TECH EXCLUSIVE
Understanding Virtual
Primary Reference Time Clock & 5G Network Timing Architectures With the rollout of 5G networking technologies, we’re seeing greater momentum in both cellular mobile operator and Long-Term Evolution (LTE) private network environments. The 5G New Radio (NR) technology leverages time division duplex technology. This requires that 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. It is important to understand time error mitigation techniques for networkbased timing delivery using Precision Time Protocol (PTP) for 5G timing architectures. Also, the concept of a virtual Primary Reference Time Clock (vPRTC) is critical for network operators to make sound infrastructure decisions. Given that timing is a critical component of the infrastructure, network-based timing architectures that use PTP for 5G fronthaul applications need time error allocation engineering to ensure the timing requirements. In wireless communications, co-channel radio interference is the most common issue related to timing. When a Global Navigation Satellite System (GNSS) (e.g. GPS, Galileo, Beidou) receiver is deployed at a cell site, it must track satellites properly to allow for time slot transmission assignment. In turn, this ensures that radios operating in adjacent or close frequencies don’t interfere with each other. If a GNSS receiver fails or stops tracking properly in a radio cluster with overlapping coverage, it can cause the radio connected to the GNSS receiver to interfere with adjacent radios as the timing degrades or accumulates phase error. Since radios typically use low cost, low performance oscillators to help cut costs, timing degradation occurs very quickly. When timing begins to degrade, either the radio needs to be removed from service or the services affected by the timing degradation need to be turned off immediately to avoid interference issues. A PTP network-based timing service can be deployed to help mitigate this type of failure scenario. In a PTP network-based timing service, the radios in the cluster are synchronized to a PTP grandmaster clock with an integrated GNSS receiver. In case of failure or tracking issues with the GNSS in the PTP grandmaster clock, the radios that are synchronized to the grandmaster clock will remain phase aligned relative to the adjacent radios. Therefore, interference is no longer an issue. High-quality oscillators deployed in the PTP grandmaster clock can help maintain time alignment to UTC for extended periods. Also, including PTP-based backup scenarios in the architecture can help maintain UTC traceable
Jim Olsen
Senior Technical Staff Engineer, Applications Microchip Technology
time in failure scenarios. This approach is very resilient and cost-effective. Other advantages include phase alignment of radio clusters in GNSS failure scenarios, the ability to bring GNSS deployment to centralized points of presence of security. In addition, good line-of-sight to the satellite constellation can be carefully engineered. The below diagram shows the distribution of PTP to 5G radio cluster over ethernet optics fronthaul technologies. Networkbased timing service delivery using PTP offers a strong case both from the business and technical points of view.
Figure 1. This shows a GNSS timing receiver with a grandmaster clock function and is an example of a distributed timing architecture in a fronthaul architecture. Timing is delivered from the grandmaster clock to the radio over the Ethernet Common Public Radio Interface (eCPRI) link.
With the evolution and advancement of timing and transport technologies, there are several enhancements and alternatives to 5G timing architectures for fronthaul applications. The objective of this article is to introduce and explore the concept of Virtual Primary Reference Time Clock (vPRTC), flesh out some of the advantages of the associated timing and transport technologies and architectures. The diagram of the PTP network-based timing delivery service architecture above leverages a distributed GNSS timing receiver. With technology advancements related to full-onpath support for PTP streams, we see the emergence of new classes of boundary clocks in switches and other devices. These reduce the time error produced by these devices in a time transfer path using PTP. This makes it possible to meet stringent timing requirements of 5G applications such as 1.5 microseconds or 260 nanoseconds without requiring the GNSS Timing receiver/PTP Grandmaster function to be in close proximity to the PTP clients in the 5G radio.
28 09 | 2021 BISinfotech
