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 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.
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For a mobile network operator, timing is everything. Not just determining when to upgrade the network to bring new services to market, but in the literal sense as well. If the radio clock loses synchronization accuracy in a radio access network (RAN), or the radios are out of synchronization, interference between cells is likely. The less accurate the clock source, the higher the probability for time shifts. Ultimately there will be performance challenges. This issue wasn’t a significant concern in legacy networks, but as we transition to 5G it becomes a really big deal.
To use available spectrum as efficiently as possible, 5G technology introduces a time division duplex (TDD) environment. Here both uplink (UL) and downlink (DL) use the same radio channel. Consider this: operators need large amounts of spectrum to deliver on the enhanced mobile broadband (eMBB) use case of 5G, amounts much greater than the 5 to 20MHz that is generally available for LTE networks. Further, most of the available wideband 5G spectrum is either in the C-Band or mmWave, which only supports TDD. This means that TDD is a key factor in enabling eMBB services.
Because a lack of synchronization in UL/DL frames further exacerbates interference problems, industry standards introduce stringent restrictions on LTE and 5G new radio (NR) TDD transmission. While the absolute time synchronization margin in a frequency division duplex (FDD) LTE environment is in the magnitude of 10µs, in the TDD radio environment it is restricted to just 1.5µs.
Download the VIAVI Timing and Sync Handbook for a deeper dive into this topic.
In addition to the absolute time error margin, another consideration is management of over-the-air synchronization requirements for advanced radio features. These include MIMO, eCIC, COMP, and location-based services. In 5G, we are moving away from a synchronized fronthaul CPRI to a packet-based fronthaul. While this approach offers a number of advantages, packet-based fronthaul introduces complexities for synchronization. Providers need different approaches depending on the topology and configuration of their networks. In most cases, we expect to see precision timing protocol (PTP) for distributing time of day (ToD), and Synchronized Ethernet (SyncE) for distributing frequency. This means that radio units (RU) will be synchronized over Ethernet.
Providers can implement various methods to meet these stringent phase and time synchronization requirements. The intent is to ensure synchronization of all nodes to the primary reference time clock (PRTC) source. However, the location of the source may vary depending on the network topology, cost, and application. By using a grand master clock synced to a satellite source and a combination of boundary clock and slave clocks, network nodes can be aligned to a common time and phase. For networks that cannot adhere to full timing support, such as networks that are not PTP aware, there are other options. For example, network operators can implement assisted partial timing support with appropriate consideration for the network topology and cost.
Lastly, it’s important to consider the use cases for frame and slot synchronization. 5G 3GPP standards defined 56 slot formats, each of which is a predefined pattern of downlink/flexible/uplink symbols during one slot. These formats allow flexibility in terms of the application supported on a 5G node B (gNB). Yet, this also creates a challenge if two networks offering different types of service are located next to each other. Interference can result even if they are synchronized in time, but their slot formats are not synchronized. Essentially, when operating a 5G or 4G LTE network in a TDD environment, we not only need frequency and phase synchronization, but also frame and slot synchronization. This avoids inter-network interference.
To learn more about what is vital for 5G-NR TDD Network Success check out this infographic, or watch this webinar, Timing is Everything, hosted by Telecoms.com analyst Wei Shi.
Synchronization is fundamental to the performance of a cellular network and the services it offers. Both 3G and 4G cellular technology required frequency synchronization, primarily to prevent interference when cells overlap. But with the introduction of 5G technology, we’ve reached a new level in terms of TDD phase and frame synchronization. Validation testing is essential to meet stricter synchronization requirements and to ensure quality of service.
For more information, please visit Timing Synchronization for Mobile Networks.