LTE system. How to compute precoder for resourse block guys!

I am studying a massive Mimo system and 3gpp LTE system and have questions about precoding in the LTE system. In LTE RB (resource block) is used to group consecutive subcarriers.  To compute precoder and reduce the complexity of computation it is possible to compute one precoder for RB.

In this post, the frequency selective channel was discussed and mentioned: "to average the second-order statistics."  Does it mean: covariance, covariance function? I have read that it is used for signal, but how to use for RB ... I dont know...

Does anyone give me more details?
Think of the RB as a subset of the OFDM channel set. That reduces the complexity of finding $N_\text{FFT}$ matrix decompositions of $N_\text{TX antennas}\cdot N_\text{RX antennas}$ matrices for each user to 
$N_\text{carriers in the individual RB}$ decompositions for each user, which might well be smaller, because at user-level, often a significant amount of TX antennas can be ignored to begin with.