Receiver structures for a fast-frequency-hopped spread spectrum mobile communications system.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Code Division Multiple Access Technologies have recently received considerable attention as a means of increasing the capacity of a mobile network. They inherently provide for increased system performance together with some resistance to the effects of the mobile channel. While direct sequence spread spectrum systems have been extensively investigated the alternative fast frequency hopped systems have largely been ignored. This thesis considers fast frequency hopped spread spectrum technology as an access method for providing a mobile communication network. Such systems have been previously used by the military to provide reliable communication between a number of parties, in a hostile interference limited channel. The multiple-access mobile communication channel has many similarities to the channel encountered in the military applications. This thesis examines the effect of the interference encountered in the mobile multiple-access communication channel on the transmitted signal. Suitable receiver structures for the channel are then derived which provide for efficient communication in the interference limited channel. These receivers operate to minimise the effect of the interference on the received signal. The multiple-access interference encountered in the channel limits the performance of conventional receivers designed to detect a single user's transmitted signal. This thesis derives a model for the effect of the multiple-access interference on these receivers, A model is provided for the effects of the time-varying frequency-selective multi-path fading encountered by the proposed system in the channel. The design of appropriate receivers for the mobile channel is then considered. Receivers are developed which minimise the effect of interference on the received signal. A prediction process is employed to minimise the effects of the multi-path fading on the received signals. An optimum multi-user receiver is developed. To minimise the effect of the multiple-access interference the optimum receiver simultaneously detects each of the signals present in the channel. The complexity of this receiver limits its application. The derivation is then extended to obtain a less complex sub-optimum receiver based on the optimal structure. This receiver employs an averaging process to approximate the effect of the multiple-access interference on the desired user's signal. A prediction process is again employed to minimise the effect of the multi-path fading channel on the received signals. The effect of the multi-path channel on the interfering users' signals is included in the averaging process. The performance of the receivers is evaluated through simulation. In the multiple-access, multi-path channel the performance of the sub-optimum receiver is found to approach that of the optimum receiver when there is a low level of multiple-access interference. As this interference increases the performance of the receiver suffers.