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Getting in Sync with Mobile Backhaul and the Challenges of 5G

Few will disagree that 5G poses an unprecedented set of operational challenges for mobile operators. Underlying these challenges is the ongoing evolution of Radio Access Network (RAN) technologies. As mobile networks eventually migrate from LTE Advanced (LTE-A) to 5G, there are three fundamental changes that will have the most significant upstream impact on mobile backhaul networks:

  • 15- to 20-fold increase in capacity (from LTE/LTE-A capacity of ~100s of Mbps to ~10 Gbps in 5G) will change the sizing requirements of the backhaul networks and drive dense 10G and 100G requirements close to the cell site.
  • Ultra-low latency of ~1 ms (round trip) will require EPC functions to be distributed and virtualized closer to cell sites, and high-touch intermediate hops (e.g., queuing, high latency functions that cause serialization delay) will need to be minimized to achieve this goal.
  • Ultra-dense nature of the network will also set unprecedented requirements for the synchronization of the cell sites as small and overlapping cell sites proliferate. It is estimated that the accuracy requirements in 5G will be three times stricter than what LTE-A requires (1.5 μs to approx 0.5 μs).

The impact of 5G and mobile backhaul has not escaped industry attention, as evidenced in the results of a recent industry survey:

The challenges of LTE-A to 5G evolution, and in particular the impact on mobile backhaul networks, is one of the key topics that we will be exploring in depth in the Coriant blog. We will begin with shining a spotlight on the critical role that synchronization will play in ensuring optimal service performance and best-in-class Quality of Experience (QoE) as backhaul networks begin the steady march toward 5G. 

For many years, the main synchronization requirements of mobile networks (2G/3G or LTE) have been a frequency accuracy of 50 ppb at the air interface. Newer LTE Time Division Duplex (LTE-TDD), LTE-A, and 5G systems also need phase/time synchronization for the following purposes:

  • Avoiding interference between overlapping cells, particularly small cells in urban environments; for LTE-A, enhanced inter-cell interference coordination (eICIC) operates in the time domain, so cells must be phase synchronized to deliver data in different sub-frames
  • Coordinated multipoint (CoMP) and multiple-input multiple-output (MIMO) transmissions, in which several base stations transmit concurrently to a single handset
  • New location-based services, multimedia broadcast services, real-time IoT applications, etc.

The stringent synchronization demands of LTE-TDD, LTE-A, and 5G are a serious challenge for 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 synchronization over packet-based mobile backhaul, two main approaches are widely deployed:

  • Physical-layer-based timing (Synchronous Ethernet, as defined in ITU-T G.8261, G.8262, and G.8264)
  • Packet-based timing (IEEE 1588v2 Precision Time Protocol [PTP]), generally in an end-to-end architecture as defined in ITU-T G.8265

For phase/time synchronization, two common approaches are:

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

Synchronization 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 also reduce the number and cost of local receivers/antennas and enable operators to extend phase synchronization to sites where GNSS is difficult to deploy.

One approach to effectively implementing synchronization across access networks is using routers with embedded synchronization capabilities. This approach delivers extremely accurate Time-of-Day and phase synchronization suitable for LTE-TDD, LTE-A, 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 and eliminates the need for an external GNSS receiver.

Below is an application example that represents the enabling of mobile backhaul for phase/time synchronization with IEEE 1588v2 full on path Boundary Clock (BC) support. In this example, each node is enabled for BC and extremely precise and robust timing is supported across the entire network, with each side prepared for all mobile technologies. PTP sync packets are transmitted on Layer 2 or Layer 3.

Where connectivity allows, IEEE 1588 masters on 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.

To further advance the flexibility of the solution and cost efficiency of deployment, 1588 BC Partial On Path Support (POPS, G.8275.2, see diagram for more detail) is being worked on. In POPS sync, intermediate router/switch hops that are not able to update the sync messages (BC unaware) can exist. This will significantly reduce the requirement for forklift upgrades of networks as equipment replacement costs are minimized.

There is a strong argument to be made for the transport network as the primary means of distributing synchronization to base stations. Through a combination of GNSS equipped cell site routers and embedded IEEE 1588v2 functionality, the demands of synchronization for LTE-TDD, LTE-A, and 5G networks can be met reliably, cost effectively, and in a way that is easily deployed and managed. The days of complex, dedicated synchronization equipment in wireless access networks are numbered.

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