5G’s advanced capabilities require a higher level of network performance expectations, from latency to the radio frequency environment. One aspect of where requirements are tightening up is in timing and synchronization.
In part, this is because the “goldilocks” midband spectrum that is prized for 5G deployment is Time Division Duplex (TDD) spectrum, where the uplink and downlink share the same frequency band and must each transmit in the right time slot, explained Sebastien Prieur, solutions manager for mobile and cloud solutions with test company EXFO, at the Test and Measurement Forum virtual event.
“TDD tends to be new to a lot of carriers, and they have had problems adapting,” said Kevin Boyle, director of transport solutions at Ericsson. “Even mediocre equipment will do a really good job at holding frequency, but TDD, I think, is difficult and requires a lot of thought into how timing is provided, how it’s distributed, [and] measuring what errors you’re getting—so you have to be very aware of that.
Operators went through a learning period of the ins and outs of synchronization with FDD and got very good at that technology, Boyle said. “Now, they’re having to do that same thing with TDD,” he added.
Timing and synchronization are also crucial to capabilities such as dynamic spectrum sharing, coordinated multi-point, industrial IoT and other mission-critical applications, according to Prieur, and the importance of timing is becoming more crucial with the move to 5G Standalone. In 5G Non-Standalone systems where the control plane relies on the 4G network, timing issues could be obscured by the ability to fall back to 4G. “As operator are now transitioning to the full 5G Standalone architecture, these timing issues are becoming more prevalent and more visible,” he said.
“Timing is critical to 5G performance,” Prieur said, adding that the impacts of timing issues are very similar to symptoms of RF interference issues. A cell site out of sync will create inter-cell interference and have RF performance issues, handover issues, data corruption and overall reduction in transmission performance. The tolerances are tight: An absolute time error of 1.5 microseconds has to be met between the user equipment and the primary reference clock. And because of the dynamic nature of mobile networks, there can be configuration issues, equipment failures, network asymmetry and other problems that increase the time error in the network or cause the 1588 timing protocol to not be delivered correctly—hence the need for test tools to validate synchronization both in the lab and in the field, when a new site is turned up or when investigating RF issues at a site—which may actually turn out to be timing issues.
“It has become hard to determine if it’s RF issue [or] if it’s a timing issue, and that’s what we are currently seeing in many cases,” Prieur said. In the U.S., he said that as much as 80% of the issues that EXFO sees are related to site configuration problems, or where cable delay is not accounted for.
“You have to be really aware of what is introducing time error,” said Boyle, re-emphasizing the point that serious consideration has to go into how timing is provided and distributed. “If you’re distributing across a network, you have to account for everything. Every node that’s in there is adding time error. You have to be aware of that. Building a timing network is not as easy as plugging in a bunch of ethernet cables, and doing some IP addressing, and it’ll work. You have to be conscious of this time error that you are getting with each hop and be very thoughtful and precise in how you are building that out.” That includes accounting for long fronthaul links in centralized or virtualized RAN set-ups, he pointed out.
Prieur says that timing issues will continue to manifest as 5G networks mature and shift from NSA to SA.
“The reality today is … we are implementing mostly Non-Standalone networks,” he said. “The core is still 4G, the evolved packet core, the control, the uplink—we still have the possibility when we are using the 5G network, to fall back on the 4G network. This is, I think, hiding a number of timing issue that we are not currently really seeing currently.” Moving to 5G Standalone is likely to reveal timing and configuration issues, he added, and more users and traffic on the 5G network will also make more apparent the impacts of timing on quality of service.
