It’s hard to believe how connected we’ve become over the past few decades. Our smartphones are the go-to gadgets for everything, from staying up to date on news and interacting with friends and social networks to making payments, taking pictures and more. We navigate with them, listen to music and make telephone calls, all of which consume bandwidth at rates considered unlikely just a few years ago.
The recent evolution of mobile networks from 4G to 5G has brought transformative changes in terms of speed and latency. As bandwidth and network capacity continue to increase, the need for precise time synchronisation across the network has become increasingly crucial. Clock synchronisation is a foundational aspect of any cellular network, ensuring that all the base
stations operate in harmony to deliver seamless communication. Unlike 4G, 5G relies on exact timing to coordinate data transmission and base station synchronisation. Any loss of clock accuracy or synchronisation will significantly impact network performance, system resilience and the overall user experience.
5G Places a Higher Priority on Base Station Clock Synchronisation
5G takes timing synchronisation to an entirely new level. 4G networks typically require a timing
accuracy of ±1.5 microseconds. However, 5G New Radio is much stricter and specifies values
between 1.5 µs and ±130 nanoseconds depending on the application, especially in dense Time
Division Duplex (TDD), Coordinated Multipoint (CoMP) and massive Multiple-Input Multiple-
Output (MIMO) deployments. Many new IoT/IIoT use cases, such as industrial automation and
autonomous driving, leverage 5G because of its ultra-low latency and high-bandwidth
capabilities.
Mitigating Loss of Clock Accuracy or Synchronisation in 5G Cellular Base Stations
While 4G base stations may utilise signals from GNSS, Synchronous Ethernet (SyncE) or IEEE
1588v2 Precision Time Protocol (PTP) for clock synchronisation, 5G may combine all these
methods for redundancy and improved robustness. The nature of 5G’s small cell architecture
necessitates more precise clock distribution that utilises boundary and transparent clocks to
support nanosecond-level synchronisation.
The Impact of Losing Synchronisation
All cellular base stations, 4G and 5G, rely on time signals from global navigation satellites to
synchronise local clocks. However, these weak signals from constellations of satellites in Low-
Earth Orbit (LEO) are extremely easy to disrupt. Over the past decade, there has been a
significant rise in GNSS jamming and spoofing activity by malicious actors and rogue nation
states. Loss of GNSS signal results in a loss of a reliable time source, causing the network to
enter a holdover state. Backup local clock sources can only reliably holdover for a short time,
leaving the network operator a finite time window to resolve the issue.
Increasingly, GNSS jamming incidents are purposefully caused through the use of cheap
jammers, such as those used by delivery drivers to avoid following a pre-planned route, through
to more sophisticated government-led methods to deter drone attacks during visits by state
leaders. Today, GNSS interference is becoming commonplace and has the potential to
significantly impact 5G network resilience.
The severity of the impact of loss of clock synchronisation in a 5G network varies from users
experiencing dropped calls, through to making secure networks open to attack from
adversaries.
- Network instability: If a base station is out of sync, it can impact time-sensitive network
functions, such as CoMP and carrier aggregation; the resulting unstable connectivity can
limit the network’s full utilisation of the wireless spectrum, lead to failed handovers
between cells and disrupt voice and data communications for mobile platforms - Loss of security: Many security protocols utilise time-based key rotations, which
necessitate precise clock synchronisation to ensure security. Desynchronisation can
impact authentication and leave the network and user applications vulnerable to
adversarial attacks - Service outage: Emergency communications and many industrial automation processes
rely on a resilient and robust cellular network, so any impact is likely to incur a financial
cost in addition to posing potential safety risks - Degraded Quality of Service (QoS): TDD transmissions rely on ultra-precise clock
synchronisation, which, if lost, will lead to cross-link interference and signal
Mitigating Loss of Clock Accuracy or Synchronisation in 5G Cellular Base Stations
Mitigating the Risks
When a GNSS signal is lost or interfered with, it is crucial a holdover clock is available to
maintain synchronisation. A master clock located on a base station can maintain holdover for
5G for up to 24 hours, but beyond that a more accurate clock source is urgently required.
To mitigate the impacts of the loss of clock synchronisation highlighted above, Microchip offers
two viable technologies that comply with the internationally recognised Recommendation ITU-
T G.8272.1, delivering a time accuracy of ±30 nanoseconds.
The enhanced Primary Reference Time Clock (ePRTC) comprises a highly accurate and reliable
physical hardware clock, such as a cesium-based atomic clock with a Global Navigation Satellite
System (GNSS) receiver. The atomic clock provides a holdover trusted time source if the
satellite signal is lost. ePRTC is aimed at on-premise locations such as 5G cellular base stations.
An alternative to ePRTC is a virtual PRTC (vPRTC). This approach provides a virtual clock
arrangement based on an ePRTC but is capable of being distributed across large networks and
remote sites. Implementing an architecture of one or two ePRTCs and distributing
synchronisation via vPRTC offers a cost-effective and practical approach to synchronisation.
Given the vulnerability of 5G base stations to GNSS disruption, industrial leaders such as
Microchip recommend building a robust and resilience time distribution network. This is
particularly the case in urban environments that are more prone to GNSS outage, a trend that is
set to continue.
A typical fibre-connected time network would utilise one or two ePRTCs with time distributed
via vPRTC to every 5G base station. Not only does this remove the notable risks associated with
GNSS outage, but also offers a more robust, resilient and cost-effective approach.
Author: Microchip
