5.1. Introduction

The transmitter is the companion of the receiver in wireless mobile stations. They operate simultaneously in a full-duplex system, but they may run in different time slots in a half-duplex system. Similar to a receiver, the architecture of a transmitter can be one of the following: superheterodyne, direct conversion, or band-pass sampling. The corresponding block diagrams of these architectures are presented in Fig. 3.1, Fig. 3.10, and Fig. 3.30, respectively. The low IF architecture may not be necessary for the transmitter since the DC offset is relatively easy to be tuned out or compensated, and the noise figure is not as critical in the transmitter as in the receiver. The selection of the transmitter architecture will depend on system performance requirements, size, cost, IC technology maturity, current consumption, etc. One thing that should be noticed is that the transmitter usually does not employ an IF SAW filter even in a superheterodyne transmitter. The cost reduction from using a direct conversion transmitter, thus, cannot count on the elimination of the IF SAW filter as is commonly done when employing a direct conversion receiver.

The signal processed and amplified in a transmitter unlike a received signal is deterministic since it is generated in the local digital base-band. The signal level in the transmitter is normally much higher than that in a receiver. The important parameters for a transmitter are the output power, especially the maximum output power, which may be seen as a counterpart of the receiver sensitivity, and...