As explained in part 1 of this series, whenever a mobile phone makes a call, the Network Operator can compose through which cell site the phone is communicating. Based on the location of the cell site, the Operator is able to evaluation the location of the phone. In a single network, the cell site has a unique identifier known as the Cell Identifier or Cell-Id. This recipe of location estimation, which uses Network measurements of the Cell-Id, is known as Network Cell-Id or Ncid.
The most definite case is when a handset is in active transportation with the network. In normal the handset will associate with the nearest base middle point and therefore this information constrains the handset to the region served by that cell site. In some cases, for instance when the base middle point involved supports a micro or pico cell with a very itsybitsy transmission range, the fact that the mobile is communicating with that base middle point provides a relatively definite constraint on the position of the handset, of the order of maybe 50 to 100 metres. Of course with a sparse distribution of cell sites, for instance in rural areas, one site may cover many tens of quadrilateral kilometres. As a effect the positional information provided by the association in the middle of base middle point and mobile is much less precise.
For an omni-directional site, the best evaluation of the handset position is the coordinates of the site. In cases where the serving base middle point is part of a multi-sector site, the coverage area of the single base middle point is likely to be more restricted. In this case it is potential to refine the estimated handset position to the notional centroid of the coverage area of the relevant sector.
In most cases, Cell-Id measurements are used in isolation to get a base point evaluation of the mobile's position. However, in some cases, multiple Cell-Id measurements may be available. In such cases the measurements can be combined to get a more definite fix.
When a handset is not in active communication, in so-called idle mode, it is not potential to identify the closest cell site. However, the network can constrain the estimated position to within a location or paging area. The network maintains this information in order to be able to focus its paging attempts when there is an incoming call destined for the handset. A location or paging area ordinarily represents a small range of cell sites. The uncertainty associated with the handset's position may be up to an order of magnitude greater than when the handset is in-call. In such cases, if a higher accuracy is required, the network may compose a brief association with the mobile to identify the nearest base middle point for greater accuracy.
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