How should C-ITS users communicate in the future?
How should C-ITS users communicate in the future?
( Source: gemeinfrei / Unsplash)

Communication

The details of V2X communication

| Author/ Editor: Holger Rosier / Florian Richert

Throughout the world, various cooperative, intelligent transport systems (C-ITS) are being developed. These systems, of course, depend on wireless and mobile communication. It is also important to consider other road users and not all areas have sufficient wireless and mobile infrastructure. So how should C-ITS users communicate with each other under such conditions?

The intelligence integrated into our transport systems must make the traffic information available for shared use with minimal latency if we are to have any chance of reducing the accident rate on our roads and motorways to zero. Thanks to modern automotive safety systems, the number of road deaths in the EU fell by 57.5% between 2001 and 2017. Although motorways are a source of daily frustration with their high traffic volumes and many road accidents, in reality, most accidents occur on rural roads and in urban areas (55% and 37% respectively). Poor visibility, mixed traffic and difficult to see obstacles are the main challenges to overcome, regardless of the driver's skill.

Mobile technologies to increase security

Since 2017, the mobile radio standard 3GPP-LTE Release 14 has offered several features. Summarized as Cellular V2X Communication (C-V2X), they describe how to use the technology that supports existing cellular LTE networks for C-ITS. The Technical Specification Group (TSG) identified three application classes: Traffic safety, traffic efficiency, and other applications.

An analysis of these three classes identifies the necessary technical requirements for C-V2X. For road safety, high service availability, high transmission reliability, and low latency are essential; possible application scenarios are, for example, collision warning systems or approaching emergency vehicles. In the field of traffic efficiency, which among other things involves the optimum use of green phases (Green Light Optimal Speed Advisory, GLOSA), lower requirements are placed on data transmission. Other applications include automated parking, information exchange about free parking spaces or special services offered by automotive OEMs to their customers. Features enable certified participants to set up application servers to process C-V2X data requests. It makes sense to use existing cellular LTE networks for V2N communication with the application servers (Figure 1). Traffic data managed by Roadside Units (RSU) can be exchanged via LTE using Vehicle-to-Infrastructure Services (V2I).

Unfortunately, the latency time associated with data transmission over a cellular network is not suitable for fast road users. It is also important to take into account traffic situations with limited or no LTE coverage - for example in tunnels or rural areas. Therefore, communication between vehicles (Vehicle-to-Vehicle, V2V), between vehicle and pedestrian (Vehicle-to-Pedestrian, V2P) and between vehicle and infrastructure (Vehicle-to-Infrastructure, V2I) must also take place without a cellular network. Besides, reception and transmission of safety-relevant data should not depend on a specific mobile network operator. C-V2X achieves this by enabling data transmission without binding to a specific mobile network operator.

Figure 1: LTE V2X defines four communication services.
Figure 1: LTE V2X defines four communication services.
(Source: Rohde & Schwarz)

In 3GPP-LTE Release 14, direct V2V, V2I and V2P communication are handled via the PC5 interface. The operation of such ad-hoc networks without a mobile communications infrastructure can take place independently of the cellular network.

Synchronization in C-V2X Scenarios

In scenarios with cellular network coverage, C-V2X-enabled vehicles can synchronize their clock signal with the eNodeB infrastructure (eNB) (Figure 2). This is important for minimizing symbol interference (ISI) in time division multiple access (TDMA) and frequency division multiple access (FDMA) systems. If no eNB is available, an alternative synchronization mechanism is required. The standard provides for different synchronization sources arranged according to preference. Global Navigation Satellite Systems (GNSS) can be accessed either directly via internal vehicle systems or indirectly via a V2V or V2I connection to a GNSS synchronized vehicle or RSU. An alternative is indirect synchronization via a connection to a C-V2X device connected to an eNB. If all this is not possible, vehicles can synchronize with each other.

Figure 2: For clock synchronization outside network coverage, C-V2X nodes use alternative sources such as eNB, GNSS or other C-V2X traffic participants..
Figure 2: For clock synchronization outside network coverage, C-V2X nodes use alternative sources such as eNB, GNSS or other C-V2X traffic participants..
(Source: Rohde & Schwarz)

Communication protocols and channels for PC5

To maintain communication via PC5, two protocol stacks were defined. The user-level protocol stack allows the exchange of user data, while the control level is used to send control data (Figure 3).

The physical layer (PHY) transmits data on the sidelink and uses 10 MHz or 20 MHz bandwidths at 5.9 GHz in ITS band 47. This type of C-ITS communication has been approved by regulatory authorities worldwide as license-free. In China, only C-V2X technology has been licensed, while in Europe all technologies are allowed. In the USA, the FCC has received an application from the regulatory authority, according to which C-V2X will in the future operate in the spectrum currently used by dedicated short-range communications (DSRC).

Media Access Control (MAC) layer manages packet flow control and resource allocation. Packet filtering on this layer ensures that only protocol data units intended for a particular V2X device are forwarded to the higher layers. The hybrid ARQ protocol (HARQ) is also implemented here.

On the radio link control layer (RLC), service data units are transmitted in the correct order and their segmentation and reassembly takes place. The Packet Data Convergence Protocol (PDCP) sublayer separates 3GPP radio access protocol layers from those of C-ITS applications. The processing of non-IP data is essential for C-ITS applications and has been supported since Release 14.

An additional layer in the control layer, the Radio Resource Control (RRC) sublayer, covers broadcast communication services. These manage communication, configure protocol services, and adjust radio parameters. The so-called "Zone Concept" uses the latitude and longitude of the vehicle to ensure that the received radio signal is within an acceptable range. This limits possible saturation, known as the near-range effect, and improves the signal-to-noise ratio (SINR) so that the radio signal can be decoded.

Figure 3.1: The protocol stack at the user level.
Figure 3.1: The protocol stack at the user level.
(Source: Rohde & Schwarz)

This MAC sublayer provides the RLC sublayer with two logical communication channels for C-V2X communication. The Sidelink Broadcast Control Channel (SBCCH) processes control level messages, the Sidelink Traffic Channel (STCH) user-level messages. These are mapped to two transport channels. The Sidelink Broadcast Channel (SL-BCH) transports higher-level control data and is mapped to the SBCCH. The Sidelink Shared Channel (SL-SCH) transports user data and is mapped to the STCH.

When operating in Autonomous Resource Selection Mode (known as Transmission Mode 4, TM4), devices may be subject to interference from other nearby C-V2X devices. As a countermeasure, the SL-SCH uses the HARQ procedure for a maximum of one retransmission. This function is not available for SL-BCH control data.

Figure 3.2: The protocol stack in the control level.
Figure 3.2: The protocol stack in the control level.
(Source: Rohde & Schwarz)

On the physical layer, these transport layers are further mapped to physical channels (SL-SCH to the Physical Sidelink Shared Channel (PSSCH) and SL-BCH to the Physical Sidelink Broadcast Channel (PSBCH)). Control information related to processing time and frequency resource allocation at the control level can be transferred to the Physical Sidelink Control Channel (PSCCH). This control information is transmitted using a robust quadrature phase-shift keying (QPSK). In contrast, user data on the PSSCH uses QPSK and 16-square amplitude modulation (16QAM).

PC5 communication also takes over LTE's general 1 ms subframe structure. With 14 Single Carrier Frequency Division Multiple Access (SC-FDMA) symbols per subframe, four are passed to a Demodulation Reference Symbol (DMRS) (Figure 4). These compensate for Doppler shifts in C-V2X communication.

Figure 4: The 1 ms PC5 subframe uses four slots for a demodulation reference symbol to improve robustness against Doppler shifts..
Figure 4: The 1 ms PC5 subframe uses four slots for a demodulation reference symbol to improve robustness against Doppler shifts..
(Source: Rohde & Schwarz)

Future-proof V2X tests

Complex C-V2X and C-ITS communication protocols, as well as traffic safety applications paired with environmental factors and mobile V2X nodes, require accurate testing under a wide variety of conditions to ensure a compliant, interoperable solution. Existing LTE measurement solutions such as the R&S®CMW500 are ideally suited for C-V2X tests (Figure 5). The Wideband Radio Communication Tester offers the entire range of functions - from static protocol conformity to operation under dynamic conditions such as fading and the effects of signal reflections. It also supports the complete stack from 3GPP-LTE-V2X Radio Access Protocols to area-dependent C-ITS applications in China, Europe, and the USA. The CMW500 is the first C-V2X test solution approved by the Global Certification Forum (GCF). The SMBV100B signal generator can be used to simulate the GNSS. Thanks to available software APIs, the devices can be integrated into existing systems and automated test environments, both for test series and long-term tests.

Figure 5: The CMW500 in combination with the SMBV100B.
Figure 5: The CMW500 in combination with the SMBV100B.
(Source: Rohde & Schwarz)

3GPP Release 14 specifies direct communication according to C-V2X PC5 for safety-critical applications (Phase I), especially for scenarios without network coverage. In Phase II of the C-V2X rollout, LTE enhanced V2X (eV2X) will become part of Release 15. The release planned for 2019 supports C-ITS applications such as cooperative perception. 5G New Radio (5G NR) is expected to be standardized in Phase III as part of 3GPP Release 16. This means that automotive application engineers will be able to use their existing test equipment in the coming years without major investment.

This article was first published in German by next-mobility.news.