By Simon Saunders, Chairman, Small Cell Forum
Interference sounds bad. But it can actually be a good thing. Managed carefully, it could improve mobile network performance by 100 fold.
The trick lies in ensuring that small cells make good use of radio spectrum, with the result that it can dramatically improve data through-put in macrocells.
The stock-in-trade of mobile operators is managing spectrum
If we didn’t have interference between cells it would imply that we were using our spectrum incredibly inefficiently and would only be able to serve a tiny number of customers with tiny amount of data.
Controlled interference management is how you get a potentially unlimited number of users into a finite spectrum. As the demand for mobile broadband grows, we need to support and more users. Managed interference is what will make mobile broadband sustainable.
Small cells play a key role in reusing spectrum efficiently. In an ideal world there would be one small cell in every household or office, and it wouldn’t threaten the outdoor macro network. That’s because an indoor cell has the exterior walls to prevent the signal leaking out. If you turn power down further you could have two small cells in one house. There is no limit providing you can get the backhaul and the costs right.
Managing spectrum in the wild
Mobile operators employ armies of radio planners to get the right level of interference between cells to ensure they get most bandwidth per user. Planning spectrum in this way is a very skilled and time consuming job. You have to perform drive tests to ensure that spectrum is where it needs to be.
Unlike current 2G and 3G macrocells, small cells can manage their spectrum without the need for manual intervention. In fact, the smarts in small cells manage spectrum and interference that the armies of radio planners can do manually. The small cell doesn’t need drive testing. It performs its own tests and has enough intelligence to dynamically adjust its own parameters. And it does this regularly as the demand changes on a daily and hourly basis. The small cell sees the small cell next door and adjusts its own power output accordingly. This is a key part of the self organizing network (SON) concept.
So where does this interference come from?
The Small Cells Forum has spent a lot of time looking at the potential impact of interference and how to mitigate it. The scenarios we’ve tested include
- co-channel deployments where the small cell and macro are on exactly the same frequency.
- closed access nodes and what happens when a mobile user, who is not registered for that small cell, gets close to it and generates and receives interference.
- low frequency bands where signals go further: that’s a good thing for coverage, but it could be a bad thing for interference.
- and small cells and interference in LTE as well as 3G.
When small cells and macrocells are on exactly the same frequency (co-channel) there are some areas of interference between the macrocell and the small cell. This happens because transmit power is turned up when a handset is at the furthest distance from a cell, and because a handset might hear a small cell better than a macrocell but not be registered to use it. In our studies on interference, we looked at three scenarios for interference:
- A macro-connected handset that is in close proximity to a small cell that it is not registered to use, causing a deadzone.
- A small cell-connected handset that is at the furthest range of the small cell
- A small cell-connected handset that is closer to a small cell in the house next door, but is not registered to use it.
These interference scenarios look like this:
Clearly we’re most concerned with how a small cell interferes with a macro network because it can impact the most users.
These are all worst case scenarios. If you were able to arrange your small cells on a slightly different frequency or straddling two macrocell channels, you get zero or much-reduced interference. Also, if you only had open small cells, it would also be avoided because the handset would roam between the strongest signals.
But many operators will need to deal with a co-channel interference caused by open and closed cells. Some have a hybrid model where closed small cells are opened up to unregistered users if they are experiencing or causing a lot of interference to the neighboring macrocell.
Managing interference in a co-channel environment
So even before we go into complicated interference management techniques, there is a host of ways of obviating interference.
1) You get dead zones because there is too much small cell power in relation to macrocell power. If the small cell was aware of that the neighbouring macrocell had a weak signal, the small cell would scale back its own power. This is one of our basic recommendations to vendors. Some vendors have their own “secret sauce” that pushes this power management even further.
2) If a small cell-connected device is very powerful and drowning out a macrocell in close proximity, we recommend that the small cell has the ability to tell the handset what the maximum power it can transmit at so as not to impact the neighboring macro and to adjust its own sensitivity to avoid its own receiver being ‘deafened’ by the strong signal.
3) If a macrocell is interfering with a small cell, unfortunately you can’t really tamper with the macrocell output. It has been manually configured to give a certain coverage level and you don’t want to change that every time a new femto is turned on. But the small cell can adjust its power to give the optimum balance between good coverage over its designated area and interference beyond its reach.
Interference can be a good thing
Don’t be put off by the potential interference between femtos and macros – it’s a good thing.
Consider the opposite scenario where there are no femtocells at all . The average 3G throughput in a metropolitan district (and these are the results of a real test) is just 50kbps. Interference impacts so much that there is less that 1mbps of shared capacity in the entire macrocell.
If you introduce femtocells into this area, macrocell performance improves substantially. Average user throughput jumps to 8 Mbps each, while the shared capacity increases to 88 Mbps – 100 times greater than when there is managed interference.
|Femto + macro||Macro-only|
|Expected available throughput per UE||7.87||0.08|
|Number of UEs per cell (34 UEs across three macro cells)||11.3||11.3|
|Expected available throughput per macro cell (= network capacity)||88.9||0.9|
The small cell & macrocell network has an air-interface capacity limit nearly 100x that of the macro alone
So it’s not just the small cell-connected users who gain a better mobile internet experience, so do the macrocell users. With less noisy devices competing for attention, interference is substantially reduced. The macrocell is no longer clogged up by the difficult-to-service indoor users.
So in summary, to get 100x throughput compared to macro-only networks, the following techniques may be applied:
- Adaptive Pilot Power Control
- Extended Tests for Dynamic Range
- Uplink power capping
- Dynamic receiver gain management
Our studies were based on simulations, although the parameters and assumptions were based on the real-world experiences of our operator and vendor members. However, since then, the techniques involved have been applied in millions of real-world femtocells and found to be effective in practice. To quote one major operator:
“We have deployed femtocells co-carrier with both the hopping channels for GSM macrocells and with UMTS macrocells. Interference isn’t a problem. We have tested femtocells extensively in real customer deployments of many thousands of femtocells, and we find that the mitigation techniques implemented successfully minimise and avoid interference. The more femtocells you deploy, the more uplink interference is reduced”
- Gordon Mansfield, Executive Director of RAN Delivery, AT&T
You can read more about interference management and the results of our studies on our web site at: http://www.smallcellforum.org/resources-white-papers (see papers numbered 012,009,008 and 003)
Professor Simon Saunders is chairman of the Small Cell Forum, the international industry association which supports, promotes and helps drive the wide-scale adoption of small cell technologies to improve coverage, capacity and services delivered by mobile networks.