Cellular System Objectives

Optimize Spectral efficiency Increase Affordability Increase Subscriber Capacity Provide Global Compatibility Provision Quality Service

Cellular networks: Why?

• Frequency reuse improves significantly the utilisation of the limited spectrum, and can offer high capacity to users.

• Use of low power transmitter can last the use of battery of mobile terminals, and reduce the danger of radiation to health and the weight of terminals.

Cellular coverage representation Ideal cell pattern circular Fictious cell pattern hexagonal Real cell pattern footprint

2R2 3.14R2

2.6R2

1.73R2

Cellsite configurations Center excited cells Edge excited cells Omni-/Sectorized cells

Basic Components of a Cellular Telephone System Cellular Mobile Phone:

A light-weight hand-held set which is an outcome of the marriage of Graham Bell’s Plain Old Telephone Technology [1876] and Marconi’s Radio Technology [1894] [although a very late delivery but very cute]

Control unit , transceiver unit (4W, 0.6 W, 1.6W)

Base Station:

A Low Power Transmitter, other Radio Equipment [Transceivers] , interface equipment, power sources, antenna systems plus a small tower

Basic Components of a Cellular Telephone System Mobile Switching Center [MSC] /Mobile

Telephone Switching Office[MTSO]

An Interface between Base Stations and the PSTN

Controls all the Base Stations in the Region and Processes User ID and other Call Parameters

A typical MSC can handle up to 100,000 Mobiles, and 5000 Simultaneous Calls

Locates mobile subscribers, Handles Handoff Requests, Call Initiation Requests, and all Billing & System Maintenance Functions

Cellular Telephone System

Cellular Telephone System

HLR

VLR

AUC

Common Air Interface (CAI)

Forward Channel

Reverse Channel

n Specifies Four Channels [Voice Channels and Control / Setup Channels]

FVC: Forward Voice ChannelRVC: Reverse Voice ChannelFCC: Forward Control ChannelRCC: Reverse Control Channel

Call Setup Procedure

Cellular Phone Codes: Special Codes are associated with each Cell Phone to identify the phone, its owner, and service provider:

Electronic Serial Number(ESN) -A Unique 32-bit Code Mobile Identification Number(MIN): A Subscriber’s Telephone NumberStation Class mark (SCM): Indicates the Max Tx Power for the User

When a Cellular Phone is turned on and Initiates a CallMonitors the Control Channels and gets hold on to the strongest oneMakes a Call Initiation Request[Dialled Called party Number, MIN , ESN and SCM automatically transmitted]Validation Procedure at MSC & Voice-Frequency pair Allocation Base Station Pages the Information for the MobileMSC Connects the Mobile with the Called Party[Another mobile/Landline Phone]Call is Established and Communication Starts

Call Setup – Mobile initiated Call Setup Procedure (Cont’d)MSC

3. Receives a Call Initiation Request from

Base Station, and Verifies that User has a Valid MIN & ESN pair

4. Locates the Called Party,

Allocates a VFPs and Instructs the Base Stations via

FCC

7. Connects the Called Party[on

PSTN]/Mobile to the Mobile

FCC

5. Pages for the Called Mobile, the Mobiles are

instructed to move to the Allocated VFPs

respectively

RCC

2. Receives a Call Initiation Request, with MIN, ESN, SCM and Called Part Number

FVC 8. Begins Voice Transmission

RVC 8. Begins Voice Reception

FCC

6. Receives [Called Mobile] the Page and Matches the MIN, the Mobiles get ready to

move to the respective Allocated VFPs

RCC1. Makes a Call Initiation Request, with MIN and

Called Part Number

FVC8. Begins Voice

Reception

RVC8. Begins Voice

Transmission

Base Station

Mobile

TIME

Call Setup – Landline to Mobile Call Setup Procedure (Cont’d)MSC

3. Receives a Call Initiation Request from

Base Station, and Verifies that User has a Valid MIN & ESN pair

4. Locates the Called Party,

Allocates a VFPs and Instructs the Base Stations via

FCC

7. Connects the Called Party[on

PSTN]/Mobile to the Mobile

FCC

5. Pages for the Called Mobile, the Mobiles are

instructed to move to the Allocated VFPs

respectively

RCC

2. Receives a Call Initiation Request, with MIN, ESN, SCM and Called Part Number

FVC 8. Begins Voice Transmission

RVC 8. Begins Voice Reception

FCC

6. Receives [Called Mobile] the Page and Matches the MIN, the Mobiles get ready to

move to the respective Allocated VFPs

RCC1. Makes a Call Initiation Request, with MIN and

Called Part Number

FVC8. Begins Voice

Reception

RVC8. Begins Voice

Transmission

Base Station

Mobile

TIME

Receives call from PSTN. Sends MIN to all BSs

Transmits page(MIN) for specifiedUser

Receives page andMatches the MIN With its own MIN

Receives MIN,ESN, SCM andPasses to MSC

Acknowledges receipt of MIN Sends ESN, SCM

Verifies validity ofMIN, ESN pairRequests BS to moveMobile to V F P

Transmits info.For mobile toMove over to V F P

Receives info. To tune to specified V F P

Connects Mobile to Called partyon PSTN

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Cellular Mobile Telephony Cellular concept

Bell Labs (1957 & 1960)

Frequency reuse Typically every 7 cells

Channel Assignment Handoff as caller moves Modified CO switch

HLR, paging, handoffs

Sectors improve reuse Every 3 cells possible

Frequency reuse concept

A

A

B

B

C

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- T.S. Rappaport, “Wireless communications, principles & practice”, ISBN 0133755363, Prentice-Hall, 1996, pp25-68.

Frequency Reuse Factor

Frequency Reuse Factor = N

Cluster formation (a group of N cells of radius ‘R’)

Each cell of a cluster should have different RF channels

Same cluster is repeated over remaining cells (FCA)

Frequency reuse concept

Distance between co-channel cells = Frequency reuse distance,

D = R (3N) Distance between adjacent cells

d = R (3)

Co-channel Reuse Ratio QArea of larger hexagon

= 2.6 D2

Area of a cell = 2.6 R2

No. of cells within the Region shown = N + 6. N/3

= 3 N

= 2.6 D2/ 2.6 R2

Q = D/R = sqrt (3 N )

Cluster size (N)= i2+j2+i.j

Reuse factor = 1/N

There exists a tradeoff in selecting cluster size (N)

A higher value is required to minimize the interference.

A lower value ensures more channels over same

network thus better radio capacity.

Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., i = 3, j = 2). (Adapted from [Oet83] © IEEE.)

19-cell reuse example (N=19)

An Example of Frequency Reuse Suppose we have spectrum for 100

voice channels

Scenario 1: a high power base station covering entire area – system capacity = 100 channels

Scenario 2: divide spectrum into 4 groups of 25 channels each; cells (1, 7), (2, 4), (3, 5), 6 are assigned distinct channel groups – system capacity = 175 channels

Suppose W = 25 MHz and B = 25 KHz/voice channel W/B = 1000 voice channels can be supported over the spectrum Scenario 1: a high power base station covering entire area (M = N =

1) system capacity n = 1000 users Scenario 2:

Coverage area divided into M = 20 cells with reuse factor N = 4 Each cluster accommodate 1000 active users 5 clusters in coverage area system capacity n = 5000 users

Scenario 3: M = 100 cells, N = 4 system capacity n = 25000 users

Scenario 4: M = 100 cells, N = 1 system capacity n = 100000 users

Frequency Reuse Example-2

Frequency Reuse - Summary

each cell allocated a group k channels a cluster has N cells with unique and disjoint channel

groups, N typically 4, 7, 12 total number of duplex channels S = kN Cluster repeated M times in a system Total number of channels that can be used (capacity)

C = MkN = MS Smaller cells higher M higher C

+ Channel reuse higher capacity+ Lower power requirements for mobiles Additional base stations required More frequent handoffs Greater chance of ‘hot spots’

Effect of cluster size N channels unique in same cluster, repeated

over clusters keep cell size same

large N : weaker interference, but lower capacity small N: higher capacity, more interference need

to maintain certain S/I level frequency reuse factor: 1/N

each cell within a cluster assigned 1/N of the total available channels

In most of the current networks, frequency reuse factor is 1.

Smaller N is greater capacity ?

Q = D/R = (3N)