How Virtual Primary Reference Time Clocks improve 5G network timing

The 5G rollout continues to gain momentum and with it comes timing issues. As timing and transport technologies evolve and advance, they offer enhancements and alternatives to 5G timing architectures for fronthaul applications. The Virtual Primary Reference Time Clock (vPRTC) offers some advantages over traditional architectures.

5G New Radio (NR) technology uses time-division duplex (TDD) technology that requires all new radio deployments to maintain phase alignment accuracy to a Universal Coordinated Time (UTC) Global Navigation Satellite System (GNSS) based timing source to within ±1.5 microseconds. Understanding time error mitigation techniques for network-based timing delivery using Precision Time Protocol (PTP) for 5G timing architectures and the vPRTC concept is critical for network operators to make sound infrastructure decisions.

The most prevalent issue related to timing in wireless communications is co-channel radio interference. The deployment of a GNSS (GPS, Galileo, Beidou) receiver at a cell site, when the receiver is tracking satellites properly, allows for the time slot transmission assignment which in turn prevents radios that operate in frequencies that are adjacent or close from interfering with each other. In a radio cluster with overlapping coverage, a GNSS receiver that fails or stops tracking properly will result in timing degradation or accumulated phase error. Those problems will cause a connected radio to interfere with adjacent radios. The timing degradation occurs quickly because radios use low cost, low performance oscillators.

To avoid interference issues, once the timing begins to degrade the radio needs to be removed from service or the services affected by the timing degradation need to be turned off immediately. To mitigate this type of failure scenario a PTP network-based timing service, where the radios in the cluster are synchronized to a PTP grandmaster clock with an integrated GNSS receiver, can be deployed.

If the GNSS in the PTP grandmaster clock fails or has tracking issues the radios that are synchronized to the grandmaster clock will remain phase aligned relative to the adjacent radios and will not have interference issues. High-quality oscillators can be deployed in the PTP grandmaster clock to maintain time alignment to UTC for extended periods, and there are PTP-based backup scenarios that can be included in the architecture to help maintain UTC traceable time in failure scenarios. The PTP grandmaster clock network-based timing services approach is very resilient and cost-effective and provides the additional benefits of phase alignment of radio clusters in GNSS failure scenarios and bringing GNSS deployment to centralized points of presence where security and good line-of-sight to the satellite constellation can be carefully engineered. Figure 1 illustrates the distribution of PTP to 5G radio clusters over optical Ethernet.

The architecture illustrated in Fig. 1 is a PTP network-based timing delivery service that leverages a distributed GNSS timing receiver. Technology advancements related to full-on-path support for PTP streams have introduced new classes of boundary clocks in switches and other devices. Boundary clocks reduce the time error produced network equipment in a time transfer path by using PTP. It is now possible to meet stringent timing requirements of 5G applications such as 1.5 µsec or 260 nsec without having the GNSS Timing receiver/PTP Grandmaster function in close proximity to the PTP clients in the 5G radio.

When using a network-based PTP timing delivery architecture for high accuracy applications such as 5G timing applications, we must eliminate or at least mitigate as many sources of time transfer error as possible. Every nanosecond of time error counts. The concepts related to time-transfer-error mitigation center around two concepts that are part of time-error-budget allocation engineering.

The first concept focuses on the GNSS source of time, consisting of a GNSS timing receiver and a PTP grandmaster clock. A GNSS timing receiver used for time transfer in telecom applications is referred to as a Primary Reference Time Clock (PRTC). There are three classifications of PRTC technology and PRTC classification is determined by how close the GNSS maintains time accuracy to UTC when tracking and extracting time for the GNSS satellite constellations.

PRTC Classification A requires the PRTC A to be within ±100 nsec of UTC. UTC is the time reference extracted for the GNSS satellite constellations, when tracking properly. PRTC Classification B requires the PRTC B to be within ±40 nsec of UTC when tracking properly. The enhanced PRTC (ePRTC) classification requires the ePRTC to be within ±30 nsec of UTC when tracking properly.

The ePRTC has an additional requirement related to GNSS vulnerability that adds a holdover specification for the ePRTC to be within 100 nsec of UTC for at least two weeks if the GNSS reception fails or is compromised. This is achieved by co-locating a cesium atomic clock reference with the GNSS timing receiver function. The ePRTC has algorithms that learn the offset between the cesium atomic clock and the GNSS UTC reference. If the reference becomes unavailable, the algorithms can compensate for the offset of the cesium and maintain UTC traceable time for an extended period.

The second concept focuses on the transport network and equipment and is referred to as full on-path support. In a full on-path support model, the PTP time stamps do not pass through the switches and routers in the path between the GNSS based PRTC quality grandmaster clocks and the end application PTP clients in the Radio Unit (RU). The PTP timestamp flow is terminated at the ingress point of the switch and regenerated with a grandmaster clock function at the egress ports of the switch. This process is referred to as a Boundary Clock (BC) function whose purpose is to mitigate the time error of the switching element by measuring and compensating for the variable delay on the time stamps introduced by the switching fabric of the switch.

BC functions recover time transfer from a PTP input and fall into the category of a Telecom-Time subordinate/client clock (T-TSC) as defined in the ITU standard G.8273.2. The Boundary Clock classification and T-TSC time error functions are bounded by a maximum allowable Constant Time error (cTE) that is the mean of the time error expressed as a single number and compared to an accuracy specification. Keep in mind that BC technology allows for mitigation of the time transfer error of the switching devices but does not allow for mitigation of time error due to the introduction of any additional network-based asymmetry.

BC technology used inside the switches has evolved over time, allowing for improvements in time transfer error when network-based timing delivery using PTP is implemented. BC technology was first introduced with a single classification that defined the maximum time error allowed for the switch that incorporated the BC function. There are now multiple BC classifications for switches that allow for much lower maximum time error classifications. These classifications let a GNSS-based grandmaster clock function be positioned at a greater distance in the network and over many more switching hops from the PTP clients in the radio unit (RU). Table 1 describes the ITU standards-based Boundary Clock/T-TSC classifications and associated cTE boundaries.

Advances in technology for both PRTC and BC functions let a network-based timing service using PTP extend the reach of the timing service from the GNSS time source to the end RU applications for both distance and number of switching hops. PRTC and BC functions maintain high levels of accuracy that allow for an alternative to distributed GNSS timing architectures where the GNSS source of time can be located in more centralized location closer to the core of the network. This concept is referred to as the virtual Primary Reference Clock (vPRTC), delivered over Ethernet/packet switching or Dense Wave Division multiplexing (DWDM) optical transport networks.

The vPRTC architecture consists of three components. The first is the GNSS time source with a PTP grandmaster clock function that can either be of PRTC B (±40ns) or ePRTC (±30ns) quality. For GNSS vulnerability issues and holdover performance, the ePRTC is recommended that adds a cesium atomic clock that’s co-located with the GNSS timing receiver. That improves the timing accuracy of the GNSS receiver relative to UTC and also provides the extended holdover capabilities, <100 nsec to UTC for a minimum of two weeks, if the GNSS signal is compromised.

The second component is the network itself and the network transport architecture between the GNSS source of time and the end RU PTP application. This transport segment of the vPRTC must provide Full-on-Path support with BC classification capabilities of class C or D for proper time-error allocation and mitigation.

The third vPRTC segment is the network edge access location where the PTP time stamp flow is delivered to the end RU PTP timing application. This location must recover and regenerate the PTP timing flow creating a vPRTC function within the PTRC A specifications of <100 nsec to UTC. This PTP timing flow is then delivered to the end RU PTP timing application over the Fronthaul network segment.

Figure 2 depicts the vPRTC concept in a Class C boundary Clock Full on Path support transport network.

Summary
Advancements in networking technologies enabling highly accurate time transfer over longer distances and longer chains of network elements let operators introduce the GNSS-based source of time for 5G timing architectures at various locations from the network edge to the core. The vPRTC architecture offers a technical advantage related to resiliency and redundancy. The vPRTC can be configured in an “east-west” configuration where there are two locations for the GNSS source of time and grandmaster function that allows for ePRTC or PRTC redundancy. In addition, this configuration supports bidirectional PTP timing flows for ring or liner-ring network architectures where a fiber cut lets the timing and traffic be delivered from the opposite direction adding. That adds a layer of resiliency and redundancy to the architecture.

As 5G networks continue to evolve, both distributed GNSS PTP timing architectures and centralized vPRTC PTP architectures will be viable commercial and technical options for global operators and 5G LTE private networks. Operators must take care in design to build the most robust and reliable timing architecture given the underlying network topology.

Jim Olsen is a solutions architect in the frequency and time systems (FTS) business unit at Microchip Technology. He has extensive experience in designing and implementing network synchronization and timing architectures in more than 50 countries. He joined Microchip in 1984 and has since served in a wide range of service, sales and marketing roles. In 2000, he transitioned to an advanced technologies role, helping identify new technologies and investment opportunities, and is currently a solutions architect for the North America region. Olsen speaks regularly at industry seminars and events and his numerous articles and whitepapers on synchronization and timing have appeared in books and trade publications.