Reduction of Multipath Errors in Spread-Spectrum Code Tracking

Exact spread-spectrum code tracking in the presence of multipath is an important issue in distance and time measurement systems. Different tracking algorithms based on feedback, feedforward or a combination of them reduce multipath effects, however, the accuracy of the tracking in presence of short-delay multipath signals is still a problem. In this paper, we consider feedback delay lock loops (DLL) and gating function schemes that have shown better performance with lower complexity under multipath. We develop a shot-noise process model for the control signal of a DLL in the presence of multipath signals and determine the range of tracking errors they cause. Then, we propose a method based on computation of parallel forward and backward correlations with the incoming signal in order to reduce tracking errors due to multipath. As will be shown, the multipath signals do not change the zero-crossing of the forward correlation output which is used to estimate the exact phase of the received signal. The new structure alleviates the problem of short-delay multipath while adding not much complexity to the system.