Femtocell: A Possible Way out of Spectrum Crunch

Date:   Thursday , April 02, 2009

Typically, the operators become spectrum deficient in dense urban areas where there may be more subscribers and hence traffic than can be handled with the available spectrum. The operators need to shrink the cell size in these areas to have more cells per sq km; and thus more capacity per sq km for a given capacity per cell. In a network with nomadic users, this approach inevitably involves deploying more infrastructure (such as cell towers and Base Transceiver Stations) thus increasing the capital expenditure (capex) and operational expenditure (opex) of the operators. The above are particularly acute in countries such as India. For example, in India each operator is allotted, on the average, about 2◊7 MHz of 2G spectrum, while the international average is about 2◊15 MHz. In the soon to be held 3G auction, spectrum planned for allocation is 2◊5 MHz for each operator while the international average is 2◊15 MHz. Due to this the spectrum starved operators are forced to use an inter-site distance of less than 150 meters in dense urban areas such as Connaught Place in New Delhi and Nariman Point in Mumbai.

A less expensive alternative to increase the capacity of the Radio Access Network is the recent concept of Femtocell - also called as home base stations.

Femtocell Architecture

Femtocells are low power access points providing wireless access to subscribers, primarily at home. The user-installed device communicates with the voice or data network over a broadband connection such as DSL or a cable modem. Femtocell uses standard mobile cellular network protocols such as GSM, CDMA, WCDMA, LTE, or Mobile WiMax to communicate with the mobile handset. Though Wi-Fi at home resembles Femtocell, it requires dual-mode handsets and is mainly meant for data services. Femtocell can deliver both in-house and mobile voice and data services with existing handsets.

The architecture of a typical 3G Femtocell approved by Third Generation Partnership Project (3GPP) is given below:

While the macro radio network connects the subscriber to the mobile core network outside the house, the hand-off to the Home Node takes place once the subscriber gets in to the house. From thereon, the subscriber generated voice and data traffic is captured by the home node and sent through the broadband connection over the Internet to the Gateway, which then passes them on through the defined interfaces to the mobile core network. It is to be noted that the Home Node incorporates both the functions of Node B as well as that of the Radio Network Controller (RNC) for radio resource management functions. The Home Node Gateway installed in the operatorís network acts as a concentrator, aggregating traffic from a number of similar Home Nodes.

Femtocell: How does it Optimize Operatorsí Spectrum

The operator, while deploying Femtocell can either allocate different frequency bands to macrocell and Femtocell users to eliminate any interference or, alternatively, serve both the macro cell and Femtocell users in the same spectrum band to maximize spectral efficiency. Considering the scarce availability of radio spectrum and for easy deployment, it is preferable that co-channel is used. However, in a co-channel scenario, the transmit powers need to be tightly controlled so that the interference between macrocell and Femtocell bands are minimized. This requires that the Femtocells auto configure and adapt to changes in the network environment through on-going measurement and self-optimization of radio parameters such as transmit and receive power levels, scrambling code (in case of WCDMA), and the neighbor list.

With a cell radius of about 10 meters and a received signal-to-noise ratio of a house portion at the cell edge at 10 dB, private Femtocell can cover usersí homes adequately well, thus minimizing interference with the macrocell users. These closed access Femtocells have a fixed number of subscribed home users and hence traffic, providing necessary security and privacy for the home users. However, there is an incentive for the operator to extend the coverage of these home-deployed Femtocells to the street just outside, by increasing the transmit power, thus enabling public access through such picocells. These open access picocells provide access to macrocell pass-by users, thus reducing the load on the macrocell and on the backhaul. However, if the number of passers-by increases, home users who pay may be starved. Hence innovative billing mechanisms need to be deployed by the operator to create an optimal number of public users accessing the picocells without degrading the quality of home users.

Since the Femtocell is a user installed easily configurable device, the operator increases capacity of the existing macrocell without any additional deployment, thus saving on capex and opex.

What is in it for the Home User?

While the operators clearly have an advantage of improving the capacity of their macrocells, and effectively using the broadband bandwidth, what does the home user gain? It is observed that due to poor in-building coverage, subscribers churn, for example, from an operator who uses 1,800 MHz band to an operator using 900 MHz, which has better in-building coverage. Femtocells reduce the last mile of the radio access network and achieve a higher Signal to Noise ratio, thus providing improved reception, lower transmit power, and hence prolonged battery life. While the home user may be willing to invest in a closed look Femtocell due to these advantages, what is the incentive to make it open access? The operators in this case need to incentivize home users to deploy picocells by fully subsidizing it or bundling it with broadband connection; or providing some form of compensation proportional to the amount of outside traffic to his or her picocell serves.

Tech Forecast

Femtocell or Picocell home base stations deployed in combination with macrocellular network have the potential of significantly reducing the total network costs. While studies have proven that the savings in costs for the operator is more for high bandwidth bursty data traffic, it has applicability even for traditional voice services in countries like India. Moreover, the Femtocell box sitting at the userís home can learn the usage pattern and hence it can provide valuable data to the operators to provide personalized service offerings. While Femto Forum (www.femtoforum.org) with over 100 members (including fixed, mobile, and integrated operators) is actively promoting the use of Femtocells, commercial deployment is still limited to certain pockets (Denver and Indianapolis in the U.S.) and the first launches in Europe and Japan are expected only this year

We estimated the possible use of Femtocell by the Indian operators for releasing macrocell bandwidth in a 3G scenario. Our simulations show that Femtocells indeed have a good growth prospect in India. With just about 2◊5 MHz allotment of 3G spectrum the operators can save substantially by investing in Femtocell deployment, thus enhancing the carrying capacity of NodeBs.

Indisputably, Femtocell is a disruptive technology that provides a win-win situation for operators as well as the subscribers. However, the rate of adoption in the following years will determine whether it will become as omnipresent as Wi-Fi.

The author is Visiting Research Fellow & Pramod Pagare, Portfolio Manager, Sasken Communication Technologies.