A new approach to multipath correction of constant modulus signals

TLDR

The paper presents an adaptive digital filtering algorithm to compensate for frequency‑selective multipath and interference on constant‑envelope modulated signals. The method exploits incidental amplitude modulation caused by multipath and interference, employing constant‑modulus performance functions and gradient‑search adaptive FIR filters to cancel these effects, and is demonstrated on FM and QPSK signals. Simulations show a 25‑dB improvement in noise power ratio with rapid convergence, and the approach also mitigates tonal interference on FM and intersymbol interference on QPSK.

Abstract

An adaptive digital filtering algorithm that can compensate for both frequency-selective multipath and interference on constant envelope modulated signals is presented. The method exploits the fact that multipath reception and various interference sources generate incidental amplitude modulation on the received signal. A class of so-called constant modulus performance functions is developed which sense this AM term but are insensitive to the angle modulation. Simple adaptive algorithms for finite-impulse-response (FIR) digital filters are developed which employ a gradient search of the performance function. One of the resulting algorithms is simulated for the example of an FM signal degraded by specular multipath propagation. Substantial improvements in noise power ratio (NPR) are observed (e.g., 25 dB) with moderately rapid convergence time. These results are then extended to include tonal interference on a FM signal and intersymbol interference on a QPSK data signal.