SATELLITE & MOBILE COMMUNICATION

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08.804 Satellite & 08.804 Satellite & 08.804 Satellite & 08.804 Satellite & MobileCommunicationMobileCommunicationMobileCommunicationMobileCommunication (T)(T)(T)(T)

KERALA UNIVERSITY BKERALA UNIVERSITY BKERALA UNIVERSITY BKERALA UNIVERSITY B----TECH 8TECH 8TECH 8TECH 8thththth SEMESTERSEMESTERSEMESTERSEMESTER

[email protected] 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 SlantRange, Eclipse, Satellite Placement. Space segment subsystems & description, Earth Station-Antenna, High Power Amplifiers, Up converter, Down converters, Monitoring and Control. Satellitelink- Basic Link and Interference analysis, Rain Induced Attenuation and Cross PolarizationInterference-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. PropagationModule II Cellular concept:-hand off strategies, Interference and system capacity-: Cell splitting,Sectoring, Repeaters, Microcells. Link budget based on path loss models. Propagationmodels(outdoor):- Longely-Rice Model, Okumura Model. Mobile Propagation:- Fading and dopplershift, impulse response model of multipath channel, parameters of multipath channel. Fading effectdue 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 UltraWideband 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 andCDMA, 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• 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:

T 2/T 2 = a 3/a 3T12/T2

2 = a13/a2

3

• 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 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 • 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 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.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 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 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 • 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 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

• 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 (noncylindricalsatellites), 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 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

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• 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 • 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– 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 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. 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 • 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 LinkLNB (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

and horizontal polarization.

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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 • 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 ConceptThe 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.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 allstation with which all

users communicate.

Entire Coverage

Area

Normal

Telephone

System

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 • 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 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 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-• 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

SwitchPSTN/ISDNPSTN/ISDN

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Wireless communication definitions

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Wide area Paging systemPaging 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

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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 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.given region.

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Tessellation (Cont’d)

• Three regular polygons that always tessellate:

– Equilateral triangle

– Square

– Regular Hexagon– Regular Hexagon

TrianglesSquares

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 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.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:

• Since the network resembles cells from a honeycomb,

the name cellular was used to describe the resulting

mobile telephone network.

Base

Station

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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 • 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 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 B to B . 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.– 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 is too large, unnecessary handoffs burden the MSC

– If is too small, there may be insufficient time to complete handoff

before a call is lost.

usable minimum,, rhandoffr PP −=∆

∆

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∆

∆

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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

– speed28-Dec-12 271

• 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 – 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.

28-Dec-12 272

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• 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.

∆

∆

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

•28-Dec-12 276

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.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.

28-Dec-12 277

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

28-Dec-12 279

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.• 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

28-Dec-12 280

• Consider a cellular system which has a total of S duplex channels.

• Each cell is allocated a group of k channels, .

• The S channels are divided among N cells.

• The total number of available radio channels

• 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

Sk <

kNS =

• 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

MSMkNC ==

N/1

28-Dec-12 281

• 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

• 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.

22jijiN ++=

28-Dec-12 282

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• 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.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

• 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

NR

DQ 3==

• A tradeoff must be made between these two objectives

28-Dec-12 285

• Let be the number of co-channel interfering cells. The signal-to-

interference ratio (SIR) for a mobile receiver can be expressed as

S: the desired signal power

: interference power caused by the ith interfering co-channel cell

base station

• The average received power at a distance d from the transmitting

0i

∑=

=0

1

i

i

iI

S

I

S

iI

• The average received power at a distance d from the transmitting

antenna is approximated by

or

n is the path loss exponent which ranges between 2 and 4.

n

rd

dPP

−

=

0

0

−=

0

0 log10)dBm()dBm(d

dnPPr TX

0d

0P :measued power

28-Dec-12 286

• When the transmission power of each base station is equal, SIR for a

mobile can be approximated as

• Consider only the first layer of interfering cells

( )∑=

−

−

=0

1

i

i

n

i

n

D

R

I

S

( )00

3)/(

i

N

i

RD

I

Sn

n

== 60 =i00 iiI

28-Dec-12 287

• 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

444

4

2)(2)(2 −−−

−

+++−=

DRDRD

R

I

S

28-Dec-12 288

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.receiving filter response desired signalresponse

desired signal interferenceinterferencesignal on adjacent channelsignal on adjacent channel

FILTER28-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 • 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.

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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

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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.

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2.7.1 Cell Splitting

• Split congested cell into smaller cells.

– Preserve frequency reuse plan.

– Reduce transmission power.

– Increase the

capacity of the

cellular system

microcell

Reduce R to R/2

microcell

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

•The microcell BS labeled G is placed half way between 2

larger stations using the same channel G

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• Transmission power reduction from to

• Examining the receiving power at the new and old cell boundary

• If we take n = 4 and set the received power equal to each other

1tP 2tP

n

tr RPP−

∝ 1]boundary cell oldat [n

tr RPP−

∝ )2/(]boundary cellnew at [ 2

16

12

tt

PP =

• 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

162t

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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

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• Interference Reduction

position of the

mobile

interference cells

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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.degrees) but rather in 120 degrees only.

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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.

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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.cell size fixed.

• Because of the benefits offered by 120 degree antenna,

these are most readily used by base station towers.

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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-

assignmentassignment

– Switch the channel to a

different zone site

• Reduce interference

– Low power transmitters

are employed

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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

MSC

MSC

MSC

Public (Wired)

Telephone

Network

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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

303

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)

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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 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

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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 tochanging. 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.

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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.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.

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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 thencontacts 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.

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

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

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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.

310

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.

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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.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.

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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 • 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.

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• 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.in this model.

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Disadvantages

• Does not provide a way of determining

corrections due to environmental factors.

• Multipath is also not considered.

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Fig. 4.23 & 4.24 (Theodore S. Rappaport: Wireless

communication principles and practice,2/e, Pearson

Education)

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 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.

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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, • 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.

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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.short period or short travel distance.

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Multipath Fading Effects

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Factors Influencing Fading

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Doppler Shift Geomerty

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• 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.

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• source is assumed to be very far away.

• The phase change in the received signal due

to the difference in path lengths is therefore

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• 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.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

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Channel issues

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Time-varying impulse response

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k is the gain of transmitter.

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.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.

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Types of Small-Scale Fading

Bs<Bc Bs>Bc

στ <Ts στ >Ts

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Bs<BD Bs>BD

Tc<Ts Tc>Ts

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.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

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• 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) & 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)

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Flat-fading (non-freq. Selective)

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Frequency selective fading

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Two independent fading issues

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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 • 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.

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Two ray NLoS multipath, resulting in Rayleigh fading

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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 • there can either be constructive or destructive

interference between the two signals.

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Above distribution is known as Rayleigh Distribution and is shown in

the figure for different σ values. It has been derived for slow fading.

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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,

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• Average Fade Duration(AFD):- average period

of time for which the received signal is below

a specified level R.

• Average fade duration,• Average fade duration,

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• Clarke’s model consider only flat fading

conditions.

• Do not consider multipath time delay or

frequency selective fading conditions.frequency selective fading conditions.

• Impulse response of the model is,

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