5G is all in the timing

In a previous blog post we talked about how Emerging RAN requirements require 5G-ready transport. In this blog we will look more closely into one aspect – timing and synchronization.

To realize the benefits of new TDD spectrum and the full potential of 5G, highly accurate time synchronization is needed almost everywhere in the network.  To ensure protection against sync loss, operators must look beyond their current sole reliance on GPS.

This article takes a look at the technology drivers, options and considerations for timing and synchronization in mobile transport when planning for 5G networks.

RAN drivers of timing and sync

Although the mobile transport network itself does not need synchronization, it can provide timing and synchronization to the RAN. The specific requirements for RAN timing and sync are dependent on the radio technology deployed and the spectrum used. In frequency-divided (FDD) spectrum, a relatively loose frequency synchronization is all that’s needed. FDD-based 5G and LTE networks can survive lengthy (> 1 hour) loss of sync. Time division duplex (TDD) spectrum such as CBRS and mmWave, however, is a different story.  Here, much tighter time and phase synchronization is required to ensure against interference between the uplink and downlink.

In addition to the spectrum requirements, RAN technologies and features rely on time sync levels of various accuracies as indicated in Figure 1.  For example, the deployment of coordinated RAN features such as CoMP requires relative time sync <= 1 μs (synchronization between neighbor radios). Beam-forming with NR-TDD, on the other hand, requires absolute time sync ~1.5 μs (synchronization across the whole network).

In general, a mix of absolute (network wide) and relative (between neighboring radios) criteria require a time sync of ~1.5 μs or less to support a network’s new spectrum and RAN features.

Figure 1 (RAN feature synchronization requirements)

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There’s also a need for increased reliability in the timing source. While today’s FDD-based LTE network can continue to operate for hours after sync loss with no degradation, in the future, loss of timing will have an immediate impact on RAN performance. Ericsson analysis of sampled North American operators showed that GPS loss of one hour or longer affected more than 15% of all sites nationwide over a 12-month period. Reliability of GNSS/GPS in urban canyons is also a major concern due to limited signal availability. This will become a bigger concern with expected urban densification and the deployment of small cells along city streets. With TDD-based spectrum eventually comprising up to 80% of total 5G network capacity, timing outages are destined to become significant performance and availability challenges, even for providers that were never affected by such events in the past.

Figure 2 (Typical macro site throughput)

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Technology options and challenges

Today in most of North America, GNSS/GPS continues to satisfy the market’s timing requirements.  However, it is only prudent for operators to have a viable backup to GPS because timing is a critical component of network performance. Further, as the number of sites grows with densifications, adding GPS to each site becomes a significant additional cost that can be mitigated through a network-based timing solution.

Time and phase sync distribution options

There are three primary options for ensuring the required availability and reliability of timing and synchronization as follows:

Ericsson’s recommendation is to move toward the GNSS + FTS option with the deployment of routers that fully support timing and sync requirements while delivering the highest level of performance.

Transport network requirements

Figure 3 illustrates the synchronization requirements in the end-to-end transport network. Ericsson proposes timing budgets allow for 1.1 µs time-of-day difference from the core of the network to the access edge, and about 400 ns from the access edge to the radio.  This gives a total budget of 1.5 µs absolute time-of-day difference between radio nodes, which meets the timing requirements of most radio applications today. For example, Carrier Aggregation and Mobile Broadcast require the absolute time-of-day difference between nodes of 3 to 5 µs.

It’s important to note, however, that some coordination features like Coordinated Multipoint (CoMP) demand much tighter timing.  This is typically on the order of 260-350 ns time-of-day difference relative to the neighboring radio nodes.  OTDOA, used for positioning for emergency services, requires relative timing as tight as 100 ns!  In all instances, accurate, stable, and reliable timing is absolutely essential.

Figure 3 (transport network synchronization requirements)

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Figure 4 indicates how the three different time and phase sync distribution options overlay onto the transport network. Option 1 shows GNSS at the individual radio sites with no network backup. Option 2 shows GNSS at the access edge sites distributed to the radio nodes using FTS. It includes backup timing from a reference clock in the core using APTS. Option 3 highlights strategic placement of GNSS receivers in the access network and the radio edge. FTS-based distribution to all nodes provides redundant clocking for all radio sites. This option can also be backed up with a reference clock elsewhere in the network and distributed using either APTS or FTS. In all cases, frequency sync distribution can be achieved using SyncE/SyncµW.

Figure 4 (how the 3 time and phase transport sync distribution options work)

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A proven Ericsson solution

Ericsson’s Router 6000 portfolio was the very first 5G site and access router portfolio on the market.  Configured with the needs of 5G radio access in mind, it bridges the gap between the transport network and the radio domain while supporting the radio-driven timing requirements imposed on the transport elements. The Router 6000’s integrated clocking delivers exceptional 5G sync and timing performance as well as other radio-first design and performance characteristics such as industry-leading throughputs, strong buffering capabilities and a footprint designed for small cell site deployments.

In the access network, Router 6000 manages a GPS receiver as an integral part of the router, and distributes timing to other elements in the network, including upstream and downstream routers, as well as to basebands at hubs and other sites. Additionally, Router 6000 is able to take timing input from network sources and utilize that as a primary or secondary timing source.  Router 6000 also has a Stratum 3E local grandmaster, meaning it can function as a boundary clock or grandmaster and can survive outages of an hour with 37 times less clock skew than typical routers with Stratum 3 clocks. This capability gives a radio network solid redundancy and the ability to survive timing outages longer without ill effects.

Click on this link to read more about the Ericsson Router 6000.

Summary

In an earlier blog, Why your 5G network needs 5G transport we discussed the many reasons why 5G requires rethinking transport network requirements.

Understanding the timing and synchronization requirements of the RAN network as it evolves is critical to ensure RAN performance and stability, and the transport network has an important role to play. The right router in the access network, i.e., a router purpose built for 5G RAN such as the Router 6000, can provide the high quality and high reliability timing and sync needed. This type of router is the most capable and cost-effective way to meet the demanding requirements of advanced RAN features and the new spectrum associated with both LTE Advanced and 5G networks.

For more information, visit Ericsson transport solutions

Acronym Guide:

APTS: Assisted Partial Timing Support (G.8275.2)
CoMP: Coordinated Multipoint
FDD: Frequency Division Duplex
FTS: Full Timing Support (G.8275.1)
GNSS: Global Navigation Satellite System
GPS: Global Positioning System
LTE: Long Term Evolution (4G radio)
NR: New Radio (5G radio)
OTDOA: Observed Time Difference of Arrival
RAN: Radio Access Network
TDD: Time Division Duplex