Post on 26-Jun-2015
description
MULTIPLEXING
Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communications channel
Advantages of multiplexing: Increase no. of channels so that more
information can be transmitted Reduce cost, because one channel can
be used to send many information signals
Multiplexing Concept
MUXDEMU
XMultiple input signals
Original input signals
Single communication channel
Multiplexer (MUX) combines all input signals into one signal
Demultiplexer (DEMUX) separate input signal into original individual signals
Frequency Division Multiplexing (FDM)
Many signals share bandwidth in one communication channel, and the combined signals will modulate a transmitter
FDM TransmitterModulato
r
Modulator
Modulator
Modulator
Linear mixer
or summ
er
Transmitter
Antenna Carrier fc1
Carrier fc2
Carrier fc3
Carrier fcn
Signal 1
Signal 2
Signal 3
Signal n
All carriers are combined into a single composite signal that modulates a transmitter
Original data modulates carriers of different frequencies
Modulated signal is transmitted
FDM TransmitterEach info signal feeds a
modulator and modulates carrier signals at different
frequencies, fc
Each modulated signal is input to linear mixer
Linear mixer adds up all the modulated signals, resulting in one
composite signal
Composite signal modulates the transmitter
FDM Receiver
Demodulator
Band pass
filter fc1
Receiver
Antenna Signal 1
Signal 2
Signal 3
Signal n
Demodulator
Band pass
filter fc2
Band pass
filter fc3
Band pass
filter fcn
Demodulator
Demodulator
Demodulator
Can be superheterodyne receiver
Received modulated signal is demodulated into composite signal
FDM ReceiverReceiver receives multiplexed signal and demodulates it to
get the composite signal
Composite signal is input to every band pass filter
Each band pass filter will pass only its corresponding signal and blocks
the others
The individual demodulator extracts the original signal from the output of
the band pass filter
FDM Applications FDM is used in
FM stereo▪ At transmitter, two microphones pick up sound from a
common source, giving sufficient difference in the two audio signals to provide more realistic reproduction of the original sound
▪ These two signals are frequency-multiplexed and transmitted by a single transmitter
Telemetry▪ The process of measurement at a distance
Telephone system▪ Voice in 0.3 – 3 kHz range modulate a subcarrier▪ 12 modulated carriers form a basic group▪ 5 basic groups form a super group▪ 10 super groups can be multiplexed into a master group
Time Division Multiplexing (TDM)
Multiple signals take turns to transmit over a single channel
One cycle of operation is called a frame
Can be used for analog and digital info signal
Basic TDM Concept
Signal 1
Signal 2
Signal 3
Signal 4
Signal 1
Signal 2
One frame
Time slots are equal for each signal
Analog TDM
Analog signals are sampled repeatedly at high rate
The sampling results in a series of pulses that vary in amplitude according to the variation in the analog signals (exact principles as PAM)
Analog TDM
At transmitter, commutator takes one sample for each input per revolution
At receiver, decommutator separates the samples and distributes them accordingly for reconstruction of the individual message
Commutator and decommutator must be synchronized, so that each pulse is distributed to the correct output line at the appropriate time
Digital TDM
Signals are sampled, quantized and encoded in such a way that the resulting digital pulses are of uniform amplitudes
Digital signals can be multiplexed at different bit rates
The digital signals are multiplexed into a single data stream at a higher bit rate than any of the input
Digital TDM
Digital multiplexing can be done bit-by-bit basis (known as bit or digit interleaving) or word-by-word basis (known as byte or word interleaving)
Digit Interleaving
Byte Interleaving
Interleaving channels with different bit rates