ch 8. cellular networks myungchul kim mckim@icu.ac.kr

Ch 8. Cellular Networks

Myungchul Kim

mckim@icu.ac.kr

Cellular Networks • Overview• 1G Analog Cellular • 2G TDMA - GSM • 2G CDMA - IS-95• 2.5G• 3G• 4G and Beyond• Cellular Engineering Issues

1G(<1Kbps)

1 Kbps

10 Kbps

100 Kbps

2 Mbps

1 Mbps

Data Rates

1980 1990 2000 2010

2G(9.6Kbps)

2.5G(10-150Kbps)

3G(144Kbps to 2Mbps)

Years

Overview

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

•Frequency allocation•Licensed •Many providers

•Multiple Access•Many users •Wide area of coverage•Traffic management

•Location management •High mobility (in cars, trains) •Multiple suppliers•Handoff management, roaming

•General principles• Handled differently by different generations

Issues Vital to cellular

Frequency AllocationCellular networks: Mostly around 900 MHz – 2GHz

100 Km 3KHzVLF - very low frequency

10 Km 30KhzLF - low frequency

1 Km 300KHzMF - medium frequency

100 m 3MHzHF - high frequency

10 m 30 MHzVHF - very high frequency

1m 300 MHzUHF - ultra high frequency

100 mm 3GHz

10 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

Multiple Access Techniques: How to allocate users

Time

Frequency Session1

Session2

Session3

Session4

Frequency Division Multiple Access (FDMA) 1G Cellular (AMPS)

TimeF

requencyTime Division Multiple Access (TDMA) 2G TDMA 3G TDMA

Session2

Session3

Session1

Session4

TimeF

requency

Code Division Multiple Access (CDMA)

All sessionsbased on a code

2G CDMA (IS-95)3G CDMA

A Cellular Network

Public SwitchedTelephoneNetwork(PSTN)

MobileTelephoneSwitchingCenter(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

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 a different

cell-id and the location database is updated. most commonly used in cellular networks. (HLR, VLR)

Neighborhood polling: Connected mobile units only move to adjacent cells Angle of arrival (AOA). the angle at which radio waves from your device

"attack" an antenna is used to calculate the location of the device. Time taken. In this case, the time taken between the device and the

antenna is used to calculate the location of the device.

Network assisted Global Positioning System (GPS). a GPS chip is

installed inside a phone and thus the location of the user is tracked.

Mobile telecommunications switching office (MTSO)

• Essentially an end office to connect calls between mobile units

• Several base stations connected to an MTSO • In a large system, many MTSOs may be connected to a

second level MTSO and so on • MTSO connected to BSs, PSTN and each other through

packet switching (ATM)• Two types of channels available between mobile unit and BS

– Control channels – used to exchange information having to do with setting up and maintaining calls

– Traffic channels – carry voice or data connection between users

Cellular System

Handoffs (typically 30 mseconds): 1. At any time, mobile station (MS) is in one cell and under the control of a BS 2. When a MS leaves a cell, BS notices weak signal3. BS asks surrounding BSs if they are getting a stronger signal4. BS transfers ownership to one with strongest signal 5. MTSO assigns new channel to the MS and notifies MS of new boss

Public SwitchedTelephoneNetwork(PSTN)

MobileTelephoneSwitchingCenter(MTSC)

Cell 1

Cell 2

HLR VLR

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 and disabled 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

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

E

A

DF

G C

B

E

A

DF

G C

BE

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 used•Frequencies may not need to be different in CDMA (soft handoff)

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 high places (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 are equidistant

(hexagonal pattern)

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

Approaches to Increase Capacity• Adding/reassigning channels - some channels

are not used• Frequency borrowing – frequencies are taken

from adjacent cells by congested cells• Cell splitting – cells in areas of high usage

can be split into smaller cells• Microcells – antennas move to buildings,

hills, and lamp posts

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 signals from BSs • It sends this info to BS with strongest control signal - passed to MTSO• Subscriber initiates call by keying in phone number and presses send

key• MTSO verifies number and authorizes user• MTSO issues message to user’s cell phone indicating send and receive

traffic channels• MTSO sends ringing signal to called party• Party answers; MTSO establishes circuit and initiates billing

information• Either party hangs up; MTSO releases circuit, frees channels,

completes billing

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 computer– can 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 stolen phones

and resell them

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 to 9.6kbps)– Forward error correction and encryption

CDPD OperationInterne

t

CDPD Provider Network

Landline Modem

CDPD Modem

Router

Router

= base interface station(special unit that connects all base stations in CDPD provider network to routers)

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 frequency allocation (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)

Advantages of Digital Communications for Wireless

• Voice, data and fax can be integrated into a single system

• Better compression can lead to better channel utilization

• Error correction codes can be used for better quality

• Sophisticated encryption can be used

Differences Between First and Second Generation Systems

• Digital traffic channels – first-generation systems are almost purely analog; second-generation systems are digital

• Encryption – all second generation systems provide encryption to prevent eavesdropping

• Error detection and correction – second-generation digital traffic allows for detection and correction, giving clear voice reception

• Channel access – second-generation systems allow channels to be dynamically shared by a number of users

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

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.

GSM (2G-TDMA)• Circuit mode data

– Transparent mode– Non-transparent mode using radio link protocol– Data rate up to 9.6kb/s

• Short message service– Limited to 160 characters

• Packet mode data: Plans for GSM Phase 2+

• Architecture specification very detailed (500 pages)

• Defines several interfaces for multiple suppliers

Mobile Station and Base Station Subsystem (BSS)

Mobile station• Mobile station communicates across Um interface (air interface) with

base station transceiver in same cell as mobile unit• Mobile equipment (ME) – physical terminal, such as a telephone or PCS

– ME includes radio transceiver, digital signal processors and subscriber 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 of mobile unit from

one cell to another within BSS, and controls paging

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

several databases• Home location register (HLR) database – stores

information about each subscriber that belongs to it• Visitor location register (VLR) database – maintains

information about subscribers currently physically in the region

• Authentication center database (AuC) – used for authentication activities, holds encryption keys

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

GSM Location Services

PublicSwitchedTelephoneNetwork(PSTN)

GatewayMTSC

VLR HLR

TerminatingMSC 1

1. Call made to mobile unit (cellular phone)2. Telephone network recognizes numberand gives to gateway MSC3. MSC can’t route further, interrogates user’s HLR4. Interrogates VLR currently serving user (roaming number request) 5. Routing number returned to HLR andthen to gateway MSC

2

34

5

5

6

6. Call routed to terminating MSC7. MSC asks VLR to correlate call to the subscriber8. VLR complies9. Mobile unit is paged10. Mobile unit responds, MSCs convey

information back to telephone

7 8

9BTS

9 10

1010 10

10

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

GSM Protocol Architecture

BSSMAP = BSS Mobile Application part BTSM = BTS management CM = Connection ManagementLAPD = Link Access Protocol, D Channel

Base TransceiverStation

MobileStation

Radio

LAPDm

RRM

Radio

LAPDm

RRM

MM

CM

64 Kbps

LAPD

BTSM

64 Kbps

MTP

SCCP

Base StationController

64 Kbps

LAPD

BTSM

BSSMAP

64Kbps

MTP

SCCP

MM

CM

BSSMAP

Mobile ServiceSwitching Center

MM = Mobility Management MTP = Message Transfer Part RRM = Radio Resources ManagementSCCP = Signal Connection Control Point

Functions Provided by Protocols

• Protocols above the link layer of the GSM signaling protocol architecture provide specific functions:– Radio resource management: controls setup,

termination and handoffs of radio channels– Mobility management: location and security

(MTSO) – Connection management: connects end users– Mobile application part (MAP): between HLR,VLR– BTS management: management base system

2G CDMA CellularIS-95 is the best known example of 2G with CDMAAdvantages of CDMA for Cellular• Frequency diversity – frequency-dependent transmission

impairments have less effect on signal• Multipath resistance – chipping codes used for CDMA

exhibit low cross correlation and low autocorrelation• Privacy – privacy is inherent since spread spectrum is

obtained by use of noise-like signals• Graceful degradation – system only gradually degrades

as more users access the system

Drawbacks of CDMA Cellular• Self-jamming – arriving transmissions from

multiple users not aligned on chip boundaries unless users are perfectly synchronized

• Near-far problem – signals closer to the receiver are received with less attenuation than signals farther away

• Soft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex than hard handoff used in FDMA and TDMA schemes

Types of Channels Supported by Forward Link

• Pilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference and provides means for signal strength comparison

• Synchronization (channel 32) - used by mobile station to obtain identification information about cellular system

• Paging (channels 1 to 7) - contain messages for one or more mobile stations

• Traffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channels

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

Wireless Network Evolution to 3rd Generation Enabling Technologies

AMPS

GSM

IS-95

GPRS

CDMA-20001XRTT

EDGE

CDMA20003XRTT(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)

• Fig 8-13

• Table 8-3

Alternatives to 3G Cellular• Major technical undertaking with many organizational and

marketing overtones.

• Questions about the need for the additional investment for 3G (happy with 2.5G)

• Wireless LAN in public places such as shopping malls and airports offer options

• Other high-speed wireless-data solutions compete with 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)

• IEEE 802.11 vs 3G Cellular

4G Systems • Wireless networks with cellular data rates of 20 Mbits/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 Internet protocol-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)

Engineering Issues

• Steps in MTSO controlled call

• TDMA design

• CDMA design

• Handoff

• Power control

• Traffic engineering

Steps in an MTSO Controlled Call between Mobile Users

• Mobile unit initialization• Mobile-originated call• Paging• Call accepted• Ongoing call• Handoff• Call blocking• Call termination• Call drop• Calls to/from fixed and remote mobile subscriber

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)

Mobile Wireless CDMA Design Considerations

• Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

• RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them– This method achieves better performance than simply

recovering dominant signal and treating remaining signals as noise

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)

• Wireless Week (www.wirelessweek.com)