Timing and synchronization for 5G TDD networks
When timing and synchronization are accurate, all 5G network elements can operate in unison. This is essential for 5G networks, which operate in the C-band and use TDD mode. Here, Sebastien Prieur Manager of RAN/Mobile Solutions at EXFO, answers questions about this aspect of 5G network performance management.
How does timing and synchronization support 5G network service reliability and user experience?
5G networks make use of C-band spectrum and rely on Time Division Duplex (TDD) mode rather than Frequency Division Duplex (FDD). In FDD, the uplink and downlink each use their own frequency so they can both transmit and receive at the same time without risk of interference. In TDD networks, the uplink and downlink share the same frequency, so the transmission of signals requires precise scheduling to avoid interference. Therefore, TDD demands that uplink and downlink transmissions be timed very precisely. This need for carefully-timed transmission covers not only user equipment, but also surrounding cell sites. If these are not in sync, interference will occur and 5G performance will degrade.
What do 5G NR TDD timing and synchronization issues look like?
Symptoms of timing issues are very similar to symptoms of RF and interference issues. Example: an out-of-sync cell site results in interference that impacts RF performance causing handover issues, data corruption and overall reduced transmission performance.
In some cases, there may be an alarm tied to RAN equipment that indicates a performance issue. This can point to a timing synchronization issue but when investigated further, differentiating between timing issues and RF interference as the root cause can be challenging, considering the similarities of the symptoms.
In the U.S., over 80% of timing and synchronization problems trace back to site configuration issues. A typical example is the failure to compensate for cable delay in the GNSS antenna connection to the edge grand master. Very often this value is not set correctly, either because it’s still on the default value from the edge grand master manufacturer, or because it hasn’t been characterized correctly for the length of cable used for the GNSS antenna. This leads directly to errors in time measurement, impacting the reliability and precision of GNSS-based systems and applications.
Timing issues can and do occur in any network using TDD, regardless of whether that network’s architecture is 5G SA or 5G NSA. But the impact of timing/sync issues is not as apparent in NSA networks because these continue to rely on a 4G LTE core and control plane. In either case, timing and synchronization issues matter and need to be fixed when and where they occur.
How should operators prepare for and address timing/synchronization for 5G TDD networks?
Synchronization requirements for 5G networks have been defined to ensure end-to-end timing (See Table 1). This means that synchronization covers all the various elements of the RAN between the macro cell site, between all the small cells, and all the way up to connected devices using the 5G network. 5G also introduces relative timing requirements between cell sites in the same region.
Table 1. In 5G networks, time error requirements between antennas become more stringent for advanced use cases
It’s important to have test tools designed to validate synchronization in both lab and field settings. In the field, best practice is to validate timing when a new 5G cell site or small cell is rolled out. This ensures that timing is accurate at turn-up, and that the network is ready for future expansion.
Tools that validate synchronization during construction of a cell site are also useful in troubleshooting scenarios when a timing issue is suspected, or when investigating RF issues.
These capabilities are part of EXFO’s FTB 5GPro test solution, which validates timing accuracy in backhaul, midhaul and fronthaul networks using 1588 PTP, or directly from the RF interface using over-the-air measurements.
