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Direct device-to-device (D2D) communication is considered to be part of fifth generation (5G) networks, while embodying vehicle-to- anything (V2X) communication into the recently defined Long Term Evolution (LTE) sidelink is already on the way. Coexistence of in-band D2D and cellular transmissions requires time synchronization between multiple links to prevent from interference and avoid using longer cyclic prefix or guard bands. This paper proposes a synchronization solution enabling interference-free coexistence of inter-cellular sidelink and existing cellular links, also considering non- synchronized base stations. Multiple synchronization sources including base stations, Global Navigation Satellite System (GNSS) and mobile users are hierarchically combined and network-instructed synchronization avoids any misalignments. Out-of-coverage users can form or join existing mobile networks by deploying the …