Multiple-Antenna Placement Delivery Array for Cache-Aided MISO Systems

We consider the cache-aided multiple-input single-output (MISO) broadcast channel, which consists of a server with <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> antennas and <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> single-antenna users, where the server contains <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> files of equal length and each user is equipped with a local cache of size <inline-formula> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> files. Each user requests an arbitrary file from library. The objective is to design a coded caching scheme based on uncoded placement and one-shot linear delivery, to achieve the maximum sum Degree-of-Freedom (sum-DoF) with low subpacketization. It was shown in the literature that under the constraint of uncoded placement and one-shot linear delivery, the maximum sum-DoF is <inline-formula> <tex-math notation="LaTeX">$\min \left\{{L+\frac {KM}{N},K}\right\}$ </tex-math></inline-formula>. However, previously proposed schemes for this setting incurred either an exponential subpacketization order in <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>, or required specific conditions in the system parameters <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula>. In this paper, we propose a new combinatorial structure called multiple-antenna placement delivery array (MAPDA). Based on MAPDA and Latin square, the first proposed scheme achieves the maximum sum-DoF <inline-formula> <tex-math notation="LaTeX">$\min \left\{{L+\frac {KM}{N},K}\right\}$ </tex-math></inline-formula> with the subpacketization of <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> when <inline-formula> <tex-math notation="LaTeX">$\frac {KM}{N}+L=K$ </tex-math></inline-formula>. Subsequently, for the general case we propose a transformation approach to construct an MAPDA from any <inline-formula> <tex-math notation="LaTeX">$g$ </tex-math></inline-formula>-regular PDA (a class of placement delivery arrays for the shared link caching problem where each integer in the array occurs <inline-formula> <tex-math notation="LaTeX">$g$ </tex-math></inline-formula> times). When <inline-formula> <tex-math notation="LaTeX">$g$ </tex-math></inline-formula>-regular PDA corresponds to the Maddah-Ali and Niesen scheme, the resulting MAPDA yields the maximum sum-DoF <inline-formula> <tex-math notation="LaTeX">$\min \left\{{L+\frac {KM}{N},K}\right\}$ </tex-math></inline-formula> with reduced subpacketization compared to the existing schemes. The general scheme can be extended to the multiple independent single-antenna transmitters (servers) corresponding to the cache-aided interference channel proposed by Naderializadeh et al. and the scenario of transmitters equipped with multiple antennas.