Modulation Schemes Effect on RF Power Amplifier Nonlinearity and RFPA Linearization Techniques

In radio communications, modulation can be described as the process of conveying a message signal by superimposing an information bearing signal onto a carrier signal by varying the signal characteristic [31–33]. Modulation is the process of changing a higher frequency signal in proportion to a lower frequency one or vise versa. The higher frequency signal is referred to as the carrier signal and the lower frequency signal is referred to as the information bearing message signal or modulating signal [31–33]. The characteristics (amplitude, frequency or phase) of the carrier signal are varied in accordance with the information bearing signal. These high-frequency carrier signals can be transmitted over the air, over fiber or coax cable. The use of high frequency signals will make the amplifier and antenna design easier for effective radio design [32, 33]. Figure 2.1 shows the up conversion of the complex-valued baseband signal x’(t) to the passband then the transmission of the real-valued bandpass signal x(t) through the communication channel [31]. After the bandpass signal goes through the channel, a down-conversion of the bandpass output Y(t) into a complex-valued baseband signal Y’(t) occurs. The baseband signal x’(t) is up-converted to the bandpass signal by amplitude, phase or frequency modulation in order to transmit it. The modulated bandpass signal x(t) can be described as