Delivering sub-microsecond time accuracy to the cellular base stations is one of the major challenges facing cellular providers as they deploy their new LTE networks. This is exacerbated by LTE-A’s stringent synchronization requirements and the growing use of small cells in 4G networks, which create unique challenges in the backhaul segment.
Before the debut of 4G, the standard way to deliver a time reference was to install a Global Navigation Satellite System, or GNSS (e.g., GPS) at every cell site. A GNSS receiver is usually referred to as a Primary Reference Time Clock (PRTC). This approach is impractical in 4G, however, given the far greater number of cell sites, the intended indoor location of part of the antennas (e.g. shopping malls), as well as the growing concern about possible jamming and spoofing. Furthermore, considerations of CapEx and OpEx render this approach highly ineffective.
Time distribution requires a stable primary counter (i.e. the Grandmaster, or GM) that counts time units based on a given standard timescale (e.g., UTC – Universal Time Coordinates), and a protocol for measuring delay between the primary counter and the client that requires the time information (e.g. the slave inside the base station). IEEE 1588-2008 (also termed PTPv2) is the widely chosen key technology for achieving this.
2G/3G backhaul synchronization distribution was mainly frequency-driven with a central GM servicing hundreds/thousands of base stations over a non-supporting backhaul. Nevertheless, accurate time distribution requires PTP on-path support from (almost) every network element between the ends, which mandates costly modification of existing network elements. In many other cases, where the mobile service provider is outsourcing its backhaul transport from third-party transport provides, this approach is simply not feasible.
A strategy to overcome that, one that is gradually gaining momentum, is to locate a relatively large number of distributed smaller PRTC/PTP Grandmasters in the access network, each servicing up to a few dozen PTP-slaves. This results in shorter time distribution chains and dramatically cuts the number of intermediate network elements that need to be enhanced with on-path PTP support. On the other hand, more PRTCs/PTP Grandmasters are required. As a result, the cost of the distributed PTP Grandmaster equipment dramatically increases the cost of the overall solution.
In an effort to meet the strict demands of this new solution, PTP Grandmasters with built-in GNSS receivers have now been miniaturized and packaged in an SFP. When plugged into an SFP port on an existing network device they instantly upgrade it with a fully featured PTP Grandmaster, a PRTC and various redundancy options to sustain operations in the event of a GNSS failure. No additional space is required at the POP for an additional PTP-GM box and power consumption is kept effectively unchanged.
This is an ideal option for 4G service providers looking for a quick, cost-effective way to bring accurate synchronization to small cell backhaul.
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