The way, the **Precoding Matrix Indicator (PMI)** is calculated depends on the vendor (vendor dependent algorithm) and may utilize e.g. the reference signals, channel estimates, etc. One of the possible schemes present in the literature is based on maximizing the sum mutual information oversubcarriers (where the selected PMI is the one that maximizes the sum among all the other PMIs). You may find the description of the algorithm in the paper entitled: Calculation of the spatial preprocessing and link adaption feedback for 3GPP UMTS/LTE