Chapter1 Part1 FoundationOfCellularCommunications V1 ForProfessors

Foundation of Cellular

CommunicationsDr. Nishith Tripathi

Dr. Jeffrey Reed

This presentation is based on Chapter 1 of our book, “Cellular Communications- A Comprehensive and Practical Guide” published by Wiley/IEEE.Visit our website, “The Wireless University,” for additional information: http://www.thewirelessuniversity.com.

http://www.thewirelessuniversity.com/

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ObjectivesAt the end of this module, you will be able to Give examples of communications

milestones Illustrate the cellular network architecture List examples of frequency bands used on

the air interface Describe the concept of frequency reuse Discuss how duplexing enables

simultaneous transmission and reception Summarize how multiple access techniques

enable users to share the same air interface Distinguish between the types of

interference

© 2015 by Tripathi, Reed, and IEEE/Wiley.

Motivationand

History

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Cellular Communications:

Motivation Look around- who does not have a cell phone? (!)

Mobile communications using a terrestrial network without any wires connected to the user device

Integral part of our lives: Phone calls, Short Message Service (SMS), emails, news, weather reports and warnings, photos, videos, and the list goes on…

Cell Phone: Ranked 2nd in the list of inventions that have changed the world (Internet: 1st)

© 2015 by Tripathi, Reed, and IEEE/Wiley. Book Reading: Section 1.1

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Selected Milestones- I


1876 The telephone is invented.1887 Hertz showed that electromagnetic waves exist.1895 Marconi proves the feasibility of radio communications.1947 The cellular concept is born at Bell Labs. 1958 Jack Kilby invents an integrated circuit.1972 Bell Labs patents its mobile communications system.1973 Motorola introduces a small mobile phone, DynaTAC.1974 Vinton Cerf & Robert Kahn use the term Internet for the first time.1977 Cellular systems are launched in Chicago, Washington, D.C., and Baltimore1979 A microwave telephone system is installed in Saudi Arabia.

The first commercial cellular system is installed by NTT in Tokyo.1983 Advanced Mobile Phone System (AMPS), the world’s first standardized & fully automated commercial cellular service, is deployed in Chicago.

YEAR EVENT

Book Reading: Section 1.2

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Selected Milestones- II Just ask those around you, “Who invented the

telephone?” More than likely, you will get the answer

“Alexander Graham Bell.” Well, Alexander Graham Bell and Elisha Gray

independently invented the telephone in 1876. Bell was awarded priority because he filed for a

patent a few hours earlier than Gray. What a difference a few hours can make…we all

remember Bell but only some of us would remember Gray.

Next time you have an idea, run (and do not walk!) and file the patent at the speed of light!!!

The Value of Time: A Lesson from the Communications History


CellularNetwork

Architecture

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Generic Network Architecture

© 2015 by Tripathi, Reed, and IEEE/Wiley. Book Reading: Section 1.3.1

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Air Interface between Device and Network

The mobile device and the radio network use the radio frequency (RF) spectrum for wireless communications.

© 2015 by Tripathi, Reed, and IEEE/Wiley. Book Reading: Sections 1.2.2 and 1.3.2

Frequency Reuse

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Heart of Cellular Communications: Frequency Reuse Cellular communications reuse the same frequency

in multiple cells, which enables the cellular network to serve numerous users simultaneously using scarce and precious RF spectrum.


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“A Hexagonal Cell” in Cellular Communications

Bell Labs considered three shapes for geometric representation of the cell: triangle, square, and hexagon.

The hexagon shape was eventually chosen because it allows the maximum number of mobiles to be connected to the Base Station (BS) in the center at the farthest distance from the BS.

The hexagon shape closely mimics the circular signal energy pattern radiating from the BS using an omnidirectional antenna.

The choice of the circular shape would have led to either significant overlap (and hence more cells to cover a given area) or areas with no coverage.

The hexagonal shape helps achieve an efficient layout for system deployment and analysis.


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Cells and Clusters


Cluster: A set of cells covering a geographic area that uses the target amount of RF spectrum.Different clusters use the same RF spectrum.

Different cells in a given cluster use different parts of the target RF spectrum.

Book Reading: Section 1.3.3

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Classification of Cells

Example classification of cells is shown below, but there are no strict or universally-accepted definitions.


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Frequency Reuse Factor & Cluster Size

Cluster size (K): Number of cells in the cluster Frequency Reuse Factor (FRF): Inverse of the

cluster size (indicates how frequently we use a given amount of spectrum)

Early deployments of second-generation (2G) cellular system called GSM used K=7.

Deployments of 3G UMTS and 4G LTE use K=1. The case of K=FRF=1 is termed “universal

frequency reuse,” where every cell in the system uses the same RF spectrum.

A higher FRF implies that more users can be supported due to availability of more radio resources but the amount of interference would be high compared to the case of a lower FRF.


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Desired Signal vs. Interference Carrier-to-Interference power (C/I) ratio

quantifies the relative strengths of desired signal and interference.


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(C/I) Ratio and Cluster Size (K)

S is the signal power measured at the MS, Rk is the co-channel interference

measured at the MS from the co-channel BS k, Nco−ch is the total number of co-channel BSs, K is the cluster size, and n is the propagation path loss exponent. Different technologies operate satisfactorily at different (C/I)

levels. For K=7 and n=4, (C/I) is 73.5 or 18.66 dB, which is more than

adequate for systems such as GSM. Technological advancements later enabled GSM to operate well for K=4 too.


See Pages 18 & 19 of the book for details.

Six first-tier of co-channel BSs


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Sectorization

Focusing of energy in a limited geographic area as opposed to all directions (“Omni-directional”)


You are looking at the most commonly deployed network configuration on the PLANET!


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Sectorization and (C/I) Sectorization reduces interference for

systems with FRF < 1 (i.e., Cluster size K > 1)


No Sectorization 120° Sectorization


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Impact of Sectorization Sectorization has different impacts on different

multiple access technologies. Sectorization reduces interference for TDMA

systems and improves the quality of communications. If the interference reduction is significant to allow higher FRF (and smaller cluster size), sectorization increases capacity of TDMA systems.

Sectorization increases both interference and capacity for FRF=1 systems such as CDMA systems due to more frequent reuse of the spectrum compared to an omnidirectional CDMA system. For example, resources are used 3 times in 3 sectors of a cell with 120° sectorization instead of just 1 time in an omnidirectional cell.


TDMA: Time Division Multiple Access CDMA: Code Division Multiple Access


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Review Exercise- IMatch the column entries (1-to-1 Mapping)

Column I Column II

1. Existence of electromagnetic waves

2. Feasibility of radio communications

3. Telephone

4. Cellular Concept

5. Internet

A. Bell Labs

B. Hertz

C. Bell (and Gray)

D. Marconi

E. Vinton Cerf & Robert Kahn


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Answers for Review Exercise- I

Match the column entries (1-to-1 Mapping)

Column I Column II

1. Existence of electromagnetic waves

2. Feasibility of radio communications

3. Telephone

4. Cellular Concept

5. Internet

A. Bell Labs

B. Hertz

C. Bell (and Gray)

D. Marconi

E. Vinton Cerf & Robert Kahn


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Review Exercise- II1. Explain the concept of frequency reuse.2. True or False? Two different clusters use the

same RF spectrum. 3. True or False? Two cells in a given cluster use

different RF spectrum.4. Assume cluster size of K=7. The Frequency

Reuse Factor (FRF) = _______.5. Assume the Frequency Reuse Factor (FRF) = ¼.

Calculate (C/I) in dB if the propagation path loss exponent is 3.5 and the number of first-tier co-channel BSs is 6.


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Answers to Review Exercise- II

1. Frequency reuse: reuse the same RF spectrum in different geographic areas.

2. True. Two different clusters use the same RF spectrum.

3. True. Two cells in a given cluster use different RF spectrum.

4. Assume cluster size of K=7. The Frequency Reuse Factor (FRF) = 1/K=1/7.

5. Assume the Frequency Reuse Factor (FRF) = ¼. K= 1/FRF=4. (C/I)= ( )n/6= ( )3.5/6 =12.9 or 10*log10(12.9)= 11.1 dB.


Duplexing

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Duplexing Duplexing: “Simultaneous” transmission and

reception of signals on two communication links (uplink and downlink)

Two methods Frequency Division Duplex/Duplexing (FDD) Time Division Duplex/Duplexing (TDD)

Majority of cellular deployments use FDD.


Downlink (Forward Link)

Uplink(Reverse Link)


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FDD vs. TDD


Tx: TransmissionRx: Reception

FDD Benefits+: Easier network deployment+: Typical spectrum is paired (both links)

TDD Benefits+: Simpler design of transceiver (fewer components)+: Cheaper spectrum (less operator interest)


Multiple Access

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Multiple Access Multiple Access: Ability of multiple users to

receive services simultaneously in a given cell or sector

Three Generic Methods Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA)

FDMA is used in first-generation cellular systems TDMA is used in 2G GSM CDMA is used in 2G IS-95 and 3G UMTS A variant of FDMA, Orthogonal Frequency

Division Multiple Access (OFDMA) , is used in LTE. OFDMA is covered in Chapter 15.


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FDMA


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TDMA


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CDMA


Interference

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Interference in Cellular Networks The receiver’s job of properly detecting the

desired signal is hampered by thermal noise and interference.

Two main types of interference Adjacent Channel Interference (ACI) Co-channel Interference (CCI)


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Adjacent Channel Interference (ACI)

ACI is addressed by means such as suitable radio channel assignments, filters, and transmit power management.© 2015 by Tripathi, Reed, and IEEE/Wiley. Book Reading: Section 1.5

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Co-channel Interference (CCI)

CCI is addressed by means such as separation of the co-channel cells (i.e., large enough cluster size K) and transmit power management.


Summary

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Points to Remember- I Examples of communications milestones include

invention of the telephone by Bell (and Gray!), discovery of the existence of electromagnetic signals by Hertz, the use of radio signals by Marconi, and the development of the cellular concept at Bell Labs.

The cellular network includes the radio access network, the core network(s), and the services network. The radio network has nodes such as BSs and BSC/RNCs and communicates with the mobile devices over the air interface. The core networks interface with the PSTN and the Internet to enable the consumer to access these networks.

Popular frequency bands for cellular communications in the U.S. include 700 MHz, 850 MHz cellular band, 1900 MHz PCS band, and 1.7 GHz/2.1 GHz AWS band.


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Points to Remember- II Frequency reuse across the geographical

regions is at the heart of cellular communications.

FDD and TDD are main duplexing methods, which support simultaneous (bi-directional) transmission and reception

FDMA, TDMA, CDMA, OFDMA multiple access techniques that enable multiple users to share the same air interface simultaneously

ACI occurs between the communication links that use adjacent radio channels, and CCI occurs between the communication links that are using the same radio channel.


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Review Exercise- IIIFill in the blanks.1) The _______ {radio access network, core network,

services network} directly communicates with the mobile device over the air interface.

2) _____________ {Duplexing, Multiple access, Frequency reuse} involves the use of the same RF spectrum in different geographic areas.

3) ___________ {Duplexing, Multiple access, Sectorization} facilitates simultaneous (i.e., bi-directional) transmission and reception using time or frequency domain.

4) _________ {Duplexing, Multiple access, Sectorization} enables multiple users to share the same RF spectrum.

5) _________ {Duplexing, Multiple access, Sectorization} focuses energy in a narrow geographic area, leading to higher quality and/or capacity.


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Answers to Review Exercise- IIIFill in the blanks.

1) The radio access network directly communicates with the mobile device over the air interface.

2) Frequency reuse involves the use of the same RF spectrum in different geographic areas.

3) Duplexing facilitates simultaneous (i.e., bi-directional) transmission and reception using time or frequency domain.

4) Multiple access enables multiple users to share the same RF spectrum.

5) Sectorization focuses energy in a narrow geographic area, leading to higher quality and/or capacity.


Chapter1 Part1 FoundationOfCellularCommunications V1 ForProfessors

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