Seamless call handover into 3G small cells

HandoverUsers of early residential femtocells suffered from a problem whereby active calls wouldn't handover (automatically switch across) when entering the home. I believe this issue has now been fully addressed and that seamless service can be provided when moving in and out of the office, home or other areas served by 3G small cells.

Read on to explain how this works...

I'll use the general term Smartphone throughout but this could be any 3G User Equipment (UE), including any 3G device from a featurephone or USB dongle to a tablet.

Hard handover in the normal 3G macro network

When actively using your smartphone, in a voice or data call, it will continually measure the signal strength and quality (signal to noise ratio) from the serving cellsite sector. It will also monitor and measure levels from adjacent sectors and nearby cellsites, both large and small, based on a list supplied by the serving cell.

This neighbour list can theoretically contain more than 32 cells, but there's a limit to how many your device can meaningfully and simultaneously scan. This list would also include 2G and 4G frequencies.

When the network determines that you would receive a better service from a different cell, it will instruct your smartphone to switch to that new site and re-route your connection. This process all happens quite seamlessly to the end user, who won't be aware of it and usually won't notice any glitches or dropouts.

The criteria used to determine where and when to make the handover are many and varied, with complex algorithms also taking into account the speed you are moving (e.g. driving in a car vs walking), capacity available in other cells and capabilities of your smartphone (e.g. frequency bands, 3G/LTE capable).

Hard handover from a 3G small cell to a macro-cell

One of the smart features of a small cell is its ability to sniff and scan its environment, detecting neighbouring 2G and 3G cells. This allows it to build up and create its own neighbour list which is broadcast to the smartphones using it. Macrocells typically have these lists externally pre-calculated by radio planning tools or radio planners and downloaded via the operations & maintenance centre.

When leaving an area served by a small cell, such as when walking out of your house mid-call, the smartphone will be continuously monitoring the signal for those neighbour cells and be instructed to make the switch to a better neighbour cell as you walk outside, after the performance of the radio link to the serving small cell degrades.

Each macrocell can be unambiguously identified by a combination of the 3G primary scrambling code and the parent RNC ID (Radio Network Controller Identity). This ensures there is never any confusion about which macrocell the smartphone would be handed off to.

So far, so good.

Too many small cells to identify

A consequence of the large numbers of small cells is that each macrocell couldn't assemble and broadcast the very long list of all small cells in its footprint. Nor would it be feasible for smartphones to identify and monitor large numbers of unique scrambling codes.

To address this problem, a small number of 3G scrambling codes (say not more than 4 or 5) can be assigned to and shared by all small cells, and these are added to the neighbour lists of each macrocell and small cell. The system can now figure out that your smartphone is getting a much better signal from a nearby small cell, but it wouldn't necessarily know which one.

Each cellsite has a unique cell identification number (Cell ID), and the measurement report messages were extended to report not just the scrambling code but the Cell ID which the smartphone saw. The network could now determine which small cell you should be handed over to, and trigger the transfer as you walked into an area covered by another macrocell or small cell.

This does require the additional reporting of the Cell ID by the smartphone, so won't work with older models. However, I understand that most recent products (say shipped within the last two years) would likely include it, but perhaps not all of the most basic ones. This is a 3GPP Release 9 enhancement, published in December 2009, so not anything very new.

Working with the unknown

While an inter-frequency handover can only take place towards a cell that is explicitly listed in the neighbour list of the serving cell (broadcast or part of the measurement control message), an intra-frequency handover may also take place to a non-listed cell (triggered by a smartphone measurement report on a "detected cell") as long as its Cell ID is known.

The consequence is that an intra-frequency hard handover can take place from a macro-cell to any of the covered or neighbouring small cells that are using the same frequency as the macro cell – provided the smartphone is capable of reporting the target Cell ID, and provided the backend network is able to route the signalling messages to the target small cell.

Gating cells at entry/exit points of buildings

Many people pass through the entrances and exits of large buildings and might expect their calls to be seamlessly handled throughout. The small cells located at these boundary points are termed "gating cells" and given special attention.

For inter-frequency handover, and because the neighbour list presented by the macro-cell is limited and so cannot list each of the potential small cell neighbours, a handover to any of the existing small cells within the macro-cell's range is not possible if many of them are around.
This drives the need to define a subgroup of small cells, which provide a bridging function and entry point into that other frequency, and which would be specifically present in the macrocell neighbour list. When it comes to selecting which cells should be gating cells, the characteristics to consider include visibility of many macrocell connected smartphones, as well as common boundaries with as many (non-gating) small cells operating (on that same frequency) as possible.

Cell reselection vs Handover

There's an important distinction between these two terms. When a smartphone is idle, not actively sending/receiving voice or data, it will go to sleep for most of the time but frequently (every 2-3 seconds) wake up to check for any incoming calls.

If you haven't moved since last time, then it can quickly listen in on the same frequency to the same cellsite and determine if the network is paging it. If so, it can then fully wake up, establish a signalling connection, and deal with the call. Usually, if you haven't moved, and the serving cell quality is good enough ("good enough" being defined by broadcast threshold parameters), the smartphone will only measure the RF-conditions of the serving cell and any other (2G or 3G) cell or frequency.

If you have moved, the smartphone may find that the quality of the serving cell has degraded below the acceptable level and will start searching for more suitable cells using the same or different 3G frequency or using 2G. If it finds a more suitable cell, the smartphone will reselect to that other cell. The key point here is that it is the smartphone itself that triggers and executes this reselection process, not the network, even though based on parameters broadcast by the network. After the reselection, the core network is only updated by the smartphone if it moved to a different zone (paging area), which helps limit the number of unnecessary signalling messages.

Cell reselection when moving indoors

One of the reasons that mobile phones might not reselect to the small cell in your home/office immediately you enter is that this cell reselection process can take some time – a few minutes or even longer – if the smartphone is still able to receive a good enough signal from the serving macrocell outside and therefore didn't bother scanning for a better alternative cell. During this period the smartphone would remain camped on to the outdoor macrocell and hence not benefit from the likely better-quality indoor small cell.

This can and has led to some customer dissatisfaction in the past. There are several remedies to this, each having their pros and cons. An example would be to modify the thresholds that define what "good enough" is, encouraging the smartphone to start searching a better cell more frequently. But this increases the smartphone "idle-mode" power consumption and reduces battery life.

Larger enterprise small cell deployments often involve tighter integration with the outdoor cellsites, where network planners actively configure these "good enough" parameters to optimise reselection as you walk through the entrance/exit doors of large buildings.

There are several additional methods which can improve this but are too detailed to include here - contact us if you'd like to discuss these further.

Summary

Mobility between macrocells and small cells is important for maximising the benefit that small cells introduce and ensuring a good customer experience. Mobility is achieved through handover and reselection – the former being triggered and managed by the network, while latter being triggered and executed by the smartphone (based on policy parameters from the network).

Both handover and reselection are important challenges for small call deployment, but solvable, and this is where Self-Organising Networks(SON) play an important part affecting not only the small cell layer, but also the macro network domain. Ultimately, SON has to address more than just the small cell or macrocell layers but co-ordinate across the entire Hetnet system.

Caveats

  • The above relates to 3G UMTS systems (used worldwide) only, not 3G CDMA (used in US and Japan) or 4G/LTE.
  • 3G also includes a soft handover mechanism. This is not widely implemented in small cells, which use hard handover, although the specifications have recently introduced a new Iurh interface that makes this possible. Proprietary solutions may also deal with handover between a centrally co-ordinated group of small cells inside the building in a different way.
  • LTE has designed small cells in from the outset, and there are no significant issues related to handover to/from small cells.
  • The term Smartphone used above should be interpreted to mean any 3G cellular device, defined in 3GPP standards terminology as the UE (User Equipment).

My thanks to those industry experts who assisted in preparing and reviewing this article.

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