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