RF Transceivers for Wireless Applications
13.1 Transmitter Architectures
13.1.1 Modulation Overview
To have a better understanding of transmitter architectures, first we briefly study the modulation types by categorizing them into two groups: constant-envelope and variable-envelope modulations.
18.104.22.168 Constant-Envelope Modulation
If the transmitter signal is specified as A(t) cos (ωct + Φ(t)), for a constant-envelope modulation, as the name implies, A(t) is fixed. This gives significant flexibility in the implementation of the transmitter because the transmitter signal is not affected by any nonlinearity in the transmitter. This especially provides considerable advantage for the power amplifier (PA) using a nonlinear class for higher efficiency. One good example of this type of modulation is the Gaussian minimum shift keying (GMSK) used in the global system for mobile communication (GSM) and general packet radio service (GPRS). Since that nonlinearity in this case does not change the zero crossing of the signal, it distorts only the amplitude of the carrier. This distortion does not produce any “spectral regrowth” and generates only harmonics of the carrier that can be substantially attenuated by the amplifier low-Q filters (LC tank).
22.214.171.124 Variable-Envelope Modulation
Although constant-envelope modulation is a very attractive option due to its relaxed linearity requirements and improved ...