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 Sidelink Control Information (SCI) message type comprises one or more format flags indicating the respective resource allocation format, obtaining a first SCI message, adjusting a size of the first SCI message based on the determined size, and transmitting the adjusted first SCI message by a first UE in a Physical Sidelink Control Channel (PSCCH) to one or more second UEs for establishing the D2D communication link.