All cellular mobile telecommunication networks are divided into cells, served by one specific base station. Each base station may serve more than one cell. The important point from a positioning and navigation perspective is that the cell where a specific User Equipment (UE) (also referred to as terminal) is located is known in the cellular system. Hence, after determination of the geographical area covered by a specific cell, it can be stated that the UE is located somewhere within said geographical area, as long as it is connected and the reported cell identity of the serving cell is equal to the cell identity of the particular geographical area.
The accuracy of the cell identity positioning method is limited by the size of the cell, something that prevents it from being used in more sophisticated navigation applications. Its main advantages include a very low response time as well as the fact that it is widely spread and always available where there is cellular coverage. The cell identity method is also straightforward to implement and has no UE impact. The advantages has lead to an interest for a development of the cell identity positioning methods. These methods are referred to as enhanced cell identity positioning methods and they aim at enhancing the accuracy of the basic cell identity method at the same time as the advantages of the method are retained.
One principle for the enhanced cell identity positioning aims at combining a cell extension model preferably by using polygons (which will be explained below) with a distance measure. The cell extension model implies that a model of the coverage of a RBS is determined. Two possibilities regarding the distance measure are round trip time (RTT) measurements and/or path loss measurements. The more accurate of these two alternatives is the round trip time measurement. The path loss measurement suffers from shadow fading effects, which results in accuracies which are of the order of half the distance to the UE. The round trip time measurement principle is depicted in FIG. 1. Briefly, the travel time of radio waves from the Radio Base Station (RBS) to the UE and back is measured. The distance (r) from RBS to UE then follows from the formula:
  r  =      c    ⁢          RTT      2      where RTT is the round trip time and where c is the speed of light.
The round trip time measurement alone defines a circle, or if the inaccuracy is accounted for, a circular strip around the RBS. By combining this information with the cell polygon, left and right angles of the circular strip can be computed.
FIG. 1 illustrates the cell identity positioning method combined with round trip time. The terminal position is determined as the intersection of the serving cell and the circular strip.
In several systems, among these the Wideband Code Division Multiple Access (WCDMA) system, Round Trip Time (RTT) can be used to identify the distance from the antenna at which a UE is positioned. This provides a distance but it is not possible to ascertain where in the circle or sector the UE is. If RTT measurements determine that the UE is for example 500 m from the base station this is along an arc in a sector or the circumference of a circle. Triangulation measurements of RTT from several base stations can be used to increase accuracy. Triangulation measurements of RTT is a method where up to three RBSs measure the distance to the same UE, or cell ID. A UE in soft handover is also in an area relatively equidistant to two base stations where the UE can be identified as present. However this method is restricted by low availability which is caused by the likelihood that three RBS are in the active set is typically 10-15%.
As stated above, a preferred representation of the geographical extension of the cell is given by the cell polygon format, as further described in 3GPP, TS 23.032, “Universal Geographical Area Description (GAD). The extension of a cell is described by 3-15 corners of a closed polygon which does not intersect itself, cf. FIG. 1. The format is two-dimensional and the corners are determined as pairs of longitudes and latitudes in the WGS84 geographical reference system.
An example of a cell polygon with corners A-E is shown in FIG. 2. The RBS is normally located close to one of the corners of the cell polygon that describes the cell coverage.
It should be noted that due to the complexity of the radio propagation, the cell polygon format is only an approximation of the extension of the true cell. The selection of the polygon format is dictated by the need to have a reasonably flexible geographical representation format, taking e.g. computation complexities and reporting bandwidths into account.
Since the polygon format approximates the cell extension (i.e. the cell coverage), the polygon is normally pre-determined in the cell-planning tool to represent the cell extension with a certain confidence. The confidence is intended to represent the probability that the UE is located within the polygon, conditioned on the fact that it is connected to the cell (RBS) that is represented by said cell polygon. The underlying off-line calculation of the cell polygon can e.g. be based on coverage simulations of varying levels of sophistication. However, the end result is normally not very reliable when the confidence of the calculated cell extension is considered. The consequence is a need to tune the confidence and the pre-calculated cell polygon for each cell, using field data. This can normally not be afforded though.
Another idea for enhanced cell identity positioning has been to use pre-calculated maps of the regions where the UE is in soft(er) handover with one or several cells. Soft(er) handover is further described in H. Holma and A. Toskala, WCDMA for UMTS—Radio access for third generation mobile communications. Chichester, UK: Wiley, 2002. This typically occurs in areas where the distances to the serving RBSs are about the same. Such areas are significantly smaller than the whole cell and whenever the UE is in such an area, there is a possibility to determine its location with a better accuracy than with the basic cell identity positioning method. Normally these maps are pre-calculated in the planning tool, exactly as the cell polygons.