“The Internet of Vehicles is a typical representative of the current integration of informatization and industrialization, and it is also an important direction of current research in all walks of life. Therefore, starting from the short-range communication technology in the United States, the 3GPP-based cellular Internet of Vehicles standard is introduced, and the standard process of the Internet of Vehicles in 3GPP is analyzed. The LTE V2X standard introduced from D2D to R14, and focuses on the scenarios and key technologies of LTE V2X.
With the development of information technology, the Internet of Vehicles has become a hot spot after the mobile Internet. On September 28, 2016, the 5G Automotive Association (5GAA, 5G Automotive Association), consisting of more than 40 companies mainly composed of car companies, operators and equipment manufacturers, was officially established. The purpose of 5GAA is to jointly define the requirements of Cellular Vehicle to X (C-V2X, Cellular Vehicle to X), jointly promote the realization of intelligent driving end-to-end solutions through multi-industry cooperation, and establish a successful IoV ecosystem. All-round cooperation in testing and sales promotion, setting standards, accelerating commercialization and promotion to the global market. This article focuses on analyzing the 4G/LTE-based V2X standards formulated by 3GPP and the emerging 5G V2X requirements.
2 V2X Standard Overview
At the beginning of 2013, LG of South Korea and Qualcomm of the United States put forward the application for the establishment of the Internet of Vehicles project in 3GPP SA1, and immediately started the research work from demand to standard formulation. At the same time, with the deep integration of informatization and industrialization, the Internet of Vehicles has given birth to a series of industrial changes. This is a new development opportunity. The China Communications Standards Association (CCSA) is also actively developing the LTE V2X standard, as shown in Figure 1:
Figure 1 Standard overview of LTE V2X
The cellular Internet of Vehicles was introduced from the 3GPP R14 version, which is the last version of the 4G technology LTE, so it is called LTE-V2X. The standard was frozen in the first quarter of 2017 and completed in the middle of the year; China’s communication standard closely follows the progress of the 3GPP standard and develops Research on the overall technical requirements of the Internet of Vehicles based on LTE system.
LTE-eV2X will be synchronized with 5G NR in R15 and will be completed around mid-2018.
Compared with LTE-V2X, LTE-eV2X mainly carries out technical enhancements for scenarios such as formation driving, enhanced driving, extended sensors, and remote control driving. LTE-V2X and LTE-eV2X are designed to meet the needs of connected vehicles in the next 10 years.
Of course, if there are new application requirements that cannot be solved through V2X and eV2X, 3GPP will also start 5G NR-V2X in the R16 phase for enhancement, including the enhancement of the Uu interface between the terminal and the base station, and the enhancement of the interface between the terminal and the terminal. Although this part of the work has not been carried out, it can be seen from the performance of 5G that 5G NR V2X will achieve shorter delay and higher reliability, but will not replace LTE V2X/eV2X, but as a supplement, provide More advanced V2X services and support interoperability with LTE V2X/eV2X.
3 V2X key technologies
As we all know, before the birth of V2X, the United States already had a mature vehicle networking communication scheme: the specific short-range communication (DSRC, Dedicated Shorted Range) scheme based on IEEE 802.11p, which is also the communication protocol of V2V. Many car companies have carried out research and evaluation work on the DSRC system for almost 10 years. Almost all car companies have identified the development direction of vehicle intelligence, but the progress is not ideal. Cellular network operators expect to enter the camp of the Internet of Vehicles, so the standard formulation is gradually carried out on the basis of learning from DSRC.
3.1 LTE D2D Technology
Throughout the development history of 3GPP, all communications are through the network. Any signaling and information transmission from user A to user B needs to pass through the base station first, and then perform processing and forwarding at subsequent nodes. There is no user A and user. B’s direct conversation. However, when it comes to the Internet of Vehicles, the scene changes, and the needs vary. For example, in the automatic driving scenarios of vehicles (such as formation driving, merging, etc.), the distance between sending and receiving vehicles is close, and the direct connection between vehicles is a better communication mode. Therefore, a brand-new bearer communication standard in the 3GPP standard system was created, namely LTE D2D (Device To Device, also known as LTE Direct). SA2 and RAN have carried out a series of standard work from requirements to architecture to specific channels, resource allocation, etc.
LTE D2D provides a new communication method for the V2V communication of the Internet of Vehicles, that is, the pass-through method based on the PC5 interface (vehicle-to-vehicle interface).
3.2 LTE MBMS
As we all know, the 5G white paper proposed by ITU mentioned that 5G has higher performance requirements than 4G: millisecond-level delay, 0.1 Gbit·s-1~1 Gbit·s-1 user experience rate and 1 million per square kilometer connection density. With the deployment of IPv6, there are dozens of IP addresses on the sole of a foot. Therefore, the traditional unicast point-to-point communication method (Unicast) based on response confirmation can no longer meet the needs of Internet of Vehicles communication, and broadcast multicast technology (MBMS, Multi Broadcast Multi Service) must be introduced to more efficiently share the surrounding information. information. Therefore, it can be seen that LTE MBMS is only a part of the Internet of Vehicles and a usage scenario of the Internet of Vehicles.
3.3 LTE V2X
As mentioned above, DSRC appeared more than ten years earlier than LTE-V, but it did not win the market. The main reason is that many problems such as congestion, interference and coverage have not been solved when vehicles communicate directly. Just like a conflict between two peers, if there is no third-party superior to arbitrate, the two peers will enter a deadlock. When LTE-V realizes direct connection, that is, when vehicles communicate directly with each other, the cellular network acts as an arbitrator, which solves the problems of congestion and interference.
The following first introduces the key technologies of LTE V2V based on PC5 direct connection:
(1) Added 4 DMRSs (DemodulaTI
(2) The decentralized scheduling technology using the semi-persistent scheduling (Semi Persistent Scheduling) method is introduced. One radio resource allocation can use multiple subframes, which reduces intra-frequency radiation, optimizes channel usage, and improves transmission efficiency.
(3) A new scheduling assignment function has been introduced. In this way, proper data resource processing can be performed in an environment with a higher density of multiple communication nodes in the vehicle, and the delay time is also improved for V2V.
(4) Clock synchronization, lack of synchronization source when the network is not covered. V2X supports both base station and Global Navigation Satellite System (GNSS) time synchronization.
(5) Professional QoS technology: V2X messages can be transmitted through non-GBR and GBR bearers: QCI 3 (GBR) and QCI 79 (non-GBR) can be used for unicast transmission of V2X messages, and QCI 75 (GBR) can only be used for unicast transmission of V2X messages. For V2X messages carried by MBMS, the transmission reliability is greatly improved through dedicated QCI.
The newly added DMRS symbols, new channel structure and scheduling allocation function through the PC5 interface enable the network to participate in the V2V communication, avoid problems such as interference and congestion, and become a de facto industrialized technology.
Although LTE-V started later than DSRC, it has good genes and excellent system design. It inherits the technical and operational advantages of mobile cellular networks. At the same time, it is supported by the Chinese and European governments, and the scale of the industry is expected to be large.
4 V2X system architecture
V2X communication has two independent and complementary working modes, namely V2X communication based on PC5 pass-through mode and V2X communication based on LTE-Uu. The working mode based on LTE-Uu can be unicast Unicast or MBMS mode. The UE can use these two working modes for reception and transmission respectively. For example, a UE can use MBMS multicast to receive V2X messages, but does not use LTE-Uu to send V2X messages. A UE can also receive V2X messages through LTE-Uu downlink unicast. Basically, V2X application servers can communicate with each other to exchange V2X information. Legal interception is applicable to V2X services.
4.1 V2X Communication Architecture Based on PC5 and LTE-Uu
The communication architecture of V2X based on PC5 and LTE-Uu is shown in Figure 2:
Figure 2 Communication architecture of V2X based on PC5 and LTE-Uu
(1) Interface network element
The Internet of Vehicles is a brand-new application of mobile cellular networks, so there are many interface network elements involved, which are explained one by one below:
V1: The interface between the V2X application (built in UE) and the V2X application server. Obviously, V1 is the interface of the application layer, and the relevant information is not defined in the scope of 3GPP.
V2: The interface between the V2X application server and the V2X Control Function. The V2X application server can connect to the V2X control functions of multiple PLMNs.
V3: The interface between the UE and the V2X control function in the home PLMN, suitable for PC5-based and LTE-Uu-based V2X communication, and LTE-Uu-based V2X communication can optionally support MBMS.
V4: The interface between the HSS and V2X control functions in the operator’s network.
V5: Interface between V2X applications in UE.
V6: Interface between V2X control functions in different PLMNs (not covered in this article).
PC5: An interface for direct D2D (Device to Device) communication on the user plane between V2X service UEs.
S6a: In the V2X scenario, during the E-UTRAN attach procedure, the S6a interface can be used to download the subscription information related to the V2X communication to the MME, or notify the MME when the subscription information in the HSS changes.
S1-MME: In V2X scenario, this interface can transfer V2X service authorization from MME to eNodeB.
LTE-Uu: Interface between UE and E-UTRAN.
(2) Functional requirements
For V2X pass-through link communication, the PC5 interface is disconnected. UEs supporting V2X direct link communication can work in two resource allocation modes:
1) Resource scheduling allocation (through link transmission mode 3), which is characterized by:
◆Before sending data, the UE must be in the RRC_CONNECTED state;
◆The UE requests transmission resources from the eNB, and the eNB schedules the transmission resources to transmit the direct link control information and data to the UE, and supports the direct link semi-persistent scheduling (SPS).
2) UE autonomous resource selection (direct link transmission mode 4), which is characterized by:
The UE autonomously selects resources from the resource pool, performs transmission format selection, and sends through link control information and data;
◆If the mapping relationship between the geographic area and the sending resource pool is configured, the UE selects the V2X direct link sending resource pool based on the geographic area where it is located;
The UE performs perception to select/or reselect resources. Based on the sensing result, the UE selects/or reselects some through link resources, and reserves a plurality of through link resources. The UE can support up to two parallel and independent resource reservation processes, and the UE can also perform a single resource selection.
For V2X direct communication, during the handover process, the configuration of the sending resource pool, including the abnormal sending resource pool configuration for the target cell, can be sent to the UE in the handover signaling to reduce sending interruptions.
In order to avoid the interruption time of V2X message reception caused by capturing the receiving resource pool broadcasted by the target cell, the synchronization configuration and the receiving resource pool configuration of the target cell can be notified to the UE in the RRC_Connected state in the handover command. For UEs in idle state, it is implemented by the UE to minimize the time of through-link transmission and reception interruptions associated with the acquisition of the target cell SIB21.
4.2 V2X Communication Architecture Based on MBMS LTE-Uu
The V2X communication architecture based on MBMS LTE-Uu is shown in Figure 3:
Figure 3 V2X communication architecture based on MBMS LTE-Uu
Most of the interfaces are the same as Figure 2, and only two different interfaces are described here:
MB2: Interface between 2X Application Server and BM-SC.
SGmb/SGi-mb/M1/M3: SGmb/SGi-mb/M1/M3 interface within the MBMS system.
(2) Functional requirements
If the UE is configured to receive V2X application server information through MBMS, it will receive local information through the corresponding broadcast channel. The local information includes address information of the local V2X application server. In addition, if the downlink uses MBMS, the local information may also include the USD (User Service Description, user service description) of the V2X application server.
The UE obtains the address of the local V2X application server according to the received information. If the UE does not receive USD during this process, the UE needs to establish a connection to the V2X application server to obtain the user’s USD information.
Because MBMS is involved, the V2X server needs to map the user’s information to information that can be identified by MBMS and then send it, that is, format matching. In addition, the UE may provide geographic location information, and the V2X application server uses this information to determine the target MBMS broadcast area. MBMS business area. The V2X application server provides the MBMS service area or cell ID to the BM-SC (multicast controller) for accurate delivery of the subsequent LTE-Uu interface. At the same time, in order to reduce the delay, MBMS needs to support a shorter delivery/update cycle.
Looking at the above analysis, the Internet of Vehicles is a brand-new network for operators, which needs to add many additional interfaces and network elements. While consolidating their own basic network construction, operators must face car companies and better integrate network information. Matching the requirements of the application layer to achieve accurate and effective delivery is a big challenge in terms of investment and technology.
With the advancement of 5G, the integration of industries will be further increased, and the functions of the V2X service platform will be improved. For example, the existing auxiliary wireless technology and the functions of the original mobile network such as scheduling and interference cancellation will be moved to the cloud. At the same time, with the 5G slicing function, Operators will provide independent networking for the Internet of Vehicles, so that the functions of the V2X vehicle terminal and the V2X platform will evolve, innovate and upgrade respectively, and accelerate the further development of the Internet of Vehicles.
This article starts with the LTE V2X standard, and details the current status of the standard and the considerations that may be based on the 5G NR evolution version. Based on the comparison with DSRC, the key technologies involved in LTE V2X, such as LTE D2D, multicast and multicast, and technical enhancements based on the LTE structure for the characteristics of the Internet of Vehicles, are described in detail. Through the analysis of this paper, it can be seen that LTE V2X technology has excellent performance, supports rich V2X services, and can be oriented to the 5G evolution route to support the evolving V2X requirements. At present, the pilot projects of the Internet of Vehicles are gradually increasing. With the emergence of the closed loop of pilot implementation, problem discovery and problem solving, it is believed that in the near future, the Internet of Vehicles will no longer stop at learning and reporting, but will, like the Internet, bring benefits to the majority of users. A whole new experience.