presentaiobnn 1.pdf

8/13/2019 Presentaiobnn 1.pdf

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Topics Discussed in Class Room

Introduction

Paging Systems

Cordless Phone

Cellular Telephone Systems


Trends in Cellular Systems

RAVILLA DILLIFaculty E & C Dept., MIT, Manipal1/25/2014

Cellular Design Concepts

Channel Assignment Strategies

Handoff Strategies

Interference and System Capacity

Mobile Communications


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Introduction


A cellular mobile comms. system uses a large no. of low-power wirelessTxs to create cells.Variable power levels allow cells to be sized according to subscriberdensity and demand within a particular region.As mobile users travel from cell to cell, their conversations are handedoff between cells.Channels (frequencies) used in one cell can be reused in another cell

some distance away.


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Wireless Communication System Definitions


Base Station: A fixed station in a mobile radio system used for radiocommunication with mobile stations.

Located at the center or on the edge of a coverage region.Consist of radio channels and transmitter and receiver antennas mountedon a tower.

Control Channel: Radio channels used for transmission of call setup, callrequest, call initiation, and other beacon or control purposes.

Forward Channel: Radio channel used for transmission of information from thebase station to the mobile.

Full Duplex systems: Communication Systems which allow simultaneous two- way communication.Half Duplex Systems: Communication systems which allow two-waycommunication by using the same radio channel for both transmission andreception.At any given time, the user can only either transmit or receive information.


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Handoff: The process of transferring a mobile station from one channel orbase station to another.

Mobile Station: A station in the cellular radio service intended for use while inmotion at unspecified locations.

Mobile stations may be hand-held personal units (portables) orinstalled in vehicles (mobiles).

Mobile Switching Center(MSC): Switching center which coordinates therouting of calls in a large service area.

In a cellular radio system, the MSC connects the cellular base stationsand the mobiles to the PSTN.

An MSC is also called a mobile telephone switching office (MTSO).


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Page: A brief message which is broadcast over the entire service area, usuallyin a simulcast fashion by many base stations at the same time.

Reverse Channel: Radio channel used for transmission of information from themobile to base station.

Roamer: A mobile station which operates in a service area (market) other

than that from which service has been subscribed.

Simplex Systems: Communication systems which provide only one-waycommunication.

Subscriber: A user who pays subscription charges for using a mobilecommunications system.

Transceiver: A device capable of simultaneously transmitting and receivingradio signals.


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Wireless Communication System


Mobile radio transmission systems classification:

1. Simplex 2. Half-Duplex 3.Full-Duplex

1. Simplex systems: Communication possible only in one direction.e.g. Paging Systems

2. Half-Duplex Systems: Its two-way communication, but user can eithertransmit or receive at the given time.

3. Full Duplex systems: allow simultaneous radio transmission and receptionbetween a subscriber and a base station.

1. FDD (Frequency Division Duplexing) 2. TDD(Time Division Duplexing)


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1. FDD (Frequency Division Duplexing) :

It provides simultaneous radio transmission channels for the MS and the BS, sothat they can simultaneously transmit and receive signals from one another.

At the BS, separate transmit and receive antennas are used to accommodatethe two separate channels.

At the MS, a single antenna is used for both transmission to and reception fromthe BS, a duplexer is used inside the MS to enable the same antenna to be usedfor simultaneous transmission and reception.

To facilitate FDD, it is necessary to separate the transmit and receivefrequencies by about 5% of the nominal RF frequency, so that the duplexer canprovide sufficient isolation while being inexpensively manufactured.


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2. TDD(Time Division Duplexing):

TDD shares a single radio channel in time, so that a portion of the time is usedto transmit from the BS to the MS, and the remaining time is used to transmitfrom the MS to the BS.

If the data transmission rate in the channel is much greater than the end- users

data rate, it is possible to store information bursts and provide the appearanceof full duplex operation to a user.

TDD is possible only with digital transmission formats and digital modulation,and is very sensitive to timing.


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Paging Systems

Paging is a radio communications system that send brief messages to asubscriber.

Paging systems operate in the simplex mode, for they broadcast signalsor messages to individuals who carry small battery-operated Rxs.

Eg: A paging company will send a radio signal that will be received by

the pager. The paging Rx has a built-in audible signaling device thatinforms the person that he is being paged.

The paging information may be an audio tone or a short printed message(numeric or alphanumeric).

The paging Rxs have a small LCD screen on which a telephone number isdisplayed.



11/92RAVILLA DILLIFaculty E & C Dept., MIT, Manipal1/25/2014

Paging Systems


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If caller wants to contact a person who has a pager, an individual dialsthe telephone number assigned to that person.

The call is received at the office of the paging company.The paging company responds with one or more signaling tones that tellthe caller to enter the telephone number which the paged personshould call.

Once the number is entered, the caller presses the pound sign key tosignal the end of the telephone entry. The calling party then hangs up.The paging system records the telephone number in a computer andtranslates this number into a serial binary-coded message.

The message is transmitted as a data bit stream to the paging Rx.The serial binary-coded message modulates the carrier of a radio Tx.Paging systems operate in the VHF and UHF frequency range.


Paging System Operation



Paging Systems

Figure: A wide area Paging System

The paging control center dispatches pages received from the PSTN

throughout several cities at the same time.


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Each paging Rx is assigned with a cap code. The paging Rx will recognize its unique code.Digital paging systems uses commonly 2 protocols:

1. POCSAG(Post Office Code Standard Advisory Group)ASK PSK Modulation Data Rate : 2.4 Kbits/SecChannel BW: 25KHz Freq. range : 150, 470, 900MHz

2. FLEX developed by Motorola.ASK FSK Modulation Data Rate: 1.6 to 6.4 Kbits /SecCRC error detection


Paging Systems



Modern Paging Systems



Cordless Telephone System

It is a full-duplex, two-way radio system, consists of1. Portable Unit 2. Base Unit



Cordless Telephones


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Base Unit :It is connected to the telephone line and receives power from the AC line.

It is a transceiver ( Tx sends the received audio signal to the Portable Unit.

Rx receives signal transmitted by the portable unit and retransmits them onthe telephone line.)

It also contains battery charger that charges the Portable Unit.It operates in frequency range : As TX 43 MHz - 46 MHz As Rx 49 MHz range.

Portable Unit : It is battery powered, fully portable and rest in the Base Unitwhere its battery can be recharged.

It operates in frequency range : As RX 43 MHz - 46 MHz As Tx 49 MHz range.

Both Units having Antennas and transceivers( use full-duplex operation), toachieve this the Tx and Rx must operate in different frequencies.


Cordless Telephones


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Frequency Allocations: The FCC has allocated 25 duplex channels. ( operatingfrequency can be changed if interference is occurring on one channel).

FM is used.Eg: Base ( Tx ) Handset ( Tx )

43.72 MHz 48.76 MHz

43.72 MHz 48.86 MHz

44.48 MHz 49.50 MHz

The max. Tx power : 500mWThe max. Usable operating range : 100 ft (varies from one part of the house toanother) Because of interference, You may also receive signals from a cordless telephone

of your neighbor. Signal quality is less than standard telephone levels. RAVILLA DILLIFaculty E & C Dept., MIT, Manipal1/25/2014

Cordless Telephones


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Advanced Cordless Telephones:4 Basic Types: 1. channels are in the range 900 MHz ( uses FM )

Advantage : Minimizes interference than common range( 46 48 MHz)2. Operates in 900 MHz range

ADPCM method of A/D conversion.Uses Digital techniques and PSK is used.

Advantage : Less sensitive to noise.Greater clarity in the signal.More reliable over long distances.

3. Operates 902 MHz 928 MHz range.Uses Spread Spectrum.

Advantage : Best quality and more secure.No Interference.

4. Operates 2.4 GHzUses Spread Spectrum.

Advantages: Longer distance communication. ( of course uses more power)RAVILLA DILLIFaculty E & C Dept., MIT, Manipal1/25/2014

Cordless Telephones


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Fig: A Cellular System


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MSC is responsible for connecting all mobiles to the PSTN in a cellular system.

Each MS communicates via radio with one of the BSs and may be handed- offto any number of BSs throughout the duration of a call.

MS contains a transceiver, an antenna, and control circuitry, and used as aportable hand-held unit.

BS consist of several Txs and Rxs which simultaneously handle full duplexcommunications and generally have towers which support several transmittingand receiving antennas.


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BS serves as a bridge between all mobile users in the cell and connects thesimultaneous mobile calls via telephone lines or microwave links to the MSC.

The MSC coordinates the activities of all of the BSs and connects the entirecellular system to the PSTN.

A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneousconversations at a time, and accommodates all billing and system maintenancefunctions, as well.


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Cellular Telephone System


Communication b/w the BS and the MS is defined by a standard common airinterface (CIA) that specifies four different channels.

FVC(Forward Voice Channels ): Channels used for voice transmission from theBS to MS.

RVC(Reverse Voice Channels): Channels used for voice transmission fromMS to the BS.

FCC(Forward Control Channels) andRCC(Reverse Control Channels:

These two channels responsible for initiating mobile calls.

Control channels are involved in call setup and moving it to an unused voicechannel.Control channels transmit and receive data messages that carry call initiationand service requests, and are monitored by MS when they do not have a callin progress.


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Overview of Cellular System

1/25/2014 ITCS RAVILLA DILLIFaculty E & C Dept., MIT, Manipal


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How a Cellular Telephone Call is made ?

When a cellular phone is turned on, it first scans the group of FCCs todetermine the one with the strongest signal, and then monitors that controlchannel until the signal drops below a usable level.

At this point it again scans the control channels in search of the strongest BSsignal.


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When a telephone call is placed to a mobile user, the MSC dispatches the

request to all BSs in the cellular system.

The mobile identification number (MIN), which is the subscribers telephonenumber, is then broadcast as a paging message over all of the forward controlchannels throughout the cellular system.

The MS receives the paging message sent by the BS which it monitors, andresponds by identifying itself over the RCC.

The BS relays the ACK sent by the MS and informs the MSC of the handshake.


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Then, the MSC instructs the BS to move the call to an unused voice channel

within the cell.

At this point the BS signals the MS to change frequencies to an unused FVC andRVC pair.

At this point another data message (called an alert ) is transmitted over the FVCto instruct the MS to ring, thereby instructing the mobile user to answer thephone.

ll l l h


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When call is in progress, the MSC adjust the transmitted power of the MS andchanges the channel of the MS and BS in order to maintain call quality as thesubscriber moves in and out of range of each BS(called Handoff ).

Special control signaling is applied to the voice channels so that the MS may becontrolled by the BS and the MSC while a call is in progress.

C ll l l h S


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How a Call is initiated by a Mobile Station ?

When a MS originates a call, a call initiation request is sent on the RCC.

MS call request includes its MIN, ESN, and MIN of the called party, and SCM.

The BS receives this data and sends it to the MSC.

The MSC validates the request, makes connection to the called party throughthe PSTN, and instructs the BS and MS to move to an unused FVC and RVC pairto allow the conversation to begin.

MIN: Mobile Identification NumberESN: Electronic Serial NumberSCM(station class mark): indicates what the maximum Tx power level for the

particular user.

C ll l T l h S


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What is Roaming ?

When a MS enters geographical area other than its home service area, it isregistered as roamer in the new service area.

MIN: Mobile Identification NumberESN: Electronic Serial Number HLR: Home Location Register

For every several minutes, the MSC issues a global command over each FCCin the system, asking for all mobiles which are previously unregistered to reporttheir MIN and ESN over the RCC.

New unregistered mobiles in the system periodically report back theirsubscriber information upon receiving the registration request.

Then MSC uses the MIN/ESN data to request billing status from the HLR foreach roaming mobile.

C ll l T l h S


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What is Roaming ?

If a particular roamer has roaming authorization for billing purposes, the MSCregisters the subscriber as a valid roamer.

Once registered, roaming mobiles are allowed to receive and place calls fromthat area, and billing is routed automatically to the subscribers home service

provider.


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Trends in Cellular Radio and Personnel Communications

Government agencies (FCC) give licenses to companies (service providers) to providecellular access in a particular geographic region.

These licenses allow the service provider to setup cellular towers in that region whichcan transmit over a prescribed band of frequencies.

The service providers must use one of the approved cellular standards for developingthe cellular network in that region.

These standards described the air interface, i.e., how cell phones and base stationsmust communicate with each other.

The first cellular systems deployed in the U.S in mid 1980s adhered to a standard calledAnalog Mobile Phone System (AMPS).

The first cellular network used analog technology. Specifically, speech was converted toan FM signal and transmitted back and forth from user phones.


Standards: Rules for a Cellular Network


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2G of CellularThe second generation (2G) of cellular networks were deployed in the early 1990s .2G cellular phones used digital technology and provided enhanced services (e.g.,messaging, caller-id, etc.).

The two standards used in U.S. are different from the 2G system used in Europe (calledGSM) and the system used in Japan.

First U.S. standard is called Interim Standard 136 (IS-136) and is based on TDMA(time-division multiple access).

Second is called IS-95 and is based on CDMA (code-division multiple access).

2.5G offer enhanced services over 2G systems (email, web-browsing, etc.).Presently, service providers are setting up third generation (3G) cellular systems.

3G offers higher data rates than 2.5G. This allows users to send/receive pictures, videoclips, etc .There are two standards for 3G, Wideband CDMA (WCDMA) and cdma2000. These twostandards have been adopted world-wide.Both are based on CDMA principles.

2.5G & 3G Cellular Systems



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First Mobile Telephone System

One and only onehigh power base

station with which allusers communicate.

Entire CoverageAreaNormal

TelephoneSystem

Wired connection



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The Core Idea: Cellular Concept

Multiple lower-power Base Stations(BS) that service mobile users within their coverage

area and handoff users to neighboring Base Stations(BS) as users move.

Instead of one base station covering an entire city, the city was broken up into cells, orsmaller coverage areas.

Each of these smaller coverage areas had its own lower-power BS.

User phones in one cell communicate with the BS in that cell.



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Three regular polygons that always tessellate:

Equilateral triangle Square Regular Hexagon

TrianglesSquares

Hexagons

Cellular Concept: 3 Core Principles

Small cells covers overall coverage area.

Users handoff as they move from one cell to another.

Frequency reuse.



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Circular Coverage Areas

Original cellular system was developed assuming base station antennas areomnidirectional, i.e., they transmit in all directions equally.

Users located outsidesome distance to thebase station receiveweak signals.

Result: base station hascircular coveragearea.



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Cell Shape

Cell

R

(a) Ideal cell (b) Actual cell

R

R R

R

(c) Different cell models



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Circles Dont Cover

Thus, ideally base stations have identical, circular coverage areas.

Problem: Circles do not tessellate.

The most circular of the regular polygons that cover is the Hexagon .

Thus, early researchers started using hexagons to represent the coverage area of a basestation, i.e., a cell.



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Hexagonal coverage

Since the network resembles cells from a honeycomb , the name cellular was used todescribe the resulting mobile telephone network.

BaseStation



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Frequency Reuse

It allows for many users to be supported at the same time.

Total spectrum allocated to the service provider is broken up into smallerbands.

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

Neighboring cells are assigned a different frequency band.

This ensures that nearby transmissions do not interfere with each other.RAVILLA DILLIFaculty E & C Dept., MIT, Manipal1/25/2014

Frequency Reuse: Process of selecting and allocating channel(frequency)groups for all the cellular BSs with in a system.


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Frequency Reuse: Example

Fig: using the same frequencies(same color)

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



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4 3 reuse mode: one group includes 3 sectors /site ,12 frequency which are distributedto 4 sites. Every site owns 3 frequency.

Frequency Reuse

A3

D2B1

C3

B2D1

D3

A2C1

B3

C2A1

B3

C2A1

A3

A1B1

D1

D3D2

C3

B2A1

C3D2

C3

C1

D2B1C2A1

A2C1

D3



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A3

C2B1

B3

A2C1

C3

B2A1

A3

C2B1

B3

A2C1

B3

A1C1

A1

A3A2

C3

B2A1

A3A3

C3

C1

B2A1B2A1

A2C1

B3

3 3 reuse mode:one group includes 3 sectors /site ,9 frequency which are distributed to 3sites. Every site owns 3 frequency.

Frequency Reuse



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Let, S : Total no. of available channelsk : no. of channels allocated to each cellN : Cluster Size (no. of Cells (each cell has same no. of unique and disjoint

channel groups)). Typically 4, 7 or 12

Therefore, total no. of available radio channels, S = kN

Frequency Reuse


Let, M : replication factor of a cluster

Then, total no. of channels or Capacity of Cellular System C = MkN = MS


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Is C depends on N ? If so, what is the relation ?

Frequency Reuse


If N reduced and the cell size kept constant, more clusters required tocover a given area. So, C increases.

But, co-channels are located close together, causes co-channelinterference.

If N is large, the ratio b/w the cell radius and the distance b/w co-channel cells to decrease, leading to weaker co-channel interference.

N depends on how much interference a MS or BS can tolerate tomaintain sufficiently quality of communications.

1/N reusing factor

Frequency Reuse


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Frequency Reuse


Fig: Method of locating co-channels in a Cellular System

How to locate Co-channels ?

1. Move i cells along any chain of hexagons 2. Turn 60 degrees counter- clockwise and move j cells.

N = i2 + ij + j2

Frequency Reuse


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Frequency Reuse


If a total of 33 MHz of bandwidth is allocated to a particular FDD cellulartelephone system which uses two 25 kHz simplex channels to provide fullduplex voice and control channels,

compute the number of channels available per cell if a system uses(a) four-cell reuse, (b) seven-cell reuse, and (c) 12-cell reuse.

If 1 MHz of the allocated spectrum is dedicated to control channels, determinean equitable distribution of control channels and voice channels in each cellfor each of the three systems.

Solution:Total available channels = 33000/50 = 660 channels

a) N = 4, Total Channels available per Cell = 660/4 = 165b) N = 7, Total Channels available per Cell = 660/7 = 95c) N = 12, Total Channels available per Cell = 660/12 = 55

Frequency Reuse


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Frequency Reuse


Solution Cont.,:

1MHz for control channels Total available control channels = 1000/50 = 20

a) N = 4, 5 Control and 160 Voice Channels available per Cell

b) N = 7,

4 Cells with 3 Control and 92 Voice Channels2 Cells with 3 Control and 90 Voice channels1 Cell with 1 Control and 92 Voice Channels

c) N = 12,

8 Cells with 2 Control and 53 Voice Channels4 Cells with 1 Control and 54 Voice Channels



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What is the objective of Frequency reuse ?


Efficient utilization of the radio spectrum ( increase cellular capacity andminimize the interference)

Two types of Channel Assignment Strategies:

1. Fixed 2. Dynamic



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1. Fixed channel assignment strategy:Each cell is allocated with a predetermined set of voice channels.

Call attempt within the cell can only be served by the unused channels inthat particular cell.

If all the channels in that cell are occupied, the call is blocked and thesubscriber does not receive service.

Borrowing strategy: a cell is allowed to borrow channels from a neighboringcell if all of its own channels are already occupied.

MSC supervises such borrowing procedures and ensures that the borrowing ofa channel does not disrupt or interfere with any of the calls in progress in thedonor cell.



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2. Dynamic channel assignment strategy:

Channels are not allocated to different cells permanently.

When a call request is made, the serving BS requests a channel from the MSC.

Then a channel is allocated based on an algorithm that takes into account theprobability of future blocking within the cell, the frequency of use of thecandidate channel, the reuse distance of the channel, and other cost functions.

Accordingly, the MSC allocates a given frequency if that frequency is not

presently in use in the cell or any other cell which falls within the minimumrestricted distance of frequency reuse to avoid co-channel interference.



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2. Dynamic channel assignment strategy:

Advantage:Channel Utilization is more.Reduce the probability of blocking(increases the capacity of the system) .

Disadvantage:MSC must collect real-time data on channel occupancy, traffic distribution,and radio signal strength indications (RSSI) of all channels on a continuousbasis.

This increases the storage and computational load on the system.


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Handoffs

Mobile phone users are by definition mobile, i.e., they move around while using thephone.

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.


A Handoff : Example


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A Handoff : Example

A user is transmitting and receiving signals from a given BS, say B 1.

Assume the user moves from the coverage area of one BS into the coverage area of asecond BS, B 2.B1 notices that the signal from this user is degrading.B2 notices that the signal from this user is improving.At some point, the users signal is weak enough at B 1 and strong enough at B 2 for a

handoff to occur .Specifically, messages are exchanged between the user, B 1, and B 2 so that communicationto/from the user is transferred from B 1 to B 2.


H d ff R i


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Handoff Region

BS i

Signal strength

due to BS j

E

X1

Signal strength

due to BS i

BS jX3 X4 X2X5 X thMS

Pmin

Pi(x) P j(x)

By looking at the variation of signal strength from either base station it is possible todecide on the optimum area where handoff can take place.


Handoff Strategies


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1/25/2014 RAVILLA DILLIFaculty E & C Dept., MIT, Manipal

Handoff: When MS moves into a different cell area while a call is in progress,the MSC automatically transfers the call to a new channel belongingto the new BS.

Try to reduce no. of Handoffs.

Handoff must be imperceptible to the users.

System designers must specify an optimum signal level at which handoff

has to be initiated. Pr minimum usable : power of minimum usable signal for acceptable voice quality

at the BS RxPr handoff : threshold power at which a handoff is made

= Pr handoff - Pr minimum usableWhat is optimum ? If is too large, unnecessary handoffs and it becomes burden to MSC.

If is too small, there is insufficient time to complete a handoff before a callis lost.

Handoff Strategies


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Case 1: Handoff is not occurred and signal is dropped below the min.acceptable level.

Call drop can be because of excess delay by the MSC in assigning a handoff.

Why Excessive delay at MSC:1. because of more computational load,2. no channels are available on any of the near by base stations.

Fig: Improper Handoff Situation

Handoff Strategies


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1/25/2014 RAVILLA DILLIFaculty E & C Dept., MIT, ManipalFig: Illustration of a Handoff scenario at Cell boundary

Handoff Strategies


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What is the role of fading in Handoff ?

Fading may cause drop in signal level.

BS monitors the signal level for a certain period of time before a handoff isinitiated.

Time to decide if a handoff is necessary depends on the vehicle speed.

Handoff Strategies


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What is dwell time ?

The time over which a call is maintained within a cell, without handoff.

It depends on interference, distance b/w the MS and BS, type of radiocoverage and speed of the user etc.,

Dwell time is important in the practical design of handoff algorithms.

Handoff Strategies


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How to determine the quality of a channel ?

1. Word Error Indicator(WEI): indicates whether the current burst was

demodulated properly in the MS.

2. Received Signal Strength Indication(RSSI): measure of received signalstrength. (typically ranges b/w 80 to 100 dB)

3. Quality Indicator(QI): related to Signal to Interference(S/I) ratio.

If WEI is good, no handoff is required.

To make the handoff decision accurately and quickly, it is desirable to use both

WEI and RSSI.

Handoff Strategies


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1. Mobile-Controlled Handoff(MCHO):

MS continuously monitors the signals of the surrounding BSs and initiates the

handoff process when criteria is met.

MCHO is used in DECT and PACS.

2. Network-Controlled Handoff(NCHO):

Surrounding BSs measure the signal from the MS, and the network initiatesthe handoff process when criteria is met.

NCHO is used in AMPS.

Handoff Strategies


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Intersystem Handoff:MS moves from one cellular system to a different cellular systemcontrolled by different MSC.

3. Mobile-Assisted Handoff(MAHO):

MS measures the received power from surrounding BSs and continuously

reports the results to the serving BS.

Handoff is initiated when the power received from neighboring BS begins toexceed the power received from serving BS by a certain level.

MAHO is faster and suited for microcellular environments where handoffs arefrequent.

MAHO is used in GSM and IS-95 CDMA.

Handoff Strategies


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Prioritizing Handoffs:

Guard Channel Concept: a fraction of the total available channels in a cell isreserved exclusively for handoff requests.

Disadvantage: no. of voice channels are reducedHowever, there is efficient spectrum utilization with dynamicchannel assignment strategies.

Queuing of handoff requests: There is a finite time interval b/w the time thereceived signal drops below the handoff threshold and the time the call isterminated due to insufficient signal level.

Queuing does not guarantee a zero probability of forced termination.

Large delays causes the received signal level to drop below the minimumrequired level to maintain communication and leads to forced termination.

The delay time and size of queue depend on traffic in the service area.

Handoff Strategies


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Practical Handoff Considerations:

role of mobile velocities :

How to design cells for high speed and low speed users ?

High speed vehicles vs Pedestrians

Handoff Strategies


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Figure: The Umbrella Cell Approach

Handoff StrategiesUmbrella cell approach :


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Umbrella cell approach :

By using different antenna heights (often on the same building or lower) anddifferent power levels, it is possible to provide large and small cells whichare co-located at a single location.

It is used to provide large area coverage to high speed users while providingsmall area coverage to users traveling at low speeds.

This approach ensures that the no. of handoffs is minimized for high speedusers and provides additional microcell channels for pedestrian users.

The speed of each user may be estimated by the BS or MSC by evaluatinghow rapidly the short-term average signal strength on the RVC changes overtime.

If a high speed user in the large umbrella cell is approaching the BS, and itsvelocity is rapidly decreasing. The BS may decide to handover the user intothe co-located microcell, without MSC intervention.

Handoff StrategiesPractical Handoff Considerations:


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Practical Handoff Considerations:Cell dragging :It results from pedestrian users that provide a very strong signal to the BS.

Such a situation occurs in an urban environment when there is a line-of-sight(LOS) radio path b/w the MS and the BS.

As the user travels away from the BS at a very slow speed, the average signalstrength does not decay rapidly.

Even when the user has traveled well beyond the designed range of the cell,the received signal at the BS may be above the handoff threshold, thus ahandoff may not be made.

This creates a potential interference and traffic management problem. sincethe user has meanwhile traveled deep within a neighboring cell.

Remedy : Handoff thresholds and radio coverage parameters must beadjusted carefully.

Handoff Strategiesff


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Hard Handoff:

Spread spectrum mobiles share the same channel in every cell.

Handoff does not mean a physical change in the assigned channel, but ratherthat a different BS handles the radio communication task.

By simultaneously evaluating the received signals from a single subscriber atseveral neighboring BSs, the MSC may actually decide which version of theusers signal is best at any moment in time.

The ability of MSC to select b/w the instantaneous received signals from avariety of BSs is called soft handoff .

Interference and System CapacitySources of interference:


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Sources of interference:another mobile in the same cell,a call in progress in a neighboring cell,other BSs operating in the same frequency band,or any non-cellular system which inadvertently leaks energy into the

cellular frequency band.

Interference on voice channels causes cross talk, where the subscriber hearsinterference in the background due to an undesired transmission.

Interference on control channels leads to missed and blocked calls due toerrors in the digital signaling.

The two major types of system-generated cellular interference1. co-channel interference2. adjacent channel interference



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Adjacent Channel Interference

Conception: The signal interference from the frequency adjacent to that of thesignal used is called adjacent channel interference .

Reduction method: accurate filtering and channel allocation (maximizingchannel intervals of the cell).

Co-Channel InterferenceConception: the interference among the signals of co-channel cells is called

co-channel interference .

Result from : Frequency reuseReduction method: co-channel cells must physically be spaced at a minimum

interval to ensure adequate isolation of transmissions .




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Interfering signals generated within the cellular system are difficult to controlin practice (due to random propagation effects).

Even more difficult to control is interference due to out-of-hand users .

In practice, the Txs from competing cellular carriers are often a significantsource of out-of-hand interference, since competitors often locatetheir BSs in close proximity to one another in order to provide comparablecoverage to customers.

Co-Channel InterferenceCo-channel Cells: Cells that use same set of frequencies.


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Interfering cells are closer by when cluster size is smaller.


Co channel Cells: Cells that use same set of frequencies.Co-channel Interference: interference b/w co-channels

Frequency Reuse


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Frequency Reuse

F1

F2

F3

F4F5

F6

F7 F1

F2

F3

F4F5

F6

F7

F1

F2

F3

F4F5

F6

F7 F1

F2

F3

F4F5

F6

F7

F1

F1

F1

F1

Ex: Set of frequency

7 cell reuse cluster


Co-Channel Interference and System Capacity


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Reuse Distance:

F1

F2

F3

F4F5

F6

F7

F1

F2

F3

F4F5

F6

F7

F1

F1

For hexagonal cells, the reuse distance is given by

R N D 3

R

R Cell radiusN the reuse pattern (the cluster sizeor the number of cells per cluster).

N R Dq 3

Reuse factor is

Cluster


D distance b/w centers of the nearest co-channel cells

--------> (1)



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Table: Co-channel Reuse Ratio for some values of N

N R

Dq 3

If q is small Large Cellular Capacity ( since cluster size N is small)

If q is large good transmission quality ( because of less co-channelinterference)



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i0 = no. of co-channel interfering cells

S desired signal power from desired BS

Ii interference power caused by the i th interfering co-channel cell BS

Signal-to-Interference ratio,

The avg. received power at a distance d from the Tx antenna,

or

P0 = power received at a close-in reference point in the far field region of the

antenna.

d0 small distance from the Tx antenna

N path loss exponent (typically, 2 to 4 in urban regions)

--------> (2)

--------> (3)



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Let the transmit power of each BS and the path loss exponent are samethroughout the coverage area, then S/I for a mobile is approximated as

Di = distance of the i th interferer from the mobile

Consider a forward link where the desired signal is the servingBS and the interference is due to co-channel BSs.

Consider the 1 st tier of BSs and they are at equal distance D b/w cell centers,then

--------> (4)

--------> (5)

Co-channel Interference


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Mobile Station

Serving Base Station

First tier co-channel

Base StationSecond tier co-channelBase Station

R

D1

D2

D3

D4

D5D6


Worst Case of Co-channel Interference


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Mobile Station

Serving Base Station Co-channel Base Station

R

D1

D2

D3

D4

D5

D6




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According to Cell geometry, if the MS is at the Cell boundary, thenthe MS is a distance D-R from the two nearest co-channelinterfering Cells and is exactly D-R/2, D, D+R/2, and D+R from

the other interfering Cells in the 1st

tier.

Figure: Illustration of the 1 st tier of co-channel cells for a cluster size, N =



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For a worst case, S/I is approximated as,assume n=4

--------> (6)



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If a signal-to-interference(S/I)ratio of 15 dB is required for satisfactoryforward channel performance of a cellular system, what is the frequencyreuse factor and cluster size that should be used for maximum capacity if thepath loss exponent is

(a) n = 4, (b) n = 3

Assume that there are six co-channel cells in the first tier, and all of them areat the same distance from the mobile. Use suitable approximations.



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Solution:(a) n=4 First, let us consider a seven-cell reuse pattern.Using Equation (1), the co-channel reuse ratio D/R = 4.583.Using Equation (5), the signal-to-noise interference ratio is given byS/l= (1/6) x (4.583) 4 = 75.3 = 18.66 dBSince this is greater than the minimum required S/I, N = 7 can be used.

(b) n=3 First, let us consider a seven-cell reuse pattern.

Using Equation (5), the signal-to-interference ratio is given byS/I = (1/6) X (4.583) 3 = 16.04 = 12.05 dB

Since this is less than the minimum required S/l, we need to use a larger N.

Using N = i 2 +ij+j2, the next possible value of N is 12, (i = j = 2).The corresponding co-channel ratio is given by Equation (1) as D/R = 6.0Using N value, the signal-to-interference ratio is given byS/I= (1/6)x(6) 3 = 36 = 15.56 dBSince this is greater than the minimum required S/I, N = 12 is used.

Power control for Reducing Interference


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The power levels at MS are controlled constantly by the serving BS.

Power control saves battery life of MS and also reduces the reverse channel S/I

in the system.

Reasons for interference


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Environmental factors:

Terrain (mountains, hills, plains, water bodies, etc.);

The quantity, heights, distribution and materials of buildings;

The vegetation of the region; Weather conditions;

Natural and artificial electromagnetic noises;

Frequency;

How MS is moved.

The transmission path is very complex, ranging from the simple line-of-sight transmissionto encountering such terrain as buildings, hills and trees. Wireless channels are extremelyunpredictable.


Text Books


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[1] Theodore S. Rappaport , Wireless Communication Principles and Practice, 2nd Edition, Pearson Education.

[2] Kamilo Feher Wireless Digital Communications, Modulation and SpreadSpectrum Applications Eastern Economy Edition.

[3] William C. Y. Lee, Mobile Cellular Telecommunications, McGraw-HillInternational.

[4] Jochen H Schiller, Mobile Communication.



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DILLI RAVILLA

Cabin No.54, FC-IV, Ground Floor, AB-5.

Contact No 9035190124

Mail Id [email protected]

1/25/2014

My ontact

RAVILLA DILLIFaculty E & C Dept., MIT, Manipal


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QUERIES ?



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