MODULATION
What is Modulation?
Modulation is a mathematically complex subject which is difficult to explain quantitatively because it is easy to get lost in all of the formulas. Fortunately, it is simple to visually understand, which is how it is covered here.
Earlier, I described a digital signal as "riding on the back of the RF." Modulation is the way the information signal (analog or digital) is made to ride on the back of the carrier signal (the RF). This is done by taking the RF signal and superimposing the information signal onto it. The act of superimposing the information signal onto the RF (carrier) signal is called modulation, and the device which does the superimposing is called a modulator. In RF systems, the RF first gets modulated and then sent through the transmitter and out the antenna. After the signal arrives at the receiver, the process is done in reverse. The received signal gets demodulated (the RF carrier gets stripped away), leaving only the information signal.
As a way to visualize modulation, think of mailing a letter as wireless communication. The envelope is the RF (the carrier) and the letter inside is the information. To get a letter (the information) from point A to point B, a letter is placed in an envelope (the signal gets modulated) and dropped into the mailbox. Once the envelope arrives, the envelope is opened (the RF is stripped away) and voila, the letter (the information) has moved from point A to point B. When something is transmitted wirelessly, two signals are sent: the RF carrier (the envelope) and the information signal (the letter). The act of combining these two is called modulation.
You will recall from Chapter 3 that the goal of every source (oscillator) is to produce a perfect sine wave, which is the RF. The reason why a perfect sine wave is needed is because modulators superimpose the information signal onto the (perfect) RF signal by making tiny modifications to it. If the RF signal is not perfect, the imperfections may be incorrectly interpreted as modifications (information), which is unwanted.
Modulators and demodulators do what they do by changing some aspect of the RF signal (a perfect sine wave) in some specific way. Technically, they aren't really considered devices or components. They are better thought of as subsystems, which are combinations of two or more components.
Types of Modulation
In the world of wireless communications today there are literally dozens of different types of modulation used (and more being created every day). The good news is that all forms of modulation fall into one of three general categories: amplitude modulation (AM), phase modulation (PM), and frequency modulation (FM), which is used to broadcast FM radio. AM and FM are older forms of modulation which have been around since the early days of wireless communication. PM is the new kid on the block and the one which is used most frequently in today's (advanced) digital wireless communication systems.
The reason AM and FM came in to being first is that the sophisticated digital chips needed to implement PM just weren't around at the beginning of wireless communication. Digital wireless communication and PM evolved as a direct result of the advances in digital semiconductor integrated circuits.
AM, FM, and PM describe the three ways in which the perfect sine wave changes as it accepts the information signal. AM changes the height of the sine wave (as time goes by), FM changes the frequency of the sine wave (as time goes by), leaving the amplitude unchanged, and PM changes the phase of successive sine waves. These changes contain the information.
Amplitude Modulation
AM changes the amplitude of the perfect sine wave RF carrier as shown in Figure 5-6. The left side of Figure 5-6 shows how an unmodulated sine wave appears. Note that the sine wave is repeated many times. The right side of Figure 5-6 shows what happens to the sine wave after it has been amplitude modulated. Notice that the sine waves are still there and that the frequency is still the same (the space between successive sine waves is unchanged), but the amplitude (height) of each successive sine wave varies. The amplitude changes (or is modulated) from sine wave to sine wave, but the frequency is unchanged. If you trace your finger over the top of the signal on the right side you will notice that it follows the path of a sine wave, which is no coincidence.
AM has been around longer than any other modulation scheme, primarily because it is easy to implement. Of course, this ease of implementation comes with a price. All RF signals pick up noise as they move around. As mentioned in Chapter 3, noise is any imperfection in the (RF carrier) sine wave. More often than not, these imperfections manifest themselves as random changes in amplitude. So when noise changes the amplitude of the RF carrier, the RF system doesn't know whether the change is intended (as a result of amplitude modulation) or unintended (as a result of noise). What all this means is that AM signals are very susceptible to being distorted by noise.
The AM depicted in Figure 5-6 is a type of "analog" AM. In analog AM, the system modulates the RF carrier with an analog signal, e.g., a sine wave. In essence, it superimposes a sine wave onto a bunch of other sine waves, as can be seen in the figure. Analog AM has been around for a long time and not much has changed—until very recently.
Binary Amplitude Shift Keying
There is a new generation of AM, which is digital in nature, known as Binary Amplitude Shift Keying (BASK). Unlike "analog" AM, which superimposes an analog (sine wave) signal onto the RF (sine wave) carrier, BASK superimposes a digital signal (like the one in Figure 1-3) onto the RF (sine wave) carrier (see Figure 5-7). Notice how the shape of the RF carrier mimics the shape of the digital signal.
Figure 5-6. An unmodulated and an amplitude modulated RF carrier.
BASK is less noise-sensitive than analog AM. BASK signals are still susceptible to random changes in amplitude from noise, but since the "smarts" of the RF system only have to differentiate between a "high" and a "low," slight changes in either amplitude (high or low) will not cause the system to misinterpret one as the other. BASK is used in today's digital wireless systems because it is less susceptible to noise.
Figure 5-7. Binary Amplitude Shift Keying (BASK)
Frequency Modulation
FM describes a second way a perfect sine wave can be made to vary (see Figure 5-8). Notice in this case that the amplitude remains the same (from unmodulated to modulated), but the frequency changes (the space between successive sine waves changes). This change in frequency (as time goes by) actually contains information, like a human voice on a cellular phone call. (As hard as it is to believe, it's true.)
FM is not as sensitive to noise as AM, which is why it came about. Like AM signals, FM signals are still susceptible to random changes in amplitude from noise, but since the "smarts" of the system is only looking for changes in frequency, the system disregards changes in amplitude. Before you start thinking that FM sounds like a free lunch—you should know better, there is also a type of noise which affects a signal's frequency, but that's another story.
Phase Modulation
PM is the third way a perfect sine wave can be made to vary. PM is similar to FM in that the amplitude is unchanged while the spacing between successive sine waves changes. Much of the digital information today is modulated onto the RF carrier by way of phase modulation.
There are many different types of PM used in digital wireless communications. The reason that so many exist is that the different modulation techniques evolved as the semiconductor technology evolved. The latest and greatest PM techniques utilized today just weren't possible with the electronic components of 10 years ago. A sampling of some of the more popular PM techniques used in digital wireless communication are shown in Table 5-3.
Figure 5-8. An unmodulated and a frequency modulated RF carrier.
| Acronym | Phase Modulation |
|---|---|
| MSK | Minimum shift keying |
| BPSK | Bi-phase shift keying |
| QPSK | Quadrature phase shift keying |
| DQPSK | Differential QPSK |
| GMSK | Gaussian minimum shift keying |
All you really need to know about the different phase modulation techniques is that they all take information in digital form and translate it into changes in the sine wave spacing (the phase) of the RF carrier, similar to that shown in Figure 5-8.
Quadrature Amplitude Modulation
The newest form of modulation used in today's digital wireless systems is Quadrature Amplitude Modulation or QAM (pronounced kwäm). QAM is simply a combination of (digital) AM and PM. It has all the noise advantages of BASK and PM and, since each successive sine wave can be modified two ways (amplitude and phase), QAM can impart a lot of information onto each sine wave of an RF carrier.
Modulators and Demodulators
RF signals get modulated by modulators, which are fairly complex devices, but they can all be represented by a simple block diagram, as shown in Figure 5-9.
Figure 5-9. Block diagram of a modulator.
At their simplest, modulators have two inputs and one output. One input is the "information" input, which can be in analog or digital form. In Figure 5-9, the information is in digital form. The other input is the RF carrier (a perfect sine wave). When the modulator gets done doing its thing, out comes a signal which is a composite of the two signals. This is the modulated output signal, and in Figure 5-9, it is BASK modulation. And, of course, demodulators do the exact opposite: they take a modulated signal and break it down into an information signal and a carrier signal. After demodulation, the carrier is no longer needed and therefore it is disregarded while the information signal is sent somewhere else in the system for further use.
How do modulators and demodulators do what they do? It depends on the type of modulation of course, but it is safe to assume that there is one or more mixers involved. In terms of their location in an RF system, modulators come before the transmitter and demodulators come after the receiver (see Figures 3-1 and 3-2).
Did You Know?
Technically, saying "QAM modulation" is redundant, because QAM modulation literally means Quadrature Amplitude Modulation modulation. The same goes for "AM modulation" and "FM modulation." But alas, it's spoken that way by convention, and because "QA modulation" just sounds too awkward.