s8 Cellular

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Ch 8. Cellular Networks

Myungchul Kim

[email protected]


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Cellular Networks Overview

1G Analog Cellular

2G TDMA - GSM 2G CDMA - IS-95

2.5G

3G 4G and Beyond

Cellular Engineering Issues


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1G(


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Cellular networks: From 1G to 3G

1G: First generation wireless cellular: Early 1980s Analog transmission, primarily speech: AMPS (Advanced Mobile

Phone Systems) and others

2G: Second generation wireless cellular: Late 1980s

Digital transmission Primarily speech and low bit-rate data (9.6 Kbps)

High-tier: GSM, IS-95 (CDMA), etc

Low-tier (PCS): Low-cost, low-power, low-mobility e.g. PACS

2.5G: 2G evolved to medium rate (< 100kbps) data

3G: future Broadband multimedia 144 kbps - 384 kbps for high-mobility, high coverage

2 Mbps for low-mobility and low coverage

Beyond 3G: research in 4G


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Frequency allocation

LicensedMany providers

Multiple Access

Many users

Wide area of coverageTraffic management

Location management

High mobility (in cars, trains)

Multiple suppliers

Handoff management, roaming

General principles

Handled differently by different generations

Issues Vital to cellular

F All i


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Frequency Allocation

Cellular networks: Mostly around 900 MHz 2GHz

100 Km 3KHzVLF - very low frequency

10 Km 30KhzLF - low frequency

1 Km 300KHzMF - medium frequency100 m

3MHzHF - high frequency

10 m 30 MHzVHF - very high frequency

1m 300 MHzUHF - ultra high frequency

100 mm 3GHz10 mm 30GHz

SHF - super high frequency

1 mm 300 GHz

EHF - extra high frequency

0.1 m 3000 GHz

THF - terribly high frequency

Source: Bekkers, R. and Smits, J., Mobile Telecommunications, Artech, 2000.

FrequencyWavelength

Radio

Waves

Micro

Waves

Infrared

X-rays

Gamma-rays


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Multiple Access Techniques: How to allocate users

Time

Frequency Session1

Session2

Session3

Session4

Frequency Division

Multiple Access (FDMA)

1G Cellular (AMPS)

Time

Frequency

Time Division

Multiple Access (TDMA)

2G TDMA

3G TDMA

Se

ssion2

Session3

Session1

Session4

Time

Fre

quency

Code DivisionMultiple Access (CDMA)

All sessions

based on a

code

2G CDMA (IS-95)3G CDMA


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A Cellular Network

Public

Switched

Telephone

Network(PSTN)

Mobile

Telephone

Switching

Center(MTSC)

Base Transceiver Station (BTS)Mobile User

Cell 1

Cell 2

Cordless connection

Wired connection

HLR VLR

HLR = Home Location Register

VLR = Visitor Location Register


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Overview of Location Services Cell-id based location.

assigned an id of the cell that you are in. cell-id is stored in a database.

As you move from one cell to another, you are assigned adifferent cell-id and the location database is updated.

most commonly used in cellular networks. (HLR, VLR)

Neighborhood polling: Connected mobile units only move toadjacent cells

Angle of arrival (AOA). the angle at which radio waves from yourdevice "attack" an antenna is used to calculate the location ofthe device.

Time taken. In this case, the time taken between the device andthe antenna is used to calculate the location of the device.

Network assisted Global Positioning System (GPS). a GPS chipis installed inside a phone and thus the location of the user is

tracked

.


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Mobile telecommunications switching

office (MTSO)

Essentially an end office to connect calls betweenmobile units

Several base stations connected to an MTSO

In a large system, many MTSOs may beconnected to a second level MTSO and so on

MTSO connected to BSs, PSTN and each otherthrough packet switching (ATM)

Two types of channels available between mobileunit and BS Control channelsused to exchange information

having to do with setting up and maintaining calls

Traffic channelscarry voice or data connectionbetween users


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Cellular System

Handoffs (typically 30 mseconds):

1. At any time, mobile station (MS) is in one cell and under the control of a BS2. When a MS leaves a cell, BS notices weak signal

3. BS asks surrounding BSs if they are getting a stronger signal

4. BS transfers ownership to one with strongest signal

5. MTSO assigns new channel to the MS and notifies MS of new boss

Public

Switched

Telephone

Network(PSTN)

Mobile

Telephone

Switching

Center(MTSC)

Cell 1

Cell 2

HLR VLR

0G Wi l


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0G Wireless Mobile radio telephones were used for military

communications in early 20th century

Car-based telephones first introduced in mid 1940s

Single large transmitter on top of a tall building

Single channel used for sending and receiving

To talk, user pushed a button, enabled transmission anddisabled reception

Became known as push-to-talk in 1950s

CB-radio, taxis, police cars use this technology

IMTS (Improved Mobile Telephone System)introduced in 1960s

Used two channels (one for sending, one for receiving)

No need for push-to-talk

Used 23 channels from 150 MHz to 450 MHz


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First-Generation Cellular

Advanced Mobile Phone Service (AMPS) invented at

Bell Labs and first installed in 1982

Used in England (called TACS) and Japan (called

MCS-L1)

Key ideas: Exclusively analog

Geographical area divided into cells (typically 10-25km)

Cells are small: Frequency reuse exploited in nearby (not

adjacent) cells

As compared to IMTS, could use 5 to 10 times more users in

same area by using frequency re-use (divide area into cells)

Smaller cells also required less powerful, cheaper,smaller

devices


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E

A

DF

G C

B

E

A

DF

G C

B

E

A

DF

G C

B

Cell Design

Cells grouped into a cluster of seven

Letters indicate frequency use

For each frequency, a buffer of two cells is used before reuse

To add more users, smaller cells (microcells) are usedFrequencies may not need to be different in CDMA (soft handoff)


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Cellular Network Organization Cell design (around 10 mile radius)

Served by base station consisting of transmitter,receiver, and control unit

Base station (BS) antenna is placed in highplaces (churches, high rise buildings) -

Operators pay around $500 per month for BS 10 to 50 frequencies assigned to each cell

Cells set up such that antennas of all neighbors areequidistant (hexagonal pattern)

In North America, two 25-MHz bands allocated toAMPS One for transmission from base to mobile unit

One for transmission from mobile unit to base


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Approaches to Increase Capacity

Adding/reassigning channels - some channelsare not used

Frequency borrowingfrequencies are taken

from adjacent cells by congested cells Cell splittingcells in areas of high usage

can be split into smaller cells

Microcellsantennas move to buildings,hills, and lamp posts


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AMPS Operation Each phone has a 32 bit serial no and 10 digit phone no in

its PROM

When a phone is turned on, it scans for control signalsfrom BSs

It sends this info to BS with strongest control signal -passed to MTSO

Subscriber initiates call by keying in phone number andpresses send key

MTSO verifies number and authorizes user

MTSO issues message to users cell phone indicating send

and receive traffic channels MTSO sends ringing signal to called party

Party answers; MTSO establishes circuit and initiatesbilling information

Either party hangs up; MTSO releases circuit, freeschannels, completes billing


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Security Issues with 1G

Analog cellular phones are insecure

Anyone with an all band radio receiver can listen in(many scandals)

Theft of airtime:

all band radio receiver connected to a computercan record 32 bit serial number and phone number

of subscribers when calling

can collect a large database by driving around

Thieves go into business - reprogram stolenphones and resell them

C ll l Di i l P k D (CDPD)


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Cellular Digital Packet Data (CDPD) Developed by IBM, mostly used in North America

Packet switching built on top of AMPS

Sends IP packets over cellular phones

CDPD base stations are connected to IP routers

Same spectrum and physical infrastructure as

analog cellular Use available cellular capacity Sniffing to find idle channels

Hops among available channels

Voice always higher priority

Share cellular infrastructure Frequencies, Towers and antennas

Raw bit rate: 19.2 kbs (actually closer to9.6kbps)

Forward error correction and encryption


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CDPD OperationInterne

t

CDPD

Provider

Network

Landline Modem

CDPD Modem

Router

Router

= base interface station

(special unit that connects all base stationsin CDPD rovider network to routers


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Second Generation Cellular

Based on digital transmission

Different approaches in US and Europe

US: divergence Only one player (AMPS) in 1G

Became several players in 2G due to competition Survivors

IS-54 and IS-135: backward compatible with AMPS frequencyallocation (dual mode - analog and digital)

IS-95: uses spread spectrum

Europe: Convergence 5 incompatible 1G systems (no clear winner)

European PTT development of GSM (uses new

frequency and completely digital communication)


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Advantages of Digital

Communications for Wireless Voice, data and fax can be integrated into a

single system

Better compression can lead to betterchannel utilization

Error correction codes can be used for better

quality

Sophisticated encryption can be used


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Differences Between First and

Second Generation Systems Digital traffic channelsfirst-generation systems

are almost purely analog; second-generationsystems are digital

Encryptionall second generation systemsprovide encryption to prevent eavesdropping

Error detection and correctionsecond-generation

digital traffic allows for detection and correction,giving clear voice reception

Channel accesssecond-generation systems allowchannels to be dynamically shared by a number of

users


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Integrating Data Over Cellular

Direct access to digital channel Voice and data using one handset

PCS 1900 (GSM-1900)

9.6 kbps circuit switched data

14.4 kbps under definition

Packet mode specified

Short message service

IS-95-based CDMA 13 kbps circuit switched data

Packet mode specified

Short message service


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GSM (Global System for Mobile Communications) Completely designed from scratch (no backward

compatability) Uses 124 channels per cell, each channel can

support 8 users through TDM (992 users max)

Some channels used for control signals, etc

Several flavors based on frequency:

GSM (900 MHz)

GSM 1800 (called DCS 1800)

GSM 1900 (called DCS 1900) - used in North America GSM 1900 phone only works in North America.

In Europe, you can transfer your SIM (Subscriber

Identity Module) card to a phone of the correct

frequency. This is called SIM-roaming.


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GSM (2G-TDMA) Circuit mode data

Transparent modeNon-transparent mode using radio link protocol

Data rate up to 9.6kb/s

Short message serviceLimited to 160 characters

Packet mode data: Plans for GSM Phase 2+

Architecture specification very detailed(500 pages)

Defines several interfaces for multiple

suppliers


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Mobile Station and Base Station Subsystem (BSS)

Mobile station

Mobile station communicates across Um interface (airinterface) with base station transceiver in same cell asmobile unit

Mobile equipment (ME)physical terminal, such as atelephone or PCS

ME includes radio transceiver, digital signal processors andsubscriber identity module (SIM)

GSM subscriber units are generic until SIM is inserted SIMs roam, not necessarily the subscriber devices

BSS BSS consists of base station controller and one or more

base transceiver stations (BTS)

BSC reserves radio frequencies, manages handoff ofmobile unit from one cell to another within BSS, andcontrols paging


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Network Subsystem CenterMobile Switching Center (MSC) is at core; consists

of several databases Home location register (HLR) databasestores

information about each subscriber that belongs toit

Visitor location register (VLR) databasemaintains information about subscribers currently

physically in the region

Authentication center database (AuC)used forauthentication activities, holds encryption keys

Equipment identity register database (EIR)keeps track of the type of equipment that exists atthe mobile station

GSM L i S i


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GSM Location Services

PublicSwitched

Telephone

Network

(PSTN)

Gateway

MTSC

VLR HLR

Terminating

MSC1

1. Call made to mobile unit (cellular phone)

2. Telephone network recognizes number

and gives to gateway MSC

3. MSC cant route further, interrogates

users HLR4. Interrogates VLR currently serving user

(roaming number request)

5. Routing number returned to HLR and

then to gateway MSC

2

34

5

5

6

6. Call routed to terminating MSC

7. MSC asks VLR to correlate call to

the subscriber

8. VLR complies

9. Mobile unit is paged10. Mobile unit responds, MSCs convey

information back to telephone

7 8

9

BTS

9 10

10 10 10

10

Legend: MTSC= Mobile Telephone Service Center, BTS = Base Transceiver StationHLR=Home Location Register, VLR=Visiting Location Register

GSM Protocol Architect re


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GSM Protocol Architecture

BSSMAP = BSS Mobile Application part

BTSM = BTS management

CM = Connection Management

LAPD = Link Access Protocol, D Channel

Base Transceiver

Station

Mobile

Station

Radio

LAPDm

RRM

RadioLAPDm

RRM

MM

CM

64 KbpsLAPD

BTSM

64 Kbps

MTP

SCCP

Base Station

Controller

64 KbpsLAPD

BTSM

BSSMAP

64Kbps

MTP

SCCP

MM

CM

BSSMAP

Mobile Service

Switching Center

MM = Mobility Management

MTP = Message Transfer Part

RRM = Radio Resources Management

SCCP = Signal Connection Control Point


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Functions Provided by Protocols

Protocols above the link layer of the GSMsignaling protocol architecture providespecific functions:

Radio resource management: controls setup,termination and handoffs of radio channels

Mobility management: location and security(MTSO)

Connection management: connects end usersMobile application part (MAP): between

HLR,VLR

BTS management: management base system


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2G CDMA Cellular

IS-95 is the best known example of 2G withCDMA

Advantages of CDMA for Cellular

Frequency diversityfrequency-dependent

transmission impairments have less effect onsignal

Multipath resistancechipping codes used forCDMA exhibit low cross correlation and low

autocorrelation Privacyprivacy is inherent since spread

spectrum is obtained by use of noise-like signals

Graceful degradationsystem only gradually

degrades as more users access the system


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Drawbacks of CDMA Cellular Self-jammingarriving transmissions from

multiple users not aligned on chip boundariesunless users are perfectly synchronized

Near-far problemsignals closer to the receiverare received with less attenuation than signalsfarther away

Soft handoffrequires that the mobile acquires

the new cell before it relinquishes the old; this ismore complex than hard handoff used in FDMAand TDMA schemes


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Types of Channels Supported by

Forward Link Pilot (channel 0) - allows the mobile unit to

acquire timing information, provides phasereference and provides means for signal strength

comparison

Synchronization (channel 32) - used by mobilestation to obtain identification information aboutcellular system

Paging (channels 1 to 7) - contain messages forone or more mobile stations

Traffic (channels 8 to 31 and 33 to 63)the

forward channel supports 55 traffic channels


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Forward Traffic Channel Processing Steps

Speech is encoded at a rate of 8550 bps

Additional bits added for error detection

Data transmitted in 2-ms blocks with forward error

correction provided by a convolutional encoder

Data interleaved in blocks to reduce effects of errors

Data bits are scrambled, serving as a privacy mask

Power control information inserted into traffic channel

DS-SS function spreads the 19.2 kbps to a rate of 1.2288

Mbps using one row of 64 x 64 Walsh matrix Digital bit stream modulated onto the carrier using QPSK

modulation scheme

Wi l N t k E l ti t 3rd G ti


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Wireless Network Evolution to 3rd GenerationEnabling Technologies

AMPS

GSM

IS-95

GPRS

CDMA-2000

1XRTT

EDGE

CDMA2000

3XRTT

(UMTS)

2.5G

3G

2G

2 Mbps

500 kbps

150 Kbps

100 Kbps

50 Kbps

10 Kbps

1999 2000 2001 2002 2003

TDMA Migration

1G-2G Migration

CDMA Migration

1980

1G

1 Kbps

W-CDMA

(UMTS)


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Fig 8-13


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Table 8-3


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Alternatives to 3G Cellular

Major technical undertaking with many

organizational and marketing overtones.

Questions about the need for the additionalinvestment for 3G (happy with 2.5G)

Wireless LAN in public places such as shoppingmalls and airports offer options

Other high-speed wireless-data solutions competewith 3G Mobitex low data rates (nominally 8 Kbps), it uses a narrowband

(2.5KHz) as compared to 30 KHz (GSM) and 5 MHz (3G).

Ricochet: 40 -128 kbps data rates. Bankruptcy

Flash-OFDM: 1.5 Mbps (upto 3 Mbps)


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IEEE 802.11 vs 3G Cellular


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4G Systems

Wireless networks with cellular data rates of 20Mbits/second and beyond.

AT&T has began a two-phase upgrade of its wireless

network on the way to 4G Access.

Nortel developing developing features for Internetprotocol-based 4G networks

Alcatel, Ericsson, Nokia and Siemens found a new

Wireless World Research Forum (WWRF) for research on

wireless communications beyond 3G. Many new technologies and techniques (multiplexing,

intelligent antennas, digital signal processing)

Industry response is mixed (some very critical)


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Engineering Issues

Steps in MTSO controlled call

TDMA design

CDMA design

Handoff

Power control

Traffic engineering


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Mobile Wireless TDMA Design

Considerations Number of logical channels (number of time slots

in TDMA frame): 8

Maximum cell radius (R): 35 km

Frequency: region around 900 MHz

Maximum vehicle speed (Vm):250 km/hr

Maximum coding delay: approx. 20 ms

Maximum delay spread (m): 10 s Bandwidth: Not to exceed 200 kHz (25 kHz per

channel)


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Mobile Wireless CDMA Design

Considerations Soft Handoffmobile station temporarily

connected to more than one base stationsimultaneously

RAKE receiverwhen multiple versions of asignal arrive more than one chip interval apart,RAKE receiver attempts to recover signals frommultiple paths and combine them This method achieves better performance than simply

recovering dominant signal and treating remainingsignals as noise


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Web sites Bekkers, R. and Smits, J., Mobile Telecommunications, Artech, 2000.

www.pcsdata.com: PCS web site

www.gsmdata.com: GSM web site

www.wlana.com wireless LAN Association

www.pcca.org) portable computers and communications association

Online Magazines

Mobile Computing & Communications (www.mobilecomputing.com)

Wireless Design Online (www.wirelessdesignonline.com)

Wireless Design & Development (www.wirelessdesignmag.com)

Wireless & Mobility (www.wireless mag.com)

Wireless Review (www.wirelessreview.com)

Wireless Systems Design (www.wsdmag.com)
http://www.pcsdata.com/http://www.gsmdata.com/http://www.wlana.com/http://www.pcca.org/http://www.mobilecomputing.com/http://www.wirelessdesignonline.com/http://www.wirelessdesignmag.com/http://www.wsdmag.com%29/http://www.wsdmag.com%29/http://www.wsdmag.com%29/http://www.wsdmag.com%29/http://www.wsdmag.com%29/http://www.wsdmag.com%29/http://www.wirelessdesignmag.com/http://www.wirelessdesignmag.com/http://www.wirelessdesignmag.com/http://www.wirelessdesignmag.com/http://www.wirelessdesignmag.com/http://www.wirelessdesignonline.com/http://www.wirelessdesignonline.com/http://www.wirelessdesignonline.com/http://www.wirelessdesignonline.com/http://www.wirelessdesignonline.com/http://www.mobilecomputing.com/http://www.mobilecomputing.com/http://www.mobilecomputing.com/http://www.mobilecomputing.com/http://www.mobilecomputing.com/http://www.pcca.org/http://www.wlana.com/http://www.gsmdata.com/http://www.pcsdata.com/

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