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for time synchronization when accessing the channel
to meet strict time synchronization requirements when accessing the channel
--- This message came from the IEEE 802.11 Working Group Reflector ---Dear all,
I propose to modify section 7.4 in the .18 document 18-20-0020 commenting on the NPRM. Summary of changes (full modified text is given at the end):
- C-V2X PC5 does not technically "require" GNSS time synchronization in the sense that it would stop working entirely without GNSS. Therefore, this should be changed to "C-V2X *uses* GNSS", and changed "requirement for GNSS" to "demand for GNSS". This statement can still be supported by Ref. [14], but this reference is optional.
- However, the studies in support of C-V2X assume that ideal time synchronization is provided through GNSS. I've now cited two of those (3GPP TR 36.885, as well as the 5GAA Waiver Request). The whole argument therefore remains essentially the same: It's *possible* (I have not seen any studies to prove otherwise) that the lack of GNSS reception in tunnels could severely impair C-V2X performance. I added the sentence "The 3GPP or 5GAA have not provided any studies to address such concerns".
- I also added that DSRC does not require a "central coordination entity" in support of DSRC.
Best Wishes,Sebastian Schiessl (u-blox)
Appendix:
Full Text Section 7.4 plus new references
Contrary to DSRC protocols, which are able to manage the access to the wireless channel in a distributed manner without requiring a central coordination entity, the C-V2X PC5 sidelink generally uses GNSS (Global Navigation Satellite System) signals like GPS for time synchronization when accessing the channel [14]. The studies cited by the 5GAA Waiver Request [New Ref 15], as well as studies conducted by the 3GPP [New Ref 16], all assume that ideal time synchronization is provided by GNSS signals. Even though this demand for GNSS does not lead to any additional costs because V2X systems already require GNSS systems for positioning, GNSS signals cannot be received in deep tunnels, which could lead to a loss of time synchronization, which could in turn reduce the efficiency and reliability of C-V2X systems in tunnels. The 3GPP or 5GAA have not provided any studies to address such concerns.
However, it is paramount for any V2X technology to provide reliable communication of safety messages in tunnels. On several past occasions, fires that resulted from traffic collisions in tunnels have spread rapidly and led to catastrophic loss of life. DSRC systems do not require time synchronization on a microsecond level and are therefore not impaired by the lack of GNSS reception in tunnels.
We note here that the lack of GNSS reception will not entirely prevent positioning. The vehicle’s position inside the tunnel could still be estimated by combining dead-reckoning systems, RADAR, LIDAR, and camera data. We acknowledge that positioning could be further improved by installing additional road-side units, which would also provide the time synchronization that is necessary for C-V2X. Nevertheless, it remains unclear whether public authorities will have sufficient funds to install these units.
[New Ref 15] 5GAA Petition for Waiver, GN Docket No. 18-357,https://ecfsapi.fcc.gov/file/11212224101742/5GAA%20Petition%20for%20Waiver%20-%20Final%2011.21.2018.pdf
[New Ref 16] 3GPP, "TR 36.885: Study on LTE-based V2X Services; (Release 14), V14.0.0", June 2016
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