tele3113 wk9tue

22 Sept. 2009TELE3113 - Multiplexing p. -1

TELE3113 Analogue and Digital Communications – Multiplexing

Wei [email protected]

School of Electrical Engineering and TelecommunicationsThe University of New South Wales


Multiplexing

Multiplexing: a number of independent signals are combined (without interfering each other) into a composite signal for transmissionover 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


Frequency Division Multiplexing (FDM)

LPF are needed before mixing operation to limit the bandwidth of the input signals


Time Division Multiplexing (TDM)

2 1N 2 1N2 1N2 1N

time

Time frameBit period

Each information signal is a PCM signal (sampled, quantized and pulse-coded)

Sampling rate at each source: fs

TDM frame rate: fs or TDM frame period: Ts=1/ fs

Bit period minimum required bandwidths

sx NfNT

T 1==

221 s

xTDM

NfT

B =≥



If each information source is a PAM signal:

time

x1(t)

x2(t)

Ts

Considering a N-channel PAM system, each signal is band-limited to B Hz so that the sampling period Ts is

BTs 2

1=

Thus, time spacing Tx between adjacent samples in the time-multiplexed waveform is

NT

T sx =

The minimum bandwidth needed for the TDM signal transmission .x

TDM TB

21

≥



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.

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-multiplexedThe bandwidth required will be 2(10+8) = 36kHz (AM) or (10+8)=18kHz (SSB)

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.



TDM with analog and digital inputs:


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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