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Employ rubidium clocks for service continuity in 4G/LTE networks

Posted: 26 Sep 2012 ?? ?Print Version ?Bookmark and Share

Keywords:IEEE 1588? 4G/LTE? rubidium? atomic clocks?

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 shows what happens to service quality when the network is not synchronized to the required specifications.

Figure 3: LTE synchronization.

Backing-up the sync signal
Depending on the type or geographical location of the network, some networks have relied heavily on GPS technology to deliver synchronization. However, GPS synchronization is susceptible to jamming and spoofing, or simple signal fades where antennas are partially blocked, which disrupt sync in the network.

Regardless of the primary technology used to synchronize the packet-based network (PTP or GPS), rubidium holdover technology can perform a critical function within the specified requirements of the base stations to support 4G/LTE services (figure 4).

Figure 4: Multiple sync technologies for 4G/LTE build out.

To ensure continuous network operations, it is recommended that service providers deploy rubidium atomic clocks in their base stations to ensure holdover for either a GPS or a PTP synched network.

Rubidium is the best technologies available in the market today to deliver holdover of up to 1.5?s (required for LTE-TDD) over a 24 hour period.

Holdover requirements, technologies
Holdover (figure 5) is achieved by equipping cellular base transceiver stations (BTS) with oscillators or atomic clocks that temporarily holdover sync signals. Holdover periods can range from several hours to several days depending on the oscillator technology (crystal or rubidium), environmental factors (temperature and temperature variation), and the quality of the implementation (algorithms that account for and adapt to the effects of aging).

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