In my previous blog, I touched upon the benefits of femtos to both operators and users. What’s all the excitement about? What makes a femto fun? For starters these plug-and-play devices enable flexible deployments, thanks to the intelligence of the chipsets (such as Qualcomm’s FSMs), which provide the brains behind the operation.
Brain power is even more relevant when femtos are used in restricted mode and share the spectrum with the macro network. As the name suggests, restricted mode allows only registered users to use the femto. For other users in the network, let’s call them macro users, who can’t access the femto, its signal appears as interference. By the same token, signal from the macro users will be interference to the femto in uplink. In some cases, the interference can be so severe as to totally jam the femto’s uplink.
So the question is, “How do you mitigate that interference?” (interference from femtos to macro users in the downlink, and from macro users to femtos in the uplink).
Fortunately, we have already figured this out. For the downlink, the basic idea is to manage femto transmit (tx) power in a way that provides good coverage in the target area, but nothing more.
For the uplink, restricting the interference getting into the femto does the trick. We are packing a set of innovative interference management algorithms into our FSM chipsets that effectively achieve this, without requiring intervention by either the user or the network operator.
Here is a brief description of each of these algorithms:
Adaptive tx power/Self-calibration: When the femto is turned “ON” it listens to, and measures the strength of nearby macro sites. Based on the coverage, the secret sauce in the self-calibration algorithm sets the tx power to a suitable level. This happens every time the femto is switched “ON” and can be considered as a coarse adjustment.
Range tuning: This mobile assisted, tx power turning algorithm is a form of fine tuning. Femto tx power is adjusted taking into account the registration attempts by (restricted) macro users, which may indicate too much coverage. This technique also factors in the signal strength measurement reports of femto users, which may indicate the need for more coverage. Unlike, the Self-calibration, range tuning is periodic. So at the end of the day for example, the range tuning algorithm can set the most optimal power level, based on measurements from the whole day.
Guest user protection: The femto is always looking for the presence of macro users in its coverage area. Whenever it detects one, and observes that the femto user is not in active state, it temporarily throttles down the power to reduce interference. A typical case for this might be an active macro user passing by your home/femto.
Home user protection: This feature protects the fetmo from being jammed in the uplink by a strong interferer (macro user). It employs adaptive attenuation (in the uplink) to pad down the effect of the interferer, while still comfortably receiving the signal from the femto users.
These techniques demonstrate that femtos are not only magical — they also have feelings for their neighbors. Importantly, the femto can truly be “plug-and-play.” Operators are well served to employ the breed that takes advantage of the aforementioned interference management techniques.
As usual, if you want more information, please check out our webpage at www.qualcomm.com/femto.