Antenna Impedance Estimation at MIMO Receivers

Abstract

This paper considers antenna impedance estimation based on training sequences at MIMO receivers.The goal is to firstly leverage extensive resources available in most wireless systems for channel estimation to estimate antenna impedance in real-time. We assume the receiver switches its impedance in a predetermined fashion during each training sequence. Based on voltage observation across the load, a classical estimation framework is developed incorporating the Rayleigh fading assumption. We then derive in closed-form a maximum-likelihood (ML) estimator under i.i.d. fading and show this same ML estimator is a method of moments (MM) estimator in correlated channels. Numerical results suggest a fast algorithm, i.e., MLE in i.i.d. fading and the MM estimator in correlated fading, that estimates the unknown antenna impedance in real-time for all Rayleigh fading channels.

Impedance Variation Detection at MISO Receivers

“…Consequently, Wu and Hughes derived a closed-form maximum-likelihood (ML) estimator for impedance, treating the channel gains as nuisance parameters, for both MISO and MIMO receivers under i.i.d. Rayleigh fading [5], [7]. They also demonstrated that these ML estimators are efficient with sufficient signal-to-noise-ratio (SNR) and diversity.…”

“…Due to time-varying near-field loading conditions, antenna impedance may change significantly. Researchers have proposed techniques to estimate the unknown antenna impedance in realtime [2]- [7]. However, it remains unclear when these estimation algorithms should be triggered.…”