An adaptive filtering PN code acquisition scheme with improved acquisition based capacity in DS/CDMA

The acquisition-based capacity of a DS/CDMA system is defined as the maximum number of simultaneous users that can achieve PN code acquisition with certain reliability. This paper introduces a novel PN code acquisition scheme utilizing an LMS adaptive filter structure to estimate the delay offset of the desired user in the presence of multiple access interference. It is shown that the adaptive filter significantly outperforms conventional matched filter acquisition schemes. It improves the acquisition-based capacity because it takes into consideration the presence of the multi-user interference while finding the optimum tap-weight setting as opposed to the matched filter which completely neglects the interference. For a PN code length of 127 chips, the matched filter can support only 6 users while the adaptive filter supports up to 40 users at a probability of acquisition failure of 1%. This is essentially the same as the capacity achieved at 10^-3 bit-error-rate