In a method of estimating a signal-to-interference ratio (SIR), a first average channel power per slot of a first channel that is under a fading environment is estimated. A second average channel power per slot of a second channel that is under a fading environment substantially the same as the first channel is estimated, wherein the first and second channels are multiplexed. A second signal power attenuation ratio of the second channel is calculated using the second average power per slot of the second channel. A third average power per slot of the first channel is calculated using the first average power per slot of the first channel and a reciprocal of the second signal power attenuation ratio of the second channel.