Achieve service continuity in 4G/LTE networks with rubidium clocks

Telecommunications networks are rapidly shifting from circuit switched to packet switched technologies to meet the exploding demand for bandwidth. The transition from TDM to packet-based networks requires a change in the synchronization architecture as the TDM layer that inherently carried the sync signal is lost, and the sync signal is broken.

Asynchronous Ethernet networks do not provide physical circuits between network elements, and consequently, synchronization of base stations must be engineered into the packet backhaul using a packet timing technology such as IEEE 1588 (PTP).

As network operators look to design the sync architecture for 4G/LTE networks, some considerations must be made for meeting must stricter synchronization requirements to support the latest mobile technologies and location based services including E911.

This will require network designers to implement a synchronization solution that can support both frequency and phase. And, they must define and architect a sync back-up in the event the primary sync signal is lost to ensure continuity of service.

IEEE 1588 (PTP)

IEEE 1588-2008 Precision Time Protocol (PTP) is a synchronization protocol that has gained traction as the technology of choice to deliver synchronization for packet-based networks because it delivers both the frequency and phase synchronization required for 4G / LTE networks. Figure 1 below shows a typical example of PTP synchronization for cellular networks.

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Figure 1: Delivery of synchronization to next generation base stations will rely on PTP grandmaster clocks deployed in the MSC/RNC. Sync packets flow from the grandmaster clock to the slave clocks in the base stations.

Mobile base stations that rely purely on frequency control, such as GSM and UMTS, have a requirement of 16 parts per billion (ppb) physical layer (G.823) clock on the E1/T1 backhaul connection (the transport interface) to lock their internal oscillators and generate the 50 ppb accuracy required to align the base stations with the mobile phones at the RF layer (the air interface).

Failure to meet the 50 ppb synchronization requirement will result in dropped calls. Figure 2 below shows the synchronization requirements for various cellular network types.

Figure 2: Synchronization Requirements

As the backhaul transitions to Ethernet, the TDM physical layer synchronization service chain is no longer available. The loss of physical layer sync has generated a requirement for base station designs to incorporate PTP slave clocks that will meet the 16 ppb requirement using packet technology. Such PTP slaves in the base stations rely on access to a carrier-grade PTP grandmaster clock deployed in the mobile switching center (MSC) or radio node controller (RNC).

With the network transition to 4G/LTE TDD (Time Division Duplex), more stringent phase synchronization is now required to support the tighter use of frequencies and emerging location based services (LBS) including E911 requirements. Figure 3 below shows what happens to service quality when the network is not synchronized to the required specifications.

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Figure 3: LTE Synchronization