Mobile Unit I

8/3/2019 Mobile Unit I

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Evolution of Mobile Radio Comm

Figure 1.1 The growth of mobile telephony as compared with other

popular inventions of the 20th

century.


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Evolution of Mobile Radio Comm

The first generation digital cellular wireless

network was the Advanced Mobile Phone

System (AMPS) The second generation wireless systems

are the popular Global System for Mobile

Communications (GSM) and Personal

Communication Service (PCS)


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Mobile Standards

AMPS - Advanced Mobile Phone System

USDC - US Digital Cellular

CDPD - Cellular Digital Packet Data

IS-95 - Interim Standard- 95

JTACKS -Japanese Total Access Cellular Systems

PDC - Pacific Digital Cellular

NTT - Nippon Telephone and Telegraph Company

PHS - Personal Handy Phone Service

ETACS - European Total Access Cellular System GSM - Global System for Mobile

CT2 - Cordless Telephone

DECT - Digital European Cordless Telephone


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Examples of Wireless

Communication System Garage Door Openers

Remote Controllers

Paging system Cordless System

Hand held walkie - talkie


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Paging System


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Disadvantages:

Wide area paging system are more

complex in design for they require a

network of telephone lines, many base

station transmitters, and large radio

towers.


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Cordless Telephone System


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Cellular or Mobile System


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Timing diagram illustrating

how a call to a mobile user

initiated by a landline

subscriber is established.


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Timing diagram illustrating how

a call initiated by a mobile

is established.


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First Generation Wireless networks

Second generation wireless networks Third Generation wireless networks

Modern Wireless CommunicationModern Wireless Communication

SystemsSystems


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Basic Concepts in a Cellular

System Frequency Reuse

Handoff

Interference System capacity

Improving Coverage and Capacity in

cellular system


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Frequency Reuse


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The design process of selecting and

allocating channel groups for all of thecellular base stations within a system is

called frequency reuse or frequency

planning

Each cellular base station is allocated a

group of radio channels that is to be used

within a small geographic area called cell

The actual radio coverage of a cell isknown as the footprint


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Concept:

Consider a cellular system which has atotal ofS channels for use and the system

is divided into N cells.

If each cell is allocated a group ofk

(k


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The N cells which collectively use the

complete set of available frequencies is

called a cluster.

A measure of capacity of the system

is

C = MkN or C = MS

here N is called the cluster size and

can have values which satisfy the equation

given below

N= i2 + ij + j2


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Method of locating co-channel cells in a cellular system. In

this example, N= 19 (i.e., I= 3, j= 2).


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Definition:

The process of transferring a

mobile from one channel or base station to

another is called as handoff.

Dwell time:

The time over which a call

maybe maintained in a cell without handoff

is called the dwell time.

Handoff


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Intersystem Handoff:

During the course of a call, ifa mobile moves from one cellular system

to a different cellular system controlled by

a different MSC then it is known asIntersystem handoff.


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Prioritizing Handoff:

Two methods discussed

1. Guard Channel concept

2. Queuing of handoff requests

Some handoff issues and their solutions

1. High speed moving vehicles

Sol: Umbrella approach

2. Pedestrian users Cell Dragging

Sol: Handoff thresholds and radio

coverage parameters should be

adjusted carefully


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Umbrella approach


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Improving Coverage and Capacity

in cellular system

Two ways

1. Reducing cell size

2. Decreasing the channel reuseratio (Q=D/R)

Techniques used to expand the cellcapacity are

1. Cell Splitting

2. Sectoring

3.Coverage zone approaches


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1. Cell Splitting


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Cell splitting is the process of subdividing the

cell into smaller cells (micro cells) each with its

own base station.

It increases the cell capacity by increasing the

number of times the channels are

used. The transmitted power and antenna height is

also reduced

The small cells are divided into more number of

cells, so more number of channels are allocated

for the cell to increase the channel capacity


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Transmit Power:

Pr[ at old cell boundary] Pt1R-nPr[ at new cell boundary] Pt2(R/2)

-n

Pt1,Pt2-Transmit powers of larger and smallerbase stations

n- Path loss component


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2. Sectoring

The technique for decreasing co-channel

interference by using directional antennas is

called sectoring.

It also used to increase the capacity of thesystem

A cell is normally partitioned into three 120

sectors or six 60 sectors.

When sectoring is employed ,the channels in aparticular cell are broken down into sectored

groups and are used within a particular sector.


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Sectoring improves S/I


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Disadvantage:

1. Increased number of handoff results

in an increased load on switching andcontrol link elements.


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The Zone Cell Concept


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In this scheme, each of three zone sites

are connected to single base station.

They share the same radio equipment.

The zones are connected by coaxial cable,

fiber optic cable or microwave link.

As a mobile travels within the cell it isserved by the zone with the strongest

signal.

As a mobile travels from one zone toanother within the cell it retains the same

channel. Thus handoff is avoided.


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Advantages:

1. Handoff is not required

2. Co-channel interference is

decreased.

3. Improved signal quality.

4. The channels are distributed in

space and time by all three zones

and are also reused in co-channel

cells in normal fashion. Thus

capacity is increased

I t f


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Interference

System capacity

Some sources of interference:

Another mobile in the same cell

A call in progress in a neighbouring cellOther base stations operating in the

same frequency band


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Two major interference

1. Co-channel interference2. Adjacent channel interference


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Indoor propagation model

It has been observed that propagation within buildings isstrongly influenced by specific features such as thelayout of the

building, the construction materials, and the building type.

Signal level vary depending on whether interior doors areopen or closed inside a building.

Some key models are given, they are Partition losses(same floor), partition losses between floors, Longdistance path loss model,

Ericsson multiple breakpoint model and Attenuation factormodel.


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Partition losses( same floor)

Hard partitions partitions that are formed

as part of the building structure

Soft partitions partitions that may movedand do not span to the building


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Partition loss (between floor)

The losses between floors of a building

are determined by the external dimensions

and materials of the building as well as thetype of construction used to create the

floors and the external surroundings.

Floor attenuation factor


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Long distance path loss model

PL (dB) = PL(d0) + 10 log (d/d0) +X

Where the value of n depends on thesurroundings and building type and X

represents normal random variable in dB

having a standard deviation of dB.


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Ericsson multiple breakpoint model

Ericsson radio system model was obtained

by measurements in a multiple floor office

building .

This model also assumes that there is 30

dB attenuation at d0 = 1m,which can be

shown to be accurate for f = 900Mhz

The path loss based on the function of

distance


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Attenuation factor model

This model provides flexibility and to reduce the standarddeviation between measured and predicted path loss toaround 4 dB as compared to 13dB

The attenuation factor model is given by

nSF represents the exponent value for the same floormeasurement.

FAF represents a floor attenuation factor for a specificobstruction encountered by a a ray drawn between thetransmitter and receiver in 3-D

The technique of drawing a single ray between thetransmitter and receiver is called primary ray tracing

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Mobile Unit I