Optimal cyclic $(r,\delta)$ locally repairable codes with unbounded length

Locally repairable codes with locality $r$ ($r$-LRCs for short) were introduced by Gopalan et al. \cite{1} to recover a failed node of the code from at most other $r$ available nodes. And then $(r,\delta)$ locally repairable codes ($(r,\delta)$-LRCs for short) were produced by Prakash et al. \cite{2} for tolerating multiple failed nodes. An $r$-LRC can be viewed as an $(r,2)$-LRC. An $(r,\delta)$-LRC is called optimal if it achieves the Singleton-type bound. It has been a great challenge to construct $q$-ary optimal $(r,\delta)$-LRCs with length much larger than $q$. Surprisingly, Luo et al. \cite{3} presented a construction of $q$-ary optimal $r$-LRCs of minimum distances 3 and 4 with unbounded lengths (i.e., lengths of these codes are independent of $q$) via cyclic codes. In this paper, inspired by the work of \cite{3}, we firstly construct two classes of optimal cyclic $(r,\delta)$-LRCs with unbounded lengths and minimum distances $\delta+1$ or $\delta+2$, which generalize the results about the $\delta=2$ case given in \cite{3}. Secondly, with a slightly stronger condition, we present a construction of optimal cyclic $(r,\delta)$-LRCs with unbounded length and larger minimum distance $2\delta$. Furthermore, when $\delta=3$, we give another class of optimal cyclic $(r,3)$-LRCs with unbounded length and minimum distance $6$.