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Exploring communications technology
Exploring communications technology

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1.6 Phase-shift keying (PSK)

The third fundamental digital modulation technique, and the most widely used in one form or another, is PSK. Its simplest form is Binary Phase-Shift Keying (BPSK).

In BPSK, 0 and 1 are represented by segments of sinusoids that differ in their phase. At the receiver, distinguishing between the two segments is easier if their phases differ by as much as possible. In BPSK the phases are separated by half a cycle (equivalent to π radians or 180°). See Figure 1.5.

Described image
Figure 1.5 BPSK
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This diagram shows a line graph. The X or horizontal axis is labelled as time, measured in seconds. The Y or vertical axis is labelled as voltage, measured in volts. Time periods are represented on the diagram by a series of equidistant dotted vertical lines. Each time period is labelled with a 1 or a 0, in the sequence 1 0 0 0 1 1 0 1 0 0 from left to right.

Here a sinusoidal signal of amplitude 1 volt is shown varying between two different phases, one in which the waveform rises to a peak at the beginning of the time period (the first phase), and one in which it descends to a trough at the beginning (the second phase). Periods in which the waveform has the first phase correspond to a 1, and periods in which the waveform has the second phase correspond to a 0. The sinusoid switches between the two phases as follows: first phase for one time period, second phase for three time periods, first phase for two time periods, second phase for one time period, first phase for one time period, second phase for two time periods.

Figure 1.5 BPSK

A BPSK-modulated signal is less susceptible to certain kinds of noise than ASK.

Activity 1.5 Self assessment

Figure 1.6 shows three examples of digitally modulated waveforms. For each example, decide which modulation scheme has been used and, based on the figures you saw earlier, work out what binary data each of these represents.

Described image
Figure 1.6 Three digitally modulated waveforms.
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This diagram shows three digitally modulated waveforms labelled A to C. Time periods are represented by a series of equidistant dotted vertical lines. The X or horizontal axes are labelled as time, measured in seconds. The Y or vertical axes are labelled as voltage, measured in volts.

In part A, a sinusoidal signal of amplitude 1 volt is shown for three time periods, then the phase changes for one time period, before reverting back to the original phase for one time period. The sinusoid then changes phase again for three time periods.

In part B, the sinusoidal signal has a small amplitude of about 0.5 volt for four time periods, then rises to an amplitude of 1 volt for two time periods. It then reverts to the smaller amplitude for one time period, and back to the higher amplitude for one time period.

In part C, a sinusoidal signal of amplitude 1 volt has a relatively high frequency for three time periods. Then the frequency lowers for two time periods, before reverting back to the original frequency for one time period. The sinusoid then changes to the lower frequency for a single time period, then back to the higher frequency for a single time period.

Figure 1.6 Three digitally modulated waveforms.

Answer

Waveform (a) is an example of a BPSK-modulated waveform representing the data:

0 0 0 1 0 1 1 1.

Waveform (b) is an example of a BASK-modulated waveform representing the data:

0 0 0 0 1 1 0 1.

Waveform (c) is an example of a BFSK-modulated waveform representing the data:

1 1 1 0 0 1 0 1.

Activity 1.6 Exploratory

This interactive activity will allow you to explore the three binary digital modulation schemes: OOK, ASK, BFSK and BPSK.

Start the activity by clicking on the image or ‘View’ link below. You will see that you are invited to ‘Create a binary data stream’. Enter a series of 0s and 1s, then click on ‘Submit’ to create a modulating waveform and use this to modulate a carrier using one of the modulation schemes. You can change the modulation scheme using the drop-down menu at the top left, and change the carrier frequency using the slider at the top right.

Try creating different modulated waveforms.

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Initially a box is displayed with text at the top, and three separate blank spaces below.

The first line of text reads ‘Create a binary data stream’. This is followed by an input box, with the greyed-out text ‘e.g. 1001011’, and next to this a button labelled ‘Submit’.

Below this is the text ‘Select binary modulation scheme:’ and a drop-down menu with the following options: ‘OOK’, ‘ASK’, ‘BFSK’ and ‘BPSK’. This is followed by a sliding scale with the text ‘Carrier frequency:’, which is labelled ‘low’ at the left-hand side and ‘high’ at the right-hand side.

The input box allows a stream of zeros and ones to be typed in. This can be any combination of zeros and ones but must be between one and ten characters in length.

Once the binary stream has been input, the drop-down menu is used to select the required binary modulation scheme.

Pressing the Submit button then generates three waveforms in the three blank spaces below.

All three waveforms have an X axis labelled time, with no units or values indicated, and a Y axis labelled voltage (V).

The first waveform is labelled ‘Data’ and illustrates the binary data stream as a series of voltage levels. The Y axis extends from 0 to 1 and the X axis is segmented into equally spaced sections, where the duration of each segment is associated with one binary data bit. Where the binary data is a ‘one’, a voltage level of ‘one’ is illustrated for the duration of the segment. Where the binary data is a ‘zero’, a voltage level of ‘zero’ is illustrated for the duration of the segment.

The second waveform is labelled ‘Unmodulated carrier’. This is a sine wave and the Y axis extends from plus one to minus one. The segments that correspond to the duration of each of the binary data bits in the first waveform are also indicated on the X axis here. The frequency of the sine wave can be varied by using the sliding scale labelled ‘Carrier frequency’ described previously. With the sliding scale at its lowest value, two full cycles are completed within each bit duration. With the sliding scale at its highest value, ten full cycles are completed within each bit duration.

The third waveform is labelled according to the binary modulation scheme that was selected using the drop-down menu. That is, either ‘On-off keying (OOK)’, ‘Amplitude shift keying (ASK)’, ‘Frequency shift keying (BFSK)’ or ‘Phase shift keying (BPSK)’. This waveform also has the segments that indicate the bit durations marked, and the Y axis extends from plus one to minus one.

If OOK has been selected from the drop-down menu, then where the bit value illustrated in the first waveform is one, the third waveform looks the same as the unmodulated carrier. Where the bit value illustrated in the first waveform is zero, the third waveform is also zero.

If ASK has been selected, then where the bit value illustrated in the first waveform is one, the third waveform looks the same as the unmodulated carrier. Where the bit value illustrated in the first waveform is zero, the third waveform looks like the unmodulated carrier in terms of phase and frequency, but the peak value is smaller – approximately 0.5.

If BFSK has been selected, then where the bit value illustrated in the first waveform is one, the third waveform looks the same as the unmodulated carrier. Where the bit value illustrated in the first waveform is zero, the third waveform looks like the unmodulated carrier in terms of phase and amplitude, but the frequency of the waveform is lower – in fact, it is half the frequency of the carrier.

If BPSK has been selected, then where the bit value illustrated in the first waveform is one, the third waveform looks the same as the unmodulated carrier. Where the bit value illustrated in the first waveform is zero, the third waveform looks like the unmodulated carrier in terms of frequency and amplitude, but the phase of the waveform is different – in fact, it is the unmodulated carrier with a phase shift of pi radians or 180 degrees.

Interactive feature not available in single page view (see it in standard view).