Ch3. The Cellular Concept Systems Design...

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Ch3. The Cellular Concept –

Systems Design Fundamentals.From Rappaport’s book

Instructor:

• Mohammed Taha O. El Astal

Early mobile systems

The objective was to achieve a large coverage area by using a single high

power transmitter with an antenna mounted on a tall tower.

It has a very good coverage area but just for a few # of users.

i.e. Bell Mobile system supported 12 users simultaneously.

3.1 Introduction: Cellular Concept

CONT.

The Cellular concept was a major breakthrough in solving the problem of

spectral congestion and user capacity.

It offered very high capacity in a limited spectrum allocation without any major

technical changes.

So, the increasing demand can be faced by reuse the channels through Tx. with

low power

3.2 Frequency Reuse

Cellular System rely on an intelligent

allocation and reuse of channels.

Each B.S. is allocated a group of radio

channels to be used within a cell.

B.S. in adjacent cells are assigned

channel groups which contain completely

different channels than neighboring cells.

To prevent large interference, optimum

antenna radiation pattern to coverage just

cell area must be designed.

channel group

1-2-3 4-5-6

7-8-9 10-11-12

13-14-15 16-17-18

19-20-21

CONT.

The real coverage of B.S. is called

footprint.

Since it is amorphous in nature, we

need to regular shape to use it in the

systematic design process.

1st choice is the circle shape, but ??

Adjacent circle can not be overlaid

upon a map without leaving gaps or

creating overlap region.

CONT.

Thus, when considering a geometric shapes which

cover an entire region without overlapping and with

equal areas, there are 3 choices :

square,

equilateral triangle ,

hexagon

since :

the cell must serve the weakest mobile with the

footprint, so the hexagon has the largest area if the

farthest distance from center is fixed, the fewest

number of shape can cover specific geographic area.

the hexagon closely approximate the circular

radiation

Two schemes

Center-excited cell

antenna type ??

Edge-excited cell

antenna type??

CONT.

S is the total number of duplex channel for the system from the regulation

body.

K is the # of channel per cell.

if there are N of cells, then :

S=KN.

if the area covered by M clusters then :

C=MS=MKN.

where C is he total capacity of the system.

M++C++ & as M--C--.

cell area fixed: N--M++ C++ but co=channel interference ++.

cell area fixed: N++M-- C-- but co=channel interference --.

Frequency reuse factor= 1/N

number of cell/ cluster

The number of cell per cluster (N)

can only have value which satisfy :

move i cells in any direction and then

move j cells in clockwise direction

CONT.

EXAMPLE 2.1:

If a total of 33 MHz of bandwidth is allocated to a particular FDD cellular

telephone system which uses two 25 kHz simplex channels to provide full

duplex voice and control channels, compute the number of channels available

per cell if a system uses (a) four-cell reuse, (b) seven-cell reuse, and (c) 12-cell

reuse. If 1 MHz of the allocated spectrum is dedicated to control channels,

determine an equitable distribution of control channels and voice channels in

each cell for each of the three systems.

Simply locate the same number of channels in each cell wherever possible

for N=4; #c=5;#v=160

for N=7;4 -#c=3-#v=92// another (4*91+3*92)

2 –#c=3-#v=90//

1-#c=2-#v=92//

for N=12 ; 8 #c=2;#v= 53

4 #c=1;#v=54

2.3 Channel Assignment strategies:

Each Cell is Assigned a

Predetermined Set of V. Ch (x)

Any request for a new call

initialization or HO beyond x

will be blocked or waited.

Borrowing Strategy

Reserve some channels for

Handoff.

channel assignment strategies

DynamicFixed

Objective: Maximize the System Capacity while Minimizing the

Interference [A Constrained Optimization Problem]

No Permanent Assignment of V. Ch.

to any Cell

Any request for a new call/ HO will

be met by a dynamic allocation of a

channel from the central pool of av.

ch. by MSC that takes into Account:

probability of future blocking,

frequency of use of candidate

channels.

reuse distance of the Channel

CONT.

Advantages of Dynamic Channel Allocation

Reduction of blocking probability

Reduction of call drop probability during Hand Off

Improvement of System Trunking Capacity [Traffic

Intensity/Channel]- All Channels are Accessible by all Cells

but

Storage and Computational Load on MSC.

MSC must Collect real-time Channel Occupancy Data.

Traffic Distribution Information

Radio Signal Strength Indications (RSSI) of all the

Channels

2.4 Handoff Definition

Definition :

When a mobile moves into a different cell

while a conversation is in progress, the

MSC automatically transfers the call to a

new channel belonging to the new base

station

Important task in any cellular radio system

Must be performed successfully,

infrequently, and imperceptible to users.

Identify a new base station

Channel allocation in new base station with

a high priority than initiation request.

CONT.

Let :

Pr,min is the minimum acceptable signal

to maintain the call .

since HO be performed successfully,

infrequently, and imperceptible to users, so

Pr,Handoff , is the signal power where to

start handoff process, must be slightly

greater than Pr,min.

Then : ∆= Pr,Handoff - Pr,min usable is the

design parameter.

∆ must choose carefully , not too large

and not too small, why??

Network Controlled Handoff (NCHO)

In first generation cellular system each

base station constantly monitors signal

strength from mobiles in its cell

Based on the measures, MSC decides if

handoff necessary

Mobile plays passive role in process

Heavy Burden on MSC

MSC

A

B

CG

F

E

D

MSC

Cell BCell A

fa

RSL A

RSL B

RSL

dbm

Distance

2.4 Handoff Styles:

Mobile Assisted Handoff (MAHO)

Present in 2nd generation systems

Mobile measures received power from

surrounding base stations and report to

serving base station.

During the idle time periods, the M.S. can

tune to other radio ch. frequencies and

measure their signal strengths.

Handoff initiated when power received

from a neighboring cell exceeds current value

by a certain level or for a certain period of

time

Faster, why??

CONT.

The Handoff types:

Hard HO : is one in which the channel

in the source cell is released and only then

the channel in the target cell is engaged.

known as (break-before-make).

Soft HO: is one in which the channel in

the source cell is retained and used for a

while in parallel with the channel in the

target cell.

known as (make-before-break).

HO Types:

Intersystem Handoff:

When ?

Mobile is at the Border of the system

what will happened?

MSC of the Serving Cell Talks to the

MSC of the Neighboring System or

Vice Versa

Handoff can take place

Call Type

Roaming is Allowed or Not?

Compatibility Issues [Standards]

User Authenticity and Call Charges

Issues

2.4.1 Prioritizing Handoff:

Guard Channel Method

A fraction of the total available channels is reserved for Handoffs

In case of fixed channel assignment, it affects system capacity

[C = M k N]

Good in case of dynamic channel assignment

Queuing Handoff Request Method

Any Handoff request, if can not be tackled immediately, it will be

placed in a queue

Does not Guarantee 100% Success for all Handoff Requests

2.4.2 Practical HO Consideration:

High Speed Vehicles:

Umbrella cell approach to tackle the issue.

Cell Dragging:

HO threshold and coverage design parameters must be chosen

very carefully.

Large Umbrella Cell

for Hi peed Traffic

Small MicroCells for

Low Speed Traffic

Site Configuration

In Jawwal Workshop.

Tuesday 13/12/2011

10:00 AM

CONT.

Antenna: is a device that transmits/receives

EM signals simultaneously

Antenna Categories:

Passive Antenna: RP Controlled by Type

and Construction of the Device, Using

Mechanical Means we can Guide the Signal

Active Antenna: RP Controlled by Type

and Construction as well as DSP Technique

of the Device

General Classification on RP

Omni Directional Antenna: Equal

Radiation in all Directions

Directional Antenna: More Radiation in a

Certain Direction

CONT.

Main Antenna Parameters :

Antenna Directivity.

Antenna Gain

Antenna Beam Width

Antenna Front-to-Back Ratio

Frequency Response

Bore Sight

Isotropic

GAIN

Type BW

M. Antenna 70 MHz

D. Antenna 25 MHz

Mob. Ant. 70 MHz

Frequency Response

824-894 MHz

824-849 MHz

824-894 MHz

0 dB

- 3 dB

GAIN

FREQUENCYfL fo fH

Typical Antenna Parameters

3.5 Interference and System Capacity

Interference: is unwanted signal which

affects the signal quality.

It is the major limiting factor in the

performance of cellular radio systems.

Interference sources in cellular systems:

another mobile in the same cell.

a Call in Progress in the Neighboring Cell.

Other base stations using the same

frequency band.

Some non-Cellular device/system leaking

energy in the cellular band.

Interference in voice channel cause a

crosstalk whereas in control channel

cause missed or blocked call due to

errors in digital signaling

CONT.

Interference is more severe in urban area due to :

greater noise floor.

high number of B.S.

A major bottle-neck in system capacity: a trade-off between system

capacity and speech quality.

Two Major Ones are:

Co-Channel Interference

Adjacent-Channel Interference

3.5.1 Co-Channel Interference & system capacity :

Co-channel Cell : are the cells using

the same set of frequencies in the given

coverage area.

The signals of the co-channel cells

interfere with each other, the effect

called co-channel interference.

Unlike thermal noise, co-channel

interference can't reduced by

increasing SNR, so what the solution??

Co-channel cells must be

physically separated by a

minimum distance to provide

sufficient isolation, (D)

if cells size are same, and also

have same Tx. power, then the

co-channel interference become a

function of R & D only.

CONT.

A

B

CG

F

E

D

A

B

CG

F

E

D

A

B

CG

F

E

DA

B

CG

F

E

D

D The Co-channel reuse factor Q is defined as :

Q = D / R = √(3 N)

As Q++co-channel I -- but ( N++ C--)

As Q--co-channel I ++ but (N-- C++)

that’s mean there is a tradeoff between C & I.

CONT.

Exercise :

If the power of propagating signal in cellular system follow : Pr(dBm)=P0-10 n

log(d/d0) , where P0 is the Tx power, and n is the path exponent, then find the

following :

1. The general equation that describe SIR at cell boundary (assume all at D

distance ).

2. simplified the equation if the interference just form first co-channel cell

layer and the distance between cells center are same .

3. if SIR must be equal or above of 18 dB and n=4 then find N.

N=7.

Exact SIR:

Suppose N from feasible values .

calculate Q.

Calculate S/I.

if larger than required then decrease N and recalculate until obtained

minimum SIR verify the condition.

if smaller then increase N until have a minimum SIR verify the condition.

CONT.

Example 3.2 :

If the power of propagating signal in cellular system follow : Pr(dBm)=P0-10 n

log(d/d0) , where P0 is the Tx power, and n is the path exponent, then find the

following :

1. The general equation that describe SIR at cell boundary (assume all at D

distance ).

2. simplified the equation if the interference just form first co-channel cell

layer and the distance between cells center are same .

3. if SIR must be equal or above of 18 dB and n=4 then find N.

N=7.

3.5.2 Channel planning for Wireless Systems:

Cellular systems spectrum :

Voice channels.

Control channels.

Voice channels can face interference more

than control channels can do , why ?

In system have N=7 for voice channels, it

must use N=21 for control channels.

3.5.3 Adjacent Channel Interference:

Adjacent-Channel Interference [ACI]:

An interference arising from energy spill-

over between two adjacent channels

ACI results from the imperfect behavior of

the Rx filters allowing nearby frequencies to

leak into the pass-band

It is more critical when near-far scenario ,

which called near-far effect.

ch1 ch2 ch3

filter

CONT.

ACI minimizing methods :

Careful filter design:

• determine the required decay slope to have

an acceptable ACI for the worst case

Intelligent Channel assignment:

• by keeping the inter-channel frequency

difference as large as possible

Example : if a M.S. at 20 times close to B.S.

than other M.S. which operate at adjacent

channel (n=4), then by 20dB/oct filter you

need to six times separation to have SIR=0

A

1,8,15,22

F

6,13,20,2

7

D

4,11,18,2

5B

2,9,16,23

C

3,10,17,2

4

G

7,14,21,2

8E

5,12,19,2

6

CONT.

Example 3.3:

3.5.4 Power control for reducing Interference:

The serving B.S. control the Tx. power of

M.S. to maintain just smallest required

power.

this help to :

prolong battery life for the subscriber unit.

reduced the reverse channel SIR in the

system.

3.6 Trunking and Grade of Service:

The Concept of Trunking :

allow large # users share small # of Ch. by

serve the user only on demand from a pool of

available Ch., then release the channel to the

pool, based on specific GOS.

Developed by Danish mathematician, A.K.

Erlang, based on a statistical behavior of

users.

Traffic intensity unit : Earlang (dimensionless)

which equal one hour call per hour or 1 minute

call per minute.

CONT.

Exercise : compute traffic intensity of a radio channel occupied for 30

minutes during one hour ??

The definition of GOS :

is a measure of the ability of a user to access a trunked system during the

busiest hour.

The GOS is a benchmark used to define the desired performance of a

particular trunked system.

Trunk terminology :

Call Set up Time: Time Required to Allocate a Trunked Radio

Channel to a Requesting User

Blocked/Lost Call: A Call that can not be Completed at the Time of

the Request, due to Congestion

Holding Time: Average Duration of a typical Call, denoted by H

Request Rate: Average Number of Call Requests/Unit Time [denoted

by ]

Traffic Intensity : Average Channel Occupancy, measured in Earling.

Trunk System Types:

1st type offers no queuing for call requests:

Immediate access to a channel if one is

available .

Else, the request is blocked and is free to try

again later

Also, called blocked calls cleared (BCC).

Pr / A/ C, 3 critical parameters.

Erlang B formula / curve.

2nd type offers queuing for call requests:

Immediate access to a channel if one is

available .

Else, the request is queuing for waiting time.

Also, called blocked calls delayed (BCD).

Erlang C formula / curve.

CONT.

the probability of queuing the call for period greater than t :

The average delay time , D :

Gives Relationships among GOS, C, and A for BCD Trunking Radio

Systems

Erlang B chart :

Gives Relationships among GOS, C, and A for BCD Trunking Radio

Systems

Erlang C :

Gives Relationships among GOS, C, and A for BCC Trunking Radio

Systems

Examples

Trunk efficiency :

Trunk efficiency: is a measure of the number of

users which can be offered a particular GOS with

a particular configuration of fixed channels.

The way in which channels are grouped can

substantially alter the number of users handled by a

trunked system.

C=10, GOS=1%A=4.46 where C=5

,GOS=1%A=1.36 which doesn’t equal A=4.46/2

that’s mean you should have as large as C in the

same cell.(60% increase than 2*5)

3.7 Improving coverage & C :

Review: What is System Capacity?

C = M k N.

N is the Cluster Size.

k is the number of Channels used per Cell.

and M is the Replications of the Cluster in the given System Coverage

Area.

In terms of Traffic Intensity, A = U Au

as U ++A ++, so the traffic offered to the system will increase which

leading to congestion [Blockage of the Calls].

Given an Allocated Spectrum [S = k N ] which is Fixed, We have to use

some Cellular Design Techniques to Improve System Capacity [ C or A as A is

a Function of C].

1.Cell Splitting :

Objective :

To increase M with keeping cluster size N and

co-channel reuse ratio, Q = D/R = sqrt(3N)

constant.

How :(In general)

By reducing the cluster coverage area (in other

words cell area).

It Maintains S/I by reducing the base station

Tx power, antenna height, and antenna down-

Tilting Mechanism.

A

B

C

D

E

F

G

C

E

D

F

E

GF

B

G

C

D

F

E

GB

C

D

CONT.

How much reduction of Tx. power?

Pr (old Cell Boundary) Pt1 R-n

Pr (new Cell Boundary) Pt2 (R/2)-n

Equating them, we get Pt2 = Pt1 / 2n

Example : There is existing system and their

engineer noticed there are increased blocking rate in

cell A.

How : By reducing the cell area: RR/2 which

result 6 new microcells, so there is additional # of

channels per unit area.

As RR/2 , also DD/2 , so R/D remain

constant.

if n=4, the new power must equal pt1/16

A

B

C

D

E

F

G

C

E

D

F

E

GF

B

G

C

D

F

E

GB

C

D

CONT.

Practical consideration in splitting :

The # HO ++.

At initial phase, which power will have to use?

Suggested solutions:

Two group of channels are created , one of them

for the large cell and the another for the small cells.

The size of these group is determined by in any

phase the system reside.

A

B

C

D

E

F

G

C

E

D

F

E

GF

B

G

C

D

F

E

GB

C

D

2.Cell Sectoring :

Objective :

To decrease N (which increase M) with

keeping co-channel reuse ratio small.

How :

Reduce N : reassign S channel to new N, which

increase M.

Since it plays with N, thus, changing Q and S/I.

To avoid co-Channel interference by using

directional Antennas.

It improves system capacity and S/I but at the

cost of decreased system trunking efficiency.

3-1

3-2

3-3

1-1

1-2

1-3

4-1

4-2

4-3

2-1

2-2

2-3

3-1

3-2

3-3

1-1

1-2

1-3

4-1

4-2

4-3

2-1

2-2

2-3

12

345

6

3.7.3+3.7.4

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