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V2x: 802.11p trumps LTE and 5G

Posted: 09 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:C-ITS? vehicle-to-infrastructure? V2I? V2V? V2x?

Something else to consider is the use of cellular modems in domains that have different safety and security requirements. LTE modems will need to support the required safety certifications that govern being able to take active control of the vehicle. If, for example, the speed limit, transmitted from the infrastructure to a vehicle with an Advanced Driving Assistance System (ADAS), is used to set the cruise control of the car, the modem will have to meet the requirements of the relevant Automotive Safety Integrity Level (ASIL), and this will make the modem hardware more expensive.

Given that the current market for cell-phone subscriptions tops out at about 8 billion subscribers worldwide, while the automotive market represents only about 100 million cars worldwide per year, the modem industry may not see a need to support automotive-specific requirements. The automotive industry has a history of moving slowly, when it comes to cellular connectivity, in part because vehicles have, traditionally, been a lower priority for the cellular industry.

Cellular for safety-related use-cases: V2V
The technical challenges faced by cellular technology for non-safety-related use-cases, with I2V and V2I communication, are relatively minor when compared to the challenges faced regarding safety-related issues and V2V communication.

The cellular network could, if available, be used for V2V communication. A car generates a message, the network receives it, and then re-transmits it to all other cars. Assuming that there is complete cellular coverage along all roads (which is not the case), the service will need to provide very high data bandwidth with very low latency. The reality, though, is that today's cellular networks don't offer this level of performance.

Some V2V use-cases require continuous information exchange (0.1 to 20Hz) among vehicles, and this generates too much data for unicast LTE networks to handle, see Table III in the appendix. According to the DoT's ITS Joint Program Office, a single car broadcasting V2V Cooperative Awareness Message (CAM), according to the E.U. standard, or the Basic Safety Message (BSM), according to the U.S. standard, generates about 0.5 GB per month, at a peak rate of 2.5 KB/second. That assumes a 256B per message, at five messages per second, and four hours of driving per day. At the receiver side, assuming 30 cars (or a peak of 300) in the area of interest, the infrastructure has to handle roughly 16 GB per month (or a peak of 750 kBytes per second) [4].

Cellular networks are historically bandwidth hungry and increase their requirements for bandwidth with every 3GPP release. More data also means more business, as MNOs typically bill based on resources used ($/bit/s/Hz). V2V traffic is, in theory, required to be supported for free, and this means MNOs will have to develop alternative business models to justify any investments in additional V2x traffic. The use of eMBMS protocols, as already part of Release 8, could mitigate the issue, but, as discussed above, they are not widely deployed.

There are some V2V use-cases that don't require high bandwidth, including event-based broadcasting of Decentralized Environmental Notification messages (DENM). The cellular network could support these use-cases, but the fact that these messages require very low latency presents a problem. Cellular systems are capable of low latency, but not in all conditions, such as when operating across multiple MNOs, across borders, or even across cells, if resources are not pre-allocated to V2x services. This is particularly true for the most critical use-case, the pre-crash warning message, which requires a latency of only 50 ms.

Another way to support V2V use-cases with cellular is to develop a direct communication technology as part of the cellular system. This is, in fact, a focus of the 3GPP's V2x study group. The approach they envision builds on top of the Device-to-Device (D2D) communication protocol, which is identified as part of Release 12 but isn't suited for V2V use-cases. The D2D protocol relies on the cellular network having the required resources assigned to the user. For example, if two nearby users want to share a file, the network lets the terminals know which time-frequency resources that can use for direct communication (figure 4). The network initializes the communication and manages the interference generated by the local D2D transmission.

Figure 4: An impression on how the cellular device-to-device communication might work for V2V communications as compared to IEEE 802.11p. In the cellular case, the network remains in full control of the direct communication to ensure a proper management of the network interference. In IEEE 802.11p, the broadcasting of the message via the random access protocol ensures a fast execution of the transmission at the expense of a less efficient use of the wireless resources.

This approach won't work for V2V use-cases that have to be fulfilled even when there's no network coverage. D2D can work in the absence of a network, but this is only allowed in emergency situations, and supported by a very slow protocol for device discovery. To make the D2D profile suitable for V2V communication, the 3GPP V2x study group has identified a number of fundamental challenges that will require changes to the signal structure (e.g. additional pilots to support a better channel estimation), and even a re-discussion of the best-suited modulation (e.g. SC-FDM or OFDM) [15,16]. Curiously enough, but maybe not too surprisingly, the technology choices being made by the 3GPP V2x study group are similar to those made by the 802.11p standard. These key changes will lead to new hardware solutions, and the associated time and costs for their development.

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