Positioning Reference Signals (PRS) in LTE

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Positioning Reference Signals (PRS) in LTE

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There has been a lot of action in Release 8 and Release 10 of UMTS (Universal Mobile Telecommunication System).  Release 8 introduces LTE (Long Term Evolution), while Release 10 defines LTE-Advanced.  Release 9 does not get much limelight amidst the comprehensive volume of work done in these two releases.  However, Release 9 does have its place in history- among other things, it provides a framework for finding the UE location (so-called UE positioning) to support a variety of location services (LCS) (e.g., emergency calls and directions to a restaurant).  Release 9 specifies a variety of UE positioning techniques essential to the offering of the LCS.  We will focus on Positioning Reference Signals (PRS) that can be used to facilitate determination of the position or location of the UE by a UE-assisted positioning technique.  A UE-assisted positioning technique implies the following: (i) the UE makes some radio signal measurements, and (ii) the network determines the UE location (e.g., latitude and longitude) by processing the measurements reported by the UE.

Let's briefly discuss the PRS now.  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.

References: TS36.211, TS 36.133, TS36.214, TS36.331, TS23.271, TS36.305

  • Hi,  really helpful article.... i had some follow-up questions...

    In the specs it says that the frequency coordinate "k" is

    determined by something like

    6*(m + Ndl_RB - Nprs_RB)...

    i am not very clear about Nprs_RB, what it actually means, isnt

    Nprs_RB and Ndl_RB are same in this case?

    Also how is this equation taking care of the fact that it shouldnt

    write over the RE's reserved for PBCH and PSS and SSS?