Adaptive LMS Filters for Cellular CDMA Overlay Situations

This paper extends and complements previous research we have performed on the performance of nonadaptive narrowband suppression filters when used in cellular codedivision multiple-access (CDMA) overlay situations. In this paper, an adaptive least mean square (ILMS) filter is applied to a cellular CDMA overlay in order to reject narrowband interference. An accurate expression for the steady-state tap-weight covariance matrix is derived for the real LMS algorithm for arbitrary statistics of the overlaid interference. Numerical results illustrate that when the ratio of the narrowband interference bandwidth to the spread spectrum bandwidth is small, the LMS filter is very effective in rejecting the narrowband interference. Furthermore, it is seen that the performance of the LMS filter in a CDMA overlay environment is not significantly worse than the performance of an ideal Wiener filter, assuming the LMS filter has had sufficient time to converge.