We rely on cost-effective, reliable and secure timing in our cellular networks. The infrastructure needed to deliver this requires proper architecture, design and management. Stricter time accuracy demands for 5G networking equipment call for reliable and robust timing architectures that promise top-notch network performance.
Introduction to Timing Requirements for Optimal Network Operation
The need for not only frequency but accurate phase and time synchronization arose as networks evolved from using communication links based on frequency division duplex (FDD) to time division duplex (TDD). To deliver accurate frequency, phase and time across the 5G network, equipment utilized in TDD networks use a combination of Global Navigation Satellite System (GNSS), Synchronous Ethernet (SyncE) and the IEEE-1588 Precision Time Protocol (PTP).
In this two-part blog post, we will review the timing design and management infrastructure used to synchronize 5G networks. The first part will discuss the timing requirements that need to be met for proper network operation. Part two will cover the design techniques used to meet these requirements.
New 5G RAN Architecture
Introduced in the 3rd Generation Partnership Project (3GPP) Release 15, the new 5G Radio Access Network (RAN) architecture splits the baseband unit (BBU) and remote radio head (RRH) into Centralized Units (CUs), Distributed Units (DUs) and Radio Units (RUs). Carriers can achieve efficiencies and cost savings throughout the network as this new RAN architecture creates a disaggregated and virtualized network.
Disaggregation brought about the enhanced common public radio interface (eCPRI), which connects the DU and RU. eCPRI provides several unique advantages compared to the CPRI links formerly used to connect the BBU and the RRH. Since eCPRI is packet based, synchronization with the RU now transpires by utilizing PTP and SyncE.
Likewise, the Open RAN (O-RAN) movement has standardized hardware and interfaces based on the 3GPP recommendations. The O-RAN Alliance has outlined four possibilities for the delivery of timing in the fronthaul network. In all four, the RU obtains timing from the DU or from a nearby Primary Reference Time Clock (PRTC). Despite the four timing methods, the essential functions required to establish timing distribution through an O-RAN network are still based on SyncE, IEEE-1588 and GNSS.
To make sure that each network component meets specific frequency, phase and time requirements, several timing recommendations have been established, guaranteeing proper end-to-end network operation. Defined by the 3GPP, the basic synchronization service requirement for time synchronization in TDD cellular networks was set to 3 µsec between base stations. The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has a set of recommendations based on the 3GPP requirement defining the absolute maximum time error (max|TE|) between a common point and the end application, translating to ±1.5 µsec.
GNSS became the primary choice for sourcing time in TDD networks through PRTCs. One tactic was to co-locate the GNSS receivers at the radio site; this requires clear line of sight to the sky for reliable operation. Therefore, radios situated indoors or in locations that block a clear line of sight cannot utilize a local GNSS source. GNSS is also susceptible to outages due to line-of-sight disruptions including weather incidents and targeted attacks from spoofing or jamming. The cost of installing and maintaining GNSS sources is difficult for carriers to handle due to the total number of planned 5G NR sites.
Due to the need for more accurate PRTCs in addition to the matter of the reliability and cost of adopting GNSS, the definition of an enhanced primary time clock, or ePRTC, was created. The ePRTC can initiate time through GNSS or another network standard time source connected to UTC. After obtaining the time, the ePRTC then uses a cesium or better atomic reference oscillator to sustain a stable, reliable and highly accurate time reference for the network. Utilizing an autonomous atomic timed reference offers a level of protection to disruptions and supports stable holdover for up to 14 days. The time accuracy of the ePRTC is ±30 nsec to UTC. This is a significant enhancement of the ±100 nsec accuracy specified for the previous PRTC. The increased accuracy reaches the difficult network requirements of 5G NR. We offer state-of-the-art solutions that are deployed worldwide for meeting these precise timing requirements.
Other critical elements to ensure the network correctly transmits time include the Telecom Boundary Clock (T-BC) and destination clock (T-TSC). The T-BC is normally located in a switch or router and oversees recovering time from upstream links and passing it to downstream links. The Ethernet equipment clock (EEC), or SyncE, inside the T-BC/T-TSC delivers a stable and accurate frequency reference traceable to the main reference clock (PRC/PRS) with a frequency accuracy of 0.01 pbb. The use of SyncE together with PTP offers various benefits for accuracy and cost improvements. The SyncE reference, which is more accurate than the local oscillator, can drive the PTP engine; this allows the PTP engine to filter larger quantities of packet delay variation (PDV), enhancing the general phase accuracy.
Basic Time-Accuracy Requirement
In TDD network operations, the end-to-end network limit for time accuracy is ±1.5 µsec (as detailed in G.8271). From this value, we develop a timing budget which specifies the performance essential for each network element, thus meeting the end-to-end limit. Defined in G.8273.2, the clock equipment specification sorts the time error into constant and dynamic time error. Constant time error (cTE) signifies an error that arises due to inherent delays in the network. These errors cannot be filtered; they accrue as time circulates through the network. Dynamic time error (dTE) is an error resulting from high- or low-frequency noise. Appropriate filtering of the network reference clocks can decrease these errors.
Figure 1. To achieve latency specifications, a network must provide ±1.5 µsec timing limits where the total extends across the network elements.
The ±1.5 µsec basic network limit is distributed between the network elements. Figure 1 shows the tolerable time-error budget for each network element for the 4G network. The PRTC with T-GM is constrained to ±100 nsec of error and each T-BC is given a max|TE| according to class level. Table 1 notes the max|TE| given for each clock class.
Table 1. G.8372.2 T-BC and T-TSC clock equipment time-error limits.
Furthermore, a cTE limit is designated for each T-BC depending on the class level. Network link asymmetries and the end application also have given max|TE| values. Networks which support up to 10 hop class A T-BC or 20 hop Class B T-BCs are adequate for meeting the basic network limit.
Advanced Time-Accuracy Requirements
The 1.5 µsec basic end-to-end requirement is identical for both 4G and 5G networks. However, some radio technologies including coordinated multipoint, carrier aggregation or massive MIMO place more rigid constraints on the time error.
Figure 2. T-BCs in 5G networks have an absolute maximum time error (max|TE|) according to class level.
Figure 2 illustrates the idea of relative time error, which explains the time error of the end applications traceable to the last common point of a radio cluster. The advanced time accuracy requirements necessary for NR deployments lowered the permissible relative time alignment error (TAE) to 130 nsec, or ±65 nsec maxTE, within a cluster.
On top of the new ePRTC, Table 1 also notes new classes of Telecom Boundary Clocks (T-BC) and destination clocks (T-TSC) that have been outlined by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) to support these stricter limits. The G.8372.2 class C and the budding class D requirements greater restrict the acceptable TE each element can create. Every class C and class D element must uphold the enhanced Ethernet Equipment Clock (eEEC) standard as defined in G.8262.1.
Keep an eye out for part two of this discussion where we will learn how to properly architect and design equipment that can meet the time-accuracy requirements. Visit our web page to learn more about 5G timing and 5G network infrastructure. Learn more about GNSS and ePRTC 5G timing architectures on our web page.
Darrin Gile, May 2, 2023