Full dimension MIMO for frequency division duplex under signaling and feedback constraints

It is a open research problem of great interest how to fully utilize the benefits of massive MIMO original designed for Time Division Duplex (TDD) also in Frequency-Division Duplexing (FDD). For efficient multi-user downlink transmission Channel State Information (CSI) has to be obtained at the receiver side and fed back to the transmitter. Due to signaling/pilot overhead scaling with the number of antennas and the feedback of the CSI the beamforming gain and spatial multiplexing gains of massive MIMO are limited. In this paper we present a novel scheme called User Non-aware Precoding and Effective Channel Estimation (UNP & ECE) for full dimension MIMO in FDD under signaling and feedback constraints. The idea is to construct a large codebook and distribute subsets of it on time-frequency resources as precoded pilots. The same precoders are also used for downlink transmission thus no additional demodulation reference signals are required. This limits the number of signaling overhead and a high quantization of the channel can be utilized. To limit the feedback we show that it is sufficient to report only the strongest stream index of a precoder subset for only a subset of Time Frequency (TF) resources. By sending just the index of the best stream the devices don't require knowledge of the applied precoders. This enables optimization of the codebook construction, the splitting in subsets, the distribution on the TF resources at the base station without awareness at the users.