The 3GPP standards body continues to approve a wide range of updates and new standards for 2G, 3G and 4G. We took at look at their recent and upcoming output with a focus on those aspects most relevant to the Small Cell industry. Interesting developments include a new LTE X2 Small Cell gateway, Carrier Wi-Fi integration, and Carrier Aggregation across different cells. It’s clear that Small Cell concepts are becoming embedded in the radio network and that a lot of attention is being given towards tighter interworking/integration with Carrier Wi-Fi.
Quick Recap on 3GPP Standards timescales and program
The 3GPP standards body publishes a raft of open standards documents which vendors incorporate and test against. Standards production is organised in batches, called Releases, with all documents updated and aligned at the completion of each full release cycle. These typically come out every 18 to 24 months, with commercial production equipment (networks and smartphones) following from around 12 to 24 months later.
Approved and working versions of standards documents are freely available on the 3GPP website, including meeting notes and proposals submitted to working groups.
With such a vast body of work, it can be quite difficult to track and determine which are related to your own field of interest. It’s also important to recognise that just because a standard is published, it doesn’t mean anyone will choose to implement it. An example of that is “Local Breakout” or LIPA, which would allow a connection from a small cell to be made directly to your home or office network, such as a printer. While I’ve seen this demonstrated, I don’t know of any significant commercial deployment to date, probably due to constraints around lawful interception.
There are also features that have been deployed that don’t require standards changes. These include some of the Cloud RAN architectures which locate some functions centrally rather than in the cellsite. As long as the external system interfaces comply (i.e. both Radio and core network), then these can be developed as a proprietary solution. Some aspects of centralised RAN are included: Inter-Node CoMP (Co-ordinated Multi-Path) deals with multiple radio heads aligned through a centralised controller.
I found a useful summary document describing Release 12 and 13 from 4GAmericas. Since it’s over 200 pages long, I’ve picked out and commented on those aspects I believe most relevant to Small Cells. 3GPP also publishes overview documents for each Release if you want even more detail.
This release is complete and was published in March 2015, ready for commercial deployment from 2016.
General system refinements: A number of Small Cell and Femtocell enhancements were made to improve integration and interworking with macrocells, making the HetNet more practical, efficient, secure and reliable. Changes have been made for both 3G and LTE, including adding Emergency Warning areas, positioning (location finding) and mobility (i.e. handover).
Carrier Aggregation and Dual Connectivity: Release 10 introduced Carrier Aggregation, where datastreams from two or more different frequency bands could be combined to achieve high speeds. Carrier Aggregation was extended to combine mixed modes, using both FDD-LTE and TD-LTE (Vodafone and Ericsson claimed the first commercial deployment in Portugal in June). Dual Connectivity has now been developed that goes a step further and allows these data streams to span between different cell sites, so a macrocell could make use of a small cell to increase capacity.
Public Safety: There is an intention to build in all the necessary features into LTE so that it can be used by first responders, such as in the USA’s dedicated LTE FirstNet and for superceding Europe’s current TETRA network. A group call feature has been introduced together with some proximity features that allow discovery of and direct connection between two mobiles without using the network. I believe this is relevant to Small Cells because we might see them embedded into first responder vehicles, such as Fire Trucks, to ensure local area service at the scene even when power supplies fail.
Wi-Fi Integration: Further progress has been made to improve network selection (using ANDSF and/or RAN control) and traffic steering between Wi-Fi and Cellular. I’d see these being most relevant where Carrier Wi-Fi is used, rather than in standalone residential or non-commercial scenarios. These features could be important for Enterprise small cells that incorporate Wi-Fi in the same box.
The initial priorities have been mapped out.
Wi-Fi integration takes a further step, with LWA (Licenced Wi-Fi Aggregation), which is effectively Carrier Aggregation across a licenced LTE band and an unlicenced Wi-Fi channel. A separate feature called NBIFOM (Network Based IP Flow Mobility) aims to harmonise voice and video traffic flows over Wi-Fi.
There is also a contentious work item to run LTE directly in the unlicenced bands currently used by Wi-Fi (probably at 5GHz initially). This would require a “Listen Before Talk” change to the LTE radio protocol to satisfy European and other regulators. We've discussed unlocking unlicenced spectrum at 5GHz and 3.5GHz, and also summarised the recent Small Cell Forum workshop on LTE-U and LAA
Indoor positioning and location, perhaps driven partly by a desire to pinpoint the location of emergency calls, even when inside buildings. This includes synchronising small cells by network listen (locking on to other cellsites) where GPS and backhaul synchronisation isn't available.
Public Safety: Another essential feature for first responders is Mission Critical Push to Talk. A new working group SA6 has been created to deal with mission critical applications such as this.
X2 Gateway: Originally the LTE architecture left out a RAN gateway and there was no equivalent of the RNC found in 3G. It was found that signalling from large deployments, especially of in-building LTE small cells, could swamp the LTE core network. So an LTE gateway was introduced which concentrated the signalling into the core network through the S1 interface. The X2 interface which directly connects between cells can now carry much more signalling and forwarded bearer traffic than was originally conceived. Looking ahead to when small cells are widespread, a second type of gateway is being introduced to perform a similar function over the X2 interface. These could potentially be quite separate units and could be implemented locally for large Enterprise RANs.
Self-Optimising Networks: SON is growing in importance. It’s becoming more essential in order to simplify management of the increasing complexity of networks, larger number of cells and increased frequency of network changes.
Is anybody out there?
Power consumption is a significant recurring cost throughout a mobile network. Release 12 had a "Green Energy" initiative aimed at reducing unnecessary power consumption. Engineers have been thinking about how to make efficient use of larger numbers of small cells, taking advantage of capacity when needed but avoiding power wastage when idle.
One energy saving measure is to switch Small Cells off when they’re not needed, such as overnight or when a venue isn’t being used. One question is – how do you efficiently turn them back on when needed. The answer is to emit a simple presence signal (Discovery Reference Signal) in standby mode. Several smartphones receiving and reporting a presence signal might persuade the network controller software to power up the small cell and make use of it. Perhaps a more difficult question is how to determine when to turn them off – smartphones may be quietly listening in idle mode for that incoming call.
Another feature is UPCON (User Plane Congestion Management) which enables identification of cells and users in congested situations to allow policy decisions to mitigate them.
How does this impact a Small Cell vendor?
I asked Kit Kilgour, Head of Standards at ip.access, for his summary of 3GPP small cell standards status:
"3G small cell standards are now quite mature and most of these updates are refinements. The larger developments relate to LTE where Small Cell concepts were embedded from an early stage. Upcoming features are mostly about increased throughput, increasing efficiency and simplifying management. Tighter and more seamless integration/interworking with Wi-Fi (or using unlicensed frequencies) is also getting a lot of attention.
“Implementing some of these features may require changes to small cell chipsets and firmware, where small cell vendors are influenced by chipset suppliers. Other features are pure software developments and would be scheduled to fit with market demand. Bear in mind that many features may also require updates to smartphones (hardware and/or software) and take time to be widely deployed.”
Much of the industry activity around Cloud RAN architectures isn’t yet being conducted through the standards bodies and will appear independently. However, some aspects of managing virtualised network elements is included in Release 13.
For those considering Small Cell deployments today, there’s no real reason to wait for any of these new features. Existing standards are mature enough already. Most of these new features could be deployed in future through a software update cycle. Only those which directly relate to the radio (e.g. LTE at a different frequency band/mode, LAA, possible Dual Connectivity) might need specific hardware. The X2 gateway, where required, could be added when deployments scale to the size that justifies it.
In the interests of simplicity I've omitted quite a few features that some would consider related. That's not to say any are unimportant to the overall network evolution, but I had to draw the line somewhere.