Evaluation of TDOA Techniques for Position Location in CDMA Systems

The Federal Communications Commission (FCC) has adopted regulations requiring wireless communication service providers to provide position location (PL) information for a user requesting E-911 service. The Time Difference of Arrival (TDOA) technique is one of the most promising position location techniques for cellular-type wireless communication systems. The IS-95 Code Division Multiple Access (CDMA) system is a popular choice for the companies deploying new cellular and PCS systems in North America. Hence, the feasibility of TDOA techniques in CDMA systems is an important issue for position location in the wireless systems of the future.

This thesis analyzes the performance of TDOA techniques in the CDMA systems. A comparison and assessment of different algorithms for finding the time difference estimates and for solving the hyperbolic equations generated by those estimates has been made. This research also considers a measure of accuracy for TDOA position location method which is shown to be more suitable for CDMA systems and more closely matches to the FCC requirements. Among the other contributions is a proposed method to perform cross-correlations to identify only the desired user's TDOA in a multiuser environment.

This thesis also evaluates the feasibility and accuracy of TDOA techniques under varying system conditions that might be encountered in real situations. This includes varying conditions of Additive White Gaussian Noise (AWGN), Multiple Access Interference (MAI), power control and loading. The effect of the mobile position and of different arrangement of base stations on TDOA accuracy is also studied. Performance comparison in AWGN and Rayleigh fading channels is made. The feasibility of using increased power levels for the 911 user in combination with interference cancellation is also studied. The effect of using a single stage of parallel interference cancellation at neighboring cell sites has also been explored. Non-ideal situations such as imperfect power control in CDMA operation has also been investigated in the context of position location. This thesis also suggests amethod to correct TDOA estimation errors in CDMA. It is shown that this improvement can give greatly improved performance even under worst-case situations. Performance comparison of results with and without that modification has also been made under various conditions.