ch 16. wireless wans. 16.1 cellular telephony designed to provide communication between two...
TRANSCRIPT
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Ch 16. Wireless WANs
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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)
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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
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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
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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
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AMPS
FM and FSK (frequency shift keying) modulation
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Second Generation• Designed for digitized voice• Digital AMPS (D-AMPS)– Backward compatible with AMPS– Same band as AMPS– PCM modulation with TDMA – FDMA
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GSM
• Global System for Mobile Communication (GSM)– European standard– Two 25 MHz bands for duplex communications
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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
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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
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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
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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
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Frequency Reuse FactorSystem Reuse Factor
AMPS (1G) 7
D-AMPS (2G) 7
GSM (2G) 3
CDMA (2G) 1
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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
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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
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Geostationary Earth Orbit (GEO)
• Satellites move at the speed of Earth’s rotation– Altitude of satellite is 35,786 Km
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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
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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
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Homework
• Exercise– 19, 20, 26