This article summarises a webinar on phase timing for small cells presented by Eric Colard of Microsemi. He explained the technical issues around phase, rather than frequency, synchronisation and alternative standard methods to achieve it. He also shared some real world experience from recent trials with operators worldwide.
Worldwide Synchronisation Evolution
Globally, 480 mobile operators in 157 countries have commercially deployed LTE and most are now planning or preparing for LTE-Advanced features that use phase. A stable phase timing network is required to support current and future applications. The backhaul network has been gracefully migrated from SONET/SDH to Ethernet, using Synchronous Ethernet to distribute frequency reference clocks.
In the past, a central Grand Master supplied a common signal that was hardwired throughout the network. Today, we now see distributed master clocks appearing almost everywhere. Typical requirements are for 50ppb frequency and 1.5us phase timing over the air, driven from 16ppb and 1.1us into the base station.
A synchronised time source
Frequency sync requires a Primary Reference Clock (PRC), whereas Timing sync requires a Primary Reference Time Clock (PRTC). The latter must come from a satellite GNSS source, such as GPS, and be traceable to Universal Co-ordinated Time (UTC).
The end-to-end Inter-Cell time error budget of 1.5us (1500nanoseconds) is split into three parts:
- A time source, with an error of up to 100n
- The transmission network, with up to 1000ns
- The small cell (eNodeB), with up to 400ns
The transmission network may have up to 10 boundary clocks with a combined total of 500ns error. The remaining allowance is split equally between dynamic time errors and network asymmetry. It is especially important that packets travelling in each direction (uplink/downlink) incur similar amounts of delay variation – if the time taken to send and receive packets varies differently, then phase timing errors would mount up rapidly.
It is this asymmetry of packet delay variation which is the biggest problem with engineering phase timing throughout a large network.
The ITU has defined two different time profile standards related to transmitting the phase sync signal.
G.8275.1, which relies on full on-path support. Each node in the backhaul transmission network must be fully aware of the phase timing component and actively support its transmission. Each router or node would have its own boundary clock that synchronises and re-generates the timebase locally. This may be feasible for new product but would otherwise require replacement or upgrade for existing routers and backhaul transmission equipment.
G.8275.2 was recently consented and only requires partial on-path support. One or more boundary clocks are installed at the most effective points in the backhaul path, with many legacy routers/nodes being unaware of the special importance of the PTP packets.
It is crucial to take into account the existing technical infrastructure and also cost for deployment. As part of this effort, it is critical to engineer the network so that asymmetry correction can be considered.
In cases where full on path support is deployed, the mitigation of uplink versus downlink asymmetries are extremely important and usually requires a manual calibration of each link which is extremely costly.
This is why Microsemi recommends considering deployment of partial On-Path support instead of placing the GrandMaster as close as possible to the small cells or eNodeB clusters. Automated asymmetry correction and APTS techniques leveraging GNSS are crucial to enable a reliable and performant time and phase deployment.
Using an Edge Clock as local Primary Reference Time Clock
This problem in backhaul networks is likely to be more prevalent when using third party broadband services for backhaul, where the underlying hardware path isn’t directly under the mobile network operator’s control.
Installing a caesium atomic clock next to every small cell isn’t feasible or cost effective. Instead, GNSS signals can be used as a reference, providing the same high level of accuracy and precision but at a fraction of the cost.
The approach Microsemi has taken is to develop a standalone edge master, integrated with a GNSS antenna, which can receive signals from multiple GNSS satellite constellations, augmented by centrally distributed data which reduces acquisition time and improves performance. This Assisted-GNSS technique allows the unit to operate deep indoors or within urban canyons where other GPS receivers may not see any signal.
The Integrated GNSS Master, similar in size and format to a Wi-Fi Access Point, acts as a local Grand Master, distributing the timing signal using SyncE and IEEE 1588 over standard Ethernet cables to up to 16 nearby small cells. These need not be very closely co-located with the small cells, but could be positioned anywhere in the building.
Some learning from recent field trials
The original indoor product had a single directional antenna, with two different variants depending whether it was wall or ceiling mounted. It was found this could cause a logistic problem with two sets of parts for stock control and spares. Installers may not know in advance whether they would be fitting the equipment vertically or horizontally until they got to site. A single product with dual antenna avoids that problem and streamlines planning & inventory.
The IGM performance considerably exceeded expectations. It works almost everywhere – even when sealed inside a locked metal trailer cabin without windows; seven satellites were regularly visible. It also worked in basements below ground level and in glass skyscrapers.
Separate product variants have been developed based on operators’ requirements to address use cases beyond enterprise small cells. In some cases, an external antenna may already be present or could be easy to install such as in a cabinet environment serving a cluster. However, rather than connect a special GPS antenna cable all the way to a basement machine room, the IGM can be fitted close by and the timing signal sent via Ethernet cable throughout the building.
An outdoor variant was also developed for use with urban and remote cell sites. This can be mounted on a pole to avoid vandalism and has been weatherproofed to protect against environmental elements.
Summary
Mobile networks are moving quickly to adopt and deploy phase timing to take full advantage of LTE-Advanced and related features. This will increase network performance and squeeze more out of precious licensed spectrum.
There are several methods of engineering the network to achieve reliable phase timing. Typically more than one will be deployed to ensure network resilience.
Where backhaul networks comprise legacy or unknown third party broadband links, deploying edge grandmaster clocks is a useful approach. These are driven from GNSS satellite signals and provide a traceable source to UTC.
For more information about Microsemi’s IGM1100 portfolio products, visit their website or click here.
You can also view the slides and/or watch a replay of a recent webinar given by Microsemi on this topic.
