QoS and service continuity in 3GPP D2D for IoT and wearables

Device-to-Device (D2D) communications have been already playing a relevant role in 3GPP for several releases of the LTE standard. This paper first shortly introduces how D2D was brought into 3GPP standards and how it evolved, starting from direct discovery and communication between devices being specified in Release 12, through UE-to-Network relaying feature introduction in Release 13, up to vehicle-to-vehicle (V2V) communications support in Release 14. Currently, the versatility of D2D communications has led to consideration of its inherent advantages also in the context of IoT and wearable devices, where the priority is to provide low power connectivity via short-range communications. In this context, there are use cases that can take advantage of UE-relayed communication that capitalizes on the LTE QoS framework. After highlighting the limitations of the currently standardized UE-to-Network relay feature and introducing the enhanced relay architecture being developed in Release 15, the paper discusses the challenges related to end-to-end QoS support in UE-relayed communications, and presents an enhancement to the current standard for dynamic split and allocation of packet delay budget in each hop of the relayed connection. Afterwards, the focus moves to QoS support in mobility scenarios, where path switch between relayed and non-relayed communication might be required while ensuring service continuity, which proves to be a challenging task. Several solutions that trade-off signaling overhead, buffer size requirements and suitability to high-mobility scenarios are presented and explained in detail.