Signals and Systems - Electrical Engineering

12.4 Application to Digital Communications 735

FIGURE 12.11
TDM system: transmitter,
channel, and receiver.

Quantizer

& encoder Channel Decoder

Commutator Decommutator

m 1 (t)

m 2 (t)

m 3 (t)

m∧ 1 (t)

m∧ 2 (t)

m∧ 3 (t)

Pulse-modulated signals have large bandwidths, and as such, when transmitted together they overlap

in frequency, interfering with each other. However, these signals only provide information at each

of the sampling times, so that one could insert in between these times other samples that will be

separated at the receiver. This is the principle oftime-division multiplexing(TDM), where pulses from

different signals are interspersed into one signal and converted into a PCM signal and transmitted. See

Figure 12.11. At the receiver, the signal is changed back into the pulse-modulated signal and separated

into the number of signals interspersed at the input. Repeaters placed between the transmitter and

the receiver regenerate a clean binary signal from a noisy binary signal along the way. The noisy signal

coming into the repeater is thresholded to known binary levels and resent. A large part of the cost of

a transmission facility is due to these repeaters that are placed about every mile along the line.

TDM allows the transmission of different types of data, and mixture of analog and digital using

different multiplexing techniques. Not to lose information, the switch at the receiver needs to be

synchronized with the switch at the transmitter.Frame synchronizationconsists in sending a synchro-

nizing signal for each frame. An additional channel is allocated for this purpose. To accommodate

more users, the width of the pulses used for each user needs to become narrower, which increases the

overall bandwidth of the multiplexed signal.

12.4.3 Spread Spectrum and Orthogonal Frequency-Division

Multiplexing

The objective of TDM is to put several users or different types of data together sharing the same

bandwidth at different times. Likewise, FDM users share part of the available bandwidth all the time.

TDM and FDM are examples of how to use bandwidth in an efficient way. In other situations, like

in quadrature-amplitude modulation (QAM), the objective is to send two messages over the same

bandwidth using the orthogonality of the carriers to recover them. In spread spectrum, the objective is

to use the orthogonality of the carriers associated with different users to share the available spectrum,

while spreading the message in frequency so that it occupies a bandwidth much larger than that of the

message. On the other hand, orthogonal frequency-division multiplexing (OFDM) is a multicarrier

system where the carriers are orthogonal.

Sharing the radio spectrum among users, or multiple access, is a basic strategy of wireless commu-

nication systems. Basic modalities are derived from FDM, TDM, and spread spectrum. In FDMA the

spectrum is shared by assigning specific channels to users, permanently or temporarily. TDMA allows

access to all of the available spectrum, but each user is assigned a time interval in which to access

it. CDMA uses spread spectrum, where a user’s message is spread or encrypted over the available