Reduced Complexity Multicast Beamforming and User Grouping for Multi-Antenna Coded Caching

A new user-grouping method for multi antenna coded caching (CC) is proposed, able to simultaneously exploit spatial multiplexing, multicasting, and global caching gains. The proposed scheme achieves the theoretical optimal sum degrees of freedom (DoF) and can be applied to networks with any number of users K, transmit antennas L, and global CC gain t. The proposed scheme comprises both multicast and unicast transmissions wherein a suitable combination of messages is chosen exploiting a tree diagram structure thus guaranteeing symmetric transmission of data for any given set of network parameters. The same structure also removes the successive interference cancellation requirement at the receivers, thus enabling a simple linear design for optimized beamformers. Moreover, even though the proposed scheme in general requires a larger subpacketization than the original single antenna CC scheme, for the particular case of $\frac{t+L}{t+1}\in \mathbb{N}$, it works with exactly the same subpacketization value. Finally, the achievability of the proposed scheme is proven, and in the simulations, it is demonstrated that the complexity of the transmitter and receiver implementations can be greatly reduced with only a modest performance loss.