Fundamentals Of CellularCommunications

Sarawuth ChaimoolWireless Communications Research Group(WCR)

Department of Electrical EngineeringKing Mongkut’s Institute of Technology North Bangkok

THAILAND

2-Ch.Sarawuth -

Lecture TopicsIntroduction

The Concept of Trunking

Determining Cellular Size

Frequency Reuse Concept

Cell Cluster Geometry

Co-channel interference

Design principles

Handoff

Cell splitting

3-Ch.Sarawuth -

IntroductionThe first attempt of mobile telecommunication was asimple interconnection of two-way mobile radio andPSTN lines (cordless telephony).

This was very simple and limited in coverage andfunctions.

Today’s mobile cellular was made possible by theadvances in electronics and DSP.

The concept of “Frequency Reuse” is the heart of mobilecellular telecommunications.

4-Ch.Sarawuth -

Isolated Mobile Systems

5-Ch.Sarawuth -

Connected Mobile Systems

Switch

6-Ch.Sarawuth -

Basic Cellular System

SwitchPSTN/ISDN

7-Ch.Sarawuth -

The Concept of TrunkingTrunking is the second concept that made mobile cellulartelecommunication possible.

Not all mobiles are active in the same time.

Conventional mobile radio systems are based onpermanent allocation of channels.

Trunked mobile radio systems are based on demandassignment of channels.

Trunking makes efficient use of the frequency spectrum.

8-Ch.Sarawuth -

Example of Trunking

4 circuit being sharedonly 4 conversations at any one time

9-Ch.Sarawuth -

Radio Coverage (non-cellular)Operators of conventional (2-way) mobile radio, radioand TV broadcast, and paging service aim to maximisearea coverage.

Radio signal coverage is proportional to:

Height of the transmitting antennaTransmitter power outputReceiver sensitivity and signal to noise ratio

Antenna height gain is the most important

10-Ch.Sarawuth -

Radio Coverage (cellular)Totally the opposite philosophy to that on non-cellularsystems

Lower the antenna as much as possible to just cover thecell area (frequency reuse)

Lower the transmit power to the level that just gives anacceptable signal

Receiver sensitivity is a considered when specifying thecell size

11-Ch.Sarawuth -

Radio Cell SizeField strength contours

Tx

S2S1

S3

S4

S5

Rx

If the threshold ofRx is S5, and Rx isthe standard receiverfor the system thenthe radius R definesthe cell size.

R

Cell size is then controlledby Tx power, Tx antennaheight, and Rx threshold.

12-Ch.Sarawuth -

Frequency Reuse SchemesTime Domain

Space Domain:

Same frequency used in two different areassimultaneously such as Broadcast Radio.Same frequency repeatedly used in the same generalarea (Cellular Mobile Systems)

13-Ch.Sarawuth -

Frequency ReuseIn mobile systems, a radio channel consists of a pair offrequencies (full-duplex)

Frequency Reuse is the core of cellular mobile radiosystems.

A radio channel using a Frequency f1 in a Cell with aRadius R can be reused at Distance D.

Users in both cells can use the same frequencysimultaneously.

Improper system planning & design can causeunacceptable level of Co-channel Interference.

14-Ch.Sarawuth -

Frequency Reuse Concept

D

f1 f1

R R

with the concept of “Frequency Reuse” comes the term“Co-channel Interference”

desiredsignal

undesired signalco-channel interference

15-Ch.Sarawuth -

Frequency Reuse (cont)Radius of each cell may be chosen to be perhaps 26km in a start-up system=> down to less than 2 kmfor a mature system.

As the traffic grows

New cells and channels are added to the system.If an irregular cell pattern is selected,

It would load to an inefficient use of the spectrumdue to its inability to reuse frequencies on accountof co-channel interference.

16-Ch.Sarawuth -

Frequency Reuse (cont)In reality

Coverage of the cell will depend on the terrain andother factors.

ideal realityFor design convenience

assume coverage area are regular polygons

achieve full coverage without dead spots

17-Ch.Sarawuth -

Cell designHexagon cell

Layout requires fewer cell and fewer transmitter sites(closely approximately a circular radiation pattern )

Layout is less expensive compared to square andtriangular cells.

18-Ch.Sarawuth -

Frequency Reuse ConceptS => total duplex channelsk => group of channels (k < S)

If the S channels are divided among N cells intounique and disjoint channel groups

S = kN

Complete set of available frequencies is called acluster.

If a cluster is replicated M times within the system,the total number of duplex channels, C => capacityof cellular system C = MkN = MS

19-Ch.Sarawuth -

Frequency Reuse (continued)N => cluster size

N is large => co-channel cells s are located much

The geometry of hexagons is such that the number ofcell per cluster, N

N = i2+ij+j2To find the nearest co- channel neighbors of aparticular cell (1) move i cells along any chain of hexagons(2) turn 60 degrees counter-clockwise and move j cells.

20-Ch.Sarawuth -

ExampleIf a total of 33 MHz of bandwidth is allocated to

a particular FDD cellular telephone system whichuses two 25 kHz simplex channels to provide fullduplex voice and control channels compute thenumber of channels available per cell if a systemuses. (a) 4-cell reuse (b) 7-cell reuse (c)12-cell reusesoln 33000/50 kHz = 660 Ch.(a) N = 4 , 660 / 4 165 Ch

(b) N = 7 , 660/7 95 Ch(c) N =12 , 660/12 55 Ch1MHz => 1000/50 = 20 Ch => 640 Ch

≈≈

≈

21-Ch.Sarawuth -

Example (cont)

a N = 4, 5 control Ch 160 voice Ch

b N = 7, 4 cell , 3 control Ch 92 voice Ch

2 cell , 3 control Ch 90 voice Ch

c N = 12, 8 cell , 2 control Ch 53 voice Ch

4 cell, 1 control Ch 54 voice Ch

22-Ch.Sarawuth -

Ideal Cells Formation

Base Station

23-Ch.Sarawuth -

Fictitious Cells Formation

Base Station

24-Ch.Sarawuth -

Real Cell Formation

Base Station

25-Ch.Sarawuth -

Frequency Reuse PatternN = 3

2

31

Note: some textbooks use K instead of N

26-Ch.Sarawuth -

12

43

Frequency Reuse PatternN = 4

27-Ch.Sarawuth -

Frequency Reuse PatternN = 7

41

36

527

28-Ch.Sarawuth -

Frequency Reuse PatternN = 7

2

43

1

7

51

6 34

25

76

29-Ch.Sarawuth -

Antenna Positioning

Base Station

30-Ch.Sarawuth -

Typical Antenna Arrangement

Tx

Rx

Rx

two Rx antennasfor diversity

31-Ch.Sarawuth -

Antenna Positioning (Country)

Base Station

32-Ch.Sarawuth -

Cell Geometry

DR

R

R

33-Ch.Sarawuth -

Distance

DR

R

34-Ch.Sarawuth -

Co-ordinate System

30oC1

C2

u

v

u1 v1

u2 v2

35-Ch.Sarawuth -

Reuse Ratio

NqRD 3==

where:D: Distance between the centres of cellsR: Radius of the cellq: Reuse ratioN: Cluster size

Assuming hexagonal shape cells of equal size

36-Ch.Sarawuth -

Exercise

NqRD 3==

As an exercise, using the cell geometry and co-ordinatesystem on the previous slides, derive the equation for thefrequency reuse ratio.

The answer to be submitted next lecture

37-Ch.Sarawuth -

3

12

4

4

2

3

3

1

1

4

21

4

2

2

4

D

R

38-Ch.Sarawuth -

Example

RND 3=

For N = 4 and R 5 km

543 ××=D

The minimum distance at which the same frequency canbe reused is approximately 3.5 times R, which is in thiscase 17.32 km

32.175464.3 =×=D

39-Ch.Sarawuth -

41

36

527

41

36

527

41

36

527

41

36

527

41

36

527

D

R

40-Ch.Sarawuth -

Example

RND 3=

For N = 7 and R 5 km

573 ××=D

The minimum distance at which the same frequency canbe reused is approximately 4.6 times R, which is in thiscase 22.91 km

91.225583.4 =×=D

41-Ch.Sarawuth -

Channel Assignment StrategiesThe way the channels are assigned inside a cell affectsthe performance of the system

especially when a change of BSs occurs

Fixed Channel Allocation Schemes (FCA)

Dynamic Channel Allocation (DCA)

Hybrid Channel Allocation (HCA)

42-Ch.Sarawuth -

Fixed Channel Allocation (FCA)

channels are divided in setsallocated to a group of cells & reassigned to other groups, according tosome reuse patternDifferent considerations are taken before the assignment ofthe channels (i.e. signal quality, distance between BSs, trafficper BS)they are fixed (i.e. a cell can not use channels that are not assigned toit)assignment of frequency sets to cells when the system is designed & does not change unless restructuredAny call attempt within the cell can only be served by the unusedchannels in that cellIf all the channels in that cell are busy, the service is blockedsimple method but does not adapt to changing traffic conditionsintroduction of new BSs supposes frequency reassignment for the complete system

BS1

BS3 BS4

BS2

10 Channels

10 Channels 10 Channels

10 Channels

43-Ch.Sarawuth -

Dynamic Channel Allocation(DCA)

BS1

BS3 BS4

BS240Channels

Channels are placed in a poolassigned to new calls depending on the carrier tointerference ratio (CIR) and other criteria.Each time a call is made the serving base station requests

a channel from the RNCThe switch then allocates a channel to the requested cell following an algorithm that takes into account the likelihood of future blocking within the cell

the frequency of use of the candidate channelthe reuse distance of the channel, and other cost functions.

The RNC only allocates a given frequency if that frequency isnot presently in use in the cell or any other cell which falls within theminimum restricted distance of frequency reuse to avoid interferencereduces the likelihood of blocking, which increases the trunking capacity ofthe system, since all the available channels in a market are accessible to all ofthe cells

44-Ch.Sarawuth -

Dynamic Channel Allocation(DCA)

Require the RNC to collect real-time data on

channel occupancytraffic distributionradio signal strength indications (RSSI) of all channels on a continuousbasis

This increases the storage and computational load on the system butprovides the advantage of increased channel utilisation and decreasedprobability of a blocked call

Allocation of channels is more complex since additional information isneeded, but is also more flexible to traffic changes (i.e. non-uniform traffic).

45-Ch.Sarawuth -

Hybrid Channel Allocation (HCA)a combination of both FCA and DCA

some channels are pre-assigned

others are shared dynamically

One of these approaches is based on the principal of borrowingchannels from a neighbouring cell when its own channels areoccupied

Known as the borrowing strategy

RNC supervises such borrowing procedures & ensures that theborrowing of a channel does not disrupt or interfere with any of thecalls in progress in the donor cell

46-Ch.Sarawuth -

Allocation ComparisonFCA better for high uniform traffic loads

Max reusability of channels is always achievedDCA performs better for non-uniform traffic loads

allocation of channels is flexibleFCA schemes behave like a no. of small groups of servers

DCA provides a way of making these small groups of serversbehave like a larger server, which is more efficient.

FCA call must always be handed off into another channelsame channel is not available in adjacent cells.

DCA the same channel can be used if interference does notoccur.

47-Ch.Sarawuth -

Allocation Comparisonvariations in traffic that are typical of microcells are not well handledin FCA.

DCA techniques perform better in microcells

Implementation complexity of DCA is higher than FCA.FCA:each cell has a number of channels and the channelselection is made independentlyDCA: the knowledge of occupied channels in other cells isnecessary (i.e. heavy signalling load).A great deal of processing power to determine optimalallocations is also required.

48-Ch.Sarawuth -

Handoff strategiesHandoff is the process used to allow a call inprogress to continue as the mobile terminal movesbetween cells.

When a mobile moves into a different cell while aconversation in progress, MSC automaticallytransfers the call to a new channel belonging to thenew base station.

49-Ch.Sarawuth -

Handoff Mechanism

F1

F4 F1 F2F1

F4F3

F3F2

F2

F1 F2 F3 F4 F1 F2

D

50-Ch.Sarawuth -

Handoff strategies(continued)1.Hard handoff occurs when the communication

to the mobile terminal is passed between disjointedradio systems, different frequency assignment, ortechnologies.

A hard handoff is a “break-before-make” processat the air interface.

2. Soft handoff occurs when the mobile terminalcommunication is passed to the target radio portwithout interrupting communication with the currentserving radio port.

Mobile terminal(MT) communicates with two radioport simultaneously.

51-Ch.Sarawuth -

Handoff strategies(cont)The handoff process consists of the following step:

1.Initiation : Either the mobile terminal or network identifiesthe need for a handoff and alerts the necessary networkelements.

2.Resource reservation : The appropriate network elementsreserve the resource necessary to support the handoff.

3.Execution :The actual handoff connection of the networkresources takes place.

4.Completion : Any unneeded network resources are freed ,and access signals are exchanged following a successfulhandoff.

52-Ch.Sarawuth -

Design ObjectivesSignal Coverage

Cover the whole area with a minimum number of cellsites.100% coverage of the whole area is impossible.

Traffic Coverage

Catering for the busy-hour traffic with an acceptablelevel of Grade of Service (GoS).Number of channels per cell and traffic load.

53-Ch.Sarawuth -

Mobile Radio Transmission Model

θ1θ2

30 -

100

m

2 km or more

Direct Path

Reflected Path

3m

54-Ch.Sarawuth -

A better Situation

θ1

θ2

30 -

100

m

2 km or more

Direct Path

Reflected Path

3m

55-Ch.Sarawuth -

Propagation Path Loss

In the mobile radio environment, the received power at theradio receiver (theoretically) is given by:

44 −− =∝ RRC αC = received carrierR = distance from Tx to Rxα = constant

Where:

The loss slope is then 40 dB per decade, i.e. a mobile movingfrom 1 to 10 km will experience 40 dB loss of signal.

Note that in Free Space (like in microwave links) this relationship is:

22 −− =∝ RRC α

The difference in power reception at two different distancesR1 and R2 will result in:

4

1

2

1

2

−

⎟⎟⎠

⎞⎜⎜⎝

⎛=

RR

CC

In real mobile radio environment, the propagation path-lossslope varies as:

γγ α −− =∝ RRC

The variable γ varies between 2 and 5 depending on the actualconditions but cannot be less than 2 (free space).

RC log10log10 γα −= dB

57-Ch.Sarawuth -

The Ratio D/R

R R

D

C1 C1

P0 P0

f1 f1

C/I = γ C/I = γ

q = D/R

58-Ch.Sarawuth -

Frequency Reuse DistanceNumber of co-channel cells in the vicinity of the centralcell,

Type of geographic terrain contour,

Antenna height, and

Transmitted power at each cell site.

59-Ch.Sarawuth -

The frequency reuse distance can be determined from:

RND 3=

Where N is the frequency reuse pattern as shown inthe next slide.

N = 4, 7, 12, and 19 are used.

60-Ch.Sarawuth -

N-D RelationshipAssuming that all cell sites transmit the same power.

Distance D in terms of R for a given K:

N = 4 D = 3.46RN = 7 D = 4.6RN = 12 D = 6RN = 19 D = 7.55R

Increase in K corresponds to increase in D

61-Ch.Sarawuth -

The ChallengeReducing Co-channel Interference to an acceptable level.

The larger the N the longer the Distance.

Increase in distance results in reduction in Co-channelInterference.

A system with a large N results in Trunking inefficiency(why ?).

The challenge is to obtain the smallest N which meetsthe required performance.

62-Ch.Sarawuth -

Co-channel InterferenceFrequency reuse is limited by co-channel interference.

The cell size is determined by the signal strength.

The receiver threshold level is adjusted to the cell size.

For a fixed cell size co-channel interference is a functionof the parameter q = D/R.

63-Ch.Sarawuth -

Co-channel Interference ReductionFactor

RDq =

)/,( ICKfD I=

The co-channel interference is a function of q, which is calledthe co-channel interference reduction factor.

The separation distance D is a function of KI and C/I

KI is the number of co-channel interfering cells in the first tierC/I is the carrier-to-interference ratio at the desired mobile

64-Ch.Sarawuth -

1

1

1

1

1

1

1

11

1

1 1

1

Interfering CellFirst tier

Second tier

D

R

65-Ch.Sarawuth -

∑=

=IK

kkI

CIC

1

C/I is calculated as:

The maximum number of K in the first tier is 6 and knowing that

γγ α −− =∝ RRC

∑=

−

−

=IK

kkD

RIC

1

γ

γ

γγ α −− =∝ DDI

Wanted signal

Interfering signal

The above equation becomes:

66-Ch.Sarawuth -

( )∑∑=

−

=

− =

⎟⎠⎞

⎜⎝⎛

=II

K

kk

K

k

k qRDI

C

11

11λ

λ

Rearranging:

and

RDq k

k =

The qk is the co-channel interference reduction factor with kth

co-channel interfering cell.

67-Ch.Sarawuth -

Example

N = 4 D = 3.46R q = 3.46

N = 7 D = 4.6R q = 4.6

N = 12 D = 6R q = 6

N = 19 D = 7.55R q = 7.55

68-Ch.Sarawuth -

Designing for TrafficEstimation of traffic load

Identification of Busy hour

What is an acceptable GoS ?

Number of channels per cell

Coping with increase traffic demand

Application of Erlang formulas

Cell Splitting techniques

69-Ch.Sarawuth -

Cell Splitting

New cell radius =Old cell radius

2

New cell area =Old cell area

2

New traffic loadUnit area

Traffic loadUnit area

4 x=

70-Ch.Sarawuth -

Cell Splitting - Method A

Original Cell

71-Ch.Sarawuth -

Cell Splitting - Method B

Original Cell

72-Ch.Sarawuth -

ReferencesGarg, Vijay K and Wilkes Joseph E, “Wireless andPersonal Communication Systems”. Prentice Hall PTR

Parsons J D and Gardiner J G, Mobile CommunicationSystems, Blackie USA Halsted Press

Lee, William C. Y., Mobile Communications Engineering,McGraw-Hill, Inc.

Lee, William C. Y., Mobile Cellular TelecommunicationsSystems, McGraw-Hill, Inc.