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Thursday, April 18, 2013

Femtocell Synchronization and Location - common technological and economic challenges

Femtocells and Small Cells rely on, at least, frequency control in all markets and on synchronisation and location in specific markets and for specific air-interfaces. Without external support, economically viable frequency references (such as TCXOs) cannot meet the long term frequency stability required by the air interface standards. Synchronisation (in this context meaning time-alignment of the transmitted frame structure within microseconds between neighbouring cells) is mandatory for all Time-Division (TD) air interfaces, CDMA2000 and EV-DO. Determination of precise location of femtocells by automatic means is required to meet regulatory and licensing requirements in some markets (but may also contribute to revenue-generating services).

This paper addresses the common technological and economic challenges in achieving synchronisation and location determination for Femto and Small Cells. It is provided as a briefing for network operators, to encourage debate amongst Femto and Small Cell vendors as far as necessary to ensure that industry requirements can be met and to alert vendors from outside the immediate industry (e.g. of time-servers, routers, modems, GPS receivers) to the specific needs and opportunities offered by the market.

The scope of the synchronisation challenge is well exemplified by the characteristics of two extreme cases, both in residential settings. A WCDMA residential femtocell (the type most widely deployed initially) requires only occasional frequency control (disciplining) of its in-built low-cost oscillator, say once or twice per day. This can be achieved by the exchange of ‘NTP’ packet timing messages over the attached broad-band connection to a server dedicated to the purpose. Over some minutes or hours any frequency error can be assessed and a correction applied. The only significant overhead for the operator is the purchase cost of a time server (thousands of dollars) supporting perhaps tens of thousands of devices.

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