An apparatus and method: access repeated copies of an OFDM output signal produced by a device in response to corresponding repeated copies of an OFDM input signal; for each copy of the OFDM output signal, time align the OFDM output symbols to the OFDM input symbols, and de-rotate a phase of the OFDM output signal with respect to the OFDM input signal; coherently sum and average the copies of the OFDM output signal; determine a variance of each copy of the OFDM output signal, and an ensemble variance of all of the copies of the OFDM output signal; discard copies of the OFDM output signal whose variance differs from the ensemble variance by more than a threshold amount to produce a qualified set of copies; determine a mean value of the qualified set of copies; and determine a total noise power of the qualified set of copies from the mean value.