The continuing evolution of radio access network technology and the introduction of packet-based mobile backhaul have created a major paradigm shift for network synchronization. Packet networks need to address synchronization requirements that were once inherently provided by the network. And they need to support increased clock accuracy and greater availability of synchronization signals as the quality of experience of mobile services is increasingly dependent on base station clocks that are aligned accurately and have high stability.
The migration of frequency synchronization from traditional E1/T1-lines to Synchronous Ethernet (SyncE) and the packet-based IEEE 1588v2 Precision Time Protocol (PTP) was a first and intermediate step only. Most mobile network operators already went through this transition. They have been driven by the need to migrate to a packet-based backhaul model to economically support the substantially increased volume of mobile data traffic. Both SyncE and 1588v2 PTP are widely adopted and enable accurate frequency synchronization of base station clocks. SyncE performance is independent of the actual network load but requires intermediate network elements to support the protocol. 1588v2 PTP frames enable frequency alignment without additional on-path support. However, frames carrying synchronization information need to be forwarded with high priority in order to minimize delay and jitter experienced.
LTE-TDD operated in unpaired spectrum and LTE-Advanced with distributed interference coordination and radio spectrum control will challenge existing synchronization architectures. Current implementations need to evolve, enabling mobile network operators to seamlessly migrate to the new technologies and enhance cell capacity, coverage and ultimately mobile user experience. The distribution of timing information for phase synchronization of the radio access network constitutes a particular challenge. Stringent phase alignment enables base stations to coordinate transmission time instants and avoid interference. In order to achieve accurate phase alignment, backhaul networks now need to actively contribute to timing distribution and provide on-path support for 1588v2 PTP. SyncE alone cannot provide information for phase alignment, since it only distributes a frequency reference. This is a new requirement for most backhaul service providers and presents new technical and economic challenges for network planners and engineers.
Meeting stringent timing requirements over backhaul paths that were not originally designed to deliver phase synchronization at the level of accuracy needed is a major challenge. Deploying GPS receivers at every base station including small cells is often not practical for technical or economic reasons. In addition, mobile network operators increasingly express reliability concerns with regard to GPS-only solutions. A new approach based on purpose-built equipment is therefore required.
The integration of grandmaster clock functionality with 1588v2 PTP fallback option into Ethernet service demarcation and pre-aggregation devices enables precise and reliable frequency and phase synchronization at the network edge. Deploying such GPS-enabled devices at macro cell sites instead of traditional solutions efficiently supports synchronization of base stations and the large-scale rollout of small cell radio base station. And mobile network operators can continue to source synchronization information following their established architecture. By moving grandmaster clock capability to the edge of the backhaul network and adding 1588v2 PTP fallback capability, synchronization paths are significantly shortened and mobile network operators can effectively meet synchronization demands while improving their customer’s quality of experience.
The availability of accurate timing information at each cell site is a crucial factor for proper operation of the radio access network. Consequently, mobile operators also want to understand the actual performance of the timing network providing this information. Delivering synchronization is not enough for enabling stable operation of the radio access network. Assured delivery with guaranteed quality of service metrics is a must – no matter whether operating the backhaul network by oneself or leasing backhaul connectivity and timing services from independent service providers.
Network timing behavior is not a stationary process. It is subject to dynamic conditions and changes over short term and longer term. Appropriate tools for cost-effective and time-efficient end-to-end management of the synchronization network during all phases of its lifecycle are therefore required – installation, turn-up testing, monitoring and troubleshooting. Mobile network operators want to identify potential problems before they cause outages and degrade user experience.