Wireless Communications

The Cellular Concept

Transferring knowledge to future leaders

Reference:Professor Johnson I Agbinya(University of the Western Cape)

2

Single Cell ‘Network’

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History of Cellular Networks

Why cellular networks?

To address requirement for greater capacity

For efficient use of frequency

To address the poor quality of non cellular mobile

networks and increases coverage

– replaces a large transmitter with smaller ones in cells

– smaller transmitting power

– each cell serves a small geographical service area

– each cell is assigned a portion of the total frequency

4

Description of a Cell

Approximated to be a hexagonal coverage

– best approximation of a circular area

Served by a base station

– low powered transceiver

– antenna system and a

– a mast

– may be divided into 6 equilateral triangles

– length of base of each triangle = 0.5R (radius)

– different groups of channels assigned to base stations

R

RR

87.02

3

5

Mathematical Description of a Cell

Area of a cell is:

Perimeter of a cell = 6R

Capacity of network = N x number of users per cell

N = number of cells in network

– we assumed cells of equal capacity - can differ in practice

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598.22

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26 R

RRx

RxAreacell

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Typical Cellular Network(For a Sparsely Populated Country)

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Multi-Cellular NetworkDensely Populated City

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Definitions

Definition of ‘channel’

depends on the system

– frequency (radio station)

– frequency band (TV transmissions)

– time slot of a frequency (GSM, GPRS)

– orthogonal code (CDMA networks)

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Structure of a Mobile Communication System

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Types of Mobile Communication Cells

The size of a cell is dictated by capacity demand

Macrocell

– large, covering a wide area

– range of several hundred kilometres (km) to ten km

– mostly deployed in rural and sparsely populated areas

Microcell

– medium cell, coverage area smaller than in macro cells

– range of several hundred metres to a couple of metres

– deployed mostly in crowded areas, stadiums, shopping malls

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Types of Mobile Communication Cells (1)

The size of a cell is dictated by capacity demand

Picocell

– small, covering a very small area

– range of several tens of metres

– low power antennas

– can be mounted on walls or ceilings

– used in densely populated areas, offices, lifts, tunnels etc

12

Means of Increasing Cells Capacity

There are several approaches for increasing

cellular system capacity including:

Cell clustering

Sectoring of cells

Cell splitting

Frequency reuse

Reduction of adjacent cell interference and co-channel

interference

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Cell Structure

F2 F3F1F3

F2F1

F3

F2

F4

F1F1

F2

F3

F4F5

F6

F7

(a) Line Structure (b) Plan Structure

Note: Fx is set of frequency, i.e., frequency group.

14

Cell Clusters

Service areas are normally divided into clusters of

cells to facilitate system design and increased

capacity

Definition

a group of cells in which each cell is assigned a different

frequency

– cell clusters may contain any number of cells, but clusters

of 3, 4, 5, 7 and 9 cells are very popular in practice

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Cell Clusters

A cluster of 7 cells

the pattern of cluster is repeated throughout the network

channels are reused within clusters cell clusters are used in frequency planning for

the network Coverage area of cluster called a ‘footprint’

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Cell Clusters (1)

A network of cell clusters in Cape Town

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Frequency Plan

Intelligent allocation of frequencies used

– Each base station is allocated a group of channels to be

used within its geographical area of coverage called a

‘cell’

Adjacent cell base stations are assigned completely

different channel groups to their neighbors

base stations antennas designed to provide just the

cell coverage, so frequency reuse is possible

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Frequency Reuse Concept

Assign to each cluster a group of radio channels to be

used within its geographical footprint

ensure this group of frequencies is completely different from

that assigned to neighbors of the cells

Therefore this group of frequencies can be reused in a

cell cluster ‘far away’ from this one

Cells with the same number have the same sets of

frequencies

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Frequency Reuse Factor

Definition

When each cell in a cluster of N cells uses one of N

frequencies, the frequency reuse factor is 1/N

Frequency reuse is an example of space diversity

multiplex access

frequency reuse limits adjacent cell interference

because cells using same frequencies are separated

far from each other

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Factors Affecting Frequency Reuse

Factors affecting frequency reuse include:

Types of antenna used

– omni-directional or sectored

placement of base stations

coverage for distance (highways) vs area (city)

capacity through macro and micro overlays

21

Excitation of Cells

Once a frequency reuse plan is agreed upon overlay the

frequency reuse plan on the coverage map and assign

frequencies

The location of the base station within the cell is referred

to as cell excitation

In hexagonal cells, base stations transmitters are either:

– centre-excited, base station is at the centre of the cell or

– edge-excited, base station at 3 of the 6 cell vertices

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Frequency Assignment Plan

Example

S duplex channels

S channels divided into N cells

Each cell allocated a group of k channels

Total # available channels S = k N

Replicate a cluster M times in the network

Total number of duplex channels, C=MkN=MS

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Characterization of Frequency Reuse

F1

F2

F3

F4F5

F6

F7

F1

F2

F3

F4F5

F6

F7

F1

F1

Reuse distance D

• For hexagonal cells, the reuse distance is given by

RND 3

R

where R is cell radius and N is the reuse pattern (the cluster size or the number of cells per cluster).

NR

Dq 3

• Reuse factor is

Cluster

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Characterization of Frequency Reuse

The cluster size or the number of cells per cluster is given by

22 jijiN

where i and j are integers.

i

j

60o

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Finding the Nearest Co-Channel

(1) Move i cells along any chain of hexagons

(2) Turn 600 counter-clockwise and move j cells, to

reach the next cell using same frequency sets

this distance D is required for a given frequency reuse

to provide enough reduced same channel interference

i.e. after every distance D we could reuse a set of

frequencies in a new cell

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Channel Assignment Strategies

Three assignment approaches

Fixed and static (most common)

Dynamic

Hybrid

Fixed channel assignment

all channels in a cell could be in use all the time

– new calls are then blocked (no channels left)

– may be solved by borrowing spare channels from nearby cells

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Channel Assignment Strategies (1)

Dynamic channel assignment

MSC allocates frequencies when a call is made

Provides high channel utilization

To do this it needs real-time information on

channel occupancy

traffic distribution and

radio signal strength indication (RSSI)

high computational load and increased storage

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Cell Splitting

Often large cells need to be split into smaller ones

because the population of users in the big cell has

increased beyond what it can support

Cell splitting increases system capacity

Is used in high density subscriber areas (e.g. Water Front)

Results to increased costs (e.g.. new base stations)

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A Split Cell

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Handover (Handoff)

Provides continuity of communication across cells

Difficulty

dropping a call before reconnecting is unacceptable

different cells use different frequencies

mobile phone users usually move from place to place and

very quickly too

therefore the current location of the mobile phone must

somehow be known and kept

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Handover Process

Parties in communication share the same channels

Received signal weakens as mobile moves out of cell

Cell site at some point requests handover to cell with

stronger signal strength

MSC switches call to new cell after allocating channels, and

informing the two mobiles of the new channels (voice and

control channels)

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Handover Process (1)

BS1

BS2

BS3

MSCPSTN Trunks

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Handover Process (2)

Handover must not be too frequent or

system is kept busy servicing handover requests

handover threshold is set with the above in mind

Minimum usable signal level is normally set to be between -

90 dBm and -100 dBm

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Setting Handover Thresholds

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Choosing Handover Margins

Handover margin

= P r handover - Pr minimum usable

If is too large unnecessary handover will occur, burdening

the MSC

If is too small, there maybe insufficient time to complete

the handover before a call is lost due to weak signals

Therefore is chosen carefully

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Interference Sources in Mobile Networks

Interference comes from many sources:

Multipath

Mobile phones in the same cell

Other users in neighboring cells

Environmental effects

base stations operating at the same frequencies

Radiation leakage from other sources into the

mobile communication band

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Types of Channel Interference

Interference is either system or environment related.

System capacity as affected by interference related

to the frequency channel used by a cellular system

is treated in this lecture. The critical ones are:

Co-channel interference

Adjacent channel interference (ACI)

Near end Far end interferenceMAI

Inter cell interference

Co-channel interference

Intra cell interference

Adjacent channel interference

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Capacity and Co-Channel Interference

Definition

Interference from co-channel cells is called co-channel

interference. Such cells use identical frequencies or channels

– Independent of transmitted power for cells of same size

– Is a function of the radius (R) of cell and distance (D) to co-

channel cell

– Cannot be overcome by increased transmission power

– Overcome by separating such cells by a required minimum

distance - by increasing the ratio Q = D/R, where Q is the co-

channel reuse ratio

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Capacity and Co-Channel Interference (1)

For hexagonal cells:

Level of Interference is quantified in terms of SIR

where SIR=signal to interference ratio and

Ii is the interference from ith co-channel cell

NR

DQ 3

0

1

i

iiI

SignalDesiredSIR

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Capacity and Co-Channel Interference (2)

For 6 interfering cells, carrier to interference ratio C/I is

If they are equidistant from the cell of interest then

RF power decreases with inverse power of distance

Decay of received power with distance is given as:

where, d0 is reference distance, P0 is Prec at d0 and is the path loss exponent and

lies between 2 and 5 for urban areas

aa DRIC 6

aRDIC 6/1

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Effects of Q on Capacity

A small value of Q is desirable for larger capacity. This

means a small value of N (small clusters)

A large value of Q improves transmission quality due to

small values of co-channel interference

In practice the value of Q is a trade off between capacity

and transmission quality

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Relationship Between Q and SIR

n is the path loss exponent

Assume all interfering base stations are equidistant (D)

from desired base station, then

For a hexagonal-shaped cell, i0 = 6, D = Di, and Q = D/R

0

1

i

i

ni

nr

D

R

I

S

00

)3()/(

i

N

i

RD

I

S nnr

6

nr Q

I

S

nr

I

SQ

1

6

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Distance between co-channel cells

D

D-R

D-R

D-R/2

D+R

D+R/2

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Distance between co-channel cells (1)

Assume a reuse ratio of 7 and cells are equidistant

from the cell in centre. The separations between the

6 outer cells and the inner one are approx. D+R, D- R, D, D - R/2 and D+ R/2, and CI is approximately:

Let n = 4 and Q=D/R, then

nnn

n

i

i

ni

nr

DRDRD

R

D

R

I

S

2220

1

444 21212

1

QQQI

S r

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Distance between co-channel cells (3)

Assume in this case the base station is centre-excited The all 6 other co-channel interfering cells are D metres away

from it, and

Area of first-tier circle is

Area of circle surrounding inner cell at centre is

Ratio of areas is equal to the number of cells that can be fitted into the first-tier of cells and is:

– Omnidirectional cells interfere with all their co-channel neighbors

– interference is reduced by sectoring

2222 3 jijiRD

2222arg 3 jijiRDkA el

2RkAsmall

NR

Djiji

A

A

small

el 332

222arg

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Distance between co-channel cells (4)

Consider base stations use sectored antennas, i.e. they are edge-

excited

Assume N=7 and antenna sectors are 120 degrees wide, the worst

case carrier to interference ration is;

i.e.

and

or C/I = 24.5 dB

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4

7.0

DRDRIC

2173 xQR

D

44217.021

1IC

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Adjacent Channel Interference (ACI)

Interference resulting from signals that are adjacent in

frequency to the desired signal is called adjacent

channel interference

ACI is caused by imperfect receiver filters that allow

radiation to leak out into the passband of adjacent cells

For omni-directional cells: isolationFilterd

dLogACI

n

c

i

10

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How to Reduce Adjacent Channel Interference

ACI is a function of the distance between the two

cells under consideration

ACI can be minimized through:

careful channel assignment

filtering

using isolation filters is not the best solution

Worst case ACI occurs when one of the mobiles is close

to the base station and the other is at the edge of the cell

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Near End Far End Interference

– In the uplink, signals are attenuated differently because they

take different paths

Signals to and from mobiles nearest to the base station are

stronger than signals from mobiles located much farther

away

– In the downlink however, mobiles at the cell edge experience

larger degradation and interference compared to mobiles close

to the base station

A

d1

d0 B

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Solution to Near Far Effect

The Mobiles experience greater interference from own

base station compared to from far base stations

Mobiles close to base stations therefore cause more

interference particularly in terms of ACI

Power control is used to mitigate “near far effect”, by

equalizing the power received by all mobiles

Absence of Power Control

ReceivedPower atBS

MS1 M

S2

MS3

With Power Control

ReceivedPower atBS

MS1

MS2

MS3

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Traffic Engineering

Problems with Connecting Phones with Switches

Many switches required - to connect n phones

together, s = (n-1)*n/2 switches are required

slow connection speeds

too many regular faults

high maintenance costs and cost of switches

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Traffic Engineering – Design Objectives

Traffic

System Capacity

Quality of Service

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Traffic Engineering - Considerations

Design for flexibility and account for low

and high traffic periods

peak traffic period occur sometimes in the

mornings and afternoons. Low traffic weekends

high traffic usually 10 to 20% of total capacity,

all users need not be directly connected

cellular systems depend on trunking to connect

a large number of users

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Trunking

In a trunked radio system, each user is allocated a

channel on a per call basis, and on termination of call,

previously occupied channel is immediately returned

to the pool of available channels

Therefore a large number of users share a small pool

of channels in a cell on a per call basis

Access is provided to each user on demand

When all channels are in use, a new user or demand is

(denied) blocked

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Unit of Traffic - Erlang

The unit of telephone traffic intensity is called the

Erlang, in honor of a Danish mathematician

Definition: One Erlang is one channel occupied

continuously for one hour. In data communications,

an 1 E = 64 kbps

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How To Estimate Telephone Traffic

Definitions, let

Au Erlangs be traffic intensity generated by each user

h be average duration of a call (hour)

is the average number of call requests per hour. Then

For a system containing U users, the total offered traffic intensity A

is

In a trunked system of C channels, the traffic intensity per channel is

hAu

uUAA

CUAA ue /

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Busy Hour

The traffic level is an average, taken over several days,

and over the busiest period. The period is usually 1

Hour, and average over that hour is called “Busy

Hour” traffic.

Example: if the circuit is said to carry 0.6 Erlangs, it will

be busy, an average, for 0.6 hours (36 min) during the

busy hour.

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Grade of Service (GoS)

A measure of the performance of a telephone system

GOS is a measure of the ability of a user to access a trunked

system during the busiest hour

Also an indication of the user not being able to secure a

channel during the busiest hour

Telephone networks are designed with specified GOS,

usually for the busiest hour. If a subscriber is able to

make a call during the busiest hour, he will be able to make

a call at any other time

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Grade of Service (1)

Definition

GOS is the probability of having a call blocked during the

busiest hour. For example, if GOS = 0.05, one call in 20 will

be blocked during the busiest hour because of insufficient

capacity

GOS is used to determine the number of channels required;

GOS could be determined by

– competition between operators (measure of good service)

– regulation - a national communication authority might

decide to impose a grade of service on its operators

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Types of Trunked Systems

Two types of trunked systems are used

(a) blocked calls cleared (Erlang B, M/M/m queue)

(b) blocked calls delayed (Erlang C formula)

Characteristics of Blocked calls Cleared Model

Call arrival rate = Poisson (exponential) distribution

Infinite number of users

Memoryless, channel requests at any time

infinite number of channels in pool

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Traffic Intensity Models

Three traffic intensity model tables are used in practice

Erlang B tables (blocked calls cleared); can over estimate

Engset formula (probability of blocking in low density areas);

used where Erlang B model fails

Erlang C tables (blocked calls delayed or held in queue

indefinitely)

Poisson tables (blocked calls held in queue for a limited time

only)

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Erlang B Formula

Determines the probability that a call is blocked

Is a measure of the GOS for trunked systems with blocked

calls cleared

Erlang B formula: GOS

k

AC

A

blockingPC

k

k

C

r

0 !

!][

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Using the Erlang B Table

The objective is to determine the number of trunks

required for a given Erlang value and a blockage

level. Three steps are required:

Locate the column with the desired blockage level; While staying in the same column, find the row with

the desired Erlang value (round off the Erlang value as necessary);

Find the number of trunks in the selected row (at the intersection);

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Planning for Cell Capacity

Assume that in a telephone network the call arrival rate is calls

per hour and the mean holding time for a call is tn (hours per call).

Example: There are 100 subscribers with the following telephone traffic

profile: 20 make 1 call/hour for 6 minutes; 20 make 3 calls/hour for half a

minute; 60 make 1 call/hour for 1 minute. The traffic they generate is:

20x1x (6/60) = 2 E

20x3x(0.5/60) = 0.5 E

60x1x(1/60) = 1 E

i.e. a total of 3.5 E. On average, each subscriber generates 35 mE.

In practice on average telephone subscribers generate between 25 to 35 mE during the busiest hour

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Planning for Cell Capacity

Example: Use the Erlang B table to compute the number of channels

required for a cell when the expected number of calls per hour is 3000,

blocking probability of 2% and the average length of a call is 1.8

minutes.

Solution: The offered traffic for this case is A = qxT/60 = 3000x1.8/60

= 90 Erlangs. Erlang B table indicates that 103 channels

are required.

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Questions and Answers