c. Combined Carrier and Radio. In situations where the use of wire lines becomes an inconvenience, such
as during tactical operations, it becomes necessary to use a radio system to carry a composite carrier signal. The
bandwidth occupied by the composite carrier signal (baseband) becomes an integrated part of the transmitted
radio spectrum. The bandwidth of the radiated signal in turn depends on the number of carrier channels being
provided.
3-3. INTEGRATED COMMUNICATION SYSTEM
a. Formulation. An integrated radiotelephone communication system is shown in figure 3-1. This diagram
illustrates how to expand traffic capacity of an existing radio facility by superimposing one device on another,
using the building-block principle. In establishing the system, a telephone carrier terminal is first superimposed
on the radio system, and the necessary telephones are then added to the system terminations. Greater message
capacity is obtained secondarily by placing the signal from a telegraph carrier terminal on one channel of each
telephone carrier terminal. Attachment of the teletypewriters to the telegraph carrier completes the overall
system.
b. System Operation. At each termination of the composite radio system, four teletypewriters together with
two telephones send and receive the message information. Messages from four teletypewriter terminal channels
(A, B, C, D) are combined, or multiplexed, on a telegraph carrier terminal. This four-channel information is
transmitted over one channel (channel 4) of the telephone carrier terminal. The facsimile signal occupies channel
3 of the telephone carrier terminal, while two telephone voice messages occupy channels 1 and 2.
3-4. FREQUENCY-DIVISION MULTIPLEXING
In a frequency-division-multiplexing (FDM) system, one amplitude- or frequency-modulated signal is
transmitted in a certain definite frequency band, and another signal is transmitted in a neighboring band above or
below the first signal. Additional bands can be added within the limitations of the transmission facilities. The
frequency bands representing the individual channel signals are selected from the composite at the receiving
terminal by channel filters. Each channel signal is distinguished from all others by the frequency band it
occupies. The final demodulation removes the intelligence from the channel signal with a locally generated
carrier which must be as close as possible to the modulating carrier frequency.
3-5. TIME-DIVISION MULTIPLEXING
Time-division multiplexing (TDM) may be explained as follows: Assume that signals from four VF telephone
channels are to be sent by pulse-modulation methods. The modulating equipment is designed to sample all four
signals, taking a sample from each of them in succession before returning to the first one. The sampling rate must
be four times as fast as would be needed to send one signal, so that each signal is sampled often enough for
satisfactory transmission. At each sampling, a pulse or group of pulses is created which registers the amplitude of
the sampled signal at that moment. Thus, bits of information from all four signals are sent in rotation by one
chain of pulses. At the receiving end, the pulses are sorted out and returned to their respective signal.
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