introduction to pcs

8/6/2019 Introduction to PCS

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INTRODUCTION TO PCS

By: Tamal Chakraborty


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Principle of Wireless Communication

By attaching an antenna of appropriate size to an

electrical circuit, the electromagnetic waves can be

broadcast efficiently and received by a receiver

some distance away. All wireless communication is based on this

principle.


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Personal Communications Services (PCS)

PCS is a technology, designed to provide themobile user with voice, data and multimediaservices at any time, at any place an in any format.

Business opportunities for such services aretremendous, since every person (not just everyhome) can be equipped, as long as the service isfairly inexpensive.

Two most common PCS technologies are: Cellular telephony

Cordless and low-tier PCS telephony


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PCS architecture

PSTN: Public Switched Telephone Network

MSC: Mobile Switching Centre

BSC: Base Station Controller

PSTN

MSC

MobilityDatabase

Base Station

BSC

Mobile Station

Base Station

Radio Network

Wire-line Transport Network


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Cellular Network Organization

Use multiple low-power transmitters (100 W or less)

Areas divided into cells

Each served by its own antenna

Served by base station consisting of transmitter, receiver,and control unit

Band of frequencies allocated to each cell

Adjacent cells are assigned different frequencies to avoid

interference and crosstalk


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Cellular Geometries

Not suitable


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Geometry of a hexagon

Say, radius = R

Length of perpendicular

from center to any side is3/2R

Distance between centersof two adjacent hexagons

= 3R Surface area = 33/2R2


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Frequency Reuse

Objective is to reuse frequency in nearby cells

10 to 50 frequencies assigned to each cell

Transmission power controlled to limit power at thatfrequency escaping to adjacent cells

The issue is to determine how many cells must intervenebetween two cells using the same frequency

Let us consider a pattern consisting of N cells, eachassigned the same number of frequencies

Each cell can have K/N frequencies, where K is the totalnumber of frequencies

For example say, K = 395, N = 7


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Cluster

Consider a cellular system which has a total of Schannels available and each cell is allocated agroup of k channels(k


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Frequency re-use and cluster

7 cell re-use pattern

f7

f7f2

f2f6

f6

f1

f5f3

f4 f1f5

f3

f4


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Cluster Radius

Say radius of a clusteris Rc

If D is the distance

between the center ofthe two clusters, i.e.the distance betweentwo cells having samefrequency, then byhexagonal geometryD = 3. Rc

D


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Network Layout Using ShiftParameters

It is a common practice touse shift parameters (i, j) tolayout a cellular system.

Move i cells along any chain

of hexagons turn counter-clockwise 60

degree move j cells along the chain

that lies on this new heading Let N be the reuse factor, i.e.

the number of cells in arepetitious pattern (i.e.cluster).


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Co-channel reuse ratio

Let 2R be the distance between thecenters of two cells

Let R be the radius of a cell

We have R = 3/2R

Let D be the distance between two

cells with same frequencies

Then the co-channel reuse ratio Qis given by Q = D/R

Figure 1

Figure 2


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Q = (3N)

By the cosine law, we have

D2 = (i.2R)2 + (j.2R)22 (i.2R) (j.2R)cos120

Substituting R = 3/2R D2 = i2.3R2 + j2.3R2 + ij.3R2

D2 = 3R2(i2 + j2 + ij) (i)

Again from Cluster geometry, D2 = 3Rc2 (ii)

Using (i) and (ii), Rc2 = R2(i2 + j2 + ij) (iii)

Now surface area of a hexagon of radius R is: SR= 33/2R2

Surface area of a cluster of N cells is N.SR

Now, since the cluster itself is a hexagon of radius Rc, its area is 33/2Rc2

Thus, 33/2Rc2 = N.SR= N 33/2R2

i.e. N = Rc2 / R2 = (i2 + j2 + ij) from (iii) Again, D = 3(i2 + j2 + ij).R from (ii) Thus co-channel reuse ratio

Q = D/R = (3N)


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How to Cope with Increasing Capacity

Adding new channels

Frequency borrowingfrequencies aretaken from adjacent cells by congested

cells Cell splittingcells in areas of high

usage can be split into smaller cells

Cell sectoringcells are divided into a

number of wedge-shaped sectors, eachwith their own set of channels

Microcellsantennas move to buildings,hills, and lamp posts


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Cellular System Overview

Base Station (BS)includes an antenna, a controller,and a number of receivers

Mobile telecommunications switching office (MTSO)

connects calls between mobile units Two types of channels available between mobile unitand BS Control channelsused to exchange information having to

do with setting up and maintaining calls Traffic channelscarry voice or data connection between

users


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Call Establishment


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Call Establishment


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Call Establishment


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Other Functions of Cellular System

Call blocking If all the traffic channels to the nearest BS are busy, then

after a certain number of failed tries a busy message isreturned to the user.

Call termination MTSO is informed and traffic channels are released.

Call drop During a connection if the BS cant maintain the required

signal strength for a certain amount of time, MTSO isinformed and the traffic channel to the user is dropped.

Calls to/from fixed and remote mobile subscriber MTSO connects to the PSTN.


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Mobile Radio Propagation Effects

Signal strength

Must be strong enough between base station and mobile

unit to maintain signal quality at the receiver

Must not be so strong as to create too much co-channelinterference with channels in another cell using the same

frequency band

Fading

Signal propagation effects may disrupt the signal and causeerrors


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Handoff Strategies Used to Determine

Instant of Handoff

Relative Signal Strength

Relative signal strength with threshold

Relative signal strength with hysteresis

Relative signal strength with hysteresis & threshold

Prediction techniques


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Power Control

Design issues making it desirable to include dynamic

power control in a cellular system

Received power must be sufficiently above the background

noise for effective communication Desirable to minimize power in the transmitted signal from

the mobile

Reduce co-channel interference, alleviate health concerns, save

battery power

In SS systems using CDMA, its desirable to equalize thereceived power level from all mobile units at the BS


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Difficulties in reverse link

The reverse link presents the most difficulty incellular systems for the following reasons:1. the base station has complete control over the power of

all the transmitted signals on the forward link. However,because of different radio propagation paths betweeneach user and the base station, the transmitted powerfrom each subscriber unit must be dynamicallycontrolled to prevent any single user from driving up the

interference level for all other users.2. Transmit power is limited by battery consumption at the

subscriber unit, therefore there are limits on the degreeto which power may be controlled on the reverse link.


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Types of Power Control

Open-loop power control

Depends solely on mobileunit

No feedback from BS

Not as accurate as closed-loop, but can react quickerto fluctuations in signalstrength


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Types of Power Control

Closed-loop powercontrol

Adjusts signal strength inreverse channel based on

metric of performance BS makes power

adjustment decision andcommunicates to mobile

on control channel


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Thank You!!

introduction to pcs

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