A CEPT Electronic Communications Committee report from 2014 that was recently posted giving practical guidance for TDD networks lists four ways to provide essential timing and synchronization for cell phone networks. From the report:
- GNSS – Main source used today, but requires view of the sky and is subject to outages. Not good for indoor small cells and some outdoor small cells.
- Network based sync including PTP – Requires careful engineering, often dedicated equipment and software, not always practical
- Cell to Cell – Will work in some scenarios, but still ultimately requires an independent UTC reference
- Loran/eLoran – UTC traceability and timing performance on a par with GNSS but not subject to GNSS jamming, possible reach indoors and underwater. It also mentions some antenna development work needed, though we understand that has progressed well since the report was published.