TABLE OF CONTENTS Arguably the most important element of any software defined radio system, whether in a base station or handset, is the linear or linearised transmitter. Receiver systems have always required a high degree of linearity, as they must possess a good strong signal handling capability, in addition to good low-noise performance. In the case of transmitters, however, a high degree of linearity is a relatively recent requirement, arising predominantly from the widespread adoption of cellular networks. Transmitters used in this type of application require a much greater degree of linearity (i.e., a much lower level of distortion) than even single-sideband (SSB) linear transmitters used in the past (e.g., for military applications). This is due to the near-far effect present in cellular systems ( [1], Chapter 1), which results in transmitter non-linearities causing significant interference to users of adjacent channels, thereby limiting system capacity. This limitation affects both uplink and downlink, depending upon which transmitter has the non-linearity problem: If it is the handset transmitter, the uplink capacity of a nearby cell will be impacted; if it is a BTS transmitter problem, the downlink capacity of a nearby cell will be impacted. Even with the high-linearity transmitters available today, many city-centre systems are currently interference limited (in terms of capacity) rather than noise limited. Highlinearity transmitters are therefore an enabling technology for many cellular systems, irrespective of the use (or otherwise) of a software defined radio-based architecture in their realisation.
In the case of a generic software defined radio...
