Ch 16. Wireless WANs

16.1 Cellular Telephony

• Designed to provide communication between two “moving” units– To track moving units (mobile station; MS), service area is

divided into small regions called cells– Each cell is controlled by network station called the base

station (BS)

Frequency-Reuse

• Cell size depends on the population of the area– High-density areas require smaller cells

• Frequency reuse– The set of available frequencies is limited– Neighboring cells cannot use the same set of

frequencies due to wireless interference near boundary– Number of cells

for a frequency reuse pattern Reuse factor

Basic Operations• Transmitting

– MS (using a setup channel) Closest BS MSC (Mobile Switching Center) Telephone central office If the callee is available, assign a voice channel

• Receiving – Telephone central office MSC (search for MS by paging) BS MS If

answers, assign a voice channel

• Handoff – If MS moves from one cell to another, MSC seeks a new cell that better

accommodates the communication of the MS– Hard handoff – one BS-MS connection at any time– Soft handoff – allow two BS-MS connections when a handoff occurs

• Roaming: extension of coverage of a service provider using other providers’ service

First Generation• Designed for voice communication• Example: Advanced Mobile Phone System (AMPS)– North America standard for analog cellular system– Unlicensed ISM 800 MHz band– Two separate analog channels

AMPS FM and FSK (frequency shift keying) modulation

Second Generation• Designed for digitized voice• Digital AMPS (D-AMPS)– Backward compatible with AMPS– Same band as AMPS– PCM modulation with TDMA – FDMA

GSM

• Global System for Mobile Communication (GSM)– European standard– Two 25 MHz bands for duplex communications

GSM

• TDMA – FDMA

Channel data (bit) rate = (1/120ms) x 26 x 8 x 156.25 = 270.8kbps

In Gaussian minimum-shift keying, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels

IS-95 CDMA• CSMA/DSSS and FDMA• Two 25 MHz bands for duplex communications• Forward transmission (BS MS)– Synchronization is required for CDMA – use GPS (will see later)

Electronic serial number

IS-95 CDMA

• 64 (forward) channels– Channel 0 is a pilot channel for synchronization

• Bit synchronization, serves as a phase reference for demodulation;• Allows the mobile station to compare the signal strength

of neighboring bases for handoff decisions.– Channel 32 gives system information to MS– Channels 1 to 7 are used for paging, to send

messages to one or more mobile stations.– Other channels are for data traffic

Reverse transmission• Reverse transmission (MS BS)– DSSS (direct sequence spread spectrum) instead of

CDMA– Normally 94 reverse channels (62 voice chs.)

• Soft handoff; frequency reuse factor = 1

Frequency Reuse FactorSystem Reuse Factor

AMPS (1G) 7

D-AMPS (2G) 7

GSM (2G) 3

CDMA (2G) 1

Third Generation• Designed to provide both digital “data and voice”– Comparable voice quality to the existing telephone net.– Data rate of 144 Kbps for moving vehicles, 384 Kbps for

pedestrians, 2 Mbps for stationary users– Support for packet- and circuit-switched data service– A band of 2 GHz with bandwidth of 2 MHz– Interface to the Internet

European; W-CDMA

North America; CDMA-2000

IMT-2000

16.2 Satellite Networks• Orbits

– Time required to make a complete trip around the Earth is determined by the distance of the satellite from the center of the Earth (Kepler’s law)

Geostationary Earth Orbit

Medium Earth Orbit

Low Earth Orbit

Geostationary Earth Orbit (GEO)

• Satellites move at the speed of Earth’s rotation– Altitude of satellite is 35,786 Km

Medium-Earth-Orbit (MEO)• Global Positioning System (GPS)

– Four satellites are visible from any point on Earth

• Trilateration is used to find a location– Three satellites are sufficient to locate

a land unit by measuring distance from each satellite

– Four satellites can be used when there is a clock offset between satellites and the land unit(Read the textbook, p 483)

• Applications of GPS– Military purpose, car navigation, clock synchronization of IS-95 CDMA

cellular system

Low-Earth-Orbit (LEO)• Cellular-like service using low-orbit satellites

• Iridium – Aim for providing direct worldwide communication using

handheld terminals• Globalstar• Teledesic– Aim for providing fiber-optic-like

communication (broadband channels,low error rate, low delay)

Intersatellite linkUser mobile linkGateway link

66 satellite network

12 orbits, 288 satellite network

Homework

• Exercise– 19, 20, 26