TABLE OF CONTENTS In the mobile radio, since the multipath fading distorts the amplitude of the carrier, the signal is sent by modulating the phase or frequency of the carrier, which has no impact on the amplitude. We call those modulations constant envelope modulations; that is, no signal is modulated on the amplitude. The distortion of carrier amplitude by other factors such as fading or nonlinear amplification will not affect the signal. Therefore, it is possible to use a nonlinear amplifier. Although, the non-linear amplifier can distort the amplitude but not the phase, as we have mentioned in the inverse-sine modulation method in Sec. 12.5. But the intermodulation introduced by the AM-PM conversion in a nonlinear amplifier can be taken care of by a few particular modulations.
Constant envelope modulation can be a linear or a nonlinear modulation in digital mobile systems. In general, constant envelope modulation is used for the nonlinear modulation (FM) in analog mobile systems.
QPSK is used for increasing the modulation efficiency from the BPSK. In BPSK (binary PSK) as mentioned previously, one symbol phase 0 or 180 at the modulation stage represents one bit. The modulation is one bit per second per Hz. In QPSK, one symbol (one of four phases) at the modulation stage represents two bits. The modulation efficiency is double. The possible combination of two bits is mapped according to the gray code the adjacent symbols (phase states) offer by one bit 00, 01, 11,...
