Eric Colard, Head of Emerging Products Frequency & Time Systems, Microchip
Using Precise Time Architecture with Enhanced Primary Reference Time Clock (ePRTC) Standard
5G networks require high-accuracy time and phase protection, which is challenging for mobile operators given that there is dependency on Global Navigation Satellite System (GNSS). GNSS can sometimes be unavailable for extended periods of time since it is vulnerable to disruption due to jamming, spoofing or natural phenomena. One way to address this is to use the Enhanced Primary Reference Time Clock (ePRTC) which helps ensure the necessary accuracy, reliability, and performance to resolve the problem. To successfully deploy the ePRTC, there needs to be solid understanding of the key elements required to build a robust and resilient architecture for precise time. This includes finding the most appropriate clock and identifying other related systems that the network operator might needs.
Keeping 5G Functioning
Let’s take the case of a 5G service-enabled mobile network that provides excellent speeds and allows for easy video downloads. If a GNSS outage causes the mobile services to be interrupted, customers are likely to blame the mobile operator for the outage. This can damage the operator’s reputation and lead to user churn.
Therefore, mobile operators as well as teams handling nation-critical infrastructure have been seeking ways to address the issue either by providing a backup for the GNSS, or reducing reliance on technology. 3G or 4G mobile networks typically relied on a frequency-based synchronization strategy, which was very well understood in the industry, widely deployed and very effective. Now, with 5G coming into play, mobile operators need to find ways to maximize the use of precious spectrum, making very stringent time and phase accuracy necessary. Precise timing helps avoiding data collision and frequency interference, and also promotes efficient use of the spectrum by minimizing guard-band size.
While the GNSS helps bring in the required level of precision, network densification due to 5G makes this a less feasible option. When a GNSS receiver is lost in a radio or base station, the particular radio or base station needs to be taken out of service quickly. This is because the lack of high-quality holdover oscillators in the radio or base station can cause interference issues. As a result, base stations have migrated to Precise Time Protocol (PTP) architectures for timing, reducing reliance on GPS. By using a very resilient architecture for precise time can help mobile operators to ensure a continuity of customer service during GNSS disruptions.
The ePRTC standard, which is a version of the primary reference time clock (PRTC) that ITU-T (ITU Telecommunication Standardization Sector) has defined for time accuracy, is suitable for this purpose. PRTC class A is designed to meet 100ns (nanosecond) accuracy relative to Coordinated Universal Time (UTC). PRTC Class B is more precise at 40ns accuracy. The highest accuracy at 30ns as defined by ITU-T G.8272.1 is possible with enhanced PRTC.
The ePRTC is highly resilient and is uniquely designed to with the ability to holdover for 14 days or more using the Cesium as a reference. It maintains a maximum deviation of only 100ns to UTC for the entire extended outage period. This is a key benefit of deploying ePRTC for a 5G mobile operator. If GPS is down, service delivery remains seamless, throughout the entire network. This ensures the required time to repair the GPS disruption or survive extended periods of GPS unavailability.
Importance of Clocks and Accessories
A quality ePRTC works on the core principle that it produces its own independent autonomous timescale to generate time. However, it does not work in isolation. It is aligned and calibrated to the GNSS signal over time. With patented measurement algorithms, a quality ePRTC engine can evaluate and measure its own autonomous timescale offset relative to GNSS.
In the ePRTC system, while the timescale is the autonomous master source of time, it relies on the Cesium clocks and GNSS to help maintain the accuracy of the ePRTC timescale.
Therefore, the ePRTC ideally should be connected to both GPS and atomic clocks (preferably two Cesium clocks to maximize resilience). In a properly weighted timescale ensemble, the ePRTC actively and seamlessly locks to two clocks. Even in the event that one atomic clock degrades in performance, the ePRTC will seamlessly de-weight such that it no longer influences the outgoing time and frequency services.
Therefore, a quality ePRTC is the key to ensure proper intelligence for ensembling and autonomous timescale functions. The ePRTC also needs to be adept at “coupling” with quality atomic clocks, especially for holdover capability. The highest quality Cesium Clock will lead to the best holdover performance of the ePRTC system itself.
Setup and Commissioning Requirements
• To successfully construct an optimized timescale system, one needs to take extreme care for setup and commissioning with due consideration for the Cesium clocks and the ePRTC system.
• Some of the commissioning verifications specified by the ITU standard include:
• ePRTC must be locked to the incoming reference time signal, and should not operate in warm-up
• There must be no failures or facility errors in the reference path. These include but are not limited to antenna failures
• Environmental conditions need to meet the operating limits specified for the equipment.
• Proper commissioning of the equipment and calibration for fixed offsets such as antenna cable length, cable amplifiers and receiver delays, the reference time signal (e.g., GNSS signal is operating within limits, as determined by the relevant operating authorities) is important.
• Multipath reflections and interference from other local transmissions, such as jamming, must be minimized to an acceptable level in cases where the reference time signal is operated over a radio system such as GNSS.
• There should be no extreme propagation anomalies, such as severe thunderstorms or solar flares.
• Time reference is GNSS and frequency reference is 1pps/10Mhz from the atomic clocks.
• The common mistake is that GNSS is set for highest priority for both time and frequency.
• This can negate the operational advantages of ePRTC since it puts the atomic clocks in a classic back-up role.
Once these commissioning requirements are considered, the next step in selecting an ePRTC solution is system validation and testing.
Validation and Testing
The 3 main phases of test and validation include:
1. 21 day “learning” period
2. 14 day “holdover” period
3. 7 day “recovery” period
During the 21-day learning period, one can determine the UTC calibration correction parameters for the ePRTC timescale and the frequency offset estimate of the local Cesium with ultra-high precision. With the continuous stream of time error measurements of the local timescale with respect to UTC that is reported by the GNSS subsystem reports, one can slowly adjust the local timescale rate. This first three-week period helps to verify whether the ePRTC is capable of meeting the time accuracy specifications by ITU-T.
We need to verify that the ePRTC can hold 100ns during the 14-day holdover period when the GNSS signal is disconnected. The level of success depends on quality of the Cesium clock.
As shown in Figure 2, the ePRTC that was tested was able to maintain time error performance limits within the 100 ns standard. It also maintained 25 ns clock class for almost the entire duration of the outage. With the use of a high-performance cesium atomic clock, the system delivered holdover performance that was four times better than required by the standard.
The recovery period helps ensure that all parameters return to normal once the GNSS is reconnected to the ePRTC unit. The key is to verify that the normal 100 percent timescale protection operation is re-established and re-convergence is successful as per Figure 3.
Importance of Holdover “Gas Gauge”
The “gas gauge” plays an important role since it allows the mobile operator to determine the duration for which the ePRTC holdover function can keep 100ns accuracy to UTC. The standard requirement is for 14 days.
The ePRTC standard ensures guaranteed delivery of consistent, highly precise phase and time as needed for 5G. It is important that it is deployed properly as part of a complete solution to ensure its smooth functioning. So, adequate validation, testing, and then commissioning the correct clock and related systems is very important.