Indirect learning Hammerstein HPA predistorter for wideband GNSS signals

Abstract

Compared with the traditional GPS L1 C/A signal, the existing and emerging new-generation wideband GNSS signals suffer more severe distortions due to the nonlinear characteristics of a high-power amplifier (HPA) in the satellite transmitter. HPA nonlinearity can lead to in-band signal distortion and out-of-band spectral regrowth, which degrade the ranging performance of a GNSS receiver. Researchers have reported power loss, correlation peak asymmetries, lock point bias as well as carrier tracking error caused by the HPA nonlinearity. However, few pre-distortion approaches that compensate for the HPA nonlinear effects have been presented in the GNSS field. We first take BDS-3 B2 asymmetric constant envelope binary offset carrier signal as an example to present the HPA nonlinear effects on wideband GNSS signals. Then, the design principle of a predistorter is investigated, and a Hammerstein predistorter is proposed to eliminate the nonlinear HPA with memory. Finally, we compare the constellation diagram, power spectrum, correlation function, code tracking bias with and without the Hammerstein predistorter. The comparison results demonstrate that a digital predistorter is a potential solution to compensate the HPA nonlinear distortions for wideband GNSS signals. This pre-distortion approach is of great significance that can effectively guarantee the nominal signal quality in space not only during the maintenance of the current GNSSs but also for the future emerging local or global navigation systems.