Multiple-antenna Placement Delivery Array for Cache-aided MISO Systems

This paper considers the cache-aided multiple-input single-output (MISO) broadcast channel (BC) consisting of one server and K users, where the server with L antennas accesses to N files and each user with single antenna has a cache of M files. The objective of this problem is to maximize the sum Degree-of-Freedom (sum-DoF) of the system. It was proved that under uncoded cache placement and one-shot zero-forcing (ZF) delivery, the maximum sum-DoF is L + KM/N. However, previously proposed schemes achieving the maximum sum-DoF either require an exponential order of K or work for some limited cases. In this paper, we propose a new combinatorial structure called multiple-antenna placement delivery array (MAPDA), which generalizes the coded caching schemes under uncoded cache placement and one-shot ZF delivery. We then propose two schemes (for the case $\frac{{KM}}{N} + L = K$ and the general case, respectively), which can achieve the maximum sum-DoF with reduced subpacketization with respect to the existing schemes. The first scheme is based on the Latin square, while the second scheme is obtained by an approach which transforms a class of schemes for the original shared-link coded caching model to the MISO system.