Network Cell ID (NCID) is the simplest positioning method in a GSM network, but delivers poor accuracy. One way to improve the accuracy is to use additional information. Systems that use this information are known as Enhanced Cell-ID or ECID. One implementation of ECID involves making changes to the network so that Round Trip Time (RTT) measurements become available. The RTT is the time that it takes for the signal to travel a round trip between the cell site and the mobile. Half the RTT multiplied by the speed of light gives the distance between the mobile and the cell site. This range estimate is noisy, partly due to fast fading and other aspects of cellular propagation, and partly due to quantisation error. In GSM, these RTT measurements are known as Timing Advance (TA). In GSM, the TA is quantised to one bit. RTT measurements are also available in 3G, but to simplify the discussion, this tutorial will focus on GSM. The same principles apply to 3G and GSM.
The standardised CID + TA method improves on the basic NCID performance by incorporating the range estimate corresponding to the Timing Advance. It should be noted that the cell site installations must be sectored for this method to yield any improvement over basic CID. In cases where the sector and TA are not known because the mobile is in idle mode, a brief connection (unnoticeable by the user) can be established, and the serving sector and the TA obtained.
In theory, the sector information localizes the mobile to within a sector centred on the pointing angle of the cell site antenna. Because the TA is rounded to the nearest bit, the accuracy of the range estimate is limited to one half a bit or 550 metres. This means that the mobile is at best localized to (for example) a 120 degree segment of an annulus some 550 meters in width. In practice, and particularly in urban areas where shadowing effects and time dispersion are greater, the angular variation as well as the range errors will be even greater. Accuracies with this method are moderately better than CID, typically in the range of a few hundred metres to a few kilometres.
Although not discussed in the standard, the accuracy can be further improved by using the signal strength values of neighbouring BTSs reported by the handset periodically in the Network Measurement Report (NMR). In GSM, such measurements are commonly referred to as CID + TA + NMR. The network measurement report is issued periodically by the mobile, every 480 ms during a call. The signal levels in the report are based on a moving average of typically 5 individual measurements over the reporting interval in an effort to reduce variations due to fast fading.
One method for using the signal strength values in the NMR is to derive a set of circular loci centered on the respective BTSs based upon knowledge of the transmission power and use of radio-propagation modelling techniques. Note that whilst this method may give the impression of precise measurements in close agreement, in practice the derived loci are very error prone due to shadowing, fast fading etc., and the uncertainty associated with each range is typically of the order of several hundred metres. A further simplification is the representation of the signal strength based locus as a circle. This is sufficient if the BTS antennas are all omnidirectional. However, in the case of sectored, directional antennas, the locus of constant received signal level will reflect the azimuthal pattern of the antenna, and more complex processing is needed to utilise the locus in the position determination.
CID+TA and CID+TA+NMR utilise existing measurements reported by GSM handsets and therefore will work with all handsets. Some changes may be necessary within the network, however, to transport the measurements from the BSC and MSC to the positioning node, which in GSM is the Serving Mobile Location Centre (SMLC). Such changes can be expensive to implement, particularly in networks built with elements from different vendors. ECID solutions can also be difficult to scale to high transaction rates.
Seeker Wireless offers a mixed user-plane and handset-based solution that provides equivalent or superior accuracy to implementations of NCID, but without the need to make complex changes to the network. More information can be found here.