D2D communications is not a small field and research fields to D2D divided into, network architecture, interoperability with the cellular network, PROSE in 3GPP specifications, etc.
I suggest you to read in Proximity Services (ProSe) in the 3GPP documentations and related papers to it (see below)
In LTE-A, the D2D communication is considered. In you need more details about D2D share radio resources with cellular user equipment (CUE) you can refer to this paper: