This post is by Ilan Tevet, Head of Service Provider Line of Business, RAD
All LTE networks need an accurate Time Of Day (TOD) reference delivered to eNodeBs and small cells. But the large, expensive and centrally-located 1588 Grandmasters typically used for this purpose do not fit all deployment scenarios or performance requirements.
Moreover, GPS receiver deployment at every cell site also might not fit all use cases, cost considerations or performance requirements.
A GPS receiver can be used to deliver local timing at every cell site with very high accuracy. But while it is impervious to network conditions such as delay and packet delay variation (jitter), it nonetheless poses several challenges to network operators.
Firstly, deploying a GPS antenna at every cell site is very expensive and involves high installation and maintenance costs. These expenses may prove prohibitive as LTE networks scale to thousands of end points with the rapid profusion of small cells to increase capacity and coverage in crowded urban areas.
Secondly, GPS is also subject to both unintentional and intentional interference, such as radio frequency interference/spectrum congestion and GPS jamming and spoofing.
Moreover, in some locations, such as dense urban areas and indoor shopping malls where there is no “clear sky view” for the GPS receiver, service coverage could be dramatically impaired.
IEEE 1588 Precision Time Protocol (PTP) defines the distribution of timing flows from a centrally-located PTP Grandmaster at the network core, so that a single Grandmaster can service hundreds or even thousands of PTP slaves. Such centralised PTP distribution is performed today end-to-end, bringing accurate frequency to the 2G/3G base stations.
However, in order to bring accurate phase/time to the new LTE/LTE-A base stations in the order of sub-microsecond, PTP would need to be distributed hop-by- hop via the backhaul network all the way to the base stations. The key challenge for this methodology are cost and operational complexity, since every single intermediate transport node would have to undergo forklifts to support PTP Boundary or Transparent Clocks.
A much more cost effective alternative is the Distributed Grandmaster solution. Based on the soon-to-be-released G.8275.2 PTP time telecom profile, a mobile demarcation or aggregation device equipped with Grandmaster capabilities is deployed at the first aggregation point in the network – a POP or hub site typically located one or two hops from the eNB or small cell. This architecture enables PTP distribution to begin closer to the cell site and involves a minimal number of hops between the Grandmaster and the end points.
In this way, the accuracy of PTP timing, TOD and synchronisation for LTE and LTE Advanced are enhanced. The result is better service coverage for small-cells, in-building and underground installations. Furthermore, costs are now lowered because hop-by-hop BC/TC support in the core/aggregation network is eliminated.
Finally, compared to the other available methodologies, integrating service demarcation and PTP Grandmaster capabilities into a single device is far less complex. It is much simpler to deploy and manage than any multi-box alternative. This lowers the cost per cell site and better guarantees flawless service delivery, which, in turn, reduces customer churn and creates sources for new revenue streams.