Under the hood of v2x communications
01 April 2020
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Many commuters regularly suffer the impact of traffic incidents on our motorways, perhaps encouraging the feeling that this is where the majority of accidents occur. With the volume of vehicles & flashing lights of the emergency services, it sticks in the mind. However, it’s actually urban & rural areas where the majority of accidents occur (37% & 55%).
This article was originally featured in the April 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
According to European Parliamentary Research Service (EPRS) data, existing efforts to reduce highway casualties, such as seatbelts, airbags and other advanced safety systems, have resulted in a drop of road fatalities in the EU by 58% between 2001 and 2017. However, as Holger Rosier, Technology Manager for Wireless at electronics T&M experts, Rohde & Schwarz tells us here, further improvements are only likely to come about through the integration of more intelligence in our safety systems, which will need to be networked to share data – and blanket coverage will be necessary. Only then can drivers benefit from innovations that overcome the challenges of poor visual conditions, blind obstacles and the mix of traffic types on our roads today.
Across the world, a range of cooperative intelligent transport systems (C-ITS) have been developed and promoted. The intent of these technologies is to enable the sharing of perception, allowing road users to be aware of road conditions they may not be able to see. Obviously, such technologies will rely upon wireless and mobile communication to function. In addition, however, it needs to consider other road users, such as pedestrians and cyclists, who participate on our roads. Furthermore, not all areas are adequately covered by existing wireless and mobile infrastructure, so how should C-ITS users communicate with one another under such conditions?
Improving safety through existing mobile technology usage
A system to enable road users and its infrastructure to share or exchange data, referred to as the Cellular-V2X Service (C-V2X), has been present since 2017 in the 3GPP-LTE Release 14 mobile communications standard. Within it, the technical approach is described that will allow C-ITS to operate, underpinned by existing LTE cellular networks. Teams known as technical specification groups (TSG) have defined three applications areas that will benefit from the technology: road safety, traffic efficiency, and other applications that are mainly related to travel convenience.
Figure 1. LTE v2x defines four communication services.
Analysis of these application classes helps to understand the technical demands required of C-V2X to support them. For example, road safety, covering use cases such as forward collision warnings or an approaching emergency vehicle, necessitate high service availability, transmission reliability and low latency. Traffic efficiency, enabling capabilities such as green light optimal speed advisory (GLOSA), have less-demanding data delivery requirements. Other potential applications include automated parking, sharing the availability of parking spaces, or unique services provided by automotive OEMs to their customers. Here, features are required to enable certified parties to set up application servers to handle C-V2X data requests and respond to them.
Use of existing LTE cellular networks to communicate with such application servers via vehicle-to-network (v2n) communication makes obvious sense (figure 1). Data collection of traffic conditions, handled by road-side units (RSU) designed for the purpose, could also share their data via LTE through vehicle-to-infrastructure services (v2i).
Of course, not all areas are adequately covered by existing cellular networks, especially rural areas, where many of the emergency response capabilities of C-V2X could have the highest impact. Existing cellular networks may have poor latency end-to-end, which would prove detrimental for highway users travelling at speed. Finally, access to services should not be limited to a network operator with whom the vehicle user has a subscription. These issues have all been addressed. Firstly, data transmission is supported without valid subscription, allowing the best available network link to be used. Secondly, communication between vehicles (vehicle-to-vehicle, v2v), vehicles and pedestrians (vehicle-to-pedestrian, v2p) and some v2i functions can be undertaken directly between the participating parties without going via the cellular network.
Within 3GPP-LTE Release 14, direct v2v, v2i and v2p communication is handled using the PC5 interface. The operation of such ad hoc networks, without the support of any mobile network infrastructure, is possible within cellular network coverage, as well as out-of-coverage.
Figure 2. Clock synchronisation in out-of-coverage scenarios require C-V2X nodes to use alternative sources, such as indirectly from an eNB (above), GNSS (bottom left), or simply other C-V2X road users (bottom right).jpg
Handling synchronisation in C-V2X scenarios
Typically, communication over cellular networks demands that all participants agree on a time-base in order that their clocks are synchronised. For mobile devices in in-coverage situations, this is achieved through their regular cellular network communication with Evolved NodeB (eNB) infrastructure (figure 2). This synchronisation is essential to minimise intersymbol interference (ISI) on frequency division multiple access (FDMA) and time division multiple access (TDMA) systems. This means that alternate synchronisation mechanisms are required as soon as the eNB becomes unavailable.
The standard provides several synchronisation sources in order of preference in such situations. Global navigation satellite systems (GNSS) are one such source, used either directly via the vehicle’s internal systems, or indirectly through a v2v or v2i connection to a vehicle or RSU that is using GNSS for its own synchronisation. Indirect synchronisation may also be achieved through connection to a C-V2X device that is, itself, connected to an eNB. Failing this, vehicles can simply synchronise with one another.
Communication protocols & channels for PC5
Two protocol stacks are defined, one in the user plane and the other in the control plane, in order that communication over PC5 can be maintained. The control plane protocol stack provides communication services for control data, while the exchange of user data via v2x applications is supported by the user plane protocol stack (figure 3).
Figure 3. The protocol stack in the user plane (above) and control plane (below)
The physical layer (PHY) transmits data on the sidelink, exploiting 10MHz or 20MHz bandwidths at 5.9GHz in radio frequency band 47. This has been made license-exempt for C-ITS communication by regulation bodies across the world. In China, permission has been granted only for C-V2X technology, while Europe is staying technology neutral. In the USA, a request has been submitted to the FCC asking for C-V2X be allowed to operate in the spectrum currently in use by dedicated short-range communications (DSRC).
Blind hybrid automatic repeat request (HARQ) without feedback is implemented in the media access control (MAC) layer. This is also where packet scheduling and resource selection occurs. Packet filtering is also implemented in this layer to ensure that only data units destined for this specific v2x device reach to the upper layers.
The radio link control (RLC) layer handles in-sequence delivery of service data units, as well as their segmentation and reassembly. Finally, the packet data convergence protocol (PDCP) sublayer separates 3GPP radio access protocol layers from those related to C-ITS applications. The support for non-IP data is essential for supporting C-ITS applications and has been present since Release 14.
For broadcast communication services, the necessary adaptation of the radio parameters is handled in a separate layer in the control plane, known as the radio resource control (RRC) sublayer.
Figure 4. The 1ms PC5 subframe uses four slots for a demodulation reference symbol, improving robustness against Doppler shift
One additional challenge lies with the potential for saturation, known as the near-far effect. Thanks to a method known as the zone concept, radio resources are assigned to vehicles depending on the geographical position. This information is then used to improve the signal to interference plus noise ratio (SINR) by keeping the radio signal within an acceptable range, resulting is an improvement in radio signal decoding.
The MAC sublayer provides two logical communication channels to the RLC sublayer for C-V2X communication. The first, the sidelink broadcast control channel (SBCCH), handles control plane messages, while the second, the sidelink traffic channel (STCH), handles user plane messages. These map to two transport channels. The sidelink broadcast channel (SL-BCH) carries higher layer control data and maps to the SBCCH. For user data, the sidelink shared channel (SL-SCH) maps to the STCH.
Due to other C-V2X devices in the vicinity, operation in autonomous resource selection mode (known as transmission mode 4, TM4) may result in the device suffering from interference. To combat this, the SL-SCH makes use of blind HARQ for a maximum of one retransmission of user data. This capability is not offered to the control data carrying SL-BCH.
At the PHY, these transport layers are further mapped to physical channels, with SL-SCH mapping to the physical sidelink shared channel (PSSCH) and SL-BCH mapping to the physical sidelink broadcast channel (PSBCH). Control information associated to the control plane handling time and frequency resource allocation is transmitted on the physical sidelink control channel (PSCCH). Such control information is transmitted using robust quadrature phase shift keying (QPSK). In contrast, user data on the PSSCH uses QPSK and order 16 quadrature amplitude modulation (16QAM).
Figure 5. Together, the CMW500 and SMBV100B are an ideal testing platform for v2x applications
PC5 communication also adopts the general 1ms subframe structure of LTE. With 14 single carrier frequency division multiple access (SC-FDMA) symbols per subframe, four are given over to a demodulation reference symbol (DMRS) (figure 4). These compensate for the challenges introduced by Doppler shifts expected in C-V2X communication.
Ensuring that testing for v2x is future-proof
With the safety of road users in mind, it’s critical that a testing approach is realised that can handle the complexity of C-V2X and C-ITS communication protocols under both static and dynamic conditions. This ensures that the end product can operate correctly under the wide variety of environmental situations it will face in the field, as well as be both interoperable and compliant to the standards defined. LTE technology test and measurement solutions, such as the R&S®CMW500, are highly suited as part of the engineering team’s C-V2X test suite (figure 5). It is the first test solution that has been approved by the Global Certification Forum (GCF) and, as a wideband radio tester, is capable of delivering results relating to both dynamic and static operating conditions. The full 3GPP LTE-V2X radio access protocols, including region dependent C-ITS application differences for China, Europe and the USA, are also supported. For the location-based element, the R&S®SMBV100B signal generator provides a simulation of GNSS. Regardless of whether the aim is to develop suites of tests, or the focus is more on long-term testing, the available APIs support integration with third-party tools and test suites or systems that have been self-developed.
For the foreseeable future, direct C-V2X PC5 for basic safety applications, termed Phase I, will rely upon LTE Release 14 for communication, especially in out-of-coverage scenarios. Phase II of the C-V2X rollout will see LTE enhanced V2X (eV2X) included in Release 15. Targeted for release in 2019, it will add support for C-ITS applications, such as cooperative perception. Support for the 5G New Radio (NR) concept is expected to be standardised in Phase III as part of 3GPP Release 16. This means that for automotive engineers developing v2x applications, their existing test and measurement investments will continue to support their needs for several years to come.
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