Microwave Engineering Europe - March 2009 - (Page 14) 14 FOCUS ON RADIO Multi-standard transceiver IC enables low cost femtocell deployment By Richard Kybett, Lime Microsystems he concept of femtocells and the advantages they offer in terms of in building coverage and operator value proposition to the end customers has been the subject of debate within the industry. Extensive trials and assessments by suppliers and operators have provided encouraging results to the point that deployment is now imminent. The future of femtocells depends in resolving a number of key challenges such as functionality and cost. Issues such as timing/synchronisation, radio interference and handover into the femtocell from a traditional macro cell have impacted the design and implementation of the radio part of femtocells. The multiple frequency bands and standards adds further complications to the relatively long supply chain, resulting in the high overall cost. The above challenges have resulted in adding more features to the femtocell transceiver IC s such as listening mode on a nearby macro BTS cell broadcast channels for receiving information such as location and timing information. These broadcast channels maybe from any of the popular modulation standards and not necessarily the same as one used by the femtocell transceiver. Borrowing resource In order to minimise cost and part count in the femtocell radio it is preferable if the radio resource from the main transceiver can be borrowed for this listening function. This requires a number of considerations in ensuring that the transceiver can be operated not only over a number of different frequency bands but also modulation standards without the need for excessive additional circuitry to meet the aggressive price points. The concept presented in this article is based on a fully reconfigurable receiver which can be used for the dominant modulation standards and has multiple wideband LNA inputs to allow direct connectivity to up to three receive band filters enabling a seamless transition to ‘listen’ mode without the need for Microwave Engineering Europe ● March 2009 ● T the addition of yet another receiver chain. The design also allows the downlink to continue transmitting its own broadcast channel while the receiver is being used independently. Femtocells have the unique characteristic that they are a piece of radio infrastructure equipment which must work seamlessly with the existing infrastructure but they are installed by and located with the end user in their home. Once powered up, the femtocell must therefore be able to self set up based on its macro environment. Hence, it must be capable of listening to both its own macro network and possibly those on other frequencies and modulation standards. This network listen mode feature requires the femtocell to utilize multiple receive paths/ ICs based on the existing single frequency and standard transceiver implementations. The problem gets even more complicated by the release of new frequency bands with no device availability. Interference management is key to the successful deployment of femtocells and the need to listen to its own downlink raises the following transceiver requirements: Figure 1: A single receive path with switch added to main receive path. Figure 2: Use of the largest blocker from the GSM900 Mobile Station. www.mwee.com http://www.mwee.com
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