Adaptive Distortion Inversion Technique for LNA's Nonlinearity Compensation in Direct RF Digitization Receivers

This paper presents the analysis of the nonlinear distortion model of the Low Noise Amplifier (LNA) for multichannel direct RF digitization receivers (DRF-RX). The distortion model gives insights into the nature of the LNA distortions and is used as the fundamental ground for distortion compensation. The paper further presents a fully-digital solution using a reference channel accompanied by a least mean square (LMS) circuit for adaptive distortion compensation. The LMS circuit uses the information from the reference channel to invert the distortion impacts caused by the LNA's nonlinearity in the main channel and eventually recover the linear component. A direct RF digitization UHF receiver with 4-QPSK channels was implemented in Matlab to evaluate the proposed method's efficiency. The performance of the compensation circuits has been studied in detail. The simulation results show that the proposed solution greatly improves the receiver performance. Compared to the receiver without compensation, the compensation circuit helps to lower the distortion components by ~20 dB and improve the bit error rate (BER) by 2 orders of magnitude.