A multi-hop relay based frequency synchronization for D2D communication in 3GPP LTE system

Device-to-device (D2D) communication has been adopted for one of the next generation technologies to support commercial/ social service and public safety application in 3-rd generation partnership project (3GPP). For D2D operation in partial network coverage (PNC) scenario, synchronization among multiple user equipments (UE) is the most important issue due to asynchronous interference which can occur among in-network coverage (INC) UEs and out-of-network coverage (ONC) UEs. Therefore, 3GPP agreed multi-hop relay based synchronization. In this method, UE considers a source type, the number of hops, and received power of D2D synchronization signal (D2DSS) to select one D2DSS among multiple D2DSSs. However, this method cannot consider actual frequency offsets of D2DSS, which are accumulated through the previous multi-hop links. For that reason, it is not suitable for multi-hop relay based frequency synchronization. Therefore, in this paper, we propose a novel frequency synchronization method where UE considers an absolute sum of frequency offsets which actually occurs in the multi-hop links of D2DSS. Also, by using frequency offset based threshold, the proposed method provides a strict condition to perform D2DSS relay. We verify that the proposed method can efficiently reduce frequency offsets among UEs in PNC scenario.