LABORATORY 4:

PAM Time Division Multiplexing (TDM) and Demultiplexing

EEE 186: COMMUNICATIONS SYSTEMS LABORATORY

Department of Electrical & Electronic Engineering

College of Engineering & Computer Science

California State University, Sacramento

FALL 2015

Time DivisionMultiplexing and Demultiplexing and Detection

Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 1800s, but found its most common application in digital telephony in the second half of the 20th century (). The most modern application of this methodology is in the Cellular GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile).

Pulse-amplitude modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period. ().

Consider the conditions at a transmitter, where two messages are to be sampled and combined into a two-channel PAM/TDM signal. If two such messages are sampled at the same rate, but at slightly different times, then these two trains of samples could be added without mutual interference, as illustrated in Figure 1.

Figure 1. Principle of a 2-channel PAM/TDM multiplexer

At the receiver side, if the timing information : frame period T and sampling width t was known, then the samples from either channel could be separated from the combined PAM/TDM signal. This process is carried out in a demultiplexer, as shown in Figure 2.

Figure 2. Principle of the PAM/TDM Demultiplexer

Part I. Simulation of PAM/TDM Multiplexer

In Simulink, set up the multiplexer system shown below in Figure 3. Assume the two input signals to be sinusoidal inputs, both having amplitude of 5 V and frequency of 3 KHz, however, with 90 degrees phase difference. The phase difference is introduced to differentiate the two signals in the common plots. Some key points are as follows:

  • In Figure 3 below, assume sampling frequency of 6 KHz in the sampling block, which can be simulated using a multiplier to combine the sinusoidal and pulse signals. The sampling period will be Ts = 1/6000 = 166.7 sec.
  • The distance between pulses should be Tn = Ts/n, where n is the number of input signals, which in this case is n = 2. Hence the distance between samples becomes Tn = Ts/n = 83.3 sec.
  • The pulse width can be chosen appropriately, for example, with a duty cycle of 10 %, the pulse width would be 8.33 sec.
  • The minimum bandwidth to transmit these samples by TDM should be: B ≥ 1/Ts = 6 KHz.

Figure 3. Simulink schematic of PAM/TDM transmitter

Part II. Simulation of PAM/TDM Demultiplexer

The Simulink TDM receiver block is shown below in Figure 4. The key issue in the recovery of input signals from TDM signals is the proper synchronization between TDM transmitter and receiver. Therefore, the clock signal in the transmitter should be passed to the receiver correctly.

Figure 4. Simulink schematic of PAM/TDM receiver

Part III. Measurement of simulated transmitter and receiver signals

(a)Plot the input signals x1(t), x2(t) in the TDM transmitter system of Figure 3.

(b)Plot the PAM/TDM signal in Figure 3 over an appropriate time period.

(c)Plot the recovered signals from the TDM receiver in Figure 4

(d)Plot the error signal between the transmitted and received signals. There should be one error plot for each signal.