mobile communication introduction

8/4/2019 Mobile Communication Introduction

1/113

Mobile Communication

(Code-EC732)Instructor: Md Jawaid Alam

Sr. LecturerJIIT, Sec-62, Noida, INDIA


2/113

Cellular Networks : Evolution(1)

First Generation

Launched in mid 1980s

Analog Systems

Analog modulation ,mostly Frequency Modulation

Voice Traffic only

FDMA/FDD multiple access

Confined to national boundaries

Examples: AMPS


3/113


Second Generation(2G)

Developed for Voice Communications Digital Systems, Digital modulation

TDMA/FDD and CDMA/FDD multiple access

Provides data rates of the order of ~9.6Kbps


4/113


Examples of Second Generation (2G)

Global System for Mobile Communication(GSM)

TDMA/FDMA

900MHz and 1800MHz

Personal Digital Communication (PDC)

Popular in Japan

IS-95 CDMA

US/South Korea


5/113


Limitations of Second Generation (2G)

Developed for Voice communication (Unsuitable

for Data Traffic)

Average rate of the order of tens of Kbps

Not suitable for internet (Packet Switched

Services)

Multiple Standards (no true global coverage)


6/113


2.5G

The effort to remove the impediments of 2G

System resulted in 2.5G.

Digital Systems

Voice + Low Data rate

Internet Access through GPRS(General Packet

Radio Service) Enhanced Data rate for Global Evolution(EDGE)


7/113


3G

Digital Modulation

Simultaneous Voice +High Speed Data

Multimegabit Internet Access

Voice Activated Call

Multimedia Transmission


8/113


Need for 4G

Present communication System are primarily designed

for one specific application, such as speech on a

mobile telephone or high data in wireless local areanetwork(WLAN)

Will integrate various networks, functions and

applications

Will create Global information multimedia Village

Will support a variety of data rates from 2G to 3Gto

3G+


9/113

TerminologyMobile: It is used to describe the radio terminal

that is attached to a high speed mobile platform.

Portable: It describes a radio terminal that can be

hand held and used by someone at walkingspeed.

Subscriber Unit: It describes a mobile or portableuser because each user pays a subscription fee touse the system and each users communicationdevice is called Subscriber Unit.


10/113

TerminologyBase Station: A fixed station in mobile radio system used

for radio communication with mobile station located atcenter or edge of coverage region and consists of radiochannels and transmitters and receivers antennasmounted on a tower.

Control Channel: Radio channel used for transmission ofcall setup, call request, call initiation and other beaconor control purpose.

Forward Channel: Radio channel used for transmission ofinformation from base station to mobile.


11/113

Terminology

Reverse Channel: Radio channel used fortransmission of information from mobile to

base station.

Subscriber: A user who pays the subscription

charges for using a mobile communication

system.

Transceiver: A device capable of simultaneously

transmitting and receiving radio signals.


12/113

Basic Concepts: Multiple Access

Multiple access schemes are used to allowmany mobile users to share a finite amount ofradio spectrum.

The sharing of the spectrum is required toachieve high capacity by simultaneously

allocating the bandwidth. Constraint: There should not be severe

performance degradation.


13/113

Frequency Division Multiple AccessScheme(FDMA)

In Base Station separate transmit and receiver antennas are used toaccommodate two separate channels.

In Subscriber Unit Single antenna is used for transmission to and receptionfrom Base Station.

A device called duplexer is used inside Subscriber Unit to enable the sameantenna to be used for simultaneous transmission and reception.

It is necessary to separate the transmit and receive frequency by about 5%of nominal RF frequency.

In US AMPS standard, the reverse channel has a frequency which is exactly45MHz lower than that of forward channel


14/113

Time Division Multiple Access(TDMA)

It shares single radio channel in time (base station tomobile and mobile to base station)

If data transmission rate in the channel is much greaterthan the end users data rate it stores informationbursts and appears like full duplex to the user

TDMA is possible only with Digital Transmissionformats and Digital Modulation

Very sensitive to timing

It used only for indoor or small area wirelessapplications where physical coverage distance is less.


15/113

Code Division Multiple

Access(CDMA)Time and bandwidth are used simultaneously bydifferent users, modulated by orthogonal or semi-orthogonal codes (e.g. spread spectrum).

7C29822.032-Cimini-9/97

Code Space

Time

Frequency


16/113

Paging System

Page: A brief message which is broadcast over theentire service area usually in a simulcast fashion bymany Base Stations at the same time.

Every cellular system has some kind of broadcastmechanism. This can be used directly for distributinginformation to multiple mobiles.

The most important use of broadcast information is to setup channels for one to one communication between themobile transceiver and the base station. This is calledpaging


17/113

Paging System

Paging system transmits the page throughout theservice area using Base Stations which broad cast thepage on a radio carrier.

Paging receivers are simple and inexpensive buttransmission system required is quite sophisticated.

Wide area paging systems consists of network oftelephone line, many base station transmitters, and

large radio towers that simultaneously broadcast apage from each base station(this is simulcasting).


18/113

18

MSC

(1)

Mobile

Switching

Center (MSC)

Cellular System


19/113

Cellular System


20/113

Cellular SystemsBasic concepts High Capacity is achieved by limiting the coverage of

each base station to small geographic region called cell

Same frequencies/timeslots/codes reused at spatially

separated locations

A switching technique called hand off enables a call toproceed uninterrupted when a user moves from one cellto another

Base station/Mobile Telephone Switching Offices(MTSO)coordinate handoff and control functions

Reuse channels to maximize the capacity


21/113

Cellular SystemsBasic concepts

Resolves the problem oflimited radio spectrum

Neighboring Base Station are assigned different group ofchannels so as to minimize the interference.

By systematically spacing the Base Station and thechannel groups may be reused as many number of timesas necessary

As demand increases, the number of base stations maybe increased thereby providing additional capacity


22/113

Forward and Reverse channels

Forward Voice Channels(FVC):Used for voicetransmission from Base Station to Mobile Station.

Reverse Voice Channels(RVC): Used for voice

transmission from Mobile Station to Base station. Forward Control Channel(FCC): Used for initiating

a call from Base Station to Mobile Station.

Reverse Control Channel(RCC): Used for initiatinga call from Mobile Station to Base Station.

The FCC and RCC are also called setup channels.


23/113

Anatomy of a Cellular Call

A cell phone, when turned on,(though not yetengaged in a call) scans the group of FCC todetermine the one with the strongest signal.

It monitors that channel until it drops below theusable threshold. It then scans for anotherchannel which is the strongest.

Control channels are defined and standardized

over the entire area of service. Typically thecontrol channel use up to 5% of the total numberof channels


24/113

A Call TO a Mobile User(1)

The MSC dispatches the request to all BS. TheMobile Identification Number(MIN) is broadcastas paging message over all FCC throughout the

service area. The MS receives the paging message from the BS

it is monitoring. It responds by identifying itselfover the RCC.

The BS conveys the handshake to the MSC. TheMSC instructs the BS to move to an unused voicechannel.


25/113

A Call TO a Mobile User(2)

The BS signals the MS to change over to an

unused FVC and RVC.

An data message ( called alert) is transmitted

over the FVC to instruct the mobile to ring!

All of these sequence of events occur in just a few

seconds, and are not noticeable to the user.

While the call is in progress, the MSC adjusts thetransmitted power in order to maintain the call

quality.


26/113

A Call FROM a Mobile User

A call initiation request is sent to the RCC.

Along with this, the MS transmits its MIN,Electronic Serial Number(ESN) and the phone

number of the called party. The MS also transmits the Station Class

Mark(SCM) which indicates the maximumtransmitted power level for the particular user.

The BS forwards the data to the MSC, whichvalidates the data and makes connection to thecalled party through the PSTN.


27/113

Call to Mobile User


28/113

Call by Mobile User


29/113

Frequency Reuse: The Need

Fixed telephone network runs wires to everyhousehold.

Suppose we give every household their ownallocation of radio spectrum for analog speech of4kHz bandwidth

12.5 million households (say Agra) X4kHz=50GHz!

Clearly impractical! No other services possible using radio transmission

Most of the spectrum unused most of the time


30/113

Frequency Reuse(2)

Cellular radio systems rely on intelligent

allocation and reuse of channels throughout

the coverage area.

Each base station is allocated a group of radio

channels to be used within the small

geographic area of its cell

Neighboring base station are given differentchannel allocation from each other.


31/113

Frequency Reuse(3)

By design of antennas, the coverage area is

limited within the cell, and the same group of

frequencies are reused to cover another cell

separated by large enough distance to keepco-channel interference within limits.

The design procedure of allocating channel

groups for all the cellular BS within a system iscalled Frequency Reuse or Frequency Planning


32/113


33/113

Frequency Reuse(4)

Cells using the same frequencies


34/113

Frequency Reuse(5)

S = Total duplex channels

k = Available channels per cell

N = Total number of cells in a cluster

Therefore, S=kN

N cells collectively use complete set ofavailable frequencies


35/113

Frequency Reuse(6)

M = No of times cluster is replicated

C= Total number of duplex channels in a sytem

Therefore, C=MkN

=> C=MS (Total channel capacity of

system)

=> C is directly proportional to M N iscalled cluster Size


36/113

Frequency Reuse(7)

If N is reduced while cell size is kept constant thenmore clusters are required to cover a given area.Hence, more capacity(large value of C)is achieved.

A large cluster size indicates that ratio between cell

radius and distance between co-channel cells is small. Conversely, small cluster size indicates that co-channel

cells are located much closer together. So, there ismore probability of interference.

From design view point smallest possible value of N isdesirable in order to maximize the capacity

Frequency reuse factorof cellular system is given by1/N

Frequency


37/113

FrequencyReuse(8)


38/113

Due to Hexagonal geometry of cell and center

line joinings are multiple of 60 degrees

So, N is given as N= i + ij + j , where I & j are

non-negative numbers.

To find the nearest co-channel neighbors of a

particular cell:

Move i cells along any chain of Hexagon

Turn 60 degree counter clockwise and move j cells

Frequency Reuse(9)


39/113

19-cell reuse example (N=19)

Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3,j= 2).

(Adapted from [Oet83] IEEE.)


40/113

Clustersize of 7

Assume a clustersize of 7. This means that the total 395voice channels are divided into groups of seven.

Thus, each cell has about 56 voice channels. This is the

most number of users that can be supported in a cell,i.e., roughly 10 square miles in normal environments.

This may/may not be sufficient based on the distribution

of users.


41/113

Clustersize of 7

To see what a system with clustersize of 7 looks like, color

a cell with color 1.

This cell (if drawn as a hexagon) has 6 neighbors. Coloreach of the seven neighbors using a different color (also

different from each other).

Now repeat this rule to get the overall reuse pattern.


42/113

Cluster size of 7, Reuse Pattern


43/113

What if we had a smaller cluster?

Now consider a system with a cluster of 4.

Then the number of voice channels per cell is 395/4,which is roughly 98.

Thus, in theory, we can hold more users per cell if thiswere true.

But there is a problem with a clustersize.


44/113

Problem with Smaller Clustersize

Interfering cells are closer by when clustersize is smaller.


45/113

Problem with Smaller Cluster size

If interfering cells are closer, then the total interferencepower will be larger.

With higher interference power, the quality of the speechsignal will deteriorate.

To reduce the interference power, we can make the cellslarger.

With larger cell, the number of users covered per unit

area reduces. So, the gain (total number of userssupported) of a smaller clustersize is not as high as wethink.


46/113

Tessellation

Three regular polygons that always tessellate:

Equilateral triangle

Square

Regular Hexagon

TrianglesSquares

Hexagons


47/113

Circular Coverage Areas

Original cellular system was developed assuming

base station antennas are omnidirectional, i.e., they

transmit in all directions equally.Users located outside

some distance to the

base station receive

weak signals.

Result: base station has

circular coverage

area.


48/113

Cell Shape

Hexagonal cells are conceptual

For most theoretical treatment, hexagonal

model of cells is universally adopted because:

Hexagonal are geometric shape that approximates

a circle(for omni directionational radiation)

Using a hexagonal geometry, fewest number of

cells can cover the entire geographical region


49/113

Channel Assignment Strategies

A scheme for increasing capacity and

minimizing interference is required.

Channel assignment strategies can be

classified as either fixed or dynamic.

The choice of the channel assignment strategy

impacts the performance of the system,

particularly how a call is managed when userishanded offfrom one cell to another.


50/113

Fixed Channel Assignment

Each cell is assigned apredeterminedset of voicechannels.

Any call attempt within the cell can only be

served by the unusedchannels inn that particularcell.

If all the channels in the cell are occupied, the callis blocked. The user does not get service.

In a variation of the fixed channel assignment, acell can borrow channels from its neighboring cellif its own channels are full.


51/113

Dynamic Channel Assignment

Voice channels are not allocated to different cellspermanently

Each time a call request is made, the BS requests

a channelfrom the MSC. MSC allocates a channel to requested cell using

an algorithm that takes into account

The likelihood of future blocking.

The frequency of use of the candidate channel The reuse distance of the channel, and

Other cost functions


52/113

Dynamic Channel Assignment

To ensure the minimum QoS, the MSC only

allocates a given frequency if that frequency is

not currently in use in the cell, or any other cell

which falls within the limiting reuse distance. DCAreduces the likelihood of blocking, thus

increasing the capacity of the system.

DCA strategies require the MSC to collect realtime data on channel occupancy and traffic

distribution on a continuous basis.


53/113

Handoff

When a mobile moves into a different cell whilethe call is in progress, the MSC automaticallytransfers the call to a new channel belonging tothe new BS.

The handoff operation involves identifying a newBS and the allocation of voice and control signalsassociated with the new BS.

Handoffs must be performed successfully, asinfrequentlyas possible, and must beimperceptible to the user.


54/113

Handoff

Handoff is made when the received signal at theBS falls below a pre-specified threshold.

In deciding when to handoff, it is important toensure that the drop in the signal level is not due

to momentary fading. In order to ensure this, the BS monitors the signal

for a certain period of time before initiatinghandoff.

The length of time needed to decide if handoff isnecessary depends on the speed at which themobile is moving.


55/113

Handoff Strategies

In the first generation analog cellular systems, the signal strengthmeasurements are made by the BS and supervised by the MSC.

There is a spare receiver ,called locator receiver,in each BS to scanand determine the signal strength of mobile users in theneighbouring cells.

In the second generation systems that use TDMA technology,Mobile Assisted Handoff(MAHO) are used.

In MAHO, every MS measures the received power from thesurrounding BS and continually report these values to thecorresponding BS.

Handoff is initiated if the signal strength of a neighboring BS

exceeds that of the current BS. During the course of a call, if a mobile user moves from one cellular

system to a different cellular system controlled by different MSC ,anintersystem handoff becomes necessary.


56/113

Handoff


57/113

Handoff

The dropped call event can happen when

There is an excessive delay by the MSC in

assigning a handoff

Threshold is set too small for the handoff time inthe system.

The excessive delay may occur during high traffic

conditions due to computational load at the MSC

or due to the fact that no channels are available.


58/113

Handoff

The time during which the call is maintainedwithin a cell, without handoff is called dwelltime.

The dwell time is governed by various factorslike

Propagation

Interference

Distance between subscriber and BS

Other time varying factors.


59/113

Handoff

For e.g. in cells which provide coverage for vehicular

highway users ,most users tend to have relatively

constant speed and travel along fixed and well

defined path with good radio coverage.

In the above instance, the dwell time for any

arbitrary user is a random variable with a distribution

that is highly concentrated about the mean dwell

time. But for the user in a dense, cluttered microcell

environments there is a large variation in the dwell

time about the mean.


60/113

Prioritizing Handoffs

Guard channel concept, where a fraction of the total availablechannels in a cell is reserved exclusively for handoff requests fromongoing calls which may be handed off into the cell.

This method reduces the total carried traffic, as fewer channels areallocated to originating calls.

Queuing of handoff requests is another method to decrease theprobability of forced termination

There is a finite time interval between the time the received signallevel drops below the handoff threshold and the time the call isterminated due to insufficient signal level.

The delay time and the size of the queue is determined from thetraffic pattern of the particular service area.

Queuing does not guarantee a zero probability of forcedtermination, since large delay will cause the received signal level todrop below the minimum required level to maintain communicationand lead to forced termination.


61/113

Practical Handoff consideration

Several schemes have been designed to

handle the simultaneous traffic of high speed

and low speed users while minimizing the

intervention from the MSC. By using different antenna height and

different poer cells, it is possible


62/113

Interference

Interference is the major limiting factor ion theperformance of cellular radio. It limits thecapacity and increases number of dropped calls.

Sources of interference Another mobile in the same cell

A call in progress in a neighbouring cell and

other BS operating in the same frequency band.

Interference is more severe in the urban areasdue to greater RF noise floor and more numberof MS and BS.


63/113

Topics Discussed :

Handoff & Handoff Strategies

Prioritizing Handoffs

Practical Handoff Considerations

Guard Channel ConceptCell Dragging

Hard Handoff & Soft Handoff

Interference & System Capacity

Co-channel Interference

Adjacent Channel Interference

Co-channel reuse Ratio


64/113

Definations

Handoff: When a mobile moves into a different cell while the call isin progress, the MSC automaticallytransfers the call to a new

channel belonging to the new BS.

Dwell Time: The time during which the call is maintained within a

cell, without handoff. MAHO: Mobile Assisted Handoff, every MS measures the received

power from the surrounding BS and continually report these values

to the corresponding BS. Handoff is initiated if the signal strength of

a neighboring BS exceeds that of the current BS.

Intersystem Handoff: When the Mobile moves from one cellularsystem to different cellular system controlled by different MSC.


65/113

Handoffs

A crucial component of the cellular concept is the notionof handoffs.

Mobile phone users are by definition mobile, i.e., they

move around while using the phone.

Thus, the network should be able to give them

continuous access as they move.

This is not a problem when users move within the same

cell. When they move from one cell to another, a handoffis

needed.


66/113

A Handoff

A user is transmitting and receiving signals from a given

base station, say B1.

Assume the user moves from the coverage area of one

base station into the coverage area of a second base

station, B2.

B1 notices that the signal from this user is degrading.

B2 notices that the signal from this user is improving.


67/113

A Handoff (Contd)

At some point, the users signal is weak enough at B1 andstrong enough at B2 for a handoff to occur.

Specifically, messages are exchanged between the user,

B1

, and B2

so that communication to/from the user is

transferred from B1 to B2.


68/113

Definations

Guard Channel Concept: where a fraction of the totalavailable channels in a cell is reserved exclusively for handoff

requests from ongoing calls which may be handed off into the

cell.

Queuing of handoff: The handoff request is queued todecrease the probability of forced termination. There is a

finite time interval between the time the received signal level

drops below the handoff threshold and the time the call is

terminated due to insufficient signal level. However, Queuingstill does not guarantee a zero probability of forced

termination.

Q i f H d ff R


69/113

Queuing of Handoff Requests


70/113


Several schemes have been designed to

handle the simultaneous traffic of high speed

and low speed users to minimize the issues of

the MSC. Addition of cell sites

Umbrell Cell Approach


71/113


Addition of cell site Approach: Practically, it is diffultto obtain new physical cell site in urban areas. Zoninglaws, ordinances and other non-technical barriersdoes not incourage cellular service providers to gofor this approach.

Umbrella cell Approach: Addition of channels and BSat the same physical location of existing cell. By usingdifferent antenna height and different power levels,it is possible to provide Large & Small cells. This

technique is called as Umbrell cell approach, used toprovide large area coverage to high speed users andsmall area coverage to users travelling at low speeds.


72/113

Umbrella Cells


73/113

Advantages of Umbrella Cell Approach

Generation of Large Umbrella cells and smallmicrocells from the same physical cell site location.

Number of handoffs is minimized for high speed

users. Provides additional microcell channels for

padestrian users.

Sophisticated Algorithms may be used for better

performance of MSCs

Load on MSC reduces.


74/113

Cell Dragging

Another practical problem in microcell system. It results from low speed users that provide very strong

signal to BS and travels away from BS. The averagesignal strength does not decay rapidly, even when the

user has traveled well beyond the designated range ofcell. Handoff may not take place. This creates potentialinterference and traffic management problems in theadjoining cell.

Solution to the cell dragging problems: Handoffthreshold and radio coverage paramenters must beadjusted carefully.


75/113

Hard & Soft Handoff

Hard Handoff Strategy: Assigning different radiochannels to the users during each handoff.

Soft Handoff Strategy: Spread spectrum mobiles sharethe same channel in each cell. The user issimultaneously connected to two or more cells duringa call and continuously make power measurements ofa list of neighboring cell sites, and determine: Whether or not to handoff the call.

When to handoff the call.

Whom to handoff the call.This ability to select between the instanteous received signal

from variety of BS is called soft handoff.


76/113

Interference

Interference is the major limiting factor in the performanceof cellular radio. It limits the capacity and increases numberof dropped calls.

Sources of interference Another mobile in the same cell

A call in progress in a neighboring cell and other BS operating in the same frequency band.

Noncellular system which leaks energy into the cellularfrequency band.

Interference causes cross talk, disturbance in voice channel,call dropping, missing and blocking of calls. It is moresevere in the urban areas due to greater RF noise floor andlarge number of BS and mobiles.


77/113

Interference

Even though the interfering signals aregenerated within the cellular system, it is

difficult to control in practice.

System generated Cellular Interference: CCI or Co-channel Interference: The interference

between the signals from the co-channel cells.

ACI or Adjacent channel interference : Theinterference between the signals from the

adjacent channels in a cell.

f


78/113

Causes of ACI

Interference resulting from signals which are adjacent infrequency to desired signal is called ACI.

Causes of ACI

Extraneous power from a signal in an adjacentchannel.

Inadequate filtering, incomplete filtering of unwantedmodulation products in FM systems,

Improper tuning, or poor frequency control, in either

the reference channel or the interfering channel, orboth.

f ff


79/113

Near-far effect

When an interferer close to BS radiates inadjacent channel, while the subscriber is far awayfrom BS, the subscriber signals will get a lot ofinterference. This problem is called as near-far

effect, where nearby transmitter captures thereceiver of subscriber.

The adjacent channel user is transmitting strong

signal in very close range to subscribers receiver,while the receiver attempts to receive weaksignals from BS on desired channel.

ff


80/113

Near Far Effect

The near-far effect occurs when mobile close to BStransmits on channel close to one being used by

weak mobile. The BS may have difficulty in

discriminating the desired mobile user.

User located away to the

base station receive

weak signals than the users

near to BS.

i i i i ACI


81/113

Minimizing ACI

Through careful filtering and intelligent channelassignment.

Using high quality filters, power control systemsand FM systems.

By keeping frequency separation between eachchannel as large as possible.

By following updated channel allocation schemes.

Avoiding the use of adjacent channels inneighboring cell sites.

C f CCI


82/113

Causes of CCI

The interference between the signals from theco-channel cells.

Causes of CCI

Undesired Transmitter Signals located far away insome other cells of the same frequencies (co-

channel signals) arrive at the receiver.

Adverse weather conditions

Poor frequency planning

Overly-crowded radio spectrum

E l f CCI


83/113

Example of CCI

D1

D6D2

D3D4

D5

C Ch l I f


84/113

Co-Channel Interference

Co-channel cells must be physically seperated byminimum distance to provide sufficient isolation

due to propogation.

When the size of each cell is approximately sameand the BS transmits same power, the CCI is

independent of transmitted power but becomes

the function of

- Radius of cells (R) and

- Distance between centers of co-channel cells (D).

Mi i i i CCI


85/113

Minimizing CCI By increasing D/R ratio, the spatial separation between the co-

channel cells relative to the coverage distance of cell is increased.

Thus co-channel interference is reduced.

This ratio (D/R) is called co-channel reuse ratio represented by Q

Where N is cluster size.

Hence, Q determines the spatial separation relative to the

coverage distance of a cell.

Smaller value of Q provides larger capacity, but higher CCI

Hence there is a tradeoffbetween the Capacity and Interference

C h l R ti T bl


86/113

Co-channel reuse Ratio Table

Si l t I t f R ti


87/113

Signal-to-Interference Ratio

The SIR (or S/I) for mobile receiver whichmonitors a forward channel is expressed as

(S/I ) = S/(Ii ) (submission i=1 to i0)

where S is desired signal power and Ii isinterference power.

i0 is number of co-channel interfering cells.

If the signal levels of co-channels are known, thenS/I ratio for the forward link can be found usingthe equation.

C l l ti f S/I


88/113

Calculation of S/I

Average received power (Pr) at a distance (d)from transmitting antenna is approximated as

Pr = Po (d/do)-n

Where Po is power received at reference pointnear to antenna at distance do

n is path loss exponent.

Now, if Di is the distance of ithinterferer from

mobile, the received power due to ithinterfering cell will be proportional to (Di)

-n

C l l ti f S/I


89/113

Calculation of S/I

Hence S/I is independent of Cell Radius.

E l f CCI


90/113

Example of CCI

D

D

D

D

D

D

Assuming all interfering BS are equidistant

Calc late cl ster si e thro gh S/I


91/113

Calculate cluster size through S/I

Assume 6 closest cells (io) are creating significant

interference and are equidistant from desired BS.

Assume path loss exponent n=4.

Subjective tests indicate that sufficient voice

quality is provided when S/I 18dB = 63.18

Using the equation

We get the cluster size N should be at least 6.49,

Thus minimum cluster size of 7 is required to meet

an S/I requirement of 18dB.

First tier of co-channel cell for N=7


92/113

First tier of co channel cell for N 7

When the mobile is at the cell boundary (point X), it

experiences worst case co-channel interference on

forward channel.

CCI Worst case (S/I Ratio)


93/113

CCI Worst case (S/I Ratio)

S/I = R-4/(2(D-R)-4 + 2(D)-4 + 2(D+R)-4)

= 1/(2(Q-1)-4 + 2(Q)-4 + 2(Q+1)-4)

CCI Average case (S/I Ratio)

Assuming user is near to centre of cell, we get

S/I = R-4/6(D)-4 = 1/6(D/R)-4

= 1/6Q-4

Power control to reduce Interference


94/113

Power control to reduce Interference

The power levels transmitted by every subscriber unitare under constant control by the serving BS. This isdone to ensure that each mobile transmits the smallestpower necessary to maintain a good quality link onreverse channel.

Power Control reduces interference.

It also provides long battery life.

It also reduces the reverse channel S/I in system.

It is especially important in spread spectrum systemsthat allow every user in every cell to share the sameradio channel .

Reduced Interference leads to higher capacity.

Improving Capacity


95/113

Improving Capacity

As the demand for service increases, system designershave to provide more channels per unit coverage area.

Common techniques are Cell Splitting, Sectoring andMicrocell Zoning

Cell Splitting increases the number of BS deployed andallows an orderly growth of cellular system.

Cell Sectoring uses directional antennas to furthercontrol interference and frequency reuse.

Microcell Zoning distributes the coverage of a cell andextends the cell boundary to hard to reach places.

Cell Splitting


96/113

Cell Splitting

Cell Splitting is a process of subdividing acongested cell into smaller cells with their own BS

A corresponding reduction in antenna height

A corresponding reduction in transmitted powerSplitting the cells reduces the cell size thus more

number of cells have to be used.

More number of cells viz. more number of clusters viz.more channels viz. higher capacity.

Cell Splitting allows a system to grow by replacing largecells by small cells without upsetting the channelallocation.

Cells are split to add channels with no


97/113

new spectrum usage

Small Cells High Density

Large Cells Low Density

Cell Splitting


98/113

Cell Splitting

The cells are split to add channels with nonew spectrum usage.

Depending on traffic patterns the smaller cells

may be activated/deactivated in order toefficiently use cell reuse resources.

A typical city cellular radio cell plan different

cell sizes and clusters.

Cell Splitting increases capacity


99/113

Cell Splitting increases capacity

Power of Smaller Cells


100/113

Power of Smaller Cells

Suppose cell radius of new cells are reduced by half.

What is the required transmit power for these new

cells?

We have Pr*at old cell boundary+ Pt1 R-nPr*at new cell boundary+ Pt2 (R/2)

-n

Pt2 = Pt1 /16 if n=4

Thus transmitted power of smaller cell shouldbe 16 times lower than the large cell.

Capacity increase


101/113

Capacity increase

Suppose a congested area is originally covered by5 Cells, each cell with 80 channels.

Capacity = 5 x 80 = 400 users

After cell splitting Rnew = R/2 Now we have 24 cells.

New Capacity = 24 x 80= 19200 users

Hence cell splitting provide strong technique tohandle more users without asking for extrabandwidth.

Cell Sectoring


102/113

Cell Sectoring

As opposed to cell splitting, where D/R is keptconstant while decreasing R, sectoring keeps R

untouched and reduces D/R.

Capacity improvement is achieved by reducingthe number of cells per cluster, thus increasing

frequency reuse. However, it will lead to higher

interference which is taken care-off.

It is necessary to reduce the relative interference

without decreasing the transmitter power.

Cell Sectoring


103/113

Cell Sectoring

The CCI may be decreased by replacing the singleomnidirectional antenna by several directionalantennas, each radiating within a specified sector.

The directional antenna transmits and receivesonly a fraction of the total number of cochannelcells. Thus CCI is reduced.

A cell is normally partitioned into three 1200

sectors, four 900

sectors or six 600

sectors. Increasing the number of sectors, CCI reduces but

increases number of handoffs.

Sectoring improves S/I


104/113

Sectoring improves S/I

Way of Cell Sectoring


105/113

Way of Cell Sectoring

Placing directional transmitters at corners

where three adjacent cells meet

Problems with sectoring


106/113

Problems with sectoring

Increase in number of antennas at each BS

Decrease in trunking efficiency due to sectoring

(dividing the bigger pool of channels into smaller

groups). Increased number of handoffs (sector-to-sector).

However, many modern BS support sectoring

and related handoff without the help of MSC. Therefore sectoring can be handled locally.

Microcell Zone Concept


107/113


The problems of sectoring can be solved byMicrocell Zone concept.

A cell is divided into microcells or zones.

Each microcell (zone) is connected to the same BS

using Fiber/microwave link. Each zone uses a directional antenna.

As a mobile travels from one zone to another, itretains the same channel, i.e no hand-off.

BS simply switches the channel to the next zoneside.

The Zone Cell Conceptlet each cell be divided into three zones


108/113

let each cell be divided into three zones



109/113


While the cell maintains a particular coveragearea, the CCI reduces because:

Large central BS is replaced by several low power

transmitters. Directional antennas are used.

Decreased CCI improves

Signal quality Capacity

Zone Cell Concept


110/113

Zone Cell Concept

Example Microcell Zone Concept


111/113

Example Microcell Zone Concept

Suppose desired S/I = 18 dB. Path loss exponent n = 4

How much capacity increase can occur if we use

Microcell Zoning of 3 zones/cell? To achieve S/I = 18 dB we need N = 7.

We use Microcell Zone concept and create 3zones within 1 cell.

This makes the cluster size N = 3.

The capacity increase factor = 7/3 = 2.33

Repeaters for range extension


112/113

Repeaters for range extension

Useful for hard to reach areas Within buildings, basements

Tunnels

Valleys

Radio transmitters, called repeaters can be used toprovide coverage in these areas.

Repeaters are bidirectional Receive signals from the BS

Amplify the signals Reradiate the signal to required area.

Received noise and interference is also reradiated.

Mobile Radio Propagation:Intoduction


113/113

Mobile Radio Propagation:Intoduction

The mobile radio channel places fundamentallimitations on the performance of wirelesscommunication systems.

Wireless communication path may be

Line of sight(LOS) Non Line Of Sight(NLOS):Obstruction by buildings,

mountain and foliage.

Radio channel are often random and time varying.

Modelling the radio channel has historically been oneof the most difficult parts of the mobile radio systemdesign.

mobile communication introduction

Documents