  Impedance matching between receive antenna and front-end significantly
impacts channel capacity in wireless channels. To implement capacity-optimal
matching, the receiver must know the antenna impedance. But oftentimes this
impedance varies with loading conditions in the antenna near-field. To mitigate
this variation, several authors have proposed antenna impedance estimation
techniques. However, the optimality of these techniques remains unclear. In
this paper, we consider antenna impedance estimation at MISO receivers over
correlated Rayleigh fading channels.
  We derive in closed-form the optimal ML estimator under i.i.d. fading and
then show it can be found via a scalar optimization in generally correlated
fading channels. Numerical results suggest a computationally efficient,
principal-components approach that estimates antenna impedance in real-time for
all Rayleigh fading channels. Furthermore, ergodic capacity can be
significantly boosted at originally poorly matched receivers with adaptive
matching using our proposed approach.
