Mutual information based calculation of the Precoding Matrix Indicator for 3GPP UMTS/LTE

This paper presents an efficient method for calculating the Precoding Matrix Indicator (PMI) at the receiver. The PMI is required for MIMO precoding in the downlink of a 3GPP UMTS/LTE system. Our method is based on maximizing the mutual information between the transmitted and received symbols with respect to the precoding matrix applied at the transmitter. The advantage of this method is that it is independent of the symbol alphabet (4/16/64 QAM) and code rate applied, which are signaled by the channel quality indicator (CQI). Although this paper only focuses on the selection of the optimal PMI, such a procedure eventually allows to decouple both problems (CQI and PMI calculation), thereby reducing complexity. The proposed method also provides means to obtain the number of useful MIMO transmission layers, signaled in form of the Rank indicator (RI), by maximizing mutual information also with respect to this value. The performance of the method is evaluated utilizing an LTE downlink physical-layer simulator.