Modulation is one of the most frequently used technical words in communications technology. One good example is that of your FM radio, where FM stands for frequency modulation. In this webpage we are going to learn the basics of modulation techniques and see how they are applied in modern cellular and communications technologies.
Length of an antenna and the wavelength of the wave to be transmitted are directly connected.
This is the very basis of the antenna technology. This fact is illustrated in Fig :1. Humans have the capability to hear sound frequency from 20 to 20 kHz, but if a radio tower transmits electromagnetic waves of the same frequency, the size of the antennas required will be really high. Here we can see the size of the antenna is proportional to the wavelength. If we had transmitted the electromagnetic wave in the frequency of sound(20-20kHz), the antenna size required would have been in the range of kilometers. This is why we need modulation; before the electromagnetic waves are transmitted they should be modulated to a high-frequency signal.
C= ƒ ×λ
for ƒ = 10kHz
3× 108 = 10 ×103 × λ
L= λ/2 = 15km
Here, L= Size of the antenna
We can understand the way we modulate the signals with a simple analogy. Try throwing a piece of paper as shown in Fig: 2A; it won’t go far. Now, tie it to a stone and throw it again as shown in Fig:2B. The second method is obviously more efficient than the first one. This is exactly how we do modulation. In place of a stone, modulation uses a high frequency signal, known as a carrier signal. As we know any signal has three basic properties: amplitude, frequency and phase. In the modulation process, one of the properties of the carrier signal is varied in accordance with the message signal.
There are mainly two types of modulation:
1. Analog Modulation
2. Digital Modulation
In analog modulation, the characteristics of the carrier signal is varied according to the instantaneous value of the message signal. Few analog modulation techniques are explaining here.
If the frequency of the carrier signal is varied according to the amplitude of the message signal, this technique is known as frequency modulation (Fig: 3). The new signal formed with this frequency modulation is capable of travelling long distances. Please note that the frequency of a carrier signal is always high, which means the modulated signal is also of high frequency and energy, thus can be easily propagated over long distances. The value of the original signal can be easily retrieved from the frequency of the modulated signal.
AM is the oldest technique and it was used in broadcasting of radio programs. In the same way we can also achieve amplitude modulation. Here the amplitude of the carrier signal is varied based on the value of the message signal (Fig: 4). AM is the oldest technique that was used in broadcasting of radio programs.
The modulation techniques, we have discussed so far, have all been analogue types; however, they are already obsolete. Analogue modulation is susceptible to noise, which degrades the quality of signals, and moreover, in today’s electronic instruments all operations are carried out in digital form, where the digital signals are either a 1 or a 0. So let’s discuss the digital modulation techniques that are currently used, more specifically, let’s see how the digital bit flow is converted to an electromagnetic wave.
In digital modulation the characteristics of the carrier signal is varied according to the discrete value of the message signal. There are several types of digital modulation techniques are available. Few digital modulation techniques are explaining here.
The first digital technique is Amplitude Shift Keying. Here, based on the digital pulses, the amplitude of the carrier signal is adjusted. High amplitude relates to 1 and low amplitude relates to 0. Following is the diagram for ASK modulated signal along with message signal as shown in Fig: 5.
The next technique is called Frequency Shift Keying. Here, based on the value of digital pulses, the frequency of the carrier signal is adjusted. In this case high frequency relates to 1 and low frequency relates to 0. Following is the diagram for FSK modulated signal along with message signal (Fig:6).
The third technique is Phase Shift Keying. Here, the phase of the carrier signal is changed by 180 degrees when the digital pulse moves from 1 to 0 or 0 to 1. PSK modulation technique is shown in below (Fig:7).
Telecommunications technology is all about increasing data transfer speed and efficiency, but if you use any of the digital modulation techniques explained previously, you wouldn’t get a high data transfer speed. However, there is a technique in physics, which, if you use it, means you can practically send up to 6 bits of information as a single electromagnetic wave. This technique is known as Quadrature Amplitude Modulation.
To understand QAM in an easy way let’s take two analogue signals. The beauty of QAM is that you can modulate these two different signals as a single signal and then transmit it (Fig: 8A), then at the receiver end you will be able to separate out the original signals (Fig: 8B) , thereby saving bandwidth.
Let’s see how this modulation is done. In QAM the first signal is amplitude modulated using a carrier wave (Fig:9). The second signal is also amplitude modulated with a carrier wave of the same frequency and amplitude, but after giving the carrier signal a 90-degree phase shift. Now, these two modulated signals are mixed together and form a single signal. We call it a multiplexed signal. The interesting thing is that on the receiver side we can easily separate out the original signals from the multiplexed signal.
In the case of digital QAM, a similar approach is used. Here instead of analogue signals, different combinations of bits are added together to produce a multiplexed signal. Let’s see how a 16-QAM works.
If you are familiar with digital technology, you know that any form of data is just a collection of 1s and 0s. For example, this image can be represented as a huge collection of 1s and 0, this is illustrated in (Fig: 10).
In 16 QAM, we can pack 4 bits together and send it as a single electromagnetic wave. Based on the values of the 4 bits, this output will have different phase angles and amplitude. This means, the phase angle and amplitude of the multiplexed signal can completely represent 4 bits of data. In 16-QAM, such 16 bit values, can be represented by adjusting the phase and amplitude of the multiplexed signal, and this single multiplexed signal is then used for the transmission. You can see how the different amplitude and phase electromagnetic signals represent various 4 bits of data as shown in Fig:11A and Fig:11B.
Using a similar technique to that used in analogue modulation, here the amplitude modulated signals are also mixed together and finally a single output is produced. As we have seen in this modulation, two carrier signals that are out of phase by 90 degrees are used. Hence the word ‘quadrature’ is used to refer to this technique, this is illustrated in Fig:12B.
If, instead of QAM, we had used a normal modulation technique to send bits of data, we would have used 4 electromagnetic signals. Thus 16 QAM increases the data transfer speed by 4 times (Fig :13).
Scientists have even achieved 64QAM, which is used in 4G communications. 64QAM uses 6 bits of data at a time thus making the data transfer speed 6 times faster compared to a normal modulation technique.
The modulation techniques are not restricted to only cellular communication and FM radio, but also have applications in television broadcasting, WiFi, optical fibers etc.
This article is written by Prerna Gupta, a post graduate in Control and Instrumentation. Currently she is working at Imajey consulting engineers pvt. ltd. as a Visual Educator. Her areas of interest are Telecommunication, Semiconductor Material and devices, Embedded systems and design. To know more about the author check this link