Multiplexing
As we know, modulation provides a method for multiplexing whereby message signals derived from independent sources are combined into a composite signal suitable for transmission over a common channel. For example, in a telephone system, multiplexing is used to transmit multiple conversations over a single long-distance line. The signals associated with different speakers are combined in such a way so that it does not interfere with each other during transmission and they can be separated at the receiving end of the system.
Multiplexing can be accomplished by separating the different message signals either in frequency or in time or through the use of coding techniques.
There are three types of multiplexing:
1. Frequency-division multiplexing.
In frequency-division multiplexing, the signals are separated by allocating them to different frequency bands. It favors the use of CW modulation. In CW modulation each message signal is able to use the channel on a continuous-time basis.
2. Time-division multiplexing.
In time-division multiplexing, the signals are allocated to different time slots within a sampling interval to separate them. It favors the use of per modulation.
3. Code-division multiplexing.
It relies upon the assignment of different codes to the individual users of the channel.
FREQUENCY-DIVISION MULTIPLEXING (FDM)
The incoming message in FDM is said to be of low-pass variety, but their spectra may or may not have non-zero values all the way down to zero frequency. It is possible that each input signal is a low-pass filter designed to remove high-frequency components that is capable of disturbing other message signals that share the common channel. The filtered signals are applied to modulators that shift the frequency ranges of the signals. It is done so as to occupy mutually exclusive frequency intervals. The carrier supply gives the carrier frequencies needed to perform these operations.
We can use any one of the methods described above for frequency modulation. Though, the most widely used method of modulation in frequency division multiplexing is single-sideband modulation. Let us take the case of voice signals. They require a bandwidth that is approximately equal to that of the original voice signal, meaning, each voice input is usually assigned a bandwidth of 4 kHz.
The bandpass filters following the modulators are used to restrict the band of each modulated wave to its prescribed range, then the resulting band-pass filter outputs are summed to form the input to the common channel. A bank of band-pass filters with their inputs connected in parallel is used at the receiving end to separate the message signals on a frequency.
TIME-DIVISION MULTIPLEXING (TDM)
Basic to the operation of a TDM system is the sampling theorem, which states that we can transmit all the information contained in a band-limited message signal by using samples of the signal taken uniformly at a rate that is usually slightly higher than the Nyquist rate. An important feature of the sampling process has to do with the conservation of time. That is, the transmission of the message samples engages the transmission channel for only a frac non of the sampling interval on a periodic basis, equal to the width T0 of a PAM modulating pulse. In this way, some of the time intervals between adjacent samples are cleared for use by other independent message sources on a time-shared basis.
In TDM, each input message signal is first restricted in bandwidth by a low-pass filter to remove the frequencies that are nonessential to an adequate representation of the signal. The low-pass filter outputs are then applied to a commutator that is usually implemented by means of electronic switching circuitry.
The function of the commutator is twofold:
(1) To take a narrow sample of each of the M input message signals at a rate of 1/T, which is slightly high er than w/pi, where w, is the cutoff frequency of the input low-pass filter.
(2)To sequentially interleave these samples inside a sampling interval Ts. The latter function is the essence of the time-division multiplexing operation. Following commutation, the multiplexed signal is applied to a pulse modulator. For example, a pulse-amplitude modulator, the purpose of which is to transform the multiplexed signal into a form suitable for transmission over the common channel. The use of time-division multiplexing introduces a bandwidth expansion factor M because the scheme must squeeze M samples derived from M independent message sources into a time slot equal to one sampling interval.
The signal is applied to a pulse demodulator at the receiving end of the system. It performs the inverse operation of the pulse modulator. A decommutator is used to distribute the narrow samples produced at the pulse demodulator output. It operates in synchronism with the commutator in the transmitter.
The timing operations of the transmitter and receiver in a TDM system are synchronized. The synchronization is essential for the satisfactory performance of the system. If a TDM system is using PAM, synchronization in that case can be achieved by inserting an extra pulse into each sampling interval on a regular basis.
A frame is defined as a combination of PAM signals and a synchronization pulse contained in a single sampling period. In PAM, the feature of a message signal that is used for modulation is its amplitude. Accordingly, a simple way of identifying the synchronizing pulse train at the receiver is to make sure that its constant amplitude is large enough to stand above every one of the PAM signals. On this basis, the synchronizing pulse train is identified at the receiver by using a threshold device set at the appropriate level. Note that the use of time synchronization in the manner described here increases the bandwidth expansion factor to M + 1, where M is the number of message signals being multiplexed.
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