Positioning Reference Signals (PRS) in LTE

The PRS are transmitted on antenna port 6 and do not use the resource elements allocated to Physical Broadcast Channel, Primary Synchronization Signal, and Secondary Synchronization Signal.  The PRS are sent in a configurable number of consecutive subframes, which could be just one subframe or as many as 5 subframes.  The E-UTRAN configures the PRS bandwidth (e.g., a certain number of resource blocks) and the periodicity of the PRS (e.g., one PRS occurrence every 160 subframes).  Within a subframe containing the PRS, the PRS are transmitted on more subcarriers and more OFDM symbols when compared to the regular cell-specific reference signals being sent on an antenna.  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 value of Cyclic Prefix.  The UE observes the PRS from different cells in the neighborhood 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 E-UTRAN processes these OTDOA measurements from the UE in an implementation-specific and non-standardized manner to estimate the UE location.

In summary, the existence of the PRS enables the UE to make OTDOA measurements.  These measurements are then used by the E-UTRAN to determine the UE location as part of a UE-assisted positioning technique.