Phase Shift Keying PSK
In phase shift keying (PSK), the phase is varied to represent binary 1 or 0. Both peak amplitude and frequency remain constant as the phase changes. For example, if we start with a phase of 0 degrees to represent binary 0, then we can change the phase to 180 degrees to send binary l. The phase of the signal during each bit is constant and its value depends on the bit (0 or 1). Figure 4.30 gives a conceptual view of PSK.
Amplitude
Amplitude
- Fig. 4.3-8 PSK
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Fig.4.3-9 PSK constellationThe above method is often called 2-PSK, or binary PSK, because two different phases (0 and 180 degrees) are used in the encoding. Figure 4.3-9 makes this point clearer by showing the relationship of phase to bit value. A second diagram, called a constellation or phase-state diagram, shows the same relationship by illustrating only the phases. PSK is neither susceptible to the noise degradation that affects ASK, nor to the bandwidth limitations of FSK. This means that smaller variations in the signal can be detected reliably by the receiver. Therefore, instead of utilizing only two variations of a signal, each representing one bit, we can use four variations and let each phase shift represent two bits (see Figure 4.3-10).
The constellation diagram for the signal in Figure 4.3-10 is given in Figure 4.3-12. A phase of 0 degrees now represents 00; 90 degrees represents 01; 180 degrees represents 10; and 270 degrees represents 12. This technique is called 4-PSK or Q-PSK. The pair of bits represented by each phase is called a dibit. We can transmit data two times as fast using 4-PSK as we can use 2-PSK.
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