Digital Signals and Modulation for communication | Soukacatv.com

Radio waves are naturally sinusoidal, with frequencies covering a wide range. They are capable of travelling through space, and are widely used for communication. This is a brief explanation of how they are able to carry information. Many of the same principles apply to other communication media, such as optical signals and electric currents.

Radio waves cover a wide range of frequencies, some of which are more suitable than others for a particular service. You can explore some uses of radio with this interactive chart.

HDMI Encoder Modulator, 16in1 Digital Headend, HD RF Modulator at Soukacatv.com

Click on the image of the electromagnetic spectrum below to learn more about the highlighted part of the spectrum (radio and microwave frequencies). You will see that this part of the spectrum is conventionally divided into bands, each covering a decade in frequency (or wavelength). Make a note of the frequencies and wavelengths and the typical uses of each band.

[The radio and microwave frequencies interactive will open in a new window. After you have viewed the interactive, click on the link 1.3 Digital signals and modulation, to return to this page.]

Generally a medium used for communication (such as radio waves) needs to be processed in some way to carry information. The process is called modulation. Two signals are combined in modulation:

·         The message signal, called the modulating signal. (Often this is non-periodic.)

·         A signal of the right frequency for transmission, called the carrier signal.

When they are combined, the modulating signal changes the carrier signal in some way, such as by changing its amplitude or frequency. This creates a new signal that contains the message information and is also at the correct transmission frequency. Note that although modulation of some kind is essential for wireless transmission, it is also used in much wired transmission, for example broadband and optical fiber.

In the next section, assume that the message to be sent is in the form of a digital signal (that is, a signal that is interpreted as a sequence of discrete values). In fact, most communications fall into this category; computer networks and almost all telephony, as well as digital TV and radio. Analogue signals such as speech are converted to digital form at one end of a communications link and back to analogue at the other. When the message signal is digital, modulation produces distinct states of the carrier wave that can be distinguished by the receiver and can be used to represent ones and zeros, or groups of ones and zeros. Next you will see some basic digital modulation schemes.

1.4 Amplitude-shift keying (ASK)

In ASK, only the amplitude of the carrier signal is modified in modulation. The simplest version is on–off keying (OOK). In OOK, either bursts of a carrier wave are transmitted or nothing is transmitted depending whether the input message signal is 1 or 0. Other versions of ASK use differing (non-zero) amplitudes to represent 1 and 0.

Figure 1.2(a) shows a digital message signal using two voltage levels. One level represents 1 and the other represents 0. The unmodulated carrier is illustrated in Figure 1.2(b). Figure 1.2(c) and (d) are the modulated waveforms using two versions of ASK. Figure 1.2(c) uses OOK, and 2(d) uses binary ASK, or BASK.

Figure 1.2 ASK: (a) data; (b) unmodulated carrier; (c) on–off keying (OOK); (d) binary amplitude-shift keying (BASK)

In OOK and BASK, the modulated carrier can take one of two different states: one state representing a 0, the other a 1. These different carrier states are what are known as symbols. If there are more than two possible carrier states – that is, more than two symbols available – then it is possible for each symbol to represent more than one bit.

Figure 1.3 shows ASK with four possible amplitude levels, or four symbols. With four symbols available, each symbol can be uniquely represented with a two-bit binary number. This is because there are just four possible two-bit binary numbers: 11, 10, 01 and 00.

Figure 1.3 ASK with four amplitude levels

If there were eight symbols, each could represent three data bits. The relationship between the number of available symbols, M, and the number of bits that can be represented by a symbol, n, is:

M = 2ⁿ

The term baud refers to the number of symbols per second, where one baud is one symbol per second.

Data rate (or bit rate) and baud are closely related.

Activity 1.4 Self assessment

·                        a.If a communications system uses 16 symbols, how many bits does each symbol represent?

·                        b.If the same system has a symbol rate of 10 000 baud, what is the data rate?

Increasing the number of bits a symbol can represent means that higher data rates can be achieved.

1.5 Frequency-shift keying (FSK)

In FSK, the frequency of the carrier signal is modified. An illustration of binary FSK, or BFSK, is given in Figure 1.4. Here, bursts of a carrier wave at one frequency or bursts of a carrier wave at a second frequency are transmitted according to whether the input data is 1 or 0.

Figure 1.4 Binary FSK

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.

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.

Figure 1.6 Three digitally modulated waveforms.

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.

1.7 Quadrature amplitude modulation (QAM)

It is possible to combine ASK, FSK and PSK. One benefit of combining different modulation methods is to increase the number of symbols available. Increasing the number of available symbols is a standard way to increase the bit rate, because increasing the number of symbols increases the number of bits per symbol. It is rare for all three methods to be combined, but very common for ASK and PSK to be combined to create Quadrature amplitude modulation (QAM).

QAM is based on the application of ASK and PSK to two sinusoidal waves of the same frequency but with a phase difference of 90°. Sinusoidal waves 90° apart are said to be in a quadrature phase relationship. It is customary to refer to one of these waves as the I wave, or in-phase wave or component, and the other as the Q wave, or quadrature wave or component (Figure 1.7).

Figure 1.7 (a) I (in-phase or sine) wave and (b) Q (quadrature or cosine) wave

You may recognize the I wave in Figure 1.7 as a sine function and the Q wave as a cosine function. These functions are said to be orthogonal to each other. If two signals are orthogonal, when they are transmitted simultaneously one can be completely recovered at the receiver without any interference from the other.

The I and Q waves remain orthogonal if either or both of them are inverted (multiplied by –1, or flipped vertically). Negative amplitudes just mean that the wave is inverted.

The set of symbols in QAM can be conveniently represented on a signal constellation diagram (Figure 1.8). This is a plot of the I and Q amplitudes with I on the horizontal axis and Q on the vertical axis. Each dot in Figure 1.8 is a symbol, as it represents a unique combination of amplitude and phase of the I and Q waves. So, in each symbol period, only one of the ‘dots’ is transmitted. As there are 16 symbols, this version of QAM is called 16-QAM.

Figure 1.8 Constellation diagram for 16-QAM.

To understand what each dot in the diagram represents, take the top left one. This represents a symbol where the Q wave is at amplitude of 3 and the I wave is at an amplitude of –3. The minus sign means the I wave is inverted (or phase shifted by 180°) relative to the I wave in Figure 1.7(a).

As the number of symbols increases, more data bits are transmitted per symbol. For example, 64-QAM is a QAM scheme with 64 symbols, and 256-QAM is a scheme with 256 symbols. 256-QAM conveys 8 bits per symbol (as 256 = 2⁸), so achieving twice the data rate of 16-QAM for the same symbol rate.

The points on the diagram in the answer to Activity 1.7 are placed at values of +/−1, 3, 5 and 7. The actual amplitudes used in practice are likely to be different; but if the spacing between constellation points remains the same (2 in this case) and we keep adding more points in this way, then we are increasing the power in the signal. The further away from the origin a constellation point is, the more power is required in the signal. Alternatively, it could be necessary to keep the maximum signal power constant whether we are using 16-QAM or 64-QAM, for instance. This would mean packing the points closer together in 64-QAM than in 16-QAM. However, if the points are closer together then adjacent symbols will be more likely to be misinterpreted at the receiver as a neighboring symbol. One of the effects of noise (which is unavoidable in communication) is to add a degree of uncertainty about which symbol has arrived at the receiver.

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Source: open.edu