#### Optimal Combinatorial Batch Codes based on Block Designs

##### Natalia Silberstein, Anna Gál

Batch codes, introduced by Ishai, Kushilevitz, Ostrovsky and Sahai, represent the distributed storage of an $n$-element data set on $m$ servers in such a way that any batch of $k$ data items can be retrieved by reading at most one (or more generally, $t$) items from each server, while keeping the total storage over $m$ servers equal to $N$. This paper considers a class of batch codes (for $t=1$), called combinatorial batch codes (CBC), where each server stores a subset of a database. A CBC is called optimal if the total storage $N$ is minimal for given $n,m$, and $k$. A $c$-uniform CBC is a combinatorial batch code where each item is stored in exactly $c$ servers. A $c$-uniform CBC is called optimal if its parameter $n$ has maximum value for given $m$ and $k$. Optimal $c$-uniform CBCs have been known only for $c\in \{2,k-1,k-2\}$. In this paper we present new constructions of optimal CBCs in both the uniform and general settings, for values of the parameters where tight bounds have not been established previously. In the uniform setting, we provide constructions of two new families of optimal uniform codes with $c\sim \sqrt{k}$. Our constructions are based on affine planes and transversal designs.

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