FC16 Cellular Principles

8/6/2019 FC16 Cellular Principles

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Fundamentals of

Communications

EE3158

Professor Ian [email protected]

www.ctr.kcl.ac.uk/members

16: Cellular Radio Principles


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EE3158 Lecture 16 Fundamentals of Communications Slide 2

Cellular Radio

Frequency Re-use

Interference limitation

Cell repeat patterns

Frequency Planning

Coverage / capacity / growth

Handover


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Radio Systems

Fixed telephone network runs wires to everyhousehold

Suppose we give every household their ownallocation of radio spectrum using analogue speech

of 4 kHz bandwidth (single sideband) 12.5 million households (UK only)x 4 kHz = 50 GHz!

Clearly impractical!

no other services possible using radio transmission

whole range of radio transmission modes to address and most of the spectrum unused most of the time!

remember Erlang and traffic statistics


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Capacity Limited

Early Mobile Radio Networks

used a single high power radio transmitter to cover a large area

few channels for many people

range limited by thermal (and man made) noise

example 100 W Tx at 30m, 30 km range, 25 kHz FM, 2 m Rx: kTB = 1.3803x10-23 x 290 x 25,000 = -130 dBm.

transmit power 10log(100/10-3) = 50 dBm

path loss over say 30 km: 40 log 30,000 - 20 log 60 = 143 dB

receive signal = +50 - 143 = -93 dBm

receive S/N ratio = 37 dB (17 dB system plus 20 dB fade margin) 1976 Bell Mobile Phone service in New York had 12 channels,

serving 543 customer, waiting list of 3,700 and market of 10million!! - CAPACITY LIMITED


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Cellular systems

Apart from the capacity limitation of these earlysystems, the other characteristic was that the carrierfrequency was only re-used many tens or hundredsof km away so that no cochannel interference

would arise. [cochannel = same frequency) Cellular systems are based on the concept of dividing

the geographic service area into a number of cellsand placing a low power transmitter in each of these,

usually at the geographic centre. The transmit frequencies are re-used across these

cells and the system becomes interference ratherthan noise limited as we shall see.


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Cellular Basics

Some consequences arise:

the need for careful radio frequency planning colouring in hexagons!

a mechanism for handling the call as the user crossesthe cell boundary - call handover

increased network complexity to route the call andtrack the users as they move around

But one significant benefit:

very much increased traffic capacity, the ability toservice many users


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Cellular System Definitions

Mobile Station

users transceiver terminal (handset, mobile)

Base Station

fixed transmitter usually at centre of cell Mobile Switching Centre

handles routing of calls in a service area

tracks user

connects to base stations and PSTN

Control Channels

radio channels for set up of call, call request etc


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Cellular System Definitions 2

Downlink or Forward Channel

radio channel for transmission of information(e.g.speech) from base station to mobile station

Uplink or Reverse Channel radio channel for transmission of information

(e.g.speech) from mobile station to base station

Handover or handoff

process of transferring mobile station from one basestation to another, may also apply to change of radiochannel within a cell


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Cellular System Definitions 3

Paging

a message broadcast over an entire service area,includes use for mobile station alert (ringing)

Roaming a mobile station operating in a service area other

than the one to which it subscribes


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Frequency Reuse

Cellular relies on the intelligent allocation and reuseof radio channels throughout a coverage area.

Each base station is allocated a group of radiochannels to be used within the small geographic area

of its cell Neighbouring base stations are given different

channel allocation from each other

If we limit the coverage area within the cell by design

of the antennas, we can re-use that same group offrequencies to cover another cell separated by alarge enough distance to keep interference levelswithin tolerable limits.


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Radio Planning

The design process of selecting and allocatingchannel frequencies for all cellular base stationswithin a system is known as frequency re-use orfrequency planning.

Most cell planning is carried out on the basis oftessellating hexagons

real cells are never hexagonal in shape

however most theoretical treatment find them a convenienttool since hexagons:

are a geometric shape that approximates a circle

tessellate a plane

represent contours of equal transmit power


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Cellular Re-use Concept

A 7 cell cluster - outlined in bold

Cells with the same letter use the same frequencygroups


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Geometry of Hexagons

Hexagonal cell geometry and axes


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Geometry of Hexagons 2

axes u,v intersect at 60o

unit scale is distance between cell centres

if cell radius to point of hexagon is R

then 2Rcos30o = 1 or

R =1

3

To find the distance of a point P(u, v) from the origin

use x - y to u - v co - ordinate transformations :

r

2

! x

2

y

2

x ! ucos 30o

y ! v usin 30o

r! (v2 uv u

2)

12


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Geometry of Hexagons 3

Using this equation to locate co-channel cells, westart from a reference cell and move i hexagonsalong the u-axis then j hexagons along the v-axis.Hence the distance between cochannel cells in

adjacent clusters is given by: D = (i2 + ij + j2)1/2

where D is the distance between cochannel cells inadjacent clusters.

and the number of cells in a cluster, N is given by D2

N = i2 + ij + j2

since i and j can only take integer values we findvalues for N


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Cell Clusters

e-use coordinates um er of

cells in re-use attern

ormalised

re eat distance

i j SQ ( )

1 1 1

1 1 3 1. 3

1 .6 6

1 3. 6

1 3 13 3.6 6

3 1 .35

1 1 .583

since D = SQ ( )


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Cochannel Cell Location

Method of locating cochannel cells

Example for N=19, i=3, j=2


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Cell Planning Example

Suppose you have 33 MHz bandwidth available,an FM system using 25 kHz channels, how manychannels per cell for 4,7,12 cell re-use?

total channels = 33,000/25 = 1320 N=4 channels per cell = 1320/4 = 330

N=7 channels per cell = 1320/7 = 188

N=12 channels per cell = 1320/12 = 110

What do we deduce? smaller clusters can carry more traffic

how much? Erlang B at 2% blocking


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Cochannel Interference

Now consider a mobile at the edge of cell,distance R from transmitter (downlink only).

Average first tier co-channel cell is distance D away

Power law ofE (typically 4 from lecture 15) Assume equal transmit powers

(wanted) signal level = k R-E,

interference (single user) = k D-E

S/I = 10 E log(D/R)

now D/R = (3N)1/2 hence S/I = 20 log 3N for E=4

SIGNAL TO INTERFERENCE LEVEL IS INDEPENDENTOF CELL RADIUS!


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Cochannel Interference2

worst case single interferer model interferingtransmitter is D-R away

S/I = 10 E log (S RT(3N)-1)

and we can computeCells / Cluster Single Interferer S/N

3 12.

15.

22.2

12 28.

an FM system requires around 18 dB minimum S/I tooperate satisfactorily so we choose N=7


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Cell Size

System performance depends on cluster size and isindependent of cell radius so what cell radius do wechoose?

Depends on traffic we wish to carry

Population density of users say P people/km2 Average busy hour traffic per user T Erlangs

So traffic is PT Erlangs / km2

If our cell has C radio channels ( and C>100) we canapproximate the Erlang B formula to give traffic in Erlangs

as E = 0.9 C

Cell supports 0.9 C / TR2 Erlangs per km2

whence R = (0.9C/TPT)1/2


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System Growth

When the system grows - more customers youneed more smaller cells to carry the trafficrequiring a new cell and frequency plan

Cell splitting need for re-tuning - tedious (and expensive) if a technician

needs to visit every base station!

Typical approach is to sectored cells e.g. tri-sector to give 21/7

or 6 sectors to give a 24/4 pattern


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System Growth 2

typical city cellular radio cell plandifferent cell sizesand clusters.


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Revision

cellular mobile uses many small cells

hexagonal planning, clusters of cells

cell repeat patterns 3,7,12 etc...

re-uses frequencies to obtain capacity is interference not noise (kTB) limited

S/I is independent of cell radius

choose cell radius to meet traffic demand N=7 is a good compromise between S/I and

capacity.

Documents

FC16 Cellular Principles