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