Blog: 5 key synchronisation challenges specific to 5G base stations

With the roll-out of 5G and initiatives such as the O-RAN alliance, the mobile equipment world is having its fair share of disruption, which certainly applies to base stations and mobile backhaul.

An explosion of devices from phones to IoT devices, combined with new applications such as mobile video streaming, industrial IoT, and automotive applications, is creating a capacity challenge for mobile networks. And, with the transition to packet-based backhaul mechanisms and the mobile backhaul moving into telecom data centres, synchronisation has become not only more important but is now also highly challenging.

Why is synchronisation important in 5G?

With 5G, the accuracy requirements are much more stringent compared to previous generation networks. The ultra-reliable low-latency communication (URLLC) networks that offer autonomous driving and enable Industry 4.0 require extremely accurate synchronisation – in the order of 100s of nanoseconds – to enable the applications to perform the required functions. In order to achieve the full potential of 5G networks with carrier aggregation for large bandwidth applications, very accurate synchronisation is required. Therefore the ‘phase synchronisation’ is vital in 5G networks.

What are the 5 key synchronisation challenges specific to base stations?

In this post we will identify the critical challenges in macro base station synchronisation and what needs to be considered when selecting synchronisation solutions.

Key challenge 1: GNSS vulnerabilities

GPS or other GNSS methods are currently the primary sources of base station synchronisation, but there is significant vulnerability around GNSS signals, intentionally or unintentionally. Signal loss can be caused by unintentional barriers such as urban canyons or atmospheric weather conditions, or intentionally for example, by signal jamming.

Important factors to consider: Systems that use GNSS based synchronisation switch to alternative synchronisation sources like precision timing protocol (PTP) or deploy holdover using ultra-stable local oscillators. Affordable quartz oscillators that can provide holdover for applications up to 24 hours are becoming available.

Key challenge 2: Decentralisation of base stations and radios

5G networks generally have a distributed radio architecture and rely on packet-based connectivity from the Distribution Units (DU) to the radios. Circuit-switched networks have been transitioning to packet-switched networks allowing base stations to use synchronous Ethernet and packet-based synchronisation techniques. Unlike traditional synchronisation techniques, packet-based solutions transfer frequency AND time, complementing the GNSS-based synchronisation solutions. However, such solutions require complex synchronisation solutions with robust reference clocks.

Important factors to consider: Traditional radios had simple VCXO-based clock recovery from physical layer clocks. But now, with distributed radios and packet connectivity, protocol layer clock recovery is needed. Even with the support of physical layer clocks, protocol layer clock recovery needs very low bandwidth servos to achieve the accuracies needed to support 5G air interface requirements. Instead of simple VCXOs, there is a shift towards selecting more powerful oscillators to enable such deployments*. 

Key challenge 3: Phase holdover

As mentioned before, in most base station synchronisation implementations, GNSS is used as the primary reference source. When GNSS signals fail, the system may switch to packet-based network synchronisation, which in turn may be supported by SyncE or traditional synchronisation. If all synchronisation sources fail, systems fall back to the local clock source to provide holdover. Depending on the system requirements, your clock source needs a certain stability within the defined holdover constraints to achieve target holdover specifications.

Important factors to consider: In a synchronisation design, the holdover time depends on the oscillator, specifically on the frequency versus temperature characteristics, ageing and the slope performances of the frequency with temperature. Select an oscillator that has the cumulative noise components within the requirements for specified holdover time. If you would like help in determining what to use for your particular application, please contact us.

Want to calculate holdover for your chosen oscillator? Try our new holdover simulator app now!

Key challenge 4: Support for various network types

5G deployment scenarios, especially on front-haul, vary significantly. Deploying over existing traditional networks poses significant challenges in synchronisation. Due to the varying nature of Packet Delay Variation (PDV), partial timing supported networks or networks with no physical layer clock support, demand packet-based networks to impose extremely low bandwidths on clock recovery Phase Locked Loops (PLLs). Macro base station synchronisation PLLs generally employ very low loop bandwidths (in the order of mHz). Such systems require very stable oscillators to implement synchronisation servos to achieve the required clock specifications.

Important factors to consider: Oscillator selection obviously needs to comply with the necessary standards at a minimum. However, the standards do not take into account specific scenarios. For example, the temperature behaviour of a synchronisation system may vary and this needs to be considered in your design in addition to what the standards may specify.

Key challenge 5: High-performance transport networks

The stringent Time Alignment Error (TAE) requirements between the DU and Radios demand minimal time error from the transport network elements. The dynamic performance of transport elements, as specified by ITU-T, is as low as 5ns. In order to achieve such low equipment error specifications, high performing reference clocks are required across all the environmental conditions.

Important factors to consider: A significant component of the intrinsic noise generation of the transport equipment comes from the behaviour of the oscillator under varying environmental conditions. The selection of reference clocks with the right performance parameters, such as temperature sensitivity, ensures minimal contribution of oscillator noise as well as compliance to TAE requirements

Conclusion

Base station and backhaul designs are currently undergoing major changes, with many factors influencing decisions around synchronisation. Selecting the right oscillator is now more complex and has consequences that reach further than ever before.

If you have any questions about your particular use case or our backhaul synchronisation products, please get in touch with us.

* For examples of this, check out our Hybrid TCXO and mini-OCXO pages.

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