Wireless WANs Chapter 16. Cellular Telephony Designed to provide communication between two mobile...

Wireless WANs

Chapter 16

Cellular Telephony

• Designed to provide communication between two mobile (moving) units, called mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land unit.

• Service provider must be to locate and track a caller, assign a channel to the call, and transfer the channel from BS to BS as the caller moves out of range.

Base station antenna (3 sector)

1/3rd of cell is covered by each sector of antenna

Cellular system

How cellular telephone systems work

• The area (a city, or a part of town) is divided into a number of cells (typically 1 to 12 miles in radius, but can be smaller for more crowded areas) and a base station is positioned within each cell

• Adjacent cells assigned different set of frequencies to avoid interference

• If a user (mobile phone/ station) is within a particular cell, the call is handled by the corresponding base station within that cell

(Cont.)

•As a user moves from one cell to another, the call is “handed over” to the base station of the other cell-This is called handoff

• The handoff is hopefully soft (transparent) to the user instead of hard handoff.

• the signal from a mobile unit to a base station (BS) is transmitted by radio waves at a certain frequency through the air

• The signal from the mobile unit arrives at the antenna of the base station

• The signal from the base station is sent to a mobile switching center (MSC) and possibly to a telephone central office through electrical wires where it is switched to the appropriate destination

• MSC responsible for connecting calls, recording call information, and billing

(Cont.)

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Figure 16.1 Cellular system

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Figure 16.2 Frequency reuse patterns

• The caller enter a phone number (7 or 10 digits)• MS scans the band seeking for a channel with strong

signal• Send the data to the closest BS using that channel• BS relays data to MSC and then to telephone central

office• If the called party is available, MSC assigns an unused

voice channel to the call and a connection is established

Transmitting

• When a mobile phone (MS) is called, the telephone central office sends the number to the MSC.

• MSC searches for the location of the MS by sending query signals to each cell in a process “paging”.

• If MS is found, MSC transmits a ring signal and assigns a voice channel to the call when MS answers.

Receiving

• A user can have access to communication or can be reached where there is a coverage.

• Neighboring service providers can provide extended coverage through a roaming contract

• Similar to snail mail between countries

Roaming

Generations of Cellular System

First generation :• was designed for voice communication using analog signals • AMPS (Advanced Mobile Phone System): one of the leading

analog cellular phone systems used in North America• Uses FDMA (frequency division multiple access) to separate

channels in a link• Operates in the ISM 800-MHZ band• Uses two separate analog channels, one for forward and one

for reverse communication• Uses FM (frequency modulation) to modulate voice channel

and FSK (frequency shift keying) modulation to modulate control channel

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Figure 16.3 Cellular bands for AMPS

Second generation:• To provide high-quality (less noise-prone) voice transmission• was designed for voice communication using digital signals • Three major digital cellular phone systems:– D-AMPS (Digital AMPS):

• Backward compatible with AMPS(Use same bandwidth as AMPS 800MHz)

• Resue factor of 7.

– GSM (Global System for Mobile Communication):• Uses two bands for duplex communication( forward , reverse)• Each uses 25 MHz band• 124 channel with 200 KHz separated by guard band • Reuse factor as low as 3 because of complex error correction

Generations of Cellular System

Second generation:– IS-95 (Interim Standard 95)• Uses two bands for duplex communication (Each band is

800 MHz or 1900 MHz divided into 20 channel of 1.228 MHz separated by guard band )• Forward uses CDMA and it needs to synchronize the

channel.• Reverse uses DSSS without without synchronization.• Reuse factor of 1 because the DSSS and CDMA will not

allow interference.

Generations of Cellular System

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Figure 16.5 Second-generation cellular phone systems

Generations of Cellular System

Third generation : now being implementedProvides both digital voice and data communication (make a call,

use the internet, download, video conference,…ITU (international telecommunication union) issued Internet

Mobile Communication 2000 (IMT -2000) which defines some criteria for third generation:• Higher-quality voice transmission• High bandwidth and data rate• Interface the internet

Generations of Cellular System

Forth generation :

There are two candidate systems are commercially deployed :Mobile WiMax in 2006Long Term Evolution (LTE) in 2009

1800 to 2300 MHz band

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Figure 16.12 IMT-2000 radio interfaces

Satellite Networks

• Is a combination of nodes (satellites, Earth station, end-user terminal , or telephone) that provides communication from one point on the Earth to another

• divides planet into cells (like cellular NW)• Provides transmission to and from any location on

Earth without requiring a huge investment in ground-base infrastructure

• Extensively used by TV networks.

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Figure 16.13 Satellite orbits

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What is the period of the Moon, according to Kepler’s law?

Example 16.1

Here C is a constant approximately equal to 1/100. The period is in seconds and the distance in kilometers.

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Example 16.1 (continued)

SolutionThe Moon is located approximately 384,000 km above the Earth. The radius of the Earth is 6378 km. Applying the formula, we get.

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According to Kepler’s law, what is the period of a satellite that is located at an orbit approximately 35,786 km above the Earth?

Example 16.2

SolutionApplying the formula, we get

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This means that a satellite located at 35,786 km has a period of 24 h, which is the same as the rotation period of the Earth. A satellite like this is said to be stationary to the Earth. The orbit, as we will see, is called a geosynchronous orbit.

Example 16.2 (continued)

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Figure 16.14 Satellite categories

Geostationary Earth Orbit Low Earth OrbitMiddle Earth Orbit

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Figure 16.15 Satellite orbit altitudes

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Table 16.1 Satellite frequency bands

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Figure 16.16 Satellites in geostationary orbit

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Figure 16.17 Orbits for global positioning system (GPS) satellites

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Figure 16.19 LEO satellite system

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Figure 16.20 Iridium constellation

Iridium system

• The Iridium system has 66 satellites in six LEO orbits, each at an altitude of 750 km.

• Iridium is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale.