A HYBRID SPREADING DESPREADING FUNCTION WITH GOOD SNR PERFORMANCE FORBAND-LIMITED DS-CDMA/

The efficiency of a direct-sequence spread-spectrum code-division mult iple-access (DS-CDMA) system depends heavily on the shape of the spect rum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achievi ng such a spectrum. One is to use a simple spreader that produces a br oad spectrum and then follow it with a precise, high order filter to b and limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple fitter to control the out -of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE recei ver. The expression includes the effects of baseband, intermediate fre quency (IF) and RF filtering as well as the effects of the spectral de nsities of the spreading/despreading functions. The analysis shows tha t the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism.