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Unit 19

PSK/QPSK System

19-1 objectives

1. To study the principle of PSK/QPSK modulation.

2. To study the principle of PSK/QPSK demodulation.

3. To implement PSK/QPSK modulator.

4. To implement PSK/QPSK demodulator.

19.2 DISCUSSION OF FUNDAMENTALS

PSK/QPSK Modulator

As mentioned in Chapter 18, phase-shift keying (PSK) modulation process may be viewed as the special caseof phase modulation (PM). The PSK modulation is shown in Figure 19-1.

In Figure 19-1, the carrier signal is a sinusoidal .wave with fixed frequency and amplitude, the modulating signal is binary information. If input information is a low (0), the carrier signal maintains its phase. If input information is a high (1): the carrier reverses its phase by 180 degrees. The pair of sinusoidal wavesthat differ only in a relative phase-shift of 180 degrees are referred to as antipodal signals This type of phase-shift keying is called binary PSK (oPSK) or phase-reverse keying (PRK)

As with BPSK, this modulation scheme is characterized by the fact that the information carried by the transmitted wave is contained in the phase. In particular, in quadriphase-shift keying (QPSK), the phase of thecarrier takes on one of four equally spaced values, such as 0°, 90°, 180°, and 270°. Each possible value of the phase corresponds to a unique pair of bits called a dibit. For example, we may choose the forgoing set of phase values to represent the Gray-coded set of dibits: 00, 01, 11, and 10. The typical waveforms of QPSK modulation are shown in Figure 19-2.

Figure 19-2 QPSK modulationNote that the phase set of PSK and QPSK mentioned above is on'y a possible choice. Other possible phase-shifts of PSK and QPSK signals are shown in Table 19-1.

Table 19-1 Possible phase-shifts of PSK and QPSK

Figure 19-3 muestra un PSK/QPSK sistema de comunicación el modulador modula la señal portadora por la entrada de la informaciony produce a PSK or QPSK señal modulada la señal moduladaes transmitida a través de medio de transmicion como cable y fibra óptica a la

input of demodulator. The demodulator receives PSK or QPSK transmitted signal and then reconstructs the original information data.
Information

Figure 19-4 shows the functional blocks of a PSK/QPSK modulator. The carrier signal generator provides a carrier signal (sinusoidal wave) to the phase switching networkand a square wave to the timing circuit. Phase-switching network provides four outputs (0°, 90°, 180°, 270°) to the inputs of data selector. The output X of data selector is determined by the select inputs A and B. There are four following cases:
1. If BA=00 (Q1=Q0=low), X=X0, the signal with phase shift 0°.
2. If BA=11 (Q1=Q0=high), X=X3, the signal with phase shift 180°.
3. If BA=01 (Q1=low,Q0=high), X=X1, the signal with phase shift 90°.
4. If BA=10 (Q1=high, Q0=low). X=X2, the signal with phase shift 70°.

The timing circuit receives the square wave (fc) from the carrier sic- :il generator output and produces two outputs fc to the load control input and signal 2fc (twice the carrier frequency) the clock input of control register as well as the sync cyclo generator these twosignals of fc and 2fc and the data rate (measured in seconds, hps) of input digital

information are used to determine whether the modulator operates in binary PSK or QPSK mode. There are three possible cases:

1. Bit rate = fc and no sync cycle generated
In this case, the data rate is equal to the carrier frequency fc and the clock frequency is twice the carrier frequency 2fc. One bit ofdigital data stream is loaded into the control register two times. The outputs Q0-Q1 of control register are therefore the same, 00 or '11. The output X of data selector is either X0 or X3 input signal. This system operates in PSK mode.

2. Bit rate=2fc and no sync cycle generated
In this case, data rate and clock frequency are' equal to twice the carrier frequency, 2fc. Two bits of data stream...