Double-Cap Differential Codebook Structure for MU MISO Systems in Correlated Channels

We propose a double-cap differential codebook (DCB) design for multiuser (MU) multiple-input-single-output systems. The MU transmission uses signal-to-leakage-and-noise ratio precoding. The double-cap DCB is designed for temporally/ spatially correlated channels and is based on a combined spherical and polar cap structure. We consider an inner spherical cap and an outer polar cap structure with different radii. The design allows us to systematically alter the radii of the caps over the feedback time depending on the previously selected codeword. The proposed double-cap DCB does not require knowledge of temporal or spatial channel correlation at the base-station, thus removing the overhead of additional feedback from the user. Simulation results show the effectiveness of the proposed double-cap DCB over a single polar cap DCB in terms of sum capacity gain and quantization errors.