An operating method of a wireless communication device operating in a narrowband Internet of things (IoT) communication standalone mode includes calculating a noise covariance of a target subframe based on signals received through unused resource elements in the target subframe, the target subframe being among a plurality of subframes included in a downlink signal, and the downlink signal being received from an external device, and estimating a signal-to-noise ratio (SNR) of the target subframe based on the noise covariance.