Properly functioning congestion control algorithms are required to coordinate the appropriate usage of the channels. Algorithms have been developed to mitigate the impact of these situations, such that resources are shared equally to keep the channels as unsaturated as possible. The standard defines two metrics that characterize the state of the channels. These are channel busy ratio (CBR) and channel occupancy ratio (COR). C-V2X modules will sense the environment for 1000 milliseconds and try to determine which resource blocks, each made up of subframes and subchannels, are transmitted by neighboring vehicles. The device then determines if there is a spare resource block at a certain frequency or a certain channel and then decides when to transmit at the least used or lowest energy segment of the spectrum. Decisions are made based on COR and CBR information that comes over the control plane, and resources are consequently allocated and transmitted.
In addition to the CBR and COR, congestion control algorithms make decisions based on various metrics and parameters, including power, channel quality indicator and range. There are various techniques the station or the device can use depending on the inputs to the algorithm. Some of the techniques that can be used are:
- Drop packet retransmission: If the retransmission feature is enabled, the station can disable it
- Drop packet transmission: The station simply drops the packet transmission, including the retransmission if enabled; this is one of the simplest techniques
- Reduce packet transmission periodicity: Extend the packet transmission interval
- Adapt transmission power: The station can reduce its transmission power; consequently, the overall CBR in the area will be reduced, and the value of the CBR limit might be increased.
Because the standards do not mandate which techniques to use, it may be beneficial to place the device under test in a crowded environment to see which mechanisms are triggered and how the device handles the messages and reacts accordingly. Testing how well the congestion control algorithms work is critical.
COEXISTENCE & INTERFERENCE TESTING
In addition to congestion stress testing, coexistence and interference testing is required to ensure the C-V2X will coexist with all the many other wireless technologies operating in and around modern vehicles (see Figure 3). 5G NR’s Frequency Range 1 (FR1) includes the frequency bands from 410 MHz to 7.125 GHz, including spectrum used by or adjacent to existing wireless communications systems, Wi-Fi and Bluetooth. Again, given the safety implications of C-V2X communications, it is critical that 5G C-V2X modules operate in this spectrum range without interference.
Comprehensive interference testing involves testing in-band and out-of-band emissions and testing the impact of the C-V2X signals on other radio signals to ensure that the 5G C-V2X signal does not cause interference with other radios in the same vehicle or other radio signals in the channel or adjacent spectrum. Interference from out-of-band emissions can degrade the reliability of C-V2X communications, directly impacting transportation safety. A U.S. Department of Transportation technical assessment found that out-of-band interference from DSRC, Wi-Fi and LTE C-V2X operating in adjacent channels can leak into the adjacent spectrum, causing concern about the reliability of C-V2X communications. C-V2X modules must operate in a shared spectrum environment without negatively impacting bandwidth. Sharing airwaves increases the responsibility of semiconductor manufacturers, automakers and equipment manufacturers to ensure the C-V2X system will coexist with existing commercial wireless infrastructure.
Much like congestion control algorithms, C-V2X systems require complex algorithms to monitor and detect other users in the spectrum and adjust the transmission and reception of signals accordingly.
5G NR’s Frequency Range 2 (FR2) extends from 24.25 to 71 GHz. Extending to mmWave frequencies enables 5G NR to access a larger contiguous bandwidth, meaning access to much more data related to traffic and road hazards to and from the cloud or to nearby vehicles. But the smaller wavelength of mmWave introduces challenges to signal quality and link budget. Factors impacting mmWave signal quality include baseband signal processing, modulation, filtering and up-conversion. Also, C-V2X modules will face signal impairments more problematic at higher frequencies and the wider channel bandwidths.
The orthogonal properties inherent in OFDM systems prevent interference between overlapping subcarriers. However, issues such as I/Q impairments, phase noise, linear (AM to AM) and nonlinear (AM to PM) compression and frequency error cause distortion in the modulated signal. Phase noise is one of the most challenging: high phase noise results in high error vector magnitude and subcarrier interference, impairing demodulation.
Operating at mmWave introduces challenges from path loss, blockage and signal propagation. Because of the shorter wavelengths at mmWave, physical obstacles in the channel—including other vehicles—will block the signal, with the severity compounded by vehicle-mounted antennas. Beamforming is a key technology for overcoming these propagation issues, making FR2 transmissions highly directional and requiring higher gain active antennas that are electrically steerable. The body of the vehicle acts as a large ground plane located near the antenna, creating a host of additional antenna testing challenges and link budget management complexities.
Overcoming the physical challenges associated with mmWave signals in C-V2X modules requires test solutions that measure and characterize signal quality accurately, without introducing new issues, to validate the C-V2X quality of service and performance on the network.
Organizations such as the 5G Automotive Association (5GAA) have created basic safety use cases for testing C-V2X devices and applications. Some of the most prominent are:
- Emergency brake light (EEBL): When a vehicle’s brakes are activated, a warning signal is sent to nearby vehicles
- Signal phase and timing from traffic lights: This is useful to determine the appropriate speed as a vehicle approaches a traffic light
- Intersection collision warning: A vehicle may send an EEBL message to warn of a potential collision risk
- Across traffic turn collision risk warning: A vehicle may send an EEBL message to warn of a potential collision risk
- Vulnerable road user protection: This alerts pedestrians or other non-vehicle users of a potential collision risk using their smartphones
- Slow vehicle warning or stationary vehicle: This provides alerts of a potential vehicle collision while in traffic jams or with other parked or stationary vehicles.
In addition to these basic safety use cases, other use cases known as “day one use cases” cover basic safety concerns. Every region has its own list of day one use cases. For example, China has approximately 17 to 20 specific to that region. Other regions may have similar collections, but they are not always identical. Various day one use cases may include forward collision warnings, left turn assist and blind spot warnings.
Like so much of 5G C-V2X technology, certification of C-V2X modules is challenging because of the evolving nature of the standards and the many organizations involved in the standardization of the technology, including 3GPP, SAE, ETSI, CCSA, IEEE, Institute of Transportation Engineers, National Electrical Manufacturers Association and European Telecommunications. ITS organizations and road operator regulators need to meet the performance criteria set by the respective standards development organizations, telecom and automotive industry governing bodies.
Typically, a C-V2X device requires Global Certification Forum (GCF) RF/protocol certification according to global cellular standards. Depending on the region where the device will operate, it likely needs to be tested to the ITS upper layer standards, such as IEEE 1609.2/3/4, SAE J2945 for North America and the applicable regional standards in Europe and China. In addition, devices likely need to pass region-specific test cases currently being developed for the ITS application layer.
Manufacturers may conclude that purchasing test equipment capable of testing future expansions of 5G NR is too great an investment. This should not be an obstacle to success. Fully audited test houses provide technical expertise and the most state-of-the-art testing technologies. Using an outside resource ensures each device being tested meets the evolving standards of C-V2X. Given the dynamic nature of C-V2X standards and test requirements, using an independent test house can help companies ensure products are tested using the latest guidelines and meet all requirements. Using a testing and certification body such as the OmniAir Consortium is one method for gaining certification for a C-V2X device in a timely and cost-effective manner, regardless of whether the testing is conducted in-house or through an independent third-party laboratory. Using an audited state-of-the-art facility for bench, field and security test ensures conformance with the latest test standards. Once the lab reports are complete, the governing body approves the findings and the device receives a certification mark to prove conformance.
As with everything else in the 5G realm, the importance of testing and validation for C-V2X cannot be overstated. Only fully audited test equipment for current and future 5G and C-V2X standards and strict adherence to the specifications will ensure certification and, ultimately, the successful performance of 5G NR C-V2X modules. While the obstacles to widespread deployment of C-V2X are not insignificant, they are surmountable with proper test equipment and methodologies. Addressing these obstacles is a critical step on the road to safer, greener and more efficient transportation—leading to fully autonomous vehicles in the future.