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Sunday, May 15, 2011

Self-Organization in Femtocells


Wireless network rollouts require careful RF planning to ensure that cell sites do not interfere with one another and that the limited spectrum is utilized to the maximum.Even after a network is operational, RF planners continue to monitor and optimize the radio network to achieve maximum efficiency. Adding a new cell site requires careful replanning and usually results in reconfiguring existing cell sites as well. Femtocells take the RF planning and cell site deployment complexity to anew level, however, and it is not humanly possible to deploy millions of femtocells inside consumers’ homes using the same old RF planning techniques. As a result, femtocells require new capabilities and have to be self-organizing in the sense that they are completely aware of their local RF environment.


With regard to licensed Spectrum, Femtocells operate in wireless network operators’ licensed spectrum, which brings distinct advantages, as previously discussed, but at the same time brings some challenges as well. One of the challenges that network operators have to address comes with licensed spectrum is the location of the frequency spectrum itself. Different wireless operators can own the same spectrum in different cities, and certainly in different countries. If a consumer packs a femtocell into his or her bag and travels to another location where the network operator does not own that same licensed spectrum, and then powers on the femtocell, the operator potentially could be considered in violation of spectrum usage. This critical requirement has led to global positioning system (GPS)-based location authentication being built into femtocells themselves. Based on the identified location, a networkoperator can lock down an unauthorized femtocell—essentially turning it into a brick. Once the femtocell is powered on, the ACS can request location information from the FAP prior to providing any configuration data. Once the ACS receives the location information from FAP, it can then determine whether to proceed with configuring the FAP or completely denying it authorization to power on and transmit.

Focusing on RF Interference, femtocells operate in the licensed spectrum they can interfere with the network operator’s existing macrocellular infrastructure. For example, if a single frequency Code Division Multiple Access (CDMA) system is being operated where the macro and femtocell network utilize the same frequency band, then the power control algorithms of the femtocell can create interference with the macrocell, thereby degrading network capacity and quality of service. In MDUs such as condominiums and apartments, particularly, multiple femtocells can interfere not only with the macrocell network, but also with each other. Some carriers are planning to deploy femtocells on a different RF than what is used for their macrocellular network and thus avoid femto/macro interference altogether, but spectrum acquisition is costly. Self-organizing femtocell networks also have to be constantly aware of their ever-changing RF environment. In MDUs, a femtocell’s neighboring RF environment could potentially change multiple times throughout the day as neighbors power on and power off their own femtocells. Femtocells can also be equipped with sniffer capabilities to enable them to be RF aware. For instance, for a very short duration a femtocell can switch its operating mode and start functioning as a handset instead of as a NodeB. In this handset configuration, the femtocell “sniffs” its RF environment and populates a list of neighboringcells, their transmit frequencies or codes, and their power levels. Based on this sniffed information, the femtocell can then compute and select its own transmit code and power levels to ensure it minimizes interference with the neighboring cells. A femtocell can switch to handset mode only if it has no active calls and can do this periodically multiple times a day. Of course the duration in which a femtocell is in handset sniffer mode has to be very short to prevent triggering handsets that were camped on the femtocell to start cell reselection procedures


In addition to the challenges and policies described above, there still can be very tricky corner cases. As an example, if a femtocell is located in a house that is very close to a macrocell tower and has direct line of sight, it could result in some big dead spots being created in the network. As users move in and out of femtocells, they are seamlessly handed off to the macrocellular network. Consider the case where a user is moving up and down his driveway. In such a scenario, the call potentially could be continuously handed off between the femtocell and the macrocell, which is usually referred to as the ping-pong effect. The ping-pong effect directly increases the signaling load on the core network as the call is being handed in and out multiple times on very short notice. Intelligent femtocells mitigate this effect by either delaying the handoff signaling trigger to the CN (core network) or by slightly increasing the transmit power to ensure the call stays parked on the femtocell.


Because femtocells carry voice calls in licensed spectrum, they will be required to provide a 911-type existing telephone infrastructure may be viable ways to achieve 911 availability. To meet E911 requirements, operators must be able to provide the location of the calling equipment to the Public Safety Answering Point (PSAP). Emergency 911 calls might put additional restrictions on open-versus-closed femtocells, too; even if a femtocell is closed, it might be required to carry emergency calls, a policy matter for regulators to address.emergency service. Alternative power sources or fallback to


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