  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.
