Modulation is the process of processing the encoded information of a signal source to make it suitable for transmission. Generally speaking, this means converting the baseband signal (source) into a band communication signal with a very high frequency relative to the baseband frequency. This band communication signal is called a modulated signal, while the baseband signal is called a modulated signal. Modulation can be accomplished by changing the fluctuation, phase, or frequency of the high-frequency carrier wave with the fluctuation of the signal. Demodulation is the process of extracting baseband signals from the carrier for processing and understanding by the intended recipient (sink).
This chapter describes various modulation techniques used in mobile communication systems. It includes analog modulation plans for the first generation mobile communication system and digital modulation plans for today’s and future systems. Because digital modulation has many advantages and is now used to replace traditional simulation systems, the focus of this chapter is on digital modulation plans. However, since imitation systems are still widely used and will continue to exist, we will first introduce imitation modulation. This chapter will describe many practical FM techniques, receiver architecture, tradeoffs in planning, and their functions under different channel loss types.
Frequency modulation (FM) is the most common analog modulation technique in mobile communication systems. During frequency modulation, the fluctuation of the modulated carrier signal remains unchanged, while the frequency changes as the modulated signal changes. In this way, FM signals contain all the information in the phase or frequency of the carrier wave. As will be seen later, simply reaching a specific minimum (FM threshold) for the acceptance signal will result in a non-linear and agile improvement in acceptance quality. In amplitude modulation (AM), there is a linear relationship between the quality of the received signal and the energy of the received signal, because AM superimposes the fluctuations of the modulated signal on the carrier wave, so that the amplitude modulated signal contains all the information in the fluctuations of the carrier wave. FM has many advantages over AM, which makes FM a better choice in many mobile communication applications.
FM has a better anti noise function than AM. Because FM signals exhibit changes in frequency rather than fluctuations, FM signals are less susceptible to atmospheric and impulse noise, which can form a rapid wobble to accept fluctuations in the signal. In addition, in FM, because changes in signal fluctuations do not carry information, as long as the received FM signal is above the FM threshold, the impact of sudden noise on the FM system is not as significant as on the AM system. In Chapter 4, we explained how small-scale fading can lead to agile fluctuations in the reception signal, which shows that FM has a better anti fading function compared to AM. In addition, in FM systems, we can compromise between bandwidth and anti noise capabilities. Unlike AM systems, FM systems can achieve better signal noise functionality by modifying the modulation index, which is the bandwidth occupied. We can see that under certain conditions, the signal to noise ratio (SNR) of an FM system can be increased by 6dB for each time the bandwidth occupied is doubled.
The ability of FM systems to exchange bandwidth for SNR may be the most important reason why they are superior to AM systems. However, AM signals occupy less bandwidth than FM signals. In modern AM systems, the sensitivity of in-band pilot tones to fading has been greatly improved because they are transmitted together with standard AM signals. Modern AM receivers can monitor pilot tones and quickly adjust the reception gain to compensate for fluctuations in the signal.
Because the envelope of the FM carrier wave does not change with the modification of the modulated signal, the FM signal is a constant envelope signal. In this way, regardless of the fluctuation of the signal, the power transmitted by the FM signal is fixed. And the constant envelope of the transmitted signal allows the use of Class C power amplifiers for RF power amplification. In amplitude modulation, because it is necessary to adhere to the linear relationship between the fluctuations of the signal and the transmitted signal, it is necessary to use low-power amplifiers such as linear class A or class AB amplifiers.
When planning a portable user terminal, the power of the amplifier is a very important issue because battery life is closely related to the power of the power amplifier. A typical Class C amplifier has a power of 70%, which means that 70% of the DC signal power at the end of the amplifier circuit is converted into the transmitted RF signal power. The rate of Class A or Class AB amplifiers is only 30 to 40%, which means that using the same battery and constant envelope FM modulation, the operation time is twice as long as using the AM method. FM has a characteristic called capture effect. The capture effect is a direct result of the agile progress of nonlinear admission quality as admission power is added. If two signals of the same frequency band appear on the FM receiver, the stronger signal will be accepted and demodulated, and the weaker signal will be discarded. This inherent ability to select the strongest signal and discard other signals gives FM systems a strong ability to resist co channel interference and provide better one-sided acceptance quality. On the other hand, in an AM system, all disturbances are accepted together, so it is necessary to remove the disturbances after demodulation.
Although FM systems have many advantages over AM systems, they also have disadvantages. In order to demonstrate its advantages in noise reduction and capture effects, FM systems require greater bandwidth (typically several times the bandwidth of AM) in transmission media. And both FM transmitting and receiving devices are more complex than AM systems. Although FM systems can tolerate the nonlinearity of certain types of signals and circuits, special attention should be paid to their phase characteristics. Both AM and FM can be demodulated using inexpensive uncorrelated demodulators. AM can be easily demodulated using an envelope detector, while FM can be demodulated using a frequency discriminator or a skew detector. AM can perform correlation demodulation using a product detector. In this case, AM performs better than FM in weak signal conditions because FM signals are only useful above the threshold.