Method and apparatus of synchronizing device to device terminals in time and frequency

A user equipment (UE) for transmitting a synchronization signal in a coverage of a base station, comprising:

• a transceiver;
• and
• a controller coupled with the transceiver and configured to control to:
• receive, from the base station, configuration information including a threshold value and information on a synchronization resource;
• measure a reference signal received power (RSRP) of a signal from the base station;
• transmit, in a subframe indicated by the information on the synchronization resource, a device-to-device synchronization signal (D2DSS) in case that the RSRP of the signal is less than the threshold value and the UE is configured to perform a non-public safety (non-PS) related D2D discovery operation;
• and
• transmit, in the subframe indicated by the information on the synchronization resource, the D2DSS and a physical sidelink broadcast channel (PSBCH), in case that the RSRP of the signal is less than the threshold value and the UE is configured to perform a public safety (PS) related D2D discovery operation,
• wherein the D2DSS includes a primary D2DSS and a secondary D2DSS,
• wherein a sequence for the primary D2DSS is generated from a Zadoff-Chu sequence and a sequence for the secondary D2DSS is generated from an m-sequence having a length of 62, and
• wherein the PSBCH includes information on a time division duplex (TDD) configuration and information on a bandwidth configuration for a D2D transmission.