Abstract
Featuring direct communications between two user equipments (UEs) without signal relay through a base station, 3GPP sidelink transmissions have manifested their crucial roles in the Long-Term Evolution (LTE) Advanced (LTE-A) for public safety and vehicle-to-everything (V2X) services. With this successful development in LTE-A, the evolution of sidelink transmissions continues in 3GPP New Radio (NR), which renders sidelink an inevitable component as well as downlink and uplink. Targeting at offering low latency, high reliability and high throughout V2X services for advanced driving use cases, a number of new sidelink functions not provided in the LTE-A are supported in NR, including the feedback channel, grant-free access, enhanced channel sensing procedure, and new control channel design. To fully comprehend these new functions, this paper therefore provides essential knowledge of 3GPP NR sidelink transmissions, including the physical layer structure, resource allocation mechanisms, resource sensing and selection procedures, synchronization, and quality-of-service (QoS) management. Furthermore, this paper also provides performance evaluation to assess the gains brought from the new control channel design. As NR sidelink transmissions have been regarded as a foundation to provide advanced services other than V2X in future releases (e.g., advanced relay), potential enhancements are also discussed to serve the urgent demand in corresponding normative works.
Highlights
The frameworks of downlink and uplink transmissions have been widely deployed in major mobile networks such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long-Term Evolution (LTE), and LTE-A to sustain commercial mobile voice/data services
FUTURE ENHANCEMENTS OF New Radio (NR) SIDELINK TRANSMISSIONS Release 16 is the first release of NR sidelink transmissions, which paves an operable foundation and the following functions can be further enhanced in future releases
In this paper, detailed operations of 3GPP NR sidelink transmissions have been thoroughly presented, including numerology, waveform, resource structure in both time and frequency domains, physical channels and reference signals, resources of PSFCH, contents of SCI for PSFCG, contents of SFCI, resource allocation for physical sidelink control channel (PSCCH)/PSSCH/PSFCH in Mode 1, resource sensing/selection and two-stage SCI in Mode 2, power control, QoS management, and synchronization procedures. It demonstrates that error propagation may be a concern in the two-stage SCI design of Mode 2, this issue can be alleviated through arranging the resources of two SCIs as close as possible
Summary
The frameworks of downlink (i.e., signals are forwarded from a base station to a UE) and uplink transmissions (i.e., signals are forwarded from a UE to a base station) have been widely deployed in major mobile networks such as Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), LTE, and LTE-A to sustain commercial mobile voice/data services. To further obtain resources for a transmitting UE to send the sidelink HARQ ACK/NACK to a gNB, a gNB may allocate one physical uplink control channel (PUCCH) occasion after the last resource in the PSSCH set for initial sidelink transmissions. When a transmitter begins launching the PSCCH, PSSCH and PSFCH, it may keep performing resource sensing If this UE finds that there are other sidelink transmissions with a higher priority occupying the reserved resources, this UE triggers the resource re-selection. A new hardware may be required rather than reusing the existing enhanced mobile broadband (eMBB) chipset to support NR sidelink transmissions
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