New optimal trade-off point for coded caching systems with limited cache size

Abstract

This paper presents a new achievable scheme for coded caching systems with $\mathsf{N}$ files, $\mathsf{K}=\mathsf{N}$ users, and cache size $\mathsf{M}=1/(\mathsf{N}-1)$. The scheme employs linear coding during the cache placement phase, and a three-stage transmissions designed to eliminate interference in the delivery phase. The achievable load meets a known converse bound, which impose no constraint on the cache placement, and is thus optimal. This new result, together with known inner and outer bounds, shows optimality of linear coding placement for $\mathsf{M} \leq 1/(\mathsf{N}-1)$ when $\mathsf{K}=\mathsf{N}\geq 3$. Interestingly and surprisingly, the proposed scheme is relatively simple but requires operations on a finite field of size at least 3.