Computationally Efficient Limited Feedback for Codebook-Based FD-MIMO Precoding

Massive multiple input multiple output (MIMO) is one of the key technologies that was introduced in the fifth generation (5G) mobile communication systems for keeping pace with the ever increasing demand for higher data rates. Fulldimension MIMO (FD-MIMO) is a special case of massive MIMO, where the base station is equipped with a uniform planar array. FD-MIMO supports beamforming in the horizontal and vertical domain. Channel state information at the transmitter is crucial for achieving gains through FD-MIMO. Since for frequency division duplex transmission channel reciprocity cannot be exploited, precoding matrix indicator (PMI) feedback is required for codebook based spatial processing. In this paper, we present a computationally efficient limited feedback scheme for calculating the PMI. We exploit the separable structure of the standardized 5G codebook and split the PMI into a horizontal and vertical indicator. The complexity for the PMI calculation is reduced by adapting the reporting interval for the vertical precoder, according to the performance achieved with the calculated PMI. The trade-off between performance and computational efficiency is investigated.