# Does the OFDM MMSE Channel Estimator Require Prior Channel Knowledge?

The OFDM MMSE channel estimator seems to require a-priori knowledge of the channel to calculate the MMSE estimate. See equation 8 here: http://cache.freescale.com/files/dsp/doc/app_note/AN3059.pdf. The autocovariance matrix explicitly includes the actual channel H or g.

However, you presumably don't have this information, otherwise why would you be estimating it? If you are able to deal with the complexity of the MMSE estimator, how do you implement it in practice without prior knowledge of what the channel is?

The autocovariance matrix $\underline R_{HH}$ is based on some statistical channel model, e.g. the Rayleigh fading channel or AWGN channel. As you have observed correctly, the channel matrix $H$ is unknown and thus $\underline R_{HH}$ cannot be calculated from $H$. It's rather an assumption based on theroetical models or earlier measurements.