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Part 1 explains the basics of amplitude modulation (AM) and frequency modulation (FM), as well as their relative strengths and weaknesses. Part 3 discusses Direct Sequence Spread Spectrum (DSSS) and Orthogonal Frequency Division Multiplex (OFDM) techniques.
Phase modulation
Another form of modulation that is widely used, especially for data transmissions, is Phase Modulation (PM). As phase and frequency are inextricably linked (frequency being the rate of change of phase), both forms of modulation are often referred to by the common term 'angle modulation'.
To explain how phase modulation works, it is first necessary to give an explanation of phase. A radio signal consists of an oscillating carrier in the form of a sine wave. The amplitude follows this curve, moving positive and then negative, and returning to the start point after one complete cycle. This can also be represented by the movement of a point around a circle, the phase at any given point being the angle between the start point and the point on the waveform as shown in Figure 3-13.
Figure 3-13. Phase modulation.
Modulating the phase of the signal changes the phase from what it would have been if no modulation were applied. In other words, the speed of rotation around the circle is modulated about the mean value. To achieve this it is necessary to change the frequency of the signal for a short time. In other words, when phase modulation is applied to a signal there are frequency changes and vice versa. Phase and frequency are inseparably linked, as phase is the integral of frequency. Frequency modulation can be changed to phase modulation by simply adding a CR network to the modulating signal that integrates the modulating signal. As such, the information regarding sidebands, bandwidth and the like also holds true for phase modulation as it does for frequency modulation, bearing in mind their relationship.
Phase shift keying
Phase modulation may be used for the transmission of data. Frequency shift keying is robust, and has no ambiguities because one tone is higher than the other. However, phase shift keying (PSK) has many advantages in terms of efficient use of bandwidth and is the form of modulation chosen for many cellular telecommunications applications.
The basic form of phase shift keying is known as Binary Phase Shift Keying (BPSK) or, occasionally, Phase Reversal Keying (PRK). A digital signal alternating between +1 and –1 (or 1 and 0) will create phase reversals – i.e. 180° phase shifts – as the data shifts state (Figure 3-14).
Figure 3-14. Binary phase shift keying.
The problem with phase shift keying is that the receiver cannot know the exact phase of the transmitted signal, to determine whether it is in a mark or space condition. This would not be possible even if the transmitter and receiver clocks were accurately linked, because the path length would determine the exact phase of the received signal. To overcome this problem, PSK systems use a differential method for encoding the data onto the carrier. This is accomplished by, for example, making a change in phase equal to a 1 and no phase change equal to a 0. Further improvements can be made upon this basic system, and a number of other types of phase shift keying have been developed. One simple improvement can be made by making a change in phase of 90° in one direction for a 1, and 90° the other way for a 0. This retains the 180° phase reversal between the 1 and 0 states, but gives a distinct change for a 0. In a basic system not using this process it may be possible to lose synchronization if a long series of zeros is sent. This is because the phase will not change state for this occurrence.
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