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Wednesday, March 25, 2015

Location-Based Services in LTE

3GPP Release 9 provides a framework for defining the UE location (so-called UE positioning) in order to support a variety of location-based services (LBSs). In Release 9, the positioning reference signals (PRSs) have been introduced to facilitate the determination of the position of the UE, referred to as a UE-assisted positioning technique. A UE-assisted positioning technique involves the following:
  • The UE makes some radio signal measurements and
  • The network determines the UE location (e.g., latitude and longitude) by processing the measurements reported by the UE. This is conducted by a separate system that processes the reported radio information to identify the UE location.

The PRS are transmitted on antenna port 6 and are sent in a configurable number of consecutive subframes of up to five subframes. The E-UTRAN configures the PRS bandwidth (e.g., a certain number of RBs) and the periodicity of the PRS (e.g., one PRS occurrence every 160 subframes). Within a subframe containing the PRS, they are transmitted on more subcarriers and more OFDM symbols when compared to the regular cell-specific reference signals being sent on an antenna. The utilization of more time–frequency resources within a subframe by the PRS can improve the quality of the UE measurements compared to the use of only the basic cell-specific reference signals.

A pseudo-random sequence is sent on the PRS, and this sequence is a function of numerous factors such as PCI (physical layer cell identity), slot number, OFDM symbol number, and the cyclic prefix. The UE observes the PRS from different neighboring cells and makes certain measurements. Examples of such measurements include OTDOA (observed time difference of arrival) measurements such as RSTD (reference signal time difference). RSTD is the relative timing difference between a neighbor cell and the reference cell. The LBS system processes these OTDOA measurements from the UE (implementation-specific algorithms) to estimate the UE location.

In order for OTDOA to function properly, it is mandatory that the eNBs are synchronized to GPS accurate time (i.e., 50 ns). The relative transmission timing of the eNBs must also be known by the LBS system. The timing must also be known by the UEs to facilitate the search for the reference symbols. The accurate relative timing information may be obtained by:

  • Asynchronous mode – Each eNB is connected to a local GPS receiver to time stamp its own transmissions. The eNB only measures its own time and it does not use GPS as a frame synchronization source.
  • Local synchronized mode – Each eNB is connected to a local GPS receiver and uses GPS to align the eNB frames in the eUTRAN network.
  •  Remote synchronized mode – The eNB is connected to a remote clock synchronization source via a network time protocol (NTP) or a precise time protocol (PTP). The synchronization signals are transmitted over the transmission network to all eNB.

The uncertainty of the position measurement is network-implementation dependent. The uncertainty of the position information is dependent on the method used, the position of the UE within the coverage area, and the activity of the UE. Several design options of the E-UTRAN system (e.g., size of cell, adaptive antenna technique, path loss estimation, timing accuracy, eNode B surveys) allow the network operator to choose a suitable and cost-effective UE positioning method for their market. The uncertainty may vary between networks as well as from one area within a network to another, based on the clutter type. The uncertainty may be hundreds of meters in some areas and only a few meters in others. In the event that a particular position measurement is provided through a UE-assisted process, the uncertainty may also depend on the capabilities of the UE. In some jurisdictions, there is a regulatory requirement for location service accuracy that is part of an emergency service .

A sample regulatory requirement for LBS accuracy is 300m for 90% percentile and 150m for 65% percentile. These values are an average accuracy requirement for a network. The accuracy for each clutter will be different due to the inter-site distance. Therefore, with dense urban and urban clutters the accuracy is expected to be better than the values mentioned above, while with suburban and rural, it will be degraded below these values.

There are many different possible uses for the positioning information. The positioning functions may be used internally by the EPS, by value-added network services, by the UE itself or through the network, and by “third party” services. It may also be used by an emergency service (which may be mandated or “value-added”), but the location service is not exclusively for emergencies.

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