In order to cater to the growing demand for mobile traffic, mobile operators are looking to deliver additional capacity and coverage through deployment of small cells as part of Heterogeneous Networks (HetNet).
Small cells is an umbrella term for low powered base-stations (access points) that operate in licensed spectrum and typically have a range from 10 meters to several hundred meters in contrast to macrocells that have a range of several tens of kilometers. In this application, we focus on synchronization solutions for small cells that are classified as "pico cells, micro cells and metro cells". Synchronization solutions for femtocells are discussed separately in Femtocell Application.
In earlier 3GPP releases, synchronization for 3G networks was delivered using TDM network or GNSS. There are two major drivers for re-architecting the synchronization delivery strategy in 4G LTE:
FDD systems require only frequency synchronization of 50 – 250ppb, however TDD systems have an additional (and far more stringent) requirement for phase alignment of less than +/-1.5μs, relative to other cells with overlapping coverage. For some LTE-Advanced features (e.g. eICIC, CoMP and MBSFN) the requirement is even more stringent at 0.5μs.
In order to choose the most optimized synchronization approach, it is important to understand the use case for small cells.
In the case of small cells deployed to enhance capacity and in dense-urban underlays which utilize LTE-A techniques such as eICIC and COMP, it will be necessary to provide phase synchronization to within 0.5μs. IEEE 1588 PTP, GNSS and macro cell sniffing are some of the plausible approaches to deliver synchronization in such scenarios.
In scenarios where small cells are deployed to enhance indoor coverage (for example dense urban venues such as stadiums, convention centers, shopping malls and multi-tenant buildings) where normally Ethernet cabling exists to provide traffic backhaul; packet based IEEE 1588 PTP synchronization techniques are a better choice. Moreover, since these small cells are deployed indoors, the GNSS signal reception will be weak and subject to multipath. In order to meet the microsecond phase requirement of LTE and LTE-A, it will be necessary to deploy the IEEE 1588 Grandmaster in the building itself or at the edge of the network (a few hops away) to deliver precise synchronization.
For outdoor coverage, distributed small cells are sometimes deployed where there is no macro cell coverage (due to their high cost of deployment). In this scenario, small cells are not part of the HetNet or macrocell umbrella, but form the first layer of coverage. In such cases, GNSS based synchronization combined with PTP synchronization can provide a resilient synchronization solution.
Given the complexity of HetNet and sync precision required by LTE and LTE-A, it is a good practice to deploy multi-sync to ensure network performance.
Managed Timing Engine Module (M64)
Managed Timing Engine Board
The Managed Timing Module (M64) is an IEEE 1588 PTP slave that can be integrated into the small electronics. It is an extremely low power solution based on a low cost oscillator. It takes in IEEE 1588 PTP as input and delivers 1PPS, frequency 5/10/20/25 MHz and Time of the Day (TOD) as output.
With additional electronics, the Timing Module also supports GNSS based synchronization. To support both GNSS and PTP with holdover, it is recommended to use the Managed Timing Board as the slave clock. The Managed Timing Board is a small form-factor standalone IEEE1588v2 GrandMaster and/or Slave integration system. In addition to being a full-fledged board, this can also be used as an evaluating kit together with various oscillators and GNSS.
For use as a HetNet synchronization source, the Managed Timing Board can be deployed as an IEEE 1588 PTP embedded Grandmaster (Edgemaster) at the macro cell aggregation point or at the edge of the Mobile Backhaul. It has the ability to act as the PRTC source with GNSS enabled.