introduction to pcs
TRANSCRIPT
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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!!