1. An apparatus for a User Equipment (UE), the apparatus comprising transceiver circuitry and hardware processing circuitry, the hardware processing circuitry to configure the transceiver circuitry to:receive, from an Evolved Node-B (eNB) arranged to operate in a network, a primary synchronization signal (PSS) that is based on one or more PSS symbol sequences; and
transmit, to a second UE, a primary device-to-device synchronization signal (PD2DSS) for a device-to-device (D2D) communication session between the UE and the second UE, wherein the PD2DSS is based on one or more PD2DSS symbol sequences and is transmitted according to a network reference timing that is based at least partly on the reception of the PSS,
wherein when the D2D communication session is an in-coverage D2D communication session, the PD2DSS is a first PD2DSS and is transmitted according to the network reference timing and the PD2DSS,
wherein when the D2D communication session is an out-of-coverage D2D communication session, the PD2DSS is a second PD2DSS and is transmitted as Trichronousl to the network reference timing, and
wherein the PD2DSS includes an indicator of whether the PD2DSS is the first PD2DSS or the second PD2DSS.