This lecture will examine how we can communicate information in digital form by modulation of a carrier waveform. Although this process is referred to as Digital Modulation (DM) it generally involves some form of AM and/or FM or PM. Hence digital modulation should be regarded as describing the ways that AM/FM/PM may be used to communicate information in terms of a pattern of ‘bits’ of information. In principle, the same forms of modulator or demodulator may be employed as when AM/FM/PM are used to send ‘analogue’ patterns. The use of digital representations of the information means that we now have to deal with a limited set of defined signal amplitudes or phases or frequencies, not the smoothly variable values which arise in an analogue representation.
Simple Binary Modulation – One Bit at a Time!
The simplest form of DM represents a series of logical ‘1’s and ‘0’s by simply switching on and off the carrier. For this reason it is often called On-Off Keying (OOK). (The term ‘keying’ goes back to the days of ‘Morse Code’ when transmissions were switched on and off by hand, using a switch called a ‘key’.) An example of this is shown in Figure 19.1. Here a ‘0’ is represented by having the carrier ‘off’ (i.e. reducing its amplitude to zero), and a ‘1’ by having the carrier ‘on’ (i.e. giving it a chosen amplitude, ).
In effect, we divide the transmitted signal into a series of timed ‘chunks’, and use these to indicate each bit in turn. Thus the example shown in Fig 19.1 shows what we might get if we use this simple on/off binary method to send a series of bits ‘01011010’. The signal pattern used during each time chunk is called a Symbol and represents some information that is being communicated by switching on or off the signal as appropriate.
Fig 19.2 zooms in to a part of the above signal pattern so we can see some of the details more clearly. We can see that – when it is non-zero – each symbol consists of an integer number (4, in this example) cycles of the chosen carrier frequency. Hence if we were using a carrier frequency of, say, 10 MHz, then in this example each symbol would have a duration of 4/107 = 0·4 microseconds. The length of each symbol is called the Useful Symbol Duration, . In between successive symbols we may also have a short ‘spacing’ period called the Guard Interval, , which more clearly separates one symbol from the next,
We can represent the on/off modulation in algebraic terms by saying that the waveform consists of a signal
where we set when the bit to be sent is a ‘1’ and when it is a ‘0’. (In Figures 19.1/2 I have chosen a value of of unity for the sake of simplicity.)
A simple alternative to the on/off modulation is to have a burst of carrier power during every symbol period, but to change the phase of the carrier to distinguish a ‘1’ from a ‘0’. An example is shown in Figure 19.3. This method is called Binary Phase Shift Keying (BPSK) modulation, and generally preferred to the simple on/off method. We can represent this in two different ways. We can either say that for a ‘0’ we now have and for a ‘1’ we have , or we can say that for a ‘0’ we have and for a ‘1’ we have . i.e. we can regard the modulation as either being an inversion of the carrier, or a change in its phase. For reasons which should become clear later, it is convenient to regard this as a change in the carrier phase.
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