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.
