Analysis of frequency hopping CDMA systems with emphasis on new coding families and improved receiving techniques.

摘要

This dissertation addresses several issues pertaining to frequency hopping CDMA systems: coding families used for address assignment, a receiving technique and its performance in a mobile environment, and application of error correction coding techniques to such systems.; The main problem in coding design is to construct large coding families that have good orthogonality properties while maintaining good synchronization capabilities. Coding families designed in this proposal are algebraically constructed which facilitates the evaluation of these properties. It is shown that their auto- and cross-hit properties are comparable to those of the coding families previously proposed, but that their size is proportional to the square of the code length. The size of previously proposed coding families was proportional to the length of the code. Increased family size allows for more users to be accommodated in the system or for an increase in the transmission rate. It also opens up the possibility of a system in which the users can have different transmission rates.; A new receiving technique based on soft decoding is presented and its performance evaluated for the forward and reverse channels, with and without fading, The forward channel is characterized by the phase coherence between transmitted signals. For the reverse channel the phase coherence does not exist because of the unequal distances between the base station and mobile users and also because of their mobility. It is shown that phase incoherence and fading degrade the capacity of the system, i.e. the maximum number of users in the system for the specified probability of error. Nevertheless, the system capacity for all four cases increases in a faster than linear manner as the length of the code used for the address assignment increases. The capacity of the systems with different transmission rates is also calculated.; Finally, the effects of the error correction coding on the system performance, mainly the symbol error probability, are investigated. The performance of both block and trellis codes is evaluated and it is shown that it is possible to reduce the error probability in the system by only increasing the complexity of the transmitter and the receiver.