Adaptive multipath equalization

DETAILED DESCRIPTION The invention meets these needs by providing method and apparatus for using a plurality (preferably four or more) of correlators to identify detailed features of a correlation function at and adjacent to the correlation peak.
In a first embodiment, the errors in location of the center point of a correlation function, formed by the received signal and a stored copy of the expected direct signal, are assumed to be strongly correlated for narrow sample spacing and wide sample spacing of the correlation function.
In a second embodiment, the multipath signal strengths and phases are estimated by a least mean squares analysis, using multiple sampling of a correlation function of an expected signal and an arriving composite signal that includes the direct signal and one or more multipath signals.
This approach assumed that the times of arrival or path delays of the direct signal and the multipath signals are determined separately.
In one method of estimating these path delays, the invention forms an error function, which is a difference between correlation functions R(.
tau.
) of "early-arriving" p(t+.
tau.
.
sub.
d) and "late-arriving" signals p(t-.
tau.
.
sub.
d) with a reference signal p(t), which may be a pseudo-random noise code signal, to determine whether multipath signals are present and to adaptively determine appropriate weighting to remove or suppress such signals.
These two signals are combined to form a single, non-coherent correlation function (complex phase not present) R(.
tau.
.
sub.
d) that achieves a local maximum for a particular time translation.
The number of slope transition points that appear in the correlation function R(.
tau.
.
sub.
d) for .
tau.
.
sub.
d .
noteq.
0 indicates the number of strong multipath signals present and their times of arrival relative to the direct or line-of-sight signal.
The apparatus determines times .
tau.
.
sub.
d where this slope changes quickly or abruptly.
A slope transition point occurs wherever this slope changes by more than a selected amount