Linear Distortion in Cellular Telephone Systems

Advisor: Prof. Michael Pursley

Third-generation cellular telephone systems will be developed over the next few years, and research is already underway for the generation beyond the third. Many of these cellular systems will employ a modulation technique known as code-division multiple access (CDMA), which is actually a form of direct-sequence spread spectrum. One project for future undergraduate research is to investigate the designs for CDMA systems and assess the effects of linear distortion on the CDMA signals. For future CDMA systems that require much higher data rates, the bandwidth limitations imposed by available technology and existing frequency allocations pose a serious problem. Use of filtering to reduce the bandwidth may produce linear distortion of the CDMA signals. Alternatively, chip waveform shaping may be used fit the CDMA signals within the allocated frequency band. The present CDMA cellular system (TIA/EIA-95-B) employs baseband filtering to shape the signal and reduce its bandwidth.

System designers need a quantitative evaluation of the impact of linear distortion and chip waveform shaping on the performance of CDMA cellular systems. The student will determine the performance of CDMA signals, which are subjected to linear distortion, including that due to bandwidth limitations, gain variations across the pass-band of the filters and amplifiers, and nonlinear phase characteristics of amplifiers and receivers. Special software for computer simulation of digital communication systems is available for the student's use in this project. This project will be carried out within the Wireless Communications program at Clemson. The Wireless Communications program has two laboratories devoted to wireless communications research. The student will work with faculty members and graduate students to understand the characteristics of the different cellular communication systems, learn how to use simulation software for research on CDMA systems, and conduct tradeoff studies that are needed in the design of new CDMA cellular communication systems.