Week 13 lecture 1 Cellular Communications (I)

Week 13 lecture 1Week 13 lecture 1

Cellular Communications Cellular Communications (I)(I)

Existing Network InfrastructureExisting Network Infrastructure

Public Switched Public Switched Telephone Network Telephone Network (PSTN): Voice(PSTN): Voice

Internet: DataInternet: Data Hybrid Fiber Coax Hybrid Fiber Coax

(HFC): Cable TV(HFC): Cable TV

Market Sectors for ApplicationsMarket Sectors for Applications

Four segments divided into two classes: Four segments divided into two classes: voice-voice-orientedoriented and and data-orienteddata-oriented, further divided into , further divided into local local and and widewide-area markets-area markets

Voice:Voice:– Local: low-power, low-mobility devices with Local: low-power, low-mobility devices with

higher QoS – cordless phones, Personal higher QoS – cordless phones, Personal Communication Services (PCS)Communication Services (PCS)

– Wide area: high-power, comprehensive coverage, Wide area: high-power, comprehensive coverage, low QoS - cellular mobile telephone servicelow QoS - cellular mobile telephone service

Data:Data:– Broadband Local and ad hoc: WLANs and WPANs Broadband Local and ad hoc: WLANs and WPANs

(WPAN-Wireless Personal Area Network)(WPAN-Wireless Personal Area Network)– Wide area: Internet access for mobile usersWide area: Internet access for mobile users

Evolution of Evolution of Voice-OrientedVoice-Oriented ServicesServices

YearYear EventEvent

Early 1970sEarly 1970s First generation of mobile radio at Bell LabsFirst generation of mobile radio at Bell Labs

Late 1970sLate 1970s First generation of cordless phonesFirst generation of cordless phones

19821982 First generation Nordic analog NMTFirst generation Nordic analog NMT

19831983 Deployment of US AMPSDeployment of US AMPS

19881988 Initiation of GSM development (new digital TDMA)Initiation of GSM development (new digital TDMA)

19911991 Deployment of GSMDeployment of GSM

19931993 Initiation of IS-95 standard for CDMAInitiation of IS-95 standard for CDMA

19951995 PCS band auction by FCCPCS band auction by FCC

19981998 3G standardization started3G standardization started

FDMA – Frequency Division Multiple AccessNMT – Nordic Mobile TelephonyAMPS – Advanced Mobile Phone SystemGSM – Global System for Mobile CommunicationTDMA – Time Division Multiple Access

IS-95 – Interim Standard 95CDMA – Code Division Multiple AccessPCS – Personal Communication SystemFCC – Federal Communication Commission

Evolution of Evolution of Data-OrientedData-Oriented ServicesServices

YearYear EventEvent

19791979 Diffused infrared (IBM Rueschlikon Lab - SwitzerlandDiffused infrared (IBM Rueschlikon Lab - Switzerland

Early 1980sEarly 1980s Wireless modem (Data Radio)Wireless modem (Data Radio)

19901990 IEEE 802.11 for Wireless LANs standardsIEEE 802.11 for Wireless LANs standards

19901990 Announcement of Wireless LAN productsAnnouncement of Wireless LAN products

19921992 HIPERLAN in EuropeHIPERLAN in Europe

19931993 CDPD (IBM and 9 operating companies)CDPD (IBM and 9 operating companies)

19961996 Wireless ATM Forum startedWireless ATM Forum started

19971997 U-NII bands released, IEEE 802.11 completed, GPRS U-NII bands released, IEEE 802.11 completed, GPRS startedstarted

19981998 IEEE 802.11b and Bluetooth announcementIEEE 802.11b and Bluetooth announcement

19991999 IEEE 802.11a/HIPERLAN-2 startedIEEE 802.11a/HIPERLAN-2 startedHIPERLAN – High Performance Radio LANCDPD – Cellular Digital Packet DataU-NII – Unlicensed National Information InfrastructureGPRS – General Packet Radio Service

Different Generations: 1GDifferent Generations: 1G 1G Wireless Systems: Analog systems1G Wireless Systems: Analog systems

– Use two separate frequency bands for forward Use two separate frequency bands for forward (base station to mobile) and reverse (mobile (base station to mobile) and reverse (mobile to base station) links: Frequency Division to base station) links: Frequency Division Duplex (FDD)Duplex (FDD)

– AMPS: United States (also Australia, southeast AMPS: United States (also Australia, southeast Asia, Africa)Asia, Africa)

– TACS: EU (later, bands were allocated to GSM) TACS: EU (later, bands were allocated to GSM) – NMT-900: EU (also in Africa and southeast NMT-900: EU (also in Africa and southeast

Asia)Asia)

– Typical allocated overall band was 25 MHz in Typical allocated overall band was 25 MHz in each direction; dominant spectra of operation each direction; dominant spectra of operation was 800 and 900 MHz bands.was 800 and 900 MHz bands.

AMPS – Advanced Mobile Phone SystemTACS – Total Access Communication SystemNMT – Nordic Mobile Telephony

2G2G

2G Wireless Systems: Four sectors 2G Wireless Systems: Four sectors

– Digital cellularDigital cellularGSM (EU/Asia): TDMAGSM (EU/Asia): TDMAIS-54 (US): TDMAIS-54 (US): TDMAIS-95 (US/Asia): CDMAIS-95 (US/Asia): CDMA

– PCS – residential applicationsPCS – residential applicationsCT-2 (EU,CA): TDMA/TDDCT-2 (EU,CA): TDMA/TDDDECT(EU):TDMA/TDDDECT(EU):TDMA/TDDPACS (US): TDMA/FDDPACS (US): TDMA/FDD

CT-2 – Cordless Telephone 2DECT – Digital Enhanced Cordless TelephonePACS – Personal Access Communication System

2G (cont’d)2G (cont’d)

2G Wireless Systems: Four sectors 2G Wireless Systems: Four sectors (cont’d)(cont’d)

– Mobile data Mobile data CDPD shares AMPS bands and site infrastructure; CDPD shares AMPS bands and site infrastructure; GPRS shares GSM’s radio system - data rates GPRS shares GSM’s radio system - data rates

suitable for Internetsuitable for Internet– WLAN – Unlicensed bands, free of charge and WLAN – Unlicensed bands, free of charge and

rigorous regulations: very attractive! rigorous regulations: very attractive! IEEE 802.11 and IEEE 802.11b use DSSS physical IEEE 802.11 and IEEE 802.11b use DSSS physical

layer;layer;HIPERLAN/1 uses GMSK; HIPERLAN/1 uses GMSK; IEEE 802.11a and HIPERLAN/2 use OFDM: next IEEE 802.11a and HIPERLAN/2 use OFDM: next

generationgeneration

CDPD – Cellular Digital Packet DataGPRS – General Packet Radio Service DSSS – Direct Sequence Spread SpectrumGMSK – Gaussian Minimum Shift KeyingOFDM – Orthogonal Frequency Division Multiplexing

3G3G

3G3G and Beyond and Beyond

– Purpose: develop an international standard Purpose: develop an international standard that combines and gradually replaces 2G that combines and gradually replaces 2G digital cellular, PCS, and mobile data digital cellular, PCS, and mobile data services, at the same time services, at the same time increasing the increasing the quality of voice, capacity of the network, and quality of voice, capacity of the network, and data rate of the mobile data services.data rate of the mobile data services.

– Radio transmission technology of choice: W-Radio transmission technology of choice: W-CDMACDMA

– 3G was envisioned to 3G was envisioned to provide multimedia provide multimedia services to users everywhereservices to users everywhere

SummarySummary

Relative coverage, mobility, and data rates of generations of cellular systemsand local broadband and ad hoc networks.

Heterogeneous Heterogeneous Cellular NetworksCellular Networks

Seamless mobility across diverse overlay Seamless mobility across diverse overlay networksnetworks

““vertical” hand-offsvertical” hand-offssoftware “agents” for heterogeneity managementsoftware “agents” for heterogeneity managementIP as the common denominator?IP as the common denominator?

High-tier

Low-tier

Satellite

High Mobility Low MobilityWide Area

Regional Area

Local Area

DesignDesign

Cellular Cellular NetworksNetworks

Cellular Network Cellular Network ArchitectureArchitecture

LocationRegister (Database)

Mobile Switching Center

MSC

Backbone Wireline Network

Base Station Controller

Base Station

MobileTerminal

RadioNetwork

Cell

BASIC BASIC ARCHITECTUREARCHITECTURE

BACKBONE TELEPHONE NETWORK

(HLR)

Mobile Switching Center

Visitor Location Register

Mobile Terminal(MT)

MSC

VLR

Local Signaling Long Distance Signaling

(MSC)

(VLR)

Home Location Register

Cellular ConceptCellular Concept The most important factor is the size and the shape of a The most important factor is the size and the shape of a

CELL.CELL. A cell is the radio coverage area by a transmitting station A cell is the radio coverage area by a transmitting station

or a BS.or a BS. Ideally,Ideally, the area covered a by a cell could be represented the area covered a by a cell could be represented

by a by a circular cell with a radius Rcircular cell with a radius R from the center of a BS. from the center of a BS. Many factors may cause reflections and refractions of the Many factors may cause reflections and refractions of the

signals, e.g., elevation of the terrain, presence of a hill or signals, e.g., elevation of the terrain, presence of a hill or a valley or a tall building and presence in the surrounding a valley or a tall building and presence in the surrounding area. area.

The actual shape of the cellThe actual shape of the cell is determined by the received is determined by the received signal strength.signal strength.

Thus, the coverage area may be a little distorted.Thus, the coverage area may be a little distorted. We need an appropriate model of a cell for the analysis We need an appropriate model of a cell for the analysis

and evaluation. and evaluation. Many posible models: Many posible models: HEXAGONHEXAGON,, SQUARE, EQUILATERAL SQUARE, EQUILATERAL

TRIANGLE.TRIANGLE.

Cell

R

(a) Ideal Cell (b) Actual Cell

R

R R

R

(c) Different Cell Models

Cell ShapeCell Shape

Size and Capacity of a Cell per Unit of Area and Size and Capacity of a Cell per Unit of Area and Impact of the Cell Shape on System Impact of the Cell Shape on System CharacteristicsCharacteristics

Cellular Concept - Cellular Concept - ExampleExample

Consider a high-power Consider a high-power transmitter that can transmitter that can support support 3535 voice channels voice channels over an area of over an area of 100 km100 km22 with the available with the available spectrumspectrum

If If 7 7 lower power lower power transmitters are used so transmitters are used so that they support that they support 30% of 30% of the channels over an area the channels over an area of 14.3 kmof 14.3 km2 2 eacheach..

Then a total Then a total 7*(30% * 35) = 7*(30% * 35) = 8080 channels are available channels are available instead of 35. instead of 35.

1

23

45

6

7

Cellular ConceptCellular Concept

If If two cells are far away from enough that two cells are far away from enough that the same set of frequencies can be used the same set of frequencies can be used in both cellsin both cells, it is called , it is called frequency reusefrequency reuse..

With With frequency reusefrequency reuse, a large area can be , a large area can be divided into small areas, each uses a divided into small areas, each uses a subset of frequencies and covers a small subset of frequencies and covers a small area.area.

With With frequency reusefrequency reuse, the system , the system capacitycapacity can be expanded without can be expanded without employing high power transmitters.employing high power transmitters.

Capacity Expansion by Capacity Expansion by Frequency ReuseFrequency Reuse

Same frequency band or channel Same frequency band or channel used in a cell can be “REUSED’ in used in a cell can be “REUSED’ in another cell as long as another cell as long as the cells are the cells are far apart and the signal strength do far apart and the signal strength do not interfere with each othernot interfere with each other..

This enhances the available This enhances the available bandwidth of each cell. bandwidth of each cell.

A group of cells that use a different set of A group of cells that use a different set of frequencies in each cell is called a frequencies in each cell is called a cell cell clustercluster..

NUMBER OF CELLS IN A NUMBER OF CELLS IN A CLUSTERCLUSTER

FREQUENCY REUSEFREQUENCY REUSEExample:Example: A typical cluster of 7 such cells and 4 A typical cluster of 7 such cells and 4 such clusters with no overlapping area such clusters with no overlapping area

F1

F2

F3F4

F5

F6

F7

F1

F2

F3F4

F5

F6

F7

F1

F2

F3F4

F5

F6

F7

F1

F2

F3F4

F5

F6

F7

|------||D|| ----------

FREQUENCY REUSE DISTANCE D

RULE to Determine RULE to Determine the Nearest Co-the Nearest Co-Channel Neighbors (with the same Channel Neighbors (with the same frequency set): frequency set): Determining the Cluster Determining the Cluster SizeSize

To find nearest co-channel To find nearest co-channel neighbors of a particular cellneighbors of a particular cell

Step 1: Move i cells along Step 1: Move i cells along any chain of hexagons;any chain of hexagons;

Step 2: Turn 60 degrees Step 2: Turn 60 degrees counterclockwise and counterclockwise and move j cells AND REACH move j cells AND REACH the co-channel.the co-channel.

i and j measure the i and j measure the number of nearest number of nearest neighbors neighbors between co-between co-channel cells channel cells

The cluster size, N,The cluster size, N, N = iN = i22+ij+j+ij+j22

i

j

If i =2 and j = 0, then N = 4If i = 2 and j = 1, then N =7

1

2

3

3

42

1

1 2

Frequency ReuseFrequency Reuse

The distance between 2 cells using the The distance between 2 cells using the same channel is known as the same channel is known as the REUSE REUSE DISTANCE DDISTANCE D..

There is a close relationship between D, There is a close relationship between D, R R (radius of each cell)(radius of each cell) and and N (the number of N (the number of cells in a cluster) cells in a cluster) -- details later-- details later

D = (sqrt 3N) . RD = (sqrt 3N) . R The REUSE FACTOR is thenThe REUSE FACTOR is then

D/R = sqrt (3N)D/R = sqrt (3N)

Frequency ReuseFrequency Reuse

Let Let N be the N be the cluster sizecluster size in terms of in terms of number of cells within it and number of cells within it and K be the K be the total number of available channelstotal number of available channels without frequency reuse.without frequency reuse.

N cells in the cluster would then N cells in the cluster would then utilize all K available channels.utilize all K available channels.

Each cell in the cluster then uses 1/N-Each cell in the cluster then uses 1/N-th of the total available channels.th of the total available channels.

NN is also referred as the is also referred as the frequency frequency reuse factorreuse factor of the cellular system. of the cellular system.

Capacity Expansion by Capacity Expansion by Frequency ReuseFrequency Reuse

Assume Assume each cell is allocated each cell is allocated JJ channels channels ((J<=KJ<=K). If the ). If the KK channels are divided among channels are divided among the the NN cells cells into unique and disjoint channel into unique and disjoint channel groups, each with groups, each with JJ channels, then channels, then K = J NK = J N

TheThe N N cells in a cluster use the complete set of cells in a cluster use the complete set of available frequenciesavailable frequencies..

The cluster can be replicated many times. The cluster can be replicated many times. LetLet M M be the number of replicated clustersbe the number of replicated clusters and and

C C be the total number of channelsbe the total number of channels in the entire in the entire system with frequency reuse, then system with frequency reuse, then C is the C is the system capacitysystem capacity and computed by and computed by

C = M J NC = M J N

Cellular System Cellular System Capacity - ExampleCapacity - Example

Suppose there are K=1001 radio channels, and each cell is Acell = 6 km2 and the entire system covers an area of Asys = 2100km2.

1. Calculate the system capacity if the cluster size is N=7. 2. How many times would the cluster of size 4 have to be

replicated in order to approximately cover the entire cellular area?

3. Calculate the system capacity if the cluster size is 4.4. Does decreasing the cluster size increase the system capacity?

Solution:1. J=K/N=143, Acluster=N*6=42km2, M (# of clusters)=2100/42=50, C=MK=50,050 chs.2. N=4, Ac=4*6=24km2, M=2100/24=87.

3. N=4, J = 1001/4 = 250 chs/cell. C = 87 * K= 87,000 chs.

4. Decrease in N from 7 to 4 increase in C from 50,050 to 87,000. Decreasing the cluster size increases system capacity. So the answer is YES!

Geometry of Hexagonal Cells Geometry of Hexagonal Cells (1)(1)How to determine the DISTANCEHow to determine the DISTANCEbetween the nearest co-channel cells ?between the nearest co-channel cells ?

Planning for Co-channel cellsPlanning for Co-channel cells D is the distance to the center of the nearest co-channel cellD is the distance to the center of the nearest co-channel cell R is the radius of a cellR is the radius of a cell

R

30o

R3

R3i

j

D

0

Geometry of Hexagonal Cells Geometry of Hexagonal Cells (2)(2)

Let Let DDnormnorm be the distance from the center of a candidate cell to the center of a be the distance from the center of a candidate cell to the center of a nearest co-channel cell, nearest co-channel cell, ““normalized”normalized” with respect to the distance between the with respect to the distance between the centers of two adjacent cellscenters of two adjacent cells, ., .

Note that Note that the normalized distance between two adjacent cells either with (i=1,j=0) or (i=0,j=1) is 1.

Let D be the “actual” distance between the centers of two adjacent co-channel cells. D = Dnorm .R3

R3


From the geometry we have

Nijji

jijD oonorm

22

2222 ))30sin(()30(cos

NDnorm

From N and Dnorm equations


RNRDD norm 33

The actual distance between the center of the candidate cell and the center of a nearest co-channel is then:

For hexagonal cells there are 6 nearest co-channel neighbors to each cell (if cluster size = 7). Co-channel cells are located in tiers.In general, a candidate cell is surrounded by 6k cells in tier k.For cells with the same size the co-channel cells in each tier lie on the boundary of the hexagon that chains all the co-channel cells in that tier.


As D is the radius between two nearest co-channel cells, the radius of the hexagon chaining the co-channel cells in the k-th tier is given by k.D.

For the frequency reuse pattern with i=2 and j=1 so that N=7, the first two tiers of co-channel cells are given in Figure (next slide).

It can be readily observed from Figure that the radius of the first tier is D and the radius of the second tier is 2.D.

Calculate Number of Cells in A Calculate Number of Cells in A ClusterCluster

A candidate cell has 6 nearest co-A candidate cell has 6 nearest co-channel cells. Each of them in turn channel cells. Each of them in turn has 6 neighboring co-channel cells. has 6 neighboring co-channel cells. So we can have So we can have a large hexagona large hexagon..

This large hexagon has radius This large hexagon has radius equal to D which is also the co-equal to D which is also the co-channel cell separation.channel cell separation.

The area of a hexagon is The area of a hexagon is proportional to the square of its proportional to the square of its radiusradius, (let , (let =2.598), =2.598),

D

R

])(3[ 2222arg

2

RjijiDA

RA

eL

Small

RNRDD norm 33

Calculate Number of Calculate Number of Cells in A ClusterCells in A Cluster

The number of cells in the large hexagon is thenThe number of cells in the large hexagon is then

)(3 22arg jijiSmall

eL

A

A

In general the large hexagon encloses the center cluster of N cells plus 1/3 the number of the cells associated with 6 other peripheral large hexagons. Hence, the total number of cells enclosed by the large hexagon is

)(

_,

,3)(6

22

31

jijiN

getweThus

NNN

Thus we proved N = f(i,j) mentioned before

Frequency Reuse Frequency Reuse RatioRatio

The The frequency reuse ratiofrequency reuse ratio, , qq, is defined as, is defined asq = D/Rq = D/R

which is also referred to as the which is also referred to as the co-channel co-channel reuse ratioreuse ratio..

Thus Thus q = D/R = sqrt(3N) q = D/R = sqrt(3N)

TradeoffTradeoff– qq increases with increases with NN..– However, However, a smaller value of a smaller value of NN has the effect of has the effect of

increasing the capacity of the cellular system increasing the capacity of the cellular system – But Smaller N can increase co-channel interferenceBut Smaller N can increase co-channel interference– Tradeoff on Tradeoff on NN

RNRDD norm 33

We assume the size of all the cells is roughly the same, as long as the cell size is fixed, We assume the size of all the cells is roughly the same, as long as the cell size is fixed, co-channel co-channel interferenceinterference will be independent of transmitted power of each cell. will be independent of transmitted power of each cell.

The The co-channel interference will become a function of qco-channel interference will become a function of q where q = D/R = sqrt (3N). where q = D/R = sqrt (3N).(q is the CO-CHANNEL REUSE RATIO and is related to the cluster size).(q is the CO-CHANNEL REUSE RATIO and is related to the cluster size).

A small value of q provides larger capacity since N is small.A small value of q provides larger capacity since N is small.

For large q, the transmission quality is better, smaller level of co-channel interferenceFor large q, the transmission quality is better, smaller level of co-channel interference..

By By increasing the ratio of D/Rincreasing the ratio of D/R,, spatial separation between co-channel cells relative to the coverage spatial separation between co-channel cells relative to the coverage distance of a cell is increased.Thus, distance of a cell is increased.Thus, interference is reducedinterference is reduced from improved isolation of RF energy from from improved isolation of RF energy from the number of cells per cluster N co-channel cells.the number of cells per cluster N co-channel cells.


Furthermore, D (distance to the center of the nearest Furthermore, D (distance to the center of the nearest cochannel cell) is a function of cochannel cell) is a function of NNII and and S/IS/I (next lecture) (next lecture)

in which in which NNII is the number of co-channel interfering cells in is the number of co-channel interfering cells in the first tierthe first tier and and S/IS/I = received signal to interference = received signal to interference ratio at the desired mobile receiver.ratio at the desired mobile receiver.

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Week 13 lecture 1 Cellular Communications (I)

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