UE Positioning function is required to provide the mechanisms to support or assist the calculation of the geographical position of a UE. UE position knowledge can be used, for example, in support of Radio Resource Management functions, as well as location-based services for operators, subscribers, and third-party service providers.

Positioning functionality provides a means to determine the geographic position and/or velocity of the UE based on measuring radio signals.  The position information may be requested by and reported to a client (e.g., an application) associated with the UE, or by a client within or attached to the core network. The position information is reported in standard formats, such as those for cell-based or geographical co-ordinates, together with the estimated errors (uncertainty) of the position and velocity of the UE and, if available, the positioning method (or the list of the methods) used to obtain the position estimate.

Several design options of the LTE E-UTRAN system (e.g., size of cell, adaptive antenna technique, pathloss estimation, timing accuracy, eNode B surveys) would allow the network operator to choose a suitable and cost-effective UE positioning method for their market.

Positioning the UE involves two main steps:
-    signal measurements
-    Position estimate and optional velocity computation based on the measurements.
The signal measurements may be made by the UE or the eNode B.

The standard positioning methods supported for E-UTRAN access are:
-    network-assisted GNSS (Global Navigation Satellites Systems) methods
-    downlink positioning
-    enhanced cell ID method.
Hybrid positioning using multiple methods from the list of positioning methods above is also supported.

E-UTRAN UE Positioning Architecture

Above figure shows the architecture in EPS applicable to positioning of a UE with E-UTRAN access.

The MME receives a request for some location service associated with a particular target UE from another entity (e.g., GMLC, eNB, or UE) or the MME itself decides to initiate some location service on behalf of a particular target UE (e.g., for an IMS emergency call from the UE). The MME then sends a location services request to an E-SMLC. The E-SMLC processes the location services request which may include transferring assistance data to the target UE to assist with UE-based and/or UE-assisted positioning and/or may include positioning of the target UE. The E-SMLC then returns the result of the location service back to the MME (e.g., a position estimate for the UE and/or an indication of any assistance data transferred to the UE). In the case of a location service requested by an entity other than the MME (e.g., UE, eNB, or E-SMLC), the MME returns the location service result to this entity.

The SLP is the SUPL entity responsible for positioning over the user plane.  

source : 3GPP 3605-900