Delivering 5G 'right on time'

In today’s mobile networks, everything needs to be ‘right on time’ – literally. Delivering 5G service to your smartphone relies on tens of thousands of pieces of network equipment, often in different locations across the country, to be synchronised to within tiny fractions of a second.

As Radio Access Networks (RANs) are evolving to support higher data rates, greater coverage and better use of the spectrum, synchronisation needs to evolve in parallel. Network synchronisation is becoming ever more challenging. Alignment of the frequencies used, the time of day, and embedded network equipment clocks; all need to be increasingly precise.

To establish network harmonisation, the network equipment clocks need to be aligned to three parameters: frequency, phase and time.

If two clocks in your home tick at the same rate, their frequency is synchronised. Alignment in frequency is called synchronisation; the second hands of the clocks could be out of step, but the gap between the hands as they rotate remains constant. If the hands of the clocks do not rotate at the same rate, they aren’t frequency synchronised. Whereas, if the second hands are always in step, they’re phase-aligned – this is true even if the clocks each show a different time. Additionally, if the clocks say the same time, they’re time-synchronised.

Mobile network base stations have strict synchronisation requirements, including requirements on the agreement of frequency and alignment of time and phase. International standards stipulate the frequency and, where required, phase limits for mobile technology. When these limits are exceeded, it can cause a range of problems. It could cause mobile phones to not hand over between cell sites, or not connect to the network; the spectrum use could be less efficient; it could even interfere with another provider’s radio spectrum.

There are two main ways to provide accurate timing to cell sites: network-based timing and local Global Navigation Satellite Systems (GNSS). Network-based timing involves GNSS clocks and backup atomic clocks combined with delivery mechanisms to transfer time across the network. It also uses boundary clocks to ensure minimal time errors between elements. This makes GNSS scalable and allows for fixes to be rolled out easily across the network as the networks evolve.

Meanwhile, local GNSS involves installing individual GNSS antennas at every cell site that communicates with satellites thousands of miles away to provide accurate time within the transmitted signals; this can be a quick solution. However, it can also create vulnerabilities in how the network operates, such as jamming or spoofing the signal. Mobile Network Operators (MNOs) must consider what option or combination fits their needs best when managing the synchronisation of their networks; there is no ‘one size fits all’ when it comes to timing.

As 5G networks evolve, we’ll be considering how we develop our atomic clock backups to provide the most secure and resilient sources, and continue to look at how timing is provided in the application space to enable services of the future. Currently, we use Caesium atomic clocks for backups, but we’ll be looking at the benefit of new, improved, optically-based atomic clocks and other atomic elements in the longer term. Looking further into the future, a lot of work is going on in the Industrial Internet of Things (IIoT), the Internet of Things (IoT) applied to industrial purposes (i.e. manufacturing and supply chain management), and the use of high-performance timing for fixed and mobile networks. I expect to see an increased use of timing for IIoT applications, new cost-effective atomic capabilities with a potential order of magnitude increase on the current atomic standards.

However, only time will tell how synchronisation in mobile networks will evolve. Synchronisation is the unspoken key that allows you to stay connected on your mobile phone; every time that you receive signal bars on your smartphone, it means that my team at the BT Labs has done their job to keep the network right-on-time.

Mike Gilson
BT's Head of Synchronisation