UE positioning is an access network function (e.g. GERAN, UTRAN, E-UTRAN). An access network may support one or more UE positioning methods, which may be same or different from another access network. In E-UTRAN the following UE positioning methods are supported:
- Cell ID positioning method
- Enhanced Cell ID based positioning method
- OTDOA positioning method
- Network assisted GNSS (A-GNSS) positioning methods
Determining the position of a UE involves two main steps:
1. Radio signal measurements
2. Position estimate computation and optional velocity computation based on the measurements
The signal measurements may be made by the UE or the E-UTRAN. Both TDD and FDD radio interface will be supported in E-UTRAN. The basic signals measured for terrestrial position methods are typically the E-UTRA radio transmissions. Also other transmissions such as general radio navigation signals including those from Global Navigation Satellites Systems can also be measured. The position estimate computation may be made in the UE or in the E-SMLC. In UE-assisted positioning the UE perform the downlink radio measurements and the E-SMLC estimates the UE position while in UE-based positioning the UE performs both the downlink radio measurements and also the position estimation. The UE may require some assistance from the network in the form of assistance data in order to perform the downlink measurements and these are provided by the network either autonomously or upon UE requesting it.
The E-UTRAN positioning capabilities are intended to be forward compatible to other access types and other position methods, in an effort to reduce the amount of additional positioning support needed in the future.
CELL ID METHOD
This is the simplest of all positioning methods but the UE position is very coarse in that only the serving cell where the UE is located is provided. As E-UTRAN and MME are involved in the mobility management (e.g. tracking area update or paging) of UEs the serving base station and serving cell of the UE is always known especially when there is signaling between the E-SMLC and the UE to query the UE position.
ENHANCED CELL ID‐BASED METHOD
In this method the position obtained by the Cell ID method is enhanced through means of use of other UE or E-UTRAN measurements to estimate the UE position with better accuracy than the Cell ID method. The measurements used may be radio resource measurements or other measurements. The E-SMLC does not configure these measurements in the UE/E-UTRAN but only queries the UE/E-UTRAN for these measurements and obtains them if available in the UE/E-UTRAN.
NETWORK ASSISTED GNSS METHODS
In network assisted GNSS methods the network provides various assistance data to the UE that are equipped with radio receivers capable of receiving GNSS signals. The UEs use the assistance data provided by the network to help perform measurements. Examples of GNSS include: GPS, Modernized GPS, Galileo, GLONASS, Space Based Augmentation Systems (SBAS) and Quasi Zenith Satellite System (QZSS). Different GNSS can be used separately or in combination to determine the position of a UE.
The OTDOA method is a downlink terrestrial positioning method. In this method the UE performs measurements of downlink signals of neighbor E-UTRAN cells. This is a good backup method for positioning the UE when satellite signals are not strong enough (e.g. indoors or bad atmospheric conditions etc). The UE receives the downlink radio transmission of four or more neighbor cells, aided by downlink reference signal transmissions from those cells and measures the time difference of arrival of the radio frames of the measured neighbor cells relative to the serving cell. These UE measurements are then used either by the UE or by the E-SMLC to estimate the UE position using a trilateration technique.
The E-UTRAN may combine two or more of the supported UE positioning methods and perform a hybrid positioning estimation to achieve a better positioning accuracy.
The UE positioning protocol is an end-to-end protocol with terminations in the UE and the E-SMLC(Enhanced Serving Mobile Location Center). This protocol is called the LTE Positioning Protocol (LPP). This is a transaction-oriented protocol with exchange of LPP messages between UE and E-SMLC where one or more messages realize each transaction. A transaction results in one activity or operation such as assistance data transfer, UE positioning capability transfer or position measurement/estimate exchange. There is a second UE positioning protocol, LPPa, with terminations in the E-UTRAN and E-SMLC that allows the exchange of information and measurements, which are useful for some specific positioning methods. Currently, the LPPa is used for the delivery of timing information that is resident only to the E-UTRAN and/or is semidynamically changing, which is required for the OTDOA positioning method. Apart from this the LPPa also supports the exchange of E-UTRAN assisted measurements that are used for the Enhanced Cell ID positioning method.