One of the areas of confusion and even misinformation relates to control of calls when entering and leaving a building. In the early days of 3G femtocells, hand-in wasn’t supported and hand-out was more by luck than judgment. The situation has evolved considerably by enhancement of the 3G standards and inherent features within the 4G standard. A combination of improved RF planning and SON automation provides a complementary toolkit to address the problem.
LTE designed for Small Cells from the outset
One of the early issues with 3G Femtocells was that they were retro-fitted into the 3G standard, whereas LTE was designed from the outset to incorporate them. Early issues were that the macrocells could not uniquely identify each small cell during the handover sequence and that not all networks were updated to support the latest features.
LTE does include both scrambling codes and unique identifiers in the handover message sequence that provide for bi-directional operation. This can be done entirely using the S1 interface, the X.2 interface improves co-ordination but isn’t mandatory.
Over time, more handsets and networks have been updated with support for these later standards features and can fully support hand-in and hand-out to Enterprise Small Cells. Either 3G or LTE can be used exclusively within buildings but we are seeing multi-mode 3G/LTE as a popular solution that supports both older and newer handsets, voice (including where VoLTE is not yet available) and high speed data. This means that today’s Enterprise systems may have to be designed to cater for handover of both 3G and LTE in parallel.
Avoiding conflicts between indoor and outdoor networks
Professor Jie Zhang, Chief Scientific Officer of Ranplan, believes that it is crucial to ensure that network design is conducted with a close partnership between indoor and outdoor planning teams. Indeed, he’s witnessed that leading operators have reorganized their planning teams to combine these activities into a single department. Individual building plans may be generated by sub-contractors onsite but are reviewed and approved by this central team to ensure they align with the wider outdoor network evolution.
This holistic approach avoids problems seen inside buildings today. Users near external windows in upper floors of tower blocks can often receive signals from many macrocells across a city landscape. Connecting to those causes widespread interference and disruption. It’s possible for handsets to “ping pong” between indoor and outdoor networks, causing unnecessary signaling and the potential for call dropouts. Handling that traffic entirely in-building is much more efficient for all concerned.
Enterprise small cells on the lower floors, close to ground level, need to control interference to outdoor. This improves call drop rates for users on the ground.
RF Planning vs SON Automation
Optimising an outdoor network to work well with indoor small cells requires some careful attention. Some aspects of this can be automated using SON software that configures parameters to make the most of the installed equipment on a tactical basis. Ensuring that the equipment itself is correctly positioned and adequately resourced is a more strategic role involving RF planning engineers using appropriate software planning tools.
Configuring the neighbor cell lists on the macro and Enterprise systems is best handled by SON. This may vary over time due to end user behaviour, handset upgrades to newer more capable models and building occupancy.
A more strategic planning task involves defining the entry and exit routes, identifying gateway cells through which most users will transit. Once inside the building, smartphones are discouraged (or blocked) from accessing external macrocells even when next a window.
Both RF design and SON are equally important and should work hand-in-hand together. Designers should be aware of the capabilities and limitations of the SON system, equally those configuring the SON should appreciate the constraints of the RF planners.
How is this mix of indoor/outdoor RF planning achieved?
Ranplan’s approach involves a comprehensive model for both outdoor and indoor environments. Both GIS (Geographic Information Systems) and detailed building data are imported. Their iBuildNet tool is thought to be the only commercial planning tool that can handle both simultaneously.
Firstly, outdoor GIS information is either purchased or constructed from openly available mapping data. Indoor 3D models can be constructed by importing CAD files supplemented by background images.
Secondly, the tool needs to import both outdoor and indoor network configurations using the standard file formats of the major outdoor RF planning tools. These are correlated with the GIS data for the building structures.
Finally, discrepancies and inaccuracies in the datasets need to be resolved. Ranplan often find there are errors or inconsistencies to be corrected and cleaned up, which provide an extra benefit for the wider entire network operation.
When each new building is to be planned, the Enterprise deployment team (often a sub-contractor) will use the tool to generate a model and RF design to be reviewed and approved as part of the normal design cycle. This can be done quickly and efficiently, ensuring both good central supervision with flexibility at the local level.
How will this equilibrium change with the introduction of new frequencies?
Today, few operators have the luxury to be able to assign dedicated spectrum exclusively for in-building use.
Potential upcoming changes that might affect this include the 3.5GHz band, both in the USA (called CBRS) and elsewhere (Band 42 and 43). There is also some TD-LTE spectrum at 2.3 and 2.6, popular in Asia but as yet mostly unused in Europe.
Where dedicated spectrum is used in-building, there is still a need to ensure good overlapping coverage with clearly identified hand-in/out routes to the outdoor networks. Shared in-building networks may need separate designs to cope with handoff to each external network, because every operator may have their own outdoor networks.
Our thanks to Professor Jie Zhang for his help in writing this article. Ranplan are a sponsor of ThinkSmallCell.
You can read more about their planning products at http://www.ranplan.co.uk