A recent report on LTE to 5G evolution from the GSA (Global mobile Suppliers Association) underlines the increasing interest in LTE-Advanced, LTE-Advanced Pro, LTE-TDD and LTE broadcast services. This is good news for end-users as new, high-bandwidth services become available. It does however create challenges for mobile network operators and connectivity providers as they need to better utilize radio spectrum. Precise synchronization of base stations at the edge of their networks is a key technical requirement to reach this objective.

Various technologies can be applied to reach this goal. There is however just one that enables the shortest and most painless path to high-quality cell synchronization. So let’s explore this route for seamlessly extending existing networks with highly precise synchronization delivery capability.

An Equation With Many Parameters

It’s good practice to initially step back, analyze the requirements and identify options for the design of a synchronization architecture: 

• Synchronization delivery can use different media: network-based synchronization using IEEE 1588 Precision Time Protocol (PTP) versus satellite-based timing selecting from GPS, GLONASS, Galileo or BeiDou
• A resilient architecture has to protect against both device failures and malicious attacks. Device redundancy in combination with different delivery technologies and oscillators with long holdover capabilities can sustain different failure scenarios. This is especially important for macro cells covering a larger serving area.
• Synchronization networks can cause significant operational cost as fault isolation is frequently a time-consuming and difficult task. Means for analyzing and assuring the quality of synchronization services will simplify operations. Space and power requirements need to be considered as well. 

Satellite-Based Versus Network-Based Timing

Global navigation satellite systems (GNSS) provide highly accurate time from space. A GNSS receiver can drive a grandmaster clock. However, there are growing concerns about satellite-based timing being used as the only source for synchronization due to many reports of problems due to distorted satellite signals caused either accidentally or deliberately by cybercriminals. In addition, GNSS receivers need line-of-sight to satellites. Small cells installed in urban canyons frequently cannot meet this requirement.

On the other hand, achieving highly precise network-based timing is no easy task. In principle, PTP can deliver very accurate synchronization, however, every network element in the data path needs to be time-aware and compensate for device-specific delay by processing PTP packets. Currently installed networks don’t provide this capability. That’s why a forklift upgrade of the communication network is needed. 

A combination of both methods solves the shortcomings of each individual approach and provides the best quality and highest availability at a reasonable cost. GNSS is used as a primary source for accurate time information. This accurate time is used to calibrate for the delay asymmetry of the packet network. In addition, selected nodes in the packet network can be enhanced with PTP processing capability as defined with ITU G.8275.2, also known as partial timing support. Those improvements enhance synchronization delivery performance of the data network significantly, so that even the stringent timing requirements of demanding LTE features can be met. 

Standalone On-Site Grandmaster Versus Base Station Integration

Integrating a GNSS receiver with a PTP slave clock in a base station would seem to be the most straightforward and easy solution. But, on closer consideration, it does throw up some serious issues:

• Frequently, multiple base stations and RAN technologies are colocated at a macro cell. With the approach suggested above, each base station has its own GNSS receiver, GNSS antenna and requires individual cabling. This means multiple components doing the same job and unnecessary installation cost.
• GNSS receivers in a base station connect to a remote antenna using coaxial cables, which are expensive to install and limited in terms of length. The integration of a GNSS antenna with an external receiver, oscillator and grandmaster could create a single component which easily delivers PTP packets using a single Ethernet cable to the local data network. 
• With indoor small cells the situation becomes even more problematic as the distance between the RF feeder and the rooftop antenna can be very long. What’s more, rooftop space needs to be rented, creating recurring cost. Again, an integrated antenna with built-in receiver simplifies this situation significantly, especially with innovative receiver design enabling window mounting rather than rooftop installation.
• There are different GNSS systems. GPS is well established while other systems are under construction. In addition, PTP is a rather new protocol with ongoing standards work in IEEE and ITU. Hence, the lifecycle of synchronization equipment is shorter than the lifecycle of small cells and radio base stations. A separation of GNSS receivers from base stations decouples innovation of mobile technologies from innovation in GNSS-based timing. This flexibility also reduces the total cost of ownership and provides better performance.
• Excluding the GNSS related technology from base station design simplifies design work, increases compactness and allows to build on best-of-breed components  

Superior Cell Site Synchronization

A miniaturized GNSS-based stand-alone grandmaster with integrated antenna solves all of the issues highlighted above. Instead of replicating functions such as GNSS receivers with dedicated antennas and cabling, a single unit featuring antenna, GNSS receiver, local oscillator and PTP grandmaster is the most efficient way for distributing accurate synchronization information to a base station or among colocated base stations / small cells. 

And there are other advantages too. Mobile network operators want to monitor synchronization quality in a consistent way. This is not an easy task with base stations from multiple vendors. A common GNSS-based grandmaster with synchronization monitoring capabilities is ideal for monitoring sync across a diverse network using a single synchronization management application. 

In case of indoor small cells, a standalone unit with integrated antenna in combination with innovative receiver can remove the need for rooftop antennas. These highly compact units can be easily mounted on windows or on antenna masts in urban canyons where less advanced integrated receivers fail to work. The same approach has also proven advantages in urban canyons in which street-level supply of GNSS signals was previously considered to be problematic.

Interested in This Solution?

Have a look at the recently launched OSA 5405 from Oscilloquartz. This ultra-compact dual-receiver design with integrated GNSS antenna, high-quality oscillator and PTP grandmaster combines all the advantages outlined above. The design is based on the experience of many mobile network operators, who are in need of efficient and manageable synchronization solutions at the edge of their network. This product is complemented by a range of OSA 5420 edge PTP grandmasters that deliver unrivalled efficiencies when installed at the first aggregation site. Such a unique portfolio enables the operation of demanding LTE features with a cost, performance and operationally optimized synchronization solution.