Interview with Stephane Daeuble, Senior Product Marketer for Small Cells, Nokia-Siemens Networks

Stephane DaeubleStephane Daeuble of Nokia Siemens Networks believes small cells will start to be widely adopted for capacity within 2-3 years. His view is that the superior technology of LTE will win out long term, but that 3G/Wi-Fi would be deployed initially over the next two years.

He reveals some insights from their live LTE small cell trial network, including the level of in-building coverage from street based metrocells, surprising sources of interference and drawbacks of the standard 3G and LTE network architecture.

How do you view the evolution of femtocell and small cells today?

The first use case of residential femtocells has been largely successful, overtaking macrocells in terms of volume but not revenue. This is a coverage play, helping customer retention and clearly has the potential to win new subscribers. We expect this to continue to pickup significantly. Recently, we've seen this now also used for multi-tenant apartments. However, Wi-Fi is the key tech for data traffic in the home, and most people have become used to it, so that today femtocells are primarily used for voice and text messaging.

Today, small cells are starting to become a tool to address traffic capacity demand. The advent of LTE has already seen traffic levels of 15Gbyte per user per month in advanced markets such as Korea, and we believe in the longer term forecasts of 30Gbyte/month/user. In these early adopter countries, LTE networks are already seeing areas of congestion and this is driving operators to deploy small cells for capacity.

Initially, this might be 1-2 small cells per macrocell in hotspots but this will quickly increase during 2013/14. By 2015-2016, we expect to see several small cells beneath each macrocell, expanding and densifying the "hotzones" with 10 small cells or more per macro. This is not spread geographically at the same ratio. In the highest traffic areas, anything up to 100 small cells per macro might be needed.

So our view is that over the coming 3-5 years, small cells will start to make a tremendous impact on the network architecture and they will be deployed in significant numbers.

Will these small cells be 3G or LTE or both?

Although LTE has become available more quickly than any preceding mobile technology, it's still not being deployed as fast as predicted in some markets. We expect there will be significant deployments of 3G and Wi-Fi small cells over the next two years, after which LTE will become more prevalent.

There is a lot of uncertainty, even disagreement, between industry analysts, who are finding it difficult to predict what types of solutions will be prevalent in the different enterprise, public indoor and outdoor markets. The ideal RF transmit power for a metrocell has been quoted at anything from 100mW to 5W so the jury is still out as to whether this will be done leveraging enterprise/public Femtos, Picos, Micros, Pico clusters type solutions, etc...

As per many analysts' forecasts, Nokia Siemens Networks don't believe there will be huge volumes of 3G metrocells deployed as they will be mostly deployed for coverage fill-in and hotspots and less suitable for future Hot Zone deployments. LTE is more efficient at managing interference in same carrier deployments making more suitable for future Hot Zone dense small cell capacity underlay. That is why we embarked on our Flexi-Zone product development with LTE in mind and built in optional Wi-Fi because it also brings lots of value for today and tomorrow.

What other concerns do you have about large scale small cell architecture?

With traditional Micro/Picos architecture, every cell is directly connected into the core network. For future networks with 10-100k LTE small cells, this will cause a huge impact on the EPC (Evolved Packet Core), with much greater signalling traffic and a higher payload. We believe this leads us to rethink the whole network architecture.

To deal with this, our Flexi Zone architecture provides a cluster controller which makes a group of small cells (up to 100) appear as a single cell to the core network, devolving the management and signalling to the local level.

Another radical change will be how operators plan and install each new small cell. Where a macrocell installation may take a day or two, we need to see small cells being deployed in an hour or so. The whole cycle time from planning to active service must be reduced to a matter of days.

How can operators learn where the pitfalls lie and how they must adapt?

Many operators are conducting their own trials today. There is considerable focus on backhaul, or what we call "streethaul", between the metrocells and the nearest macrocell. A variety of wireless technologies are being considered for this purpose - some available today and some that will still need some additional innovation work to make them suitable for street level deployments.

For our part, we wanted to understand the real world issues as much as possible. We installed our own live LTE/Wi-Fi small cell network in Chicago and have been running trials of many aspects for over a year. We've gained a lot of practical insight during this time and have many anecdotes. For example, we found a sudden drop in performance which we later tracked down to the Wi-Fi backhaul being affected by many obstacles such as leaves on trees, buses and even Christmas decorations. Macro/Small Cells co-existence (interference, mobility) has also been a major issue that we had to address with Flexi Zone innovations.

Overall, we've identified a total of 15 challenges which need to be overcome, ranging from backhaul, performance/scalability, O&M, mobility, QoS through to ease of installation.

What spectrum frequency do you expect metrocells to operate at?

Generally speaking, the higher frequencies will be used for small cells. Recently 1800MHz has become key for macrocells (as deployed by EE in the UK and Telstra in Australia), with largescale widespread use of the 700/800 spectrum released by the digital dividend [Ed Note: i.e. from switching off analogue TV].

The 2600MHz band is very good for small cells – there is plenty of it and it is good for capacity. This will probably be the most common global band for LTE small cells.

We also see 2100MHz being used in Korea and Japan, and the 3500MHz band is likely to be used later.

Of course, for any of these bands to be successful, there must be cheap and plentiful devices. As always, this is a bit of a "chicken and egg" scenario.

Will metrocells fulfil indoor coverage/capacity needs, or will we also require indoor small cells?

I think we'll need both outdoor and indoor small cells to handle in-building service properly. Our results in Chicago (at 700MHz) achieved very good indoor penetration. Other trials, even at 2.6GHz have also had good results.

We saw 1 to 3 wall and 5 to 6 storey height penetration. This is partly because the angle of a street mounted metrocell is more favourable for in-building penetration than a higher up macro. This means we would reach fairly deep inside, typically covering the most important "trading" areas of shops/restaurants/etc...

The beauty of deploying on lightpoles/lampposts is that there only needs to be negotiations with one company or local authority to gain access to thousands of poles. By contrast, you may need to negotiate individually with every single shop or retailer, involving a lot of time and cost. Larger businesses and premises are different – shopping malls, large enterprises, airports etc. would clearly be worthwhile addressing with more specific solutions.

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More details of Nokia Siemens Networks' Flexi Zone can be found on their website

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