Multirate Baseband Filter Optimization in Code Division Multiple Access Digital Telephony

Abstract

The commercial demand for a greater number of cellular telephone channels has prompted the cellular industry to try alternative methods to increase the bandwidth efficiency. The two methods that have emerged are Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). TDMA promises a bandwidth efficiency gain of 3-6 over conventional analog Frequency Division Multiple Access (FDMA) techniques, and CDMA promises a bandwidth efficiency gain of 10-20 over analog FDMA [4]. While the CDMA system offers greater bandwidth efficiency, it is also more complex and therefore a more expensive technology. To facilitate the commercial use of the two technologies, the Telecommunications Industry Association (TIA) in conjunction with the Electronic Industries Association (EIA), have developed the IS-54 interim standard for TDMA systems, and the IS-95 interim standard for CDMA systems. Within these documents are the specifications of the Requirements for Mobile Station TDMA/CDMA Operation and the Requirements for Base Station TDMA/CDMA Operation. One of the more sophisticated components of the CDMA system is a 47th order baseband lowpass Finite Impulse Response (FIR) filter. This filter has a high complexity factor which also means it has a high monetary cost. This thesis investigates the use of multirate techniques to decrease the filter complexity of the baseband FIR filter. The proposed filters meet the IS-95 baseband filter specifications. At the same time, the filters reduce the overall filter complexity by a factor of about 1.5 for a single rate Interpolated FIR, and by about 2.5 for a single stage multirate filter.