A Robust Nonlinear RLS Type Adaptive Filter for Second-Order-Intermodulation Distortion Cancellation in FDD LTE and 5G Direct Conversion Transceivers

TL;DR

This paper introduces a robust nonlinear RLS adaptive filter designed to cancel second-order intermodulation distortion in FDD LTE and 5G direct conversion transceivers, improving receiver performance in the presence of transmitter leakage.

Contribution

It proposes a new nonlinear Wiener model RLS adaptive filter with enhanced numerical stability for IMD2 cancellation in direct conversion receivers.

Findings

Effective IMD2 cancellation demonstrated in simulations

Robust algorithm maintains stability with highly correlated signals

Measurement results confirm improved receiver signal quality

Abstract

Transceivers operating in frequency division duplex experience a transmitter leakage (TxL) signal into the receiver due to the limited duplexer stop-band isolation. This TxL signal in combination with the second-order nonlinearity of the receive mixer may lead to a baseband (BB) second-order intermodulation distortion (IMD2) with twice the transmit signal bandwidth. In direct conversion receivers, this nonlinear IMD2 interference may cause a severe signal-to-interference-plus-noise ratio degradation of the wanted receive signal. This contribution presents a nonlinear Wiener model recursive least-squares (RLS) type adaptive filter for the cancellation of the IMD2 interference in the digital BB. The included channel-select-, and DC-notch filter at the output of the proposed adaptive filter ensure that the provided IMD2 replica includes the receiver front-end filtering. A second, robust version of the nonlinear RLS algorithm is derived which provides numerical stability for highly correlated input signals which arise in e.g. LTE-A intra-band multi-cluster transmission scenarios. The performance of the proposed algorithms is evaluated by numerical simulations and by measurement data.