In the mobile multipath environment the received signal can be adversely affected by unwanted, random envelope modulation. With AM-type systems these envelope variations are impressed directly on to the receiver's demodulated output. Automatic Gain Control (AGC) can be used to reduce the unwanted envelope fading without distorting the required modulation. This thesis begins by analysing mobile radio propagation in some detail and develops worst-case deterministic test signals to study the performance of AGC. The dynamic performance of a variety of AGC systems is then described along with their application to mobile radio receivers. Initially, conventional feedback AGC systems are discussed. These are shown to be capable of suppressing deep, unwanted fading only if it occurs at rates well below that of the wanted envelope modulation. The effects of time delay and the use of coherent envelope detectors are analysed and shown to worsen the dynamic performance of feedback AGC. Feedforward AGC systems, which can possess greatly improved dynamics, are then introduced. A form of feedforward AGC is developed which can achieve a specified dynamic performance with the narrowest of all control bandwidths. Finally, the effects of signal-carrier decorrelation, noise and interference are shown to limit the ultimate performance of all AGC systems.
|Date of Award||1982|