Department of Electrical and Computer Engineering
Ph.D. Thesis Abstract
Efficient Access and Interference Management for CDMA Wireless Systems
Code Division Multiple Access (CDMA) has emerged as a promising access scheme to meet the ever growing capacity demand of wireless services. This thesis investigates several issues related to the characterization and enhancement of the capacity of circuitswitched and packetswitched CDMA systems.
In circuit-switched CDMA systems, there exists continuous communication between a user and the base station. The capacity is limited by the mutual interference between the users and can be enhanced by efficient interference management. This thesis investigates interference management techniques that combine receiver based temporal and spatial processing and transmit power control.
A nonlinear programming approach is used to develop approximate solutions for the optimum multiuser bit detection problem. This approach is helpful in understanding the convergence properties of various iterative multiuser detectors. A new detector which approximates the optimum detector is derived.
Next, this thesis investigates the problem of combined multiuser detection and receiver beamforming for CDMA systems with base station antenna arrays. In this case, it is possible to further increase system capacity by employing both temporal (multiuser detection) and spatial (beamforming) filtering. Filter structures are derived and analyzed and the performance of the proposed schemes are compared.
Power control is an interference management technique that balances the received powers of the users so that no one user creates excessive interference to another. This thesis investigates the improvement possible by adding transmit power control to temporalspatial filtering. Iterative power control algorithms are derived and analyzed where transmit powers and filters of the users are updated to achieve minimum total transmit power of all users while guaranteeing each user its required quality of service.
When advanced receivers, such as the temporalspatial filters discussed in this thesis, are not available at the base station, cell sectorization is an effective interference management method to reduce the interference each user experiences. This thesis studies simple spatial capacity enhancement techniques and gives the solution of the joint transmit power control and cell sectorization problem where sectors are designed in response to the spatial density of users.
This thesis provides an introduction for the characterization of the user capacity of connectionless packetswitched CDMA systems. In such systems, users keep no connection to the base station unless they are actively sending a packet. Thus, users and their timings have to be acquired each time they try to send a packet. No signature sequence assignment is assumed. This thesis investigates the multiuser access capacity of such systems and concludes that the connectionless system considered is limited by the efficiency of the timing acquisition process.
Ph.D. Dissertation Director: Professor Roy D. Yates
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