  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.
