You have heard the old saying “splitting hairs” referring to arguing about very small differences or unimportant details. There is splitting hairs and then there is splitting cells – cell splitting to be exact and it is occurring all over the world as wireless service providers attempt to add ubiquitous coverage and more traffic capacity.

Ever since the early days of AMPS, macrocell site design was based on the number of cells and a frequency reuse factor and coverage range was 5 to 10 km or more. In the 1990s the use of microcells, those access points with coverage of less than 1 km, included 1×3 base stations and outdoor repeaters employed to add coverage to weak spots. With the rise in distributed antenna systems (DAS), the notion of picocells became more popular for in-building systems. Picocells, by contrast, typically cover areas less than 200 meters in length. In 2007, AT&T sold the first shoebox size femtocell whose range is typically less than 100 meters. Unlike many passive DAS systems who are backhauled “over-the-air” to a macro cell, an AT&T femtocell (branded as a microcell) has a unique IP address and traffic is passed through the subscriber’s broadband connection to an access gateway installed in the AT&T network where the radio network controller (RNC) also resides to perform air interface operations.

Femtocells are pretty much “plug and play”, self configurable and revenue generators. Femtocells are more like a mini base station which is different from typical DAS that merely reradiates traffic off of a macro cell. Download speeds with femtocells can be up to about 3.5 Mbps depending on the backhaul speed employed. However, more common speeds on AT&Ts network are less than 300 kbps. Femtocells are required to comply with FCC E911 requirements. When a customer registers their femtocell, the physical address they provide is geocoded by the E911 database which then tries to validate the PSTN.

A 3G femtocell must be associated with an active cell phone number stored in the subscriber identification module (SIM). This is also called the “MSISDN” and is managed by the subscriber through an on-line account management. This allows management of a “white list” of MSISDNs approved to access the femtocell. The subscriber provided white list of MSISDNs is associated with the corresponding mobile subscriber identifier (IMSI) by the provisioning system. The 3G femtocell maintains this whitelist of associated IMSIs and processes standard UMTS authentication requests for devices allowed. Femtocells are compatible with some Wi-Fi routers. Figure 1 below shows logical architecture of a 3G femtocell and its connectivity to the network via the internet.

Figure 1: Logical Architecture of a 3G FemtoCell and its connectivity to the network
Figure 1: Logical Architecture of a 3G FemtoCell and its connectivity to the network

AT&T 3G femtocell is actually branded as a “microcell” and is a UMTS access point for use in residential or small business settings. It supports mobile voice and data and uses any broadband connection to connect to the AT&T mobility network. Femtocells provide coverage and capacity for homes and small offices and enable high quality 3G services while offloading a macro cell.

What are the potential impacts of femtocells on interference? In most cases, 3G Femtocells have to share spectrum with the macro network (GSM and/or UMTS), and the 3G femtocells and macro cells can interfere with each other. Minimizing the negative impact on the macro network, GSM and UMTS, is a key requirement when selecting possible frequencies for the 3G femtocells. Transmissions from users on 3G Femtocells increase the noise floor of the macro cell, effectively reducing its coverage and capacity.

In general, 3G Femtocell users transmit lower power than macro users due to their proximity to the 3G femtocell. However, in areas with strong macro coverage, a 3G Femtocell user at the edge of the 3G Femtocell cell coverage can transmit at higher power than a macro user in the same location. This can significantly increase the macro cell noise floor and reduce macro cell coverage. As part of the auto configuration the 3G femtocell also determines its own maximum transmit power between 1 mW and 250 mW. In areas with low macro coverage on the selected frequency, the 3G Femtocell will choose a lower transmit power to limit interference to the macro network.

Femtocells are 3G-only devices and as such cannot handover to 2G or EDGE handsets. The femtocell has a network listen function (NWL) that scans the macro network pilot channels and “assigns” the appropriate macro neighbors it sees. Scanning is done on the 850 and 1900 MHz bands, both 2G and 3G technologies. Some users of femtocells place the box near a window which has line-of-site to a macro cell and as such the potential for interference can occur. Interference can also occur when two femto cells are placed too close to each other. Once a user establishes a call on a femtocell they may handover to the macro cell. On the contrary, a mobile phone talking to the macro cell may not hand into a femto cell. Hence one can see where drop calls can occur driving up to a femtocell is possible.

Troubleshooting or tracking performance of individual femtocell access points remotely is not realistic for the wireless carrier so proper installation of the femtocell is important for customer satisfaction. The gps antenna (internal or external) must be positioned to receive the satellite signal so that the unit can register on the AT&T network. An internal picture of an AT&T femtocell is show below in Figure 2. The big question is how to utilize the femtocell for coverage and capacity enhancements without splitting hairs. Maybe the next hair splitting will result in an “attocell”.

Figure 2: Interior View of an AT&T FemtoCell showing GPS, 850 MHz and 1900 MHz antenna ports
Figure 2: Interior View of an AT&T FemtoCell showing GPS, 850 MHz and 1900 MHz antenna ports

LBA Technology, Inc. is the exclusive US distributor for Kathrein / Schomandl RF Test Equipment including an RF power meter that is useful for diagnosing problems with femtocells and other wireless equipment that operates in the 2-6 GHz range.

About The Author

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LBA Group, Inc. has 60 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The group is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection and EMC equipment for broadcast, industrial and government users worldwide; the professional consultancy Lawrence Behr Associates and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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