Investigation of Optimum Double Codebook Design for Downlink MIMO in LTE-A

Due to the limited feedback amount, the codebook and feedback design is very important. In order to achieve better tradeoff between overhead and performance, the double codebook structures are proposed and discussed, where a final precoder W is determined by two precoders W₁ and W₂, which are selected from two different codebooks. In this paper, we give an overview of several double codebook structures and evaluate them based on system level simulations (SLS). We consider two frameworks to determine a final precoder: W₁W₂ and W₂W₁ where W₁ and W₂ track long-term/wideband and short term/subband channel properties, respectively. It is shown that under 4Tx assumption, double codebooks achieve gains compared with Rel.8 codebook when considering the multi-user (MU) MIMO and do not suffer in the case of single-user (SU) MIMO. Both W₁W₂ and W₂W₁ provide nearly the same gains over that for the Rel. 8 codebook with uniform linear array (ULA) deployment. Furthermore, W₁W₂ framework performs better than W₂W₁ framework with cross-polarized array (CPA) deployment.