V2X within the 3GPP standards – Physical Layer, Radio Resource Allocation, Synchronization, and QoS Management

Introduction

This post is a continuation of the 3GPP cellular vehicle-to-anything (C-V2X) standards description, whereas the former one, focused on general and architectural aspects, which you can find here. With this part, we will present a more detailed description of the sidelink radio interfaces proposed in LTE-V2X (Release 14/15) and its successor – NR-V2X. The focus will be mostly on the physical layer and radio resource allocation aspects, with some information on the synchronization possibilities and Quality-of-Service (QoS) management, also provided.

V2X Physical Layer

The physical layer of the sidelink uses the same technology as the Uu interface used in the uplink, that is the Single-Carrier Frequency Division Multiple Access (SC-FDMA) method for LTE-V2X and the Orthogonal Frequency Division Multiple Access (OFDMA) for NR-V2X. The sidelink frame structure is organized in radio frames, each with a duration of 10 ms.

These are further divided into 10 subframes, comprising a number of slots. With LTE-V2X each subframe contains 14 symbols grouped into two slots. The situation is slightly different with NR-V2X, where the number of slots per subframe and the subcarrier spacing is flexible, with the former one being a multiple of 15 kHz. Here the number of slots depends on the selected numerology, with each slot comprising 14 or 12 time-domain symbols, depending on the cyclic prefix length. Table I summarizes the available numerology configurations.

Id	Sub-carrier spacing [kHz]	CP length	OFDM symbols per slot	Slot duration[μs]	Number of slots in a subframe	Maximum carrier bandwidth [MHz]
0	15	Normal	14	1000	1	50
1	30	Normal	14	500	2	100
2	60	Normal/Extended	14/12	250	4	200
3	120	Normal	14	125	8	400

Table I Supported numerologies in NR-V2X [1]

The available radio resources are grouped into Resource Blocks (RBs) comprising 12 subcarriers in frequency and a single subframe, in the case of LTE-V2X, or a single slot in NR-V2X. These RBs are further grouped into sub-channels that comprise RBs only within the same subframe/slot and are the smallest unit for data transmission and reception. A single transmission may use one or multiple sub-channels.

Not all radio resources within the specified channel are available for sidelink transmission. Only a subset of the RBs can be used for V2V communications, which is known as a resource pool. A resource pool comprises several contiguous or non-contiguous slots in time, configured as a bitmap, and L contiguous sub-channels comprising a set of consecutive RBs in the frequency domain, as shown in Fig. 1.

The available resources are used to carry different physical channels:

• Physical Sidelink Shared Channel (PSSCH) – that carries data organized in Transport Blocks (TBs). A TB can use one or several sub-channels depending on the specified number of RBs per sub-channel, the utilized Modulation and Coding Scheme (MCS), and the size of the data packet.
• Physical Sidelink Control Channel (PSCCH) – that carries the control information, including Sidelink Control Information (SCI) associated with a TB, that contains the scheduling assignment (RBs occupied by the TB, selected MCS, etc.).
• Physical Sidelink Broadcast Channel (PSBCH) – used to disseminate information for supporting synchronization in sidelink, that is carried inside the sidelink synchronization signal block (S-SSB).
• Physical Sidelink Feedback Channel (PSFCH) – available only in NR V2X, and used to carry the feedback on successful or failed sidelink transmission.

Within these physical channels, several special-purpose physical signals are transmitted, such as the reference signals (demodulation reference signal – DMRS, sidelink channel state information reference signal – SL-CSI or sidelink phase-tracking reference signal – SL-PT-RS) or synchronization signals (sidelink primary synchronization signal – S-PSS and sidelink secondary synchronization signal – S-SSS).

Resource Allocation in V2X

The allocation of resources available within the resource pool can be performed in one of the two available modes. The first one (denoted Mode 3 in LTE-V2X and Mode 1 in NR-V2X) relies on the involvement of the eNodeB/gNodeB that is responsible for the management of resources, so the communicating vehicles need to be in-network coverage. In LTE-V2X with Mode 3, two scheduling options are available: dynamic scheduling, where the resources are requested individually for each transmitted data block, and semi-persistent scheduling, where the eNodeB reserves the resources for transmission of multiple data blocks.

With NR-V2X also two options are possible: the dynamic grant which works in a similar way as the dynamic scheduling in LTE-V2X, and the configured grant, where the gNodeB allocates a set of resources for transmission of several blocks of data. Two configured grants are possible: type 1, where the UE can immediately start using resources after receiving the grant, and type 2, where the use of resources must be activated by the gNodeB with dedicated information and can be later similarly deactivated.

The other mode of resource allocation – Mode 4 in LTE-V2X and Mode 2 in NR-V2X – relies on UEs autonomously selecting the resources. In LTE-V2X a sensing-based semi-persistent scheduling (SB-SPS) algorithm is applied, where the candidate resources (a group of adjacent sub-channels within the same subframe) are chosen from a subset of available resources that were identified as unoccupied in the sensing process within a Selection Window (SW). If the sensing-based resource selection is triggered at time n, the UE will consider the sidelink sensing measurements performed during the sensing window [n+T0; n+Tproc,0], where T0 is preconfigured and Tproc,0 is a user-specific parameter.

SW is a time window that includes the subframes in the range [n+T1, n +T2], where T1 is the processing time for transmission, and T2 is left to UE implementation but must be included within the range T2min≤T2≤100 (10≤T2min≤20). After selecting candidate resources, the allocation is kept fixed in frequency and periodic in time (with period Prsvp) for a number of consecutive data block transmissions, which is determined by the Reselection Counter (RC) parameter. The procedure for SB-SPS is shown in Fig.2.

Mode 2 uses a similar approach, but with two allocation types possible. With the dynamic scheme, the resources are allocated only for the next data block and its eventual retransmission, while the semi-persistent scheme performs allocation for the number of blocks determined by the value of RC.

V2X Synchronization

In a V2X system, there can be a variety of UEs which are deriving synchronization from different sources, depending on their network connection status (in-coverage or out-of-coverage). For NR V2X sidelink, there are two main sources for synchronization, namely GNSS and a gNB or eNB. If these are unavailable, a UE can use a SyncRef UE (a UE transmitting S-SSB) or its own internal clock as its synchronization reference. A UE selects its synchronization reference based on the different priorities of gNB/eNB, GNSS, and SyncRef UEs. If multiple SyncRef UEs are available, then selection among them is based on their priority indicated by their synchronization ID (SLSS ID), which is higher for the in-coverage UEs.

A SyncRef UE transmits the synchronization signals within the S-SSB, which consists of the PSBCH with associated DMRS, S-PSSS, and S-SSS. The S-PSS and S-SSS are referred to as the sidelink synchronization signal (SLSS) and are used for time and frequency synchronization.

In LTE-V2X the synchronization block is transmitted every 160 ms in the central 72 subcarriers of the sidelink bandwidth. S-PSS and S-SSS are transmitted in the central 62 subcarriers of the bandwidth and in the same subframe as PSBCH. S-PSS is in the 2nd and 3rd symbols of the subframe, and SSSS in the 12th and 13th symbols. Fig. 3 illustrates the structure of the LTE-V2X synchronization block.

The NR-V2X S-SSB is also transmitted based on 160 ms periodicity, however, there can be multiple transmissions of S-SSB, with the period, with the number of S-SSB  blocks depending on subcarrier spacing and frequency range. S-PSS and S-SSS use an M-sequence and a Gold sequence, respectively, transmitted in 127 subcarriers of the S-SSB bandwidth, in the same slots as PSBCH. S-PSS is in the 2nd and 3rd symbols of the slot, S-SSS is in the 4th and 5th symbols, and PSBCH and its DMRS are in each of the remaining symbols. The structure of NR-V2X S-SSB is shown in Fig. 4.

QoS Management in V2X

V2X communications in C-V2X provide QoS support. With LTE-V2X sidelink communications, the QoS handling specified by ProSe is used, where the QoS management is performed on a per-packet basis. Each V2X packet from the Application Layer is associated with a priority value (Proximity Service Per-Packet Priority -PPPP) and, eventually, a reliability value (Proximity Service Per-Packet Reliability – PPPR).

The mapping between the data from lower layers (PHY and Layer 2) to the values of the Sidelink Radio Bearer (SLRB) configuration is UE-implementation dependent. Therefore, there are no unified rules for managing QoS among UEs in LTE-V2X sidelink transmission.

With NR-V2X sidelink communications, a more advanced 5G QoS model is used, which is the same as the one defined for NR Uu. QoS management in NR V2X SL is based on the QoS Flows, that are associated with the QoS requirements of the V2X applications. The association of each application to a flow is based on QoS Profiles, which are specified using QoS parameters and QoS characteristics. A dedicated PC5-RRC sublayer is introduced in NR V2X to support QoS profiling and management of unicast V2X communications. Thus, QoS management here is configured by the network, and it is no longer dependent on UE implementation.

Summary

With the presented summary of C-V2X radio interfaces, we can clearly see their capability of fulfilling the requirements for V2X services. While LTE-V2X is an extension of the LTE standard to support basic V2X services, allowing only for broadcast communications, NR-V2X provides flexibility and configurability to support various communication scenarios.

The use of the flexible configuration of the physical resources grid, following the concept of different numerologies, allows for use of various bandwidths and optimization of processing latency using shorter slots. The different defined resource allocation modes, accounting for the possibility of operation in-coverage and out-of-coverage, provide tools for safe and efficient selection of sub-channels used for transmission, minimizing the risk of collision.

Furthermore, with the possibility of providing synchronization information over sidelink, the out-of-coverage operation is further supported. Finally, the improved QoS management introduced in NR-V2X provides a tool for guaranteeing the fulfillment of the requirements set for different V2X applications.

References

[1] 3GPP, “TS 38.300 NR and NG-RAN Overall Description, v16.5.0,” 3GPP, Tech. Spec., Mar. 2021. (3GPP)

[2] L. Gallo and J. Haerri, „Unsupervised Long- Term Evolution Device-to-Device: A Case Study for Safety-Critical V2X Communications,” in IEEE Vehicular Technology Magazine, vol. 12, no. 2, pp. 69-77, June 2017

[3] M. Castaneda Garcia et. al, “A Tutorial on 5G NR V2X Communications”, IEEE Communication Surveys and Tutorials 2021

[4] Z. Ali, S. Lagén, L. Giupponi and R. Rouil, „3GPP NR V2X Mode 2: Overview, Models and System-Level Evaluation,” in IEEE Access, vol. 9, pp. 89554-89579, 2021

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