Modulation is the process of processing the coded information of the signal source to make it suitable for transmission. Generally speaking, this means that the baseband signal (source) is transformed into a band communication signal with a very high frequency relative to the baseband frequency. This band communication signal is called modulated signal, and the baseband signal is called modulated signal. Modulation can be accomplished by changing the fluctuation, phase or frequency of the high-frequency carrier with the change of signal fluctuation. Demodulation is the process of extracting the baseband signal from the carrier for the predetermined receiver (sink) to process and understand.
This chapter describes various modulation skills used in mobile communication system. It includes analog modulation schemes for the first generation mobile communication system and digital modulation schemes for current and future systems. Because digital modulation has many advantages and has been used to replace the traditional imitation system, this chapter focuses on the digital modulation scheme. However, because the imitation system is still widely used and will continue to exist, the imitation modulation is introduced first. This chapter will describe many practical FM skills, receiver architecture, trade-offs in planning, and their functions under different channel loss types.
Frequency modulation (FM) is the most common imitation modulation skill in mobile communication system. During frequency modulation, the fluctuation of the modulated carrier signal remains unchanged, and the frequency changes with the change of the modulated signal. In this way, the FM signal contains all the information in the phase or frequency of the carrier. As will be seen later, as long as the reception signal reaches a specific minimum value (FM threshold), the reception quality will make nonlinear and rapid progress. In amplitude modulation (AM), there is a linear relationship between the quality of the received signal and the energy of the received signal, because amplitude modulation superimposes the fluctuation of the modulation signal on the carrier, so that the amplitude modulation signal contains all the information in the fluctuation of the carrier. FM has many advantages over am, which makes FM better choice in many mobile communication applications.
FM has better anti noise function than am. Because the FM signal shows the change of frequency rather than the change of fluctuation, the FM signal is less vulnerable to the influence of atmosphere and impulse noise, which will form the agile fluctuation of the received signal. In addition, in FM, because the change of signal fluctuation does not carry information, as long as the received FM signal is above the FM threshold, the impact of sudden noise on FM system is not as great as that on am system. In Chapter 4, we explain how small-scale fading leads to the agile shaking of the received signal. Therefore, FM has better anti fading function than am. In addition, in the FM system, we can make a compromise between bandwidth and anti noise function. Different from the AM system, the FM system can obtain better signal-to-noise function by changing the modulation index, that is, the occupied bandwidth. We can see that under certain conditions, the signal-to-noise ratio (SNR) of FM system can be increased by 6dB every time the bandwidth occupied by FM system is doubled.
The ability of FM system to exchange bandwidth for SNR may be the most important reason why it is superior to am system. However, AM signals occupy less bandwidth than FM signals. In the modern am system, because the in band pilot tone is transmitted together with the standard AM signal, its sensitivity to fading has been greatly improved. Modern AM receiver can monitor the pilot tone and quickly adjust the reception gain to compensate for the fluctuation of the signal.
Because the envelope of FM carrier does not change with the change of modulation signal, 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 a fixed value. And the constant envelope of the transmitted signal allows the use of class C power amplifier in RF power amplification. In amplitude modulation, because it is necessary to adhere to the linear relationship between signal and transmission signal fluctuation, it is necessary to use low-power amplifiers such as linear class A or class ab.
When planning a portable user terminal, the power of the power amplifier is a very important problem because the battery service time is closely related to the power of the power amplifier. The power of a typical class C amplifier is 70%, that is, 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 AB amplifiers is only 30 ~ 40%, which means that with the same battery, the operation time of constant envelope FM modulation is twice as long as that of AM method. FM has a characteristic called capture effect. The capture effect is the direct result of the rapid progress of nonlinear admission quality with the addition of admission power. 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 lose other signals makes the FM system have a strong ability to resist co channel interference and provide better reception quality. On the other hand, in the AM system, all disturbances are accepted together, so it is necessary to remove the disturbances after demodulation.
Although FM system has many advantages over am system, it also has disadvantages. In order to reflect its advantages in noise reduction and capture effect, FM system needs to occupy more bandwidth in the transmission medium (generally several times of AM). And FM transmitting and receiving equipment are more complex than am system. Although the FM system can tolerate the nonlinearity of specific types of signals and circuits, special attention should be paid to its phase characteristics. Both AM and FM can be demodulated with low-cost uncorrelated demodulators. Am can be easily demodulated by envelope detector, while FM can be demodulated by frequency discriminator or skew detector. Am can use the product detector for correlation demodulation. In this case, because the FM signal is only useful when it is above the threshold, the function of AM is better than FM under the condition of weak signal.