  An estimation method of Radio Frequency fingerprint (RFF) based on the
physical hardware properties of the nonlinearity and in-phase and quadrature
(IQ) imbalance of the transmitter is proposed for the authentication of
wireless orthogonal frequency division multiplexing (OFDM) devices. Firstly,
the parameters of the nonlinearity of the transmitter and finite impulse
response (FIR) of the wireless multipath channel are estimated with a
Hammerstein system parameter separation technique. Secondly, the best IQ
imbalance parameter combination estimation is obtained with a searching
algorithm and the applied conjugate anti-symmetric pilots. Thirdly, the
estimations of the nonlinear coefficients and IQ imbalance parameter
combination are considered as a novel RFF, the features are extracted from the
novel RFF, and a k-Nearest Neighbor (k-NN) classifier is used to classify the
communication devices with the features. It is demonstrated with the numerical
experiments of five transmitters that the novel RFF eliminates the adverse
effect of the wireless channel and is therefore stable. The proposed RF
fingerprinting method is helpful for the high-strength authentication of the
OFDM communication devices with subtle differences from the same model and same
series.
