Multiplexing and Data Rate

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• Topic: Multiplexing, Time-division multiplexing, Data transmission
• Pages : 13 (2284 words )
• Published : December 17, 2011

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Multiplexing
CHAPTER 4

Outline
* Frequency Division Multiplexing(FDM)
* Synchronous Time Division Multiplexing
* Statistical Time Division Multiplexing

Multiplexing
* Set of techniques that allows the simultaneous transmission of multiple signals across a single link * allows several transmission sources to share a larger transmission capacity

Channel = portion of a link that carries a transmission between a given pairs of lines

2 CATEGORY OF MULTIPLEXING

Frequency Division Multiplexing

* FDM – numerous signals are combined for
transmission on a single communications line or
channel. Each signal is assigned a different
frequency (subchannel) within the main channel.
* Useful bandwidth of medium exceeds required
bandwidth of channel
* Channel allocated even if no data
Frequency Division Multiplexing Diagram

* Each signal is modulated to a different carrier frequency * Carrier frequencies separated by guard bands (unused bandwidth) – to prevent interference so signals do not overlap. 3
FDM System
FDM is an analog multiplexing
technique that combines signals.

FDM process

FDM Demultiplexing

Example 1
Assume that a voice channel occupies a bandwidth of
4 KHz. We need to combine three voice channels into a
link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show
the configuration using the frequency domain without
the use of guard bands.
Solution
Shift (modulate) each of the three voice channels to a
different bandwidth, as shown in next figure

Example 2
Five channels, each with a 100-KHz bandwidth, are to
be multiplexed together. What is the minimum
bandwidth of the link if there is a need for a guard
band of 10 KHz between the channels to prevent
interference?
Solution
For five channels, we need at least four guard bands.
This means that the required bandwidth is at least
5 x 100 + 4 x 10 = 540 KHz,
as shown in next Figure.

Example 3
Four data channels (digital), each transmitting at 1
Mbps, use a satellite channel of 1 MHz. Design an
appropriate configuration using FDM
Solution
The satellite channel is analog. We divide it into four
channels, each channel having a 250-KHz bandwidth.
Each digital channel of 1 Mbps is modulated such that
each 4 bits are modulated to 1 Hz. One solution is 16-
QAM modulation. Figure 6.8 shows one possible
configuration.

Analog Carrier Systems
* AT&T (USA)
* Hierarchy of FDM schemes
* Group
1. 12 voice channels (4kHz each) = 48kHz
2. Range 60kHz to 108kHz
* Supergroup
1. 60 channel
2. FDM of 5 group signals on carriers between 420kHz and 612 kHz * Mastergroup
1. 10 supergroups

Analog Hierarchy

To maximize the efficiency infrastructure,
multiplexed signals from lower
bandwidth lines onto higher-bandwidth
signals

FDM of Three Voiceband Signals

FDM Applications

* AM (530 – 1700KHz) – shared with all radio stations * FM uses a wider band (88 – 108MHz) – each station needs more bandwidth, 200KHz 2. Television Broadcasting
* Each TV channel has own bandwidth of 6 Mhz
3. 1st Generation of Cellular telephones
* Voice signal 3KHz (300 – 3300Hz) channels
* Bt = 10 x Bm , therefore each channel has 30KHz
channels
* each user has been allocated two 30KHz
channel, therefore 60KHz.
Example 4
The Advanced Mobile Phone System (AMPS) uses two
bands. The first band, 824 to 849 MHz, is used for
sending; and 869 to 894 MHz is used for receiving. Each
user has a bandwidth of 30 KHz in each direction. The 3-
KHz voice is modulated using FM, creating 30 KHz of
modulated signal. How many people can use their
cellular phones simultaneously?
Solution
Each band is 25 MHz. If we divide 25 MHz into 30 KHz,
we get 833.33. In...