Timing is of the essence

Editorial Type: Technology Focus Date: 2015-11-01 Views: 2,935 Tags: Networking, Coriant
Mikko Hannula, Director of Product Management at Coriant explains why significant upgrades to the mobile backhaul transport network are needed to enable LTE-TDD, LTE-Advanced and 5G services

It's the skeleton in the closet. With the advent of more sophisticated mobile network infrastructure designed for higher bandwidth, greater spectral efficiency, and new services, today's frequency synchronisation technology alone just won't cut it. A major upgrade in mobile network synchronisation is required.

For many years the main synchronisation requirement of mobile networks (2G/3G or LTE) has been a frequency accuracy of 50 parts per billion at the air interface. Newer LTE Time Division Duplex (LTE-TDD), LTE Advanced (LTE-A), and 5G systems also need phase/time synchronisation in order to:

• Avoid interference between overlapping cells, particularly small cells in urban environments. For LTE-A, enhanced inter-cell interference coordination (eICIC) operates in the time domain, meaning cells must be phase synchronised to deliver data in different sub-frames.
• Coordinate multipoint (CoMP) and multiple inputs - multiple outputs (MIMO) transmissions, in which several base stations transmit concurrently to a single handset.
• Enable new location-based services, multimedia broadcast services, real-time IoT applications, etc.

The stringent synchronisation demands of LTE-TDD, LTE-A and 5G are a serious challenge for mobile network planners. Without a robust and cost-effective solution that far exceeds current implementations, the promise of next generation services will remain unfulfilled.

To support frequency synchronisation over packet-based mobile backhaul, two main approaches are widely deployed:

• Physical layer-based timing (Synchronous Ethernet).
• Packet-based timing (IEEE 1588v2 Precision Time Protocol), generally in an end-to-end architecture.

For phase/time synchronisation, two common approaches are:

• Distributed local primary reference time clocks (PRTCs), typically using GNSS (GPS).
• Packet-based time synchronisation using IEEE 1588v2 PTP.

Synchronisation provided to cell sites by a dedicated 1588 master clock or GNSS receiver and antenna at each site, whether standalone or built into the base station, can be costly as networks grow and small cells proliferate. Installation complexity or physical site limitations can become prohibitive. Local GNSS receivers are also vulnerable to inconsistent satellite reception and signal-jamming.

Even with local GNSS receivers, a back-up timing source is usually still needed. This is motivating the rollout of IEEE 1588 phase-and time-enabled packet networks. Using IEEE 1588v2 to distribute timing can reduce the number and cost of local receivers/antennas and enable operators to extend phase synchronisation to sites where GNSS is difficult to deploy.

Synchronisation can be effectively implemented across access networks using equipment with embedded synchronisation capabilities as shown in figure 1, above. These network elements deliver extremely accurate time-of-day and phase synchronisation that is suitable for LTE-TDD, LTE-Advanced, and 5G networks. For networks that are not IEEE 1588 capable, some routers offer a cost-effective integrated GNSS SFP that acts as a local IEEE 1588 PRTC master at a cell site or aggregation site, thus eliminating the need for an external GNSS receiver.

Where connectivity allows, IEEE 1588 Masters at different aggregation sites can provide redundancy to other sites. Synchronous Ethernet provides holdover accuracy to further protect against GNSS outages. Because it is integrated with the router, this solution can also be managed by the router's network management system. The combination of managed, integrated GNSS modules and IEEE 1588v2 functionality offers flexible deployment models with optimal cost and performance.

There is a strong argument for using the transport network as the primary means of distributing synchronisation to base stations. Through a combination of GNSS-equipped cell site routers and embedded IEEE 1588v2 functionality, the demands of synchronisation for LTE-TDD, LTE-Advanced, and 5G networks can be met reliably and cost-effectively, and in a way that is easily deployed and managed.

The days of complex, dedicated synchronisation equipment in wireless access networks are numbered. NC