1. A method of estimating a covariance matrix in a communication system, the method comprising: i) obtaining a plurality of waveforms, the plurality of waveforms associated with a reception time; ii) determining whether a packet is detected during the reception time; iii) determining whether an automatic gain control setting changes during the reception time; and iv) estimating a covariance matrix based on the plurality of waveforms, wherein when it is so determined that a packet is detected during the reception time or an automatic gain control setting changes during the reception time, at least operations i)-iii) are repeated.