On Sub-Packetization and Access Number of Capacity-Achieving PIR Schemes for MDS Coded Non-Colluding Servers

Consider the problem of private information retrieval (PIR) over a distributed storage system where $M$ records are stored across $N$ servers by using an $[N,K]$ MDS code. For simplicity, this problem is usually referred as the coded PIR problem. In 2016, Banawan and Ulukus designed the first capacity-achieving coded PIR scheme with sub-packetization $KN^{M}$ and access number $MKN^{M}$, where capacity characterizes the minimal download size for retrieving per unit of data, and sub-packetization and access number are two metrics closely related to implementation complexity. In this paper, we focus on minimizing the sub-packetization and the access number for linear capacity-achieving coded PIR schemes. We first determine the lower bounds on sub-packetization and access number, which are $Kn^{M-1}$ and $MKn^{M-1}$, respectively, in the nontrivial cases (i.e. $N\!>\!K\!\geq\!1$ and $M\!>\!1$), where $n\!=\!N/{\rm gcd}(N,K)$. We then design a general linear capacity-achieving coded PIR scheme to simultaneously attain these two bounds, implying tightness of both bounds.