Pilot symbol design for multiuser CDMA systems

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Due to the development of advanced processor architecture and better fabrication technologies of chips mobile devices have shrinked in size and improved in processing power. These pocket devices are now able run data hungry applications like browsing internet capturing and transferring video and audio files. To keep up with these advances in the devices new efficient algorithms on physical layer have to be developed to increase the capacity of the system and improve the utilization of available bandwidth. Code Division Multiple Access system support the best data rate amongst all the existing multiple access technologies, the performance of CDMA is further improved by using Multiuser detection. Most of the research focus for Multiuser CDMA system has been on the detection and channel estimation algorithms. There is a ceiling to the improvement of system performance for any give estimation algorithm. To further improve the performance of the system, the pilot sequences have to be optimized.There are lots of algorithms available for the design of training sequences for single user systems; these pilots cannot be directly applied for multiuser system as they degrade the correlations of the PN sequences and consequently the system performance. The preexisting training sequence design algorithms based on LS estimates cannot be directly used for CDMA systems as these pilot sequences are very long and the search space for optimization becomes very large. The time domain (TD), frequency domain (FD) based approaches of pilot design are also unable to reduce the search space for optimization to a manageable size. A novel approach for design of multiuser pilot training sequences is proposed in this thesis; this method optimizes the existing training/pilot sequence to improve the channel estimation. It is further confirmed via simulation that performance of both the LS and MMSE algorithms improves.