the object.
• Elevation is measured vertically from that
point on the horizon up to the object.

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Wideband Receiver
• A duplicate receiver is provided so that if one
fails, the other is automatically switched in.
• The combination is referred to as a redundant
receiver, meaning that although two are
provided, only one is in use at a given time.
Refer fig.7.14 and 7.16(Dennis Roody, Satellite
communication,2/e, McGraw Hill)

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• Directional beams are usually produced by means of
reflector-type antennas. Eg:-Paraboloidal reflector
• Gain of a paraboloidal reflector relative to an isotropic
radiator, G=ηI(πD/λ)2
• λ -wavelength of the signal
•D-reflector diameter ηI-aperture efficiency
•3dB beamwidth, Ɵ3dB=70 λ/D
•Gain can be increased and the beamwidth made narrower
by increasing the reflector size or decreasing the
wavelength.

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AOCS
• Attitude- orientation of satellite in space
• Attitude control-ensure the directional
antennas point in the proper directions.
• Disturbance torques-forces which alter the
attitude. Eg:-gravitational fields of earth &
moon, solar radiation
• Station keeping:- maintaining a satellite in its
correct position using thrusters.

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• Sensors- measures satellite’s orientation in space
and of any tendency for this to shift. Eg:-Infrared
sensors(horizon detectors)
• With 4 such sensors, one for each quadrant-any
shift in orientation is detected by one or other of
the sensors, and a corresponding control signal is
generated, which activates a restoring torque.
• Attitude maneuver-a shift in attitude is required,
this is executed. The control signals needed to
achieve this maneuver is transmitted from earth
station.

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• Controlling torques may be generated by passive or
active attitude control.
• 3 axes which define satellite’s attitude are roll, pitch
and yaw.
• In spin stabilization (cylindrical satellites), mechanically
balanced about one of the axes and is set spinning
around this axis.
• Also achieved by a spinning fly wheel (noncylindrical
satellites), rather than by spinning the satellite itself.
• If the average momentum referred as momentum bias
is zero, this is termed as momentum wheel or reaction
wheel.

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• If each axis is stabilized by a reaction wheel,
called as 3 axis stabilization.
• The wheel is attached to the rotor, which
consists of a permanent magnet providing the
magnetic field for motor action.
• The stator of the motor is attached to the
body of the satellite. Thus the motor provides
the coupling between the flywheel and the
satellite structure.

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• The demands on the attitude and orbit control system (AOCS) differ during
the two main phases of the mission- the orbit-raising phase and the
operational phase.
• Two types of attitude control systems are in common use-
1. spin stabilization and
2. Three-axis stabilization (Momentum wheel stabilization)
• The specifications of the attitude-control system depend on the desired
spacecraft pointing accuracy which is a function of the satellite antenna
beam width.
• The attitude control may be either active or passive.
• A passive attitude-control system maintains the attitude by obtaining an
equilibrium at the desired orientation without the use of active attitude
devices. Eg:- Spin stabilization
• An active control system maintains the attitude by the use of active
devices in the control loop. Eg:- Momentum wheel stabilization

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Mobile Satellite Systems

• Like cellular systems, except that the base
stations (i.e., satellites) move as will as
mobile devices
• Satellite coverage attractive for areas of
world not well served by existing terrestial
infrastructure: ocean areas, developing
countries.

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• Mobile Satellite Systems
• Geostationary Systems
– INMARSAT
– MSAT
• Big “LEO” Systems
– ARIES
– ELLIPSO
– IRIDIUM
– ODYSSEY
• Little “LEO” Systems
– Orbcomm
– LEOSAT
– STARNET
– VITASAT

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Inmarsat

• is a British satellite telecommunications
company, offering global, mobile services.
• It provides telephony and data services to
users worldwide, via portable or mobile
terminals which communicate to ground
stations through
eleven geostationary telecommunications
satellites.

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• Carrier to Noise Ratio (C/N)
The ratio of the received carrier power and the noise power in a given bandwidth, expressed in
dB. This figure is directly related to G/T and S/N; and in a video signal the higher the C/N, the
better the received picture.
• G/T
A figure of merit of an antenna and low noise amplifier combination expressed in dB. "G" is
the net gain of the system and "T" is the noise temperature of the system. The higher the
number, the better the system.
• dBW:
• decibels with respect to one Watt. A Logarithmic representation of a power level reference to
1W of power.
• Figure of Merit:
• A Figure of merit is a quantity used to characterize the performance of a device relative to
other devices of the same type. In engineering, figures of merit are often defined for particular
materials or devices in order to determine their relative utility for an application.
• The overall Earth station figure of merit is defined as the ratio of receive gain to system noise
temperature expressed in decibels per Kelvin
• e.g. G/T is a measure of the performance of a downlink station expressed in units of dB/K,
depending on the receive antenna and low noise amplifier

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• An isotropic radiator is an antenna which radiates in all
directions equally.
• Effective Isotropic Radiated Power (EIRP) is the amount of
power the transmitter would have to produce if it was
radiating to all directions equally.
• A measure of the strength of the signal radiated by an
antenna.
• The calculation of received signal based on transmitted
power and all losses and gains involved until the receiver is
called “Link Power Budget”, or “Link Budget”.
• The received power Pr is commonly referred to as “Carrier
Power”, C.

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• The satellite link is probably the most basic in
microwave communications since a line-of-
sight path typically exists between the Earth
and space.
• This means that an imaginary line extending
between the transmitting or receiving Earth
station

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Design of the Satellite Link
LNB (LOW NOISE BLOCK DOWN
CONVERTER)
A device mounted in the dish, designed to
amplify the satellite signals and convert
them from a high frequency to a lower
frequency. LNB can be controlled to
receive signals with different polarization.
The television signals can then be carried
by a double-shielded aerial cable to the
satellite receiver while retaining their high
quality. A universal LNB is the present
standard version, which can handle the
entire frequency range from 10.7 to 12.75
GHz and receive signals with both vertical
and horizontal polarization.

Critical Elements of the Satellite Link

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Interference Analysis

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Refer 5.4,5.5,5.6 (Dennis Roody, Satellite

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Refer 12.9.2 (Dennis Roody, Satellite

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Interference Analysis (contd..)
• Refer 5.4, 5.5, 5.6, 12.9.2, 13.1, 13.2, 13.2.1,
13.2.2, 13.2.3. (Dennis Roody, Satellite

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• Eb/No (Energy per bit per Noise Power Density)
• Is the performance criterion for any desired BER
• It is the measure at the input to the receiver
• Is used as the basic measure of how strong the
signal is
• Directly related to the amount of power
transmitted from the uplink station
• Eb/No = (C/N)T + Noise BW – Information Rate

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The Cellular Concept

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Basic Concept

• Cellular system developed to provide mobile telephony:
telephone access “anytime, anywhere.”

• First mobile telephone system was developed and
inaugurated in the U.S. in 1945 in St. Louis, MO.

• This was a simplified version of the system used today.

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System Architecture
• A base station provides coverage (communication
capabilities) to users on mobile phones within its
coverage area.
• Users outside the coverage area receive/transmit signals
with too low amplitude for reliable communications.
• Users within the coverage area transmit and receive
signals from the base station.
• The base station itself is connected to the wired
telephone network.

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First Mobile Telephone System

One and only one
high power base
station with which all
users communicate.

Normal
Telephone Entire Coverage
System Area

Wired connection

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Problem with Original Design
• Original mobile telephone system could only support a
handful of users at a time…over an entire city!

• With only one high power base station, users phones
also needed to be able to transmit at high powers (to
reliably transmit signals to the distant base station).

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Improved Design
• Over the next few decades, researchers at AT&T Bell Labs
developed the core ideas for today’s cellular systems.

• Although these core ideas existed since the 60’s, it was
not until the 80’s that electronic equipment became
available to realize a cellular system.

• In the mid 80’s the first generation of cellular systems
was developed and deployed.

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The Core Idea: Cellular Concept
• The core idea that led to today’s system was the cellular
concept.
• The cellular concept: multiple lower-power base
stations that service mobile users within their coverage
area and handoff users to neighboring base stations as
users move. Together base stations tessellate the system
coverage area.

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Cellular Concept
• Thus, instead of one base station covering an entire city,
the city was broken up into cells, or smaller coverage
areas.

• Each of these smaller coverage areas had its own lower-
power base station.

• User phones in one cell communicate with the base
station in that cell.

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3 Core Principles
• Small cells tessellate overall coverage area.

• Users handoff as they move from one cell to another.

• Frequency reuse.

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Basic Cellular System

PSTN/ISDN Switch

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Wireless communication definitions

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Wide area Paging system
Paging is usually one way. It can be Numeric, alphanumeric or a voice
message. They are used to notify a subscriber that they need to call back or
get in touch with somebody. Some applicatios are:
New headlines
Stock quoattions
Fax
Network management
Distance and coverage:
Inside a building
Simple : 2 to 5 Kms
Wide area paging : worldwide coverage.
Concept is simple but the transmission systems are quite complicated

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Cordless handsets

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Cellular system Concept

MSC is also called Mobile telephone switching office (MTSO)
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• A basic system comprises :
• Cellular subscriber phones
• Base station
• Mobile switching center
• The cellular network is connected to public telephone network.
• High capacity is achieved by limiting the coverage of each base station transmitter to a small
geographic area is called cell so that the same radio channels may be reused by another base station
located some distance away. A sophisticated switching technique called a handoff enables a call to
proceed uninterrupted when the user moves from one cell to another.

• Each cell uses different freq channels.
• Cellaular systems use standard freq plan. The voice and control channels are defined. Normally 95%
of channels are used for information communication while only 5% are used for signaling purposes.
• Switching system, called handoff, enables call to proceed uninterrupted when the user moves from
one cell to another.
• Typical MSC handles 100,000 cellular users and 5,000 simultaneous conversations at a time.

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Tessellation
• Some group of small regions tessellate a large region if
they cover the large region without any gaps or overlaps.

• There are only three regular polygons that tessellate any
given region.

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Tessellation (Cont’d)
• Three regular polygons that always tessellate:
– Equilateral triangle
– Square
– Regular Hexagon

Triangles
Squares
Hexagons
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Circular Coverage Areas
• Original cellular system was developed assuming base
station antennas are omnidirectional, i.e., they transmit
in all directions equally.
Users located outside
some distance to the
base station receive
weak signals.

Result: base station has
circular coverage
area.

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Circles Don’t Tessellate
• Thus, ideally base stations have identical, circular
coverage areas.
• Problem: Circles do not tessellate.
• The most circular of the regular polygons that tessellate
is the hexagon.
• For a given distance between the center of a polygon and
its farthest perimeter points, the hexagon has the largest
area of the three.
• Thus, early researchers started using hexagons to
represent the coverage area of a base station, i.e., a cell.

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Thus the Name Cellular
• With hexagonal coverage area, a cellular network is
drawn as:

Base
Station

• Since the network resembles cells from a honeycomb,
the name cellular was used to describe the resulting
mobile telephone network.
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Handoffs
• A crucial component of the cellular concept is the notion
of handoffs.
• Mobile phone users are by definition mobile, i.e., they
move around while using the phone.
• Thus, the network should be able to give them
continuous access as they move.
• This is not a problem when users move within the same
cell.
• When they move from one cell to another, a handoff is
needed.

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A Handoff
• A user is transmitting and receiving signals from a given
base station, say B1.

• Assume the user moves from the coverage area of one
base station into the coverage area of a second base
station, B2.

• B1 notices that the signal from this user is degrading.
• B2 notices that the signal from this user is improving.

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A Handoff (Cont’d)
• At some point, the user’s signal is weak enough at B1 and
strong enough at B2 for a handoff to occur.
• Specifically, messages are exchanged between the user,
B1, and B2 so that communication to/from the user is
transferred from B1 to B2.

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2.4 Handoff Strategies
• When a mobile moves into a different cell while a conversation is in
progress, the MSC automatically transfers the call to a new channel
belonging to the new base station.
• Handoff operation
– identifying a new base station
– re-allocating the voice and control channels with the new base station.
• Handoff Threshold
– Minimum usable signal for acceptable voice quality (-90dBm to -100dBm)
– Handoff margin cannot be too large or too small.
– If ∆ = Pr ,handoff − Pr ,minimum burden the MSC
is too large, unnecessary handoffs usable
– If is too small, ∆ there may be insufficient time to complete handoff
∆
before a call is lost.
∆

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• Handoff must ensure that the drop in the measured signal is not due to
momentary fading and that the mobile is actually moving away from the
serving base station.
• Running average measurement of signal strength should be optimized so that
unnecessary handoffs are avoided.
– Depends on the speed at which the vehicle is moving.
– Steep short term average -> the hand off should be made quickly
– The speed can be estimated from the statistics of the received short-term
fading signal at the base station
• Dwell time: the time over which a call may be maintained within a cell
without handoff. (Avg. time having a smooth conversation before going for a
handoff.)
• Mean Dwell time- fixed, well-defined path of constant speed. Eg:- Highway
users
• Dwell time depends on
– propagation
– interference
– distance
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– speed 271

• RSSI of reverse voice channels
• Locator Receiver in each BS controlled by MSC
• Monitor the signal strength of MUs in neighboring cells and report all
RSSI values to the MSC
• Handoff measurement
– In first generation analog cellular systems, signal strength measurements
are made by the base station and supervised by the MSC.
– In second generation systems (TDMA), handoff decisions are mobile
assisted, called mobile assisted handoff (MAHO)
– Every MU measures the received power from BSs and continually reports
the results to the serving BS.
– A handoff is initiated when the power received from the neighboring BS
begins to exceed that of current BS by a certain level for a certain period
of time.
• Intersystem handoff: If a mobile moves from one cellular system to a
different cellular system controlled by a different MSC.

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Prioritizing Handoffs
• Guard Channel for handoff requests
• Queuing of handoff requests

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Practical Handoff Consideration

• Different type of users
– High speed users need frequent handoff during a call.
– Low speed users may never need a handoff during a call.
• Microcells to provide capacity, the MSC can become burdened if high
speed users are constantly being passed between very small cells.
• Minimize handoff intervention
– handle the simultaneous traffic of high speed and low speed users.
• Large and small cells can be located at a single location (umbrella cell)
– different antenna height
– different power level
• Cell dragging problem: pedestrian users provide a very strong signal
to the base station
– The user may travel deep within a neighboring cell

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• Handoff for first generation analog cellular systems
– 10 secs handoff time
– ∆ is in the order of 6 dB to 12 dB
• Handoff for second generation cellular systems, e.g., GSM
– 1 to 2 seconds handoff time
– mobile assists handoff
– ∆ is in the order of 0 dB to 6 dB
– Handoff decisions based on signal strength, co-channel interference, and
adjacent channel interference.
• IS-95 CDMA spread spectrum cellular system
– Mobiles share the channel in every cell.
– No physical change of channel during handoff
– MSC decides the base station with the best receiving signal as the service
station

•
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Frequency Reuse
• Extensive frequency reuse allows for many users to be
supported at the same time.

• Total spectrum allocated to the service provider is broken
up into smaller bands.

• A cell is assigned one of these bands. This means all
communications (transmissions to and from users) in this
cell occur over these frequencies only.

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Frequency Reuse (Cont’d)
• Neighboring cells are assigned a different frequency
band.

• This ensures that nearby transmissions do not interfere
with each other.

• The same frequency band is reused in another cell that is
far away. This large distance limits the interference
caused by this co-frequency cell.

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Example of Frequency Reuse

Cells using the same frequencies

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2.2 Frequency Reuse
• Each cellular base station is allocated a group of radio channels within
a small geographic area called a cell.
• Neighboring cells are assigned different channel groups.
• By limiting the coverage area to within the boundary of the cell, the
channel groups may be reused to cover different cells.
• Keep interference levels within tolerable limits.
• Frequency reuse or frequency planning

•seven groups of channel from A to G
•footprint of a cell - actual radio
coverage
•omni-directional antenna v.s.
directional antenna

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• Consider a cellular system which has a total of S duplex channels.
• Each cell is allocated a group of k channels, k < S.
• The S channels are divided among N cells.
• The total number of available radio channels
S = kN
• The N cells which use the complete set of channels is called cluster.
• The cluster can be repeated M times within the system. The total
number of channels, C, is used as a measure of capacity
C = MkN = MS
• The capacity is directly proportional to the number of replication M.
• The cluster size, N, is typically equal to 4, 7, or 12.
• Small N is desirable to maximize capacity.
• The frequency reuse factor is given by
1/ N

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• Only certain cluster sizes and cell layout are possible.
• The geometry of hexagon is such that the number of cells per cluster,
N, can only have values which satisfy
N = i 2 + ij + j 2
• Co-channel neighbors of a particular cell, eg, i=3 and j=2 and N=19.
• To find the co-channel neighbours of a particular cell,
(a) move i cells along any chain of hexagons
(b) turn 600 conuter clockwise and move j cells.

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2.5 Interference and System Capacity
• Sources of interference
– another mobile in the same cell
– a call in progress in the neighboring cell
– other base stations operating in the same frequency band
– noncellular system leaks energy into the cellular frequency band
• Two major cellular interference
– co-channel interference
– adjacent channel interference

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2.5.1 Co-channel Interference and System
Capacity
• Frequency reuse - there are several cells that use the same set of
frequencies
– co-channel cells
– co-channel interference
• To reduce co-channel interference, co-channel cell must be separated
by a minimum distance.
• When the size of the cell is approximately the same and the BSs
transmit the same power,
– co-channel interference is independent of the transmitted power
– co-channel interference is a function of
• R: Radius of the cell
• D: distance between the centers of the nearest co-channel cells
• Increasing the ratio Q=D/R, the interference is reduced.
• Q is called the co-channel reuse ratio
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• For a hexagonal geometry
D
Q= = 3N
R

• A small value of Q provides large capacity
• A large value of Q improves the transmission quality - smaller level of
co-channel interference
• A tradeoff must be made between these two objectives

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• Let i0 be the number of co-channel interfering cells. The signal-to-
interference ratio (SIR) for a mobile receiver can be expressed as
S S
= i0
I
∑I
i =1
i

S: the desired signal power
I i : interference power caused by the ith interfering co-channel cell
base station
• The average received power at a distance d from the transmitting
antenna is approximated by

−n
d 
Pr = P0  
d  d0
or  0
d  P0 :measued power
Pr (dBm ) = P0 (dBm ) − 10n log 
d  XT
 0
n is the path loss exponent which ranges between 2 and 4.
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• When the transmission power of each base station is equal, SIR for a
mobile can be approximated as
S R−n
= i0
I
∑ (Di )−n
i =1
• Consider only the first layer of interfering cells
S ( D / R )n
= =
( 3N )n

i0 = 6
I i0 i0

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• For hexagonal geometry with 7-cell cluster, with the mobile unit being
at the cell boundary, the signal-to-interference ratio for the worst
case can be approximated as

S R −4
=
I 2 ( D − R ) − 4 + 2( D + R ) − 4 + 2 D − 4

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2.5.2 Adjacent Channel Interference
• Adjacent channel interference: interference from signals which are
adjacent in frequency to the desired signal.
– Imperfect receiver filters allow nearby frequencies to leak into the
passband
– Performance degrade seriously due to near-far effect.
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28-Dec-12 289

• The near-far problem is a condition in which a receiver captures a
strong signal and thereby making it impossible for the receiver to
detect a weaker signal.
• Consider a receiver(BS) and two transmitters(MUs), one close to the
BS, the other far away. If both transmitters transmit simultaneously
and at equal powers, the SNR for the farther transmitter is much
lower.
• This makes the farther transmitter more difficult to detect.
• Adjacent channel interference can be minimized through careful
filtering and channel assignment.
• Keep the frequency separation between each channel in a given cell
as large as possible
• If the frequency reuse factor is large (ie, small N), a channel
separation greater than six channel bandwidth separations is needed
to bring the adjacent channel interference to an acceptable level.

28-Dec-12 290

2.5.3 Power Control for Reducing Interference
• Ensure each mobile transmits the smallest power necessary to
maintain a good quality link on the reverse channel
– long battery life
– increase SIR
– solve the near-far problem

28-Dec-12 291

2.7 Improving Capacity in Cellular Systems
• Methods for improving capacity in cellular systems
– Cell Splitting: subdividing a congested cell into smaller cells.
– Sectoring: directional antennas to control the interference and frequency
reuse.
– Coverage zone : Distributing the coverage of a cell and extends the cell
boundary to hard-to-reach places.

28-Dec-12 292

2.7.1 Cell Splitting
• Split congested cell into smaller cells.
– Preserve frequency reuse plan.
– Reduce transmission power.
– Increase the
capacity of the Reduce R to R/2
cellular system
microcell

28-Dec-12 293

•The microcell BS labeled G is placed half way between 2
larger stations using the same channel G

Illustration of cell splitting within a 3 km by 3 km square

28-Dec-12 294

• Transmission power reduction from Pt1 to Pt 2
• Examining the receiving power at the new and old cell boundary
Pr [at old cell boundary ] ∝ Pt1R − n
Pr [at new cell boundary ] ∝ Pt 2 ( R / 2) − n

• If we take n = 4 and set the received power equal to each other
Pt1
Pt 2 =
16
• The transmit power must be reduced by 12 dB in order to fill in the
original coverage area.
• Problem: if only part of the cells are splited
– Different cell sizes will exist simultaneously
• Handoff issues - high speed and low speed traffic can be
simultaneously accommodated

28-Dec-12 295

2.7.2 Sectoring
• Decrease the co-channel interference and keep the cell radius R
unchanged
– Replacing single omni-directional antenna by several directional antennas
– Radiating within a specified sector

28-Dec-12 296

• Interference Reduction

position of the
mobile

interference cells

28-Dec-12 297

Directional Antenna
• One way to get more capacity (number of users) while
maintaining cell size is to use directional antenna.
• Assume antenna which radiates not in alldirections (360
degrees) but rather in 120 degrees only.

28-Dec-12 298

Directional Antenna (Cont’d)
• Because these directional antenna only receive signals in
particular direction, the amount of interference power
they receive assuming a clustersize of 7 is reduced by
1/3.

• With less interference power, the speech quality is much
better than it needs to be.

• So we can reduce the clustersize (increase interference
power) and still have good speech quality.

28-Dec-12 299

Directional Antenna
• Trials show that in systems with 120 degree antenna, the
clustersize can be as small as 3.

• This allows more users to be supported, while keeping
cell size fixed.

• Because of the benefits offered by 120 degree antenna,
these are most readily used by base station towers.

28-Dec-12 300

2.7.3 Microcell Zone Concept
• Antennas are placed at the outer edges of the cell
• 3 antennas at 3 corners and all are connected to the BS
• Any channel may be assigned to any zone by the base station
• Mobile is served by the zone with the strongest signal.
• Handoff within a cell
– No channel re-
assignment
– Switch the channel to a
different zone site
• Reduce interference
– Low power transmitters
are employed

28-Dec-12 301

Complete Cellular Network
A group of local base stations are connected (by wires) to
a mobile switching center (MSC). MSC is connected to
the rest of the world (normal telephone system).
MSC

Public (Wired)
Telephone MSC
Network

MSC
MSC
28-Dec-12 302

Terminologies involved in Cellular Phone Systems

1) Mobile Identification Number (MIN): Subscriber’s Telephone No.

2) Electronic Serial Number (ESN): Serial No. of the Mobile

3) Station Class Mark(SCM): It indicates the maximum Transmitter
power level for a particular user.
International Mobile Subscriber identity number ( IMSI) Ex: GSM
First 3 digit ( Mobile country code : MCC); next 2 mobile Network code ( MNC); Next 10
Mobile subscriber Identity no.( MSIC)
262 02 454 275 1010 ( Germany; Optus commun; MSIC ) (India-404,405) (Airtel 02-Punjab,03
Himachal Prade, 10- Delhi NCR 900 MHz)

28-Dec-12 303

Cell Phone Codes
Electronic Serial Number (ESN) - a unique 32-
bit number programmed into the phone when
it is manufactured
Mobile Identification Number (MIN) - a 10-
digit number derived from your phone's
number
System Identification Code (SID) - a unique 5-
digit number that is assigned to each carrier by
the FCC

28-Dec-12 304

All cell phones have special codes associated
with them. These codes are used to identify
the phone, the phone's owner and the service
provider.

When you first power up the phone, it listens for an SID on
the control channel. The control channel is a special
frequency that the phone and base station use to talk to
one another about things like call set-up and channel
changing. If the phone cannot find any control channels to
listen to, it knows it is out of range and displays a "no
service" message.
When it receives the SID, the phone compares it to the SID
programmed into the phone. If the SIDs match, the phone
knows that the cell it is communicating with is part of its
home system.
Along with the SID, the phone also transmits a registration
request, and the MTSO keeps track of your phone's
location in a database -- this way, the MTSO knows which
cell you are in when it wants to ring your phone.

28-Dec-12 305

The MTSO gets the call, and it tries to find you. It looks in its
database to see which cell you are in.
The MTSO picks a frequency pair that your phone will use in
that cell to take the call.
The MTSO communicates with your phone over the control
channel to tell it which frequencies to use, and once your
phone and the tower switch on those frequencies, the call is
connected. Now, you are talking by two-way radio to a
friend.
As you move toward the edge of your cell, your cell's base
station notes that your signal strength is diminishing.
Meanwhile, the base station in the cell you are moving
toward (which is listening and measuring signal strength on
all frequencies, not just its own one-seventh) sees your
phone's signal strength increasing. The two base stations
coordinate with each other through the MTSO, and at some
point, your phone gets a signal on a control channel telling it
to change frequencies. This hand off switches your phone to
the new cell.

28-Dec-12 306

As you travel, the signal is passed from cell to cell.
Let's say you're on the phone and you move from one
cell to another -- but the cell you move into is covered by
another service provider, not yours. Instead of dropping
the call, it'll actually be handed off to the other service
provider.
If the SID on the control channel does not match the SID
programmed into your phone, then the phone knows it
is roaming. The MTSO of the cell that you are roaming in
contacts the MTSO of your home system, which then
checks its database to confirm that the SID of the phone
you are using is valid. Your home system verifies your
phone to the local MTSO, which then tracks your phone
as you move through its cells. And the amazing thing is
that all of this happens within seconds.

28-Dec-12 307

Timing Diagram when a call is made by a Landline User to a Mobile

28-Dec-12 308

Timing Diagram when a call is made by a Mobile user to a Landline User

28-Dec-12 309

Roaming
• All cellular systems provide a service called roaming.

– This allows subscribers to operate in service areas other than the
one from which service is subscribed.

– When a mobile enters a city or geographic area that is different
from its home service area, it is registered as a roamer in the new
service area.

– Periodically, the MSC issues a global command over each
FCC in the system, asking for all mobiles which are previously
unregistered to report their MIN and ESN over the RCC for billing
purposes.

– If a particular mobile user has roaming authorization for billing
purposes, MSC registers the subscriber as a valid roamer.

28-Dec-12 310

Outdoor Propagation Model
• Radio transmission in a mobile communication
system often takes place over irregular terrain.
• The terrain profile may vary from a simple curved
earth profile to a highly mountainous profile.
• Presence of trees, buildings and other obstacles may
also taken into account.
• A number of propagation models are available to
predict the path loss over irregular terrain.

28-Dec-12 311

Longely-Rice Model
• Applicable to point-to-point communication
systems in the frequency range from 40MHz
to 100GHz, over different kinds of terrain.
• Point-to-point mode prediction-When a
detailed terrain profile is available, the path
specific parameters can be easily determined.
• Area mode prediction- If the terrain profile is
not known, the method provides techniques
to estimate the path-specific parameters.

28-Dec-12 312

• Certain modications over the rudimentary
model like an extra urban factor (UF) due to
urban clutter near the reciever is also included
in this model.

28-Dec-12 313

Disadvantages
• Does not provide a way of determining
corrections due to environmental factors.
• Multipath is also not considered.

28-Dec-12 314

Fig. 4.23 & 4.24 (Theodore S. Rappaport: Wireless
communication principles and practice,2/e, Pearson
Education)

28-Dec-12 318

Multipath Propagation
• In wireless telecommunications, multipath is the propagation
phenomenon that results in radio signals reaching the
receiving antenna by two or more paths.
• Causes of multipath include atmospheric ducting, ionospheric
reflection and refraction, and reflection from water bodies
and terrestrial objects such as mountains and buildings.
• The effects of multipath include constructive and destructive
interference, and phase shifting of the signal.

28-Dec-12 319

Multipath Fading
• Multipath signals are received in a terrestrial
environment, i.e., where different forms of
propagation are present and the signals arrive at the
receiver from transmitter via a variety of paths.
• Therefore there would be multipath interference,
causing multipath fading.
• Adding the effect of movement of either Tx or Rx or
the surrounding clutter to it, the received overall
signal amplitude or phase changes over a small
amount of time. Mainly this causes the fading.

28-Dec-12 320

Fading
• The term fading, or, small-scale fading, means
rapid fluctuations of the amplitudes, phases,
or multipath delays of a radio signal over a
short period or short travel distance.

28-Dec-12 321

Multipath Fading Effects

28-Dec-12 322

Factors Influencing Fading

28-Dec-12 323

Doppler Shift Geomerty

28-Dec-12 324

• a mobile moving at a constant velocity v, along
a path segment length d between points X and
Y, while it receives signals from a remote BS
source S.
• The difference in path lengths travelled by the
wave from source S to the mobile at points X
and Y is ∆l = d cosƟ = v ∆ t cosƟ , where ∆ t is
the time required for the mobile to travel
from X to Y.

28-Dec-12 325

• source is assumed to be very far away.
• The phase change in the received signal due
to the difference in path lengths is therefore

28-Dec-12 326

• The small scale variations of a mobile radio signal can be
considered as impulse response of the mobile radio
channel.
• Mobile radio channel may be modelled as a linear filter
with time varying impulse response in continuous time.
• consider the channel impulse response (time varying
impulse response) h(d,t) and x(t), the transmitted signal.
• The received signal y(d,t) at any position d

28-Dec-12 327

Channel issues

28-Dec-12 328

Time-varying impulse response

28-Dec-12 331

k is the gain of transmitter.

28-Dec-12 333

Parameters of Mobile Multipath Channel (Ref: 5.4)
• Multipath delay spread- due to the different
multipath waves which have propagation delays
which vary over different spatial locations of the
receiver.
• Coherence BW- the range of frequencies over
which we get a flat response of the channel.
• Doppler Spread- Spectral broadening of the signal
at the receiver due to doppler shift.
• Coherence time- time duration over which 2
signals arriving at the receiver have a strong
correlation.

28-Dec-12 334

Types of Small-Scale Fading

Bs<Bc Bs>Bc
στ <Ts στ >Ts

Bs<BD Bs>BD
Tc<Ts Tc>Ts

28-Dec-12 335

Flat Fading

• Such types of fading occurs when the bandwidth of the
transmitted signal is less than the coherence bandwidth of the
channel.
• Equivalently if the symbol period of the signal is more than
the delay spread of the channel, then the fading is flat fading.
• Bs-Signal BW
• Bc-Coherence BW
• Ts-Symbol (signal) period
• Tc- Coherence time
• στ- rms delay spread
• Bd- Doppler spread
28-Dec-12 336

• Over time, the received signal r(t) varies in gain,
but the spectrum of transmission is preserved.
• But in freq. selective fading, the received signal
includes multiple versions of the transmitted
waveform, which are attenuated (faded) &
delayed in time, and hence the received signal is
distorted.
• Refer fig. 5.12 & 5.13(Theodore S. Rappaport:
Wireless communication principles and
practice,2/e, Pearson Education)
28-Dec-12 338

Flat-fading (non-freq. Selective)

28-Dec-12 339

Frequency selective fading

28-Dec-12 342

Two independent fading issues

28-Dec-12 345

Statistical models for multipath propagation

• Many multipath models have been proposed
to explain the observed statistical nature of a
practical mobile channel.
• The most popular of these models are
Rayleigh model, which describes the NLoS
propagation.
• The Rayleigh model is used to model the
statistical time varying nature of the received
signal.

28-Dec-12 346

Two ray NLoS multipath, resulting in Rayleigh fading

28-Dec-12 347

Rayleigh Fading Model
• Let there be two multipath signals S1 and S2
received at two different time instants due to
the presence of obstacles as shown in Figure.
• there can either be constructive or destructive
interference between the two signals.

28-Dec-12 348

Above distribution is known as Rayleigh Distribution and is shown in
the figure for different σ values. It has been derived for slow fading.

28-Dec-12 349

2nd order parameters of fading
• Level Crossing Rate(LCR):- expected rate at which the Rayleigh
fading envelope, normalized to the local rms signal level,
crosses a specified level ‘R’ in a positive going directon.
• The number of level crossings /second,

28-Dec-12 351

• Average Fade Duration(AFD):- average period
of time for which the received signal is below
a specified level R.
• Average fade duration,

28-Dec-12 352

• Clarke’s model consider only flat fading
conditions.
• Do not consider multipath time delay or
frequency selective fading conditions.
• Impulse response of the model is,

28-Dec-12 355

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