Electrical & Computer Engineering, Department of


Date of this Version

August 2007


A Dissertation Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements for the Degree of Doctor of Philosophy. Major: Engineering (Electrical Engineering). Under the Supervision of Professor Lance C. Pérez Lincoln, Nebraska: August, 2007.
Copyright © 2007 Chen Xia.


A collision frequency shift keyed system (CFSK) using M-ary FSK signaling is investigated. CFSK is a wireless multiple access system in which users access a channel without cooperation. This results in collisions when users simultaneously transmit signals in the same frequency band. However, analysis has shown that the CFSK system has the potential to achieve greater capacities than binary multiple access systems.

In this dissertation, bounds on the multiuser capacity of the noisy synchronous and asynchronous CFSK channel are derived. Analytical models are designed for investigating these capacity results. It is shown that the capacity of the noisy synchronous CFSK channel is greater than the capacity of multiuser direct-squence (DS) and frequency-hopping (FH) CDMA systems. The capacity of the asynchronous CFSK channel is significantly high in the over-loaded case and this capacity is greater than most asynchronous CDMA systems.

Multiuser detection on the CFSK channel is then investigated. Due to the extremely high complexity of the optimal multiuser detector, iterative multiuser detectors are considered. Three simplified iterative multiuser detectors are derived, the wide sense most probable (WSMP) combinations detector, the narrow sense most probable (NSMP) combinations detector and the relaxation detector. The WSMP detector utilizes the K-most probable combinations (K-MPC) and the k-most probable frequencies (k-MPF) metric. Simulations show that these detectors can achieve near single-user performance under a variety of load conditions. The relaxation detector, in particular, offers excellent performance.

Randomly generated convolutional codes are considered for use in the CFSK system. The choice of code parameters for different detectors and different transmission rates are discussed. Simulation results show that with the proper choice of code parameters, it is possible to achieve performance near that of uniquely decodable codes. In addition, the achieved rates surpass the rates for most CDMA systems.