It is important to understand though that the RI and PMI are only recommendations and the network does not need to follow the RI/PMI provided by the terminal when selecting the actual transmission rank and precoder matrix to be used for transmission to the terminal. When not following the terminal recommendation, the network must explicitly inform the terminal what precoder matrix is used for the downlink transmission.
Open-loop precoding does not rely on any detailed precoder recommendation being reported by the terminal and does not require any explicit network signaling of the actual precoder used for the downlink transmission. Instead, the precoder matrix is selected in a predefined and deterministic way known to the terminal in advance. One of the uses of open-loop precoding is in high-mobility scenarios where accurate feedback is difficult to achieve due to the latency in the PMI reporting
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