The Optimal Placement Delivery Arrays
  In wireless networks, coded caching is an effective technique to reduce network congestion during peak traffic times. Recently, a new concept called placement delivery array (PDA) was proposed to characterize the coded caching scheme. So far, only one class of PDAs by Maddah-Ali and Niesen is known to be optimal. In this paper, we mainly focus on constructing optimal PDAs. Firstly, we derive some lower bounds. Next, we present several infinite classes of PDAs, which are shown to be optimal with respect to the new bounds. 
Minquan Cheng, Jing Jiang, Qifa Yan, Xiaohu Tang, and Haitao Cao
cs.IT math.IT