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Use rubidium clocks to achieve service continuity in 4G/LTE networks

Posted: 26 Apr 2012     Print Version  Bookmark and Share

Keywords:IEEE 1588  4G/LTE  rubidium  atomic clocks 

Telecommunications networks are rapidly transitioning 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 synchronisation 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, synchronisation of base stations must be engineered into the packet backhaul using a packet timing technology such as IEEE 1588 Precision Time Protocol (PTP).

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

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

Figure 1: Delivery of synchronisation 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).

Figure 2: Synchronisation requirements.

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

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