Positioning Reference Signal PRS LTE

Positioning Reference Signal (PRS) is taken into consideration in one of the LTE release 9 features to determine the location of User Equipment (UE) based on radio access network information. Now you might be thinking that what is the necessity of PRS,  if we have a GPS technology already built in smartphones and in other cellular equipment. Just think of it  J (GPS may not be accurate always and GPS services may not be available all around the geographical areas , also the accuracy of functioning GPS depends on money you have paid for the services and the quality of GPS device).  The end user application of this PRS feature could be supporting location based services which can be navigation (direction to hotel etc.), emergency call etc.  

Process of finding UE location using PRS:

The overall process of finding UE locations are based on three major steps.

Step 1. UE receive PRS from cells (Reference cell and Neighbor cells) 

Step 2. Based on received PRS, UE may measure observed time difference of arrival (OTDA) and report RSTD (mentioned in my previous article of LTE UE Measurement RSRP RSSI RSRQ RSTD) to cell.

Step 3. Based on UE reported reference signal time difference (RSTD), eNodeB may calculate the longitude and latitude of the UE (which can be based on any specific algorithm, not standardized).

Positioning Reference Signal is transmitted in downlink subframes (as per higher layer configuration, discussed later in this article) on antenna port 6.  The PRS should not be sent on resource element used for PBCH, PSS or SSS. The PRS sequence will be generated on the basis of slot number, OFDM symbol number, cell ID, normal CP or extended CP.

Position of PRS in terms of OFDM symbol (Resource Element):

If both MBSFN (Multicast Broadcast Single Frequency Network) and normal downlink subframes are configured for PRS, the OFDM symbol configured for PRS uses the same cyclic prefix as subframe 0.

If only MBSFN subframe is configured for PRS, the OFDM symbol configured for PRS will use extended cyclic prefix.

The starting position of PRS OFDM symbol in a subframe will be identical to those in a subframe in which all OFDM symbols have the same CP length as PRS OFDM symbols; iff the subframe is configured for PRS transmission (PRS subframe configuration explained below). For more detail of mapping PRS resource element into resource grid, please refer 3GPP 36.211 section 6.10.4.2 for both normal CP and extended CP.

PRS subframe configuration (PRS periodicity, PRS subframe, PRS Configuration Index, Number of Consecutive PRS subframe):

From specs 36.211 of release 9, the configurations of PRS subframe are explained below where:         Nprs is number of consecutive downlink subframe with PRS (Configured by higher layers may be 1,2,4 or 6 subframes)

Iprs is the PRS Configuration Index (can be any value between 0-2399, values 2400 to 4095 are reserved)

Tprs is the periodicity of PRS in terms of subframes. This could be one value among 160, 320, 640 or 1280 depending on configuration of Iprs.

Dprs is the delta PRS subframe offset (can be Iprs, Iprs-160, Iprs-480 or Iprs-1120), depending on Iprs configuration index.

Please refer below table for composed values of Iprs, Tprs, Dprs:

Now take one example to understand the PRS subframe configuration:

Suppose the Nprs is configured by higher layers is 2 and Iprs is configured as 160. Hence from the above table the value of Dprs will be 0. The PRS instances for the first DL subframe of Nprs shall satisfy below formula:

(10*Nf+ ceiling_func_of Ns/2-Dprs) mod Tprs=0 --------Equation 1

Where Nf is system frame number and Ns is slot number.

Hence put the values of Tprs and Dprs in the above equation:

(10*Nf+ ceiling_func_of Ns/2-0) mod 320=0------------Equation 2

Hence for all the values of Nf and Ns which satisfy the Equation 2, will be the first downlink subframe which carry PRS. Hence Equation 2 satisfied for Nf value 32 and Ns (slot) value 0 (that is subframe #0). Hence the first subframe which carries PRS will be subframe 0 of system frame 32 and subframe 1 of system frame 32 (because Nprs is configured as 2 from higher layer for accuracy of consecutive PRS).

Questions are welcome.