Multicast Beamformer Design for Mimo Coded Caching Systems

Coded caching (CC) techniques have been shown to be conveniently applicable in multi-input multi-output (MIMO) systems. In a K-user network with spatial multiplexing gains of L at the transmitter and G at every receiver, if each user can cache a fraction γ of the file library, a total number of GKγ+ L data streams can be served in parallel. In this paper, we focus on improving the finite-SNR performance of MIMOCC systems. We first consider a MIMO-CC scheme that relies only on unicasting individual data streams, and then, introduce a decomposition strategy to design a new scheme that delivers the same data streams through multicasting of G parallel codewords. We discuss how optimized beamformers could be designed for each scheme and use numerical simulations to compare their finite-SNR performance. It is shown that while both schemes serve the same number of streams, multicasting provides notable performance improvements. This is because, with multicasting, transmission vectors are built with fewer beamformers, leading to more efficient usage of available power resources.

1. INTRODUCTION

Wireless networks are under mounting pressure to bear exponentially increasing volumes of multimedia content [1] and to support the imminent emergence of new applications such as wireless immersive viewing [2], [3]. For the efficient delivery of such multimedia content, coded caching (CC) was proposed in [4] as a means of increasing the data rates by exploiting cache content across the network. In a single-stream downlink network of K users, each capable of caching a portion γ ≤ 1 of the entire file library, CC enables boosting the achievable rate by a factor of Kγ + 1 through multicasting carefully created codewords such that each user can use its cache content to remove unwanted parts from the received signal. As a result, with CC, the achievable degrees of freedom (DoF) is increased from one to Kγ + 1. The multiplicative factor t ≡ Kγ is also called the coded caching gain.