The mobile telephone system (II)

1. Unit 4 Electronic systems
Lesson 11 The mobile telephone System
(II)

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Lesson 11 The mobile telephone
System (II)
• Backgrounds
• Text tour
• Language in use
– Vocabulary

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• Terminology
– TDM
– FDM
– PCM
– Spread Spectrum Communication
Backgrounds

4. TDM
• Time-division multiplexing (TDM) is a method of
transmitting and receiving independent signals over a
common signal path by means of synchronized switches at
each end of the transmission line so that each signal
appears on the line only a fraction of time in an alternating
pattern.
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5. FDM
• In Telecommunications, frequency-division multiplexing
(FDM) is a technique by which the total bandwidth
available in a communication medium is divided into a
series of non-overlapping frequency bands, each of which
is used to carry a separate signal.
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6. PCM
• Pulse-code modulation (PCM) is a method used to
digitally represent sampled analog signals. It is the
standard form of digital audio in computers, compact discs,
digital telephony and other digital audio applications.
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7. Spread Spectrum Communication
• Spread-spectrum techniques are methods by which a
signal (e.g. an electrical, electromagnetic or acoustic
signal) generated with a particular bandwidth is
deliberately spread in the frequency domain, resulting in a
signal with a wider bandwidth.
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Text tour
• Outline
– D-AMPS -The Digital Advanced Mobile Phone
System
– GSM - The Global System for Mobile
Communications
– CDMA - Code Division Multiple Access

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• D-AMPS is the second generation of AMPS
– Fully digital
– International Standard: IS-54 and IS-136
• D-AMPS and AMPS share…
– 30 kHz channel
– upstream channels in the 1850–1910 MHz range
– downstream channels in the 1930–1990 MHz range
• D-AMPS differs in …
- Compression the vocal system to get BW from the standard
56kbps PCM to 8kbps or less (done by vocoder).
- Done in telephone rather than the base station or end office
to reduce the no. of bits sent over the air link.
-The gain is much in D-AMPS.
D-AMPS
The Digital Advanced Mobile Phone System

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Figure 4.2. (a) A D-AMPS channel with 3 users.
(b) A D-AMPS channel with 6 users.
D-AMPS
The Digital Advanced Mobile Phone System

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• GSM is similar to D-AMPS in …
– cellular system
– FDM is used
– transmitting on one frequency and receiving on a higher
frequency
– a single frequency pair is split by TDM into time slots
shared by multiple mobiles.
GSM
The Global System for Mobile Communications

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• GSM is different from D-AMPS in…
– the GSM channels are much wider (200 kHz vs. 30 kHz)
– hold relatively few additional users (3 vs. 8)
– GSM has a much higher data rate per user
GSM
The Global System for Mobile Communications

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Figure 4.3. GSM uses 124 frequency channels,
each of which uses an eight-slot TDM system.
GSM
The Global System for Mobile Communications

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Figure4.4. A portion of the GSM framing structure.
GSM
The Global System for Mobile Communications

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• Completely different from D-AMPS and GSM
• An analogy
– TDM is …
– FDM is …
– CDMA is …
• An example
• Practical considerations for CDMA
 Capacity
 Power Levels: e.g. WiFi / Mobile network
 Receiver: Senders identity
CDMA
Code Division Multiple Access

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• Vocabulary
– modulation, approximation, spectrum,
scheme, complicate
Language in use

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Vocabulary

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Modulation in dictionary
• The variation of a property of an electromagnetic wave or
signal, such as its amplitude, frequency or phase.
• The process by which some characteristic of one wave is
varied in accordance with another wave or signal. This
technique is used in modems to make business machine
signals compatible with communication facilities.

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Modulation in text
• On a D-AMPS mobile phone, the voice signal picked up
by the microphone is digitized and compressed using a
model that is more sophisticated than the delta modulation
and predictive encoding schemes.

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Modulation in use
• adaptive delta modulation
• amplitude modulation
• angle modulation
• binary pulse-code modulation
• differential pulse code modulation

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Approximation in dictionary
• The act, process or result of approximating.
• An inexact result adequate for a given purpose.

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Approximation in text
• To a first approximation, GSM is similar to D-AMPS.
Both are cellular systems.

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Approximation in use
• polynomial approximation
• closest rational approximation
• difference approximation
• differential approximation
• digital approximation
• engineering approximation
• exponential approximation

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Spectrum in dictionary
• A range of values of a quantity or set of related quantities.
• A broad sequence or range of related qualities, ideas, or
activities:
– the whole spectrum of 20th-century thought.
– the whole spectrum of industry

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Spectrum in text
• Increasing the amount of information to be sent from b
bits/sec to mb chips/sec can only be done if the bandwidth
available is increased by a factor of m, making CDMA a
form of spread spectrum communication (assuming no
changes in the modulation or encoding techniques).

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Spectrum in use
• absorption spectrum
• electromagnetic spectrum
• energy density spectrum
• frequency spectrum
• infrared spectrum
• phase spectrum
• power spectrum
• power density spectrum
• upper sideband spectrum

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Scheme in dictionary
• A systematic plan of action.
• A secret or devious plan; a plot.
• An impractical or unrealistic plan:
• An orderly combination of related parts:
• A chart, a diagram, or an outline of a system or an object.

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Scheme in text
• Nevertheless, CDMA is a clever scheme that is being
rapidly introduced for wireless mobile communication.

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Scheme in use
• block scheme
• classification scheme
• code scheme
• color scheme
• command coding scheme
• decision scheme
• educational scheme
• equivalent scheme
• labeling scheme
• proof scheme

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Complicate in dictionary
• To make or become complex or perplexing.
• To twist or become twisted together.
• Complex, intricate, and involved.

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Complicate in text
• CDMA also has many other complicating factors that have
been glossed over here.

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Complicate in use
complex, complicated, intricate, involved, knotty
• complex
– the complex mechanism of a watch
• complicated
– The old view of Galileo was delightfully uncomplicated.
• intricate
– The child was perplexed by the intricate plot of the story.
• involved
– These are involved financial affairs.
• knotty
– This is a knotty passage.

34. Thank You
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