1. TELE3113 Analogue and Digital
Communications – Multiplexing
Wei Zhang
w.zhang@unsw.edu.au
School of Electrical Engineering and Telecommunications
The University of New South Wales
TELE3113 - Multiplexing 22 Sept. 2009 p. -1

2. Multiplexing
Multiplexing: a number of independent signals are combined (without
interfering each other) into a composite signal for transmission
over a communication channel
share the resources among users
maximize the total bandwidth utilization of transmission channel
Multiplexer: combine the multiple signals into one composite (multiplexed) signal
Demultiplexer: separate individual signals from the multiplexed signal
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3. Frequency Division Multiplexing (FDM)
LPF are needed
before mixing
operation to limit
the bandwidth of
the input signals
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4. Time Division Multiplexing (TDM)
Each information signal is a PCM signal
(sampled, quantized and pulse-coded)
Bit period Time frame
N 2 1 N 2 1 N 2 1 N 2 1
time
Sampling rate at each source: fs
TDM frame rate: fs or TDM frame period: Ts=1/ fs
T 1 1 Nf
Bit period Tx = s = minimum required bandwidth BTDM ≥ = s
N Nf 2Tx 2 p. -4
TELE3113 - Multiplexing 22 Sept. 2009 s

5. Time Division Multiplexing (TDM)
If each information source is a PAM signal:
x1(t)
x2(t)
Ts time
Considering a N-channel PAM system, each signal is band-limited to B Hz so that the sampling
period Ts is 1
Ts =
2B
Thus, time spacing Tx between adjacent samples in the time-multiplexed waveform is
Ts
Tx =
N
1
The minimum bandwidth needed for the TDM signal transmission BTDM ≥ .
2Tx
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6. Time Division Multiplexing (TDM)
Example:
In a two-channel PAM system, channel 1 handles 0-8 kHz signals and channel 2
handles 0-10 kHz signals. The two channels are sampled at equal intervals of time
using very narrow pulse at the lowest frequency that is theoretically adequate. The
sampled signals are time-multiplexed before transmission.
In order to sample channel 2 adequately, we must take samples at least 20-kHz rate.
So, the TDM frame interval Ts is 1/(20kHz) = 50µs.
As there are only two channels, each of them can at most occupy ½ of the frame
interval. Thus, bit period Tx= Ts/2=25µs the clock rate of the TDM system is
1/Tx=40kHz.
The minimum requirement of the channel bandwidth is 1/(2Tx)= 20kHz
If these channels were frequency-multiplexed, instead of time-multiplexed
The bandwidth required will be 2(10+8) = 36kHz (AM) or (10+8)=18kHz (SSB)
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7. Time Division Multiplexing (TDM)
TDM with analog and digital inputs:
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8. Digital Multiplexing for Digital Telephone
• Voice bandwidth ~4kHz sampling rate=8kHz
• 8-bit per PCM word (voice channel)
• Twenty-four 8-bit voice channels are time-multiplexed to give 192 bits
per frame.
• One extra bit is inserted to give frame synchronization, yielding 193
bits/frame.
• Each channel is sampled at an 8-kHz rate resulting in a frame interval
of 125 µsec.
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