When at least one of device-to-device UEs in connections of a group of D2D UEs is outside of the coverage area of mobile network of an access point, an information exchange mechanism established directly between the D2D UEs, without involving a third party, is provided. A resource allocation method for a device-to-device (D2D) communication link between two or more D2D user equipments (UEs) in a mobile communication network is provided according to an example. The method includes a first D2D UE indicating resource allocation information for the D2D communication link in a resource allocation message, wherein the resource allocation message is mapped into a resource allocation format; wherein the resource allocation format comprises one or more format flags; wherein one or more resource allocation messages are differentiated in the resource allocation format in accordance to the one or more format flags; and the first D2D UE transmitting the resource allocation format in a Physical Sidelink Control Channel (PSCCH) to one or more additional D2D UEs.