MIMO adaptive codebook for closely spaced antenna arrays

We discuss the design of a double structure codebook, in which the final precoding codeword is obtained as the product of codewords from two separate codebooks. The first codeword W
₁
, representing long-term and wideband channel properties, limits the signal space and the second one W
₂
, representing short-term and narrowband properties, allows to pick any vector within this space. We study optimization of both codebooks, and discuss low complexity codeword selection algorithms to allow increased quantization granularity for the first codeword. We show that in case of eight transmit antennas at the transmitter our optimized codebooks outperform the codebook in Release 10 3GPP LTE as well as the adaptive codebook of spherical caps.