LTE OTDOA Positioning Reference Signals

36.211 §6.10.4 Downlink PRS

OTDOA is a feature to determine the location of a User Equipment based on radio access network information
Downlink OTDOA assistance provides the UE the cell PRS position for intra or inter-frequency RSTD measurements
Positioning reference signals are transmitted on antenna port 6
The PRS sequences use QPSK modulation (2 bits per symbol)

Cell-specific Reference Signal
Observed Time Difference Of Arrival
Physical Cell Identity
Physical Resource Block
Positioning Reference Signal
Resource Element
Reference Signal Time Difference Measurement
Serving Mobile Location Center
User Equipment

Graphical view of PRS in one radio frame (10 ms)

Bandwidth = 6 PRBs, Cyclic prefix = Normal, Antennas = 2, PCI = 0, IPRS index = 12, NPRS subframes = 4

OTDOA frame

SMLC sends LPP (36.355) OTDOA ProvideAssistanceData message to the UE

LPP OTDOA-NeighbourCellInfoElement
   physCellId INTEGER (0..503)
   cellGlobalId ECGI OPTIONAL
   earfcn INTEGER (0..65535) OPTIONAL
   cpLength ENUMERATED { normal, extended }
   antennaPortConfig ENUMERATED { ports-1-or-2, ports-4 } OPTIONAL
   slotNumberOffset INTEGER (0..19) OPTIONAL
   prs-SubframeOffset INTEGER (0..1279) OPTIONAL
   expectedRSTD INTEGER (0..16383)
   expectedRSTD-Uncertainty INTEGER (0..1023)
   prsInfo PRS-Info OPTIONAL
      prs-Bandwidth ENUMERATED { n6, n15, n25, n50, n75, n100 }
      prs-ConfigurationIndex INTEGER (0..4095)
      numDL-Frames ENUMERATED { sf-1, sf-2, sf-4, sf-6 }
      prs-MutingInfo-r9 CHOICE { po2 BIT STRING (SIZE(2)), po4 BIT STRING (SIZE(4)),
                                 po8 BIT STRING (SIZE(8)), po16 BIT STRING (SIZE(16)) } OPTIONAL

SMLC sends LPPa (36.455) OTDOA InformationRequest message to the eNbs

LPPa OTDOACell-Information-Item
   pCI INTEGER (0..503)
   cellId ECGI
   tAC TAC
   eARFCN INTEGER (0..65535)
   pRS-Bandwidth ENUMERATED { bw6, bw15, bw25, bw50, bw75, bw100 }
   pRS-ConfigurationIndex INTEGER (0..4095)
   cPLength ENUMERATED { normal, extended }
   numberOfDlFrames NumberOfDlFrames ENUMERATED { sf1, sf2, sf4, sf6 }
   numberOfAntennaPorts ENUMERATED { n1-or-n2, n4 }
   pRSMutingConfiguration CHOICE { two BIT STRING (SIZE(2)), four BIT STRING (SIZE(4)),
                                   eight BIT STRING (SIZE(8)), sixteen BIT STRING (SIZE(16)) }

UE measures Reference Signal Time Difference

For instance, with three eNbs (two dimensional coordinates)
   Reference eNb1 (x1, y1), neighbor eNb2 (x2, y2) and eNb3 (x3, y3)

Distance between the UE (x, y) and the reference eNb1
   d1 = sqrt( (x1 - x)2 + (y1 - y)2 )
   d1 = c * t1 (c is the speed of light)

The UE measures the Time Difference Of Arrival of PRS between the reference eNb1 and the two neighbor eNb2 and eNb3
   t2 - t1 = (d2 - d1) / c
   t3 - t1 = (d3 - d1) / c

The radio frame of all the eNbs on one frequency layer need to be synchronized in time, so that the PRS occasions of all the eNbs are aligned in time

UE sends LPP (36.355) OTDOA ProvideLocationInformation to the SMLC

The UE reports the relative timing difference between neighbor cells and the RSTD reference cell

LPP OTDOA-SignalMeasurementInformation
  systemFrameNumber BIT STRING (SIZE (10))
  physCellIdRef INTEGER (0..503)
  cellGlobalIdRef ECGI OPTIONAL
  referenceQuality OTDOA-MeasQuality OPTIONAL
  neighbourMeasurementList NeighbourMeasurementList
    physCellIdNeighbour INTEGER (0..503)
    cellGlobalIdNeighbour ECGI OPTIONAL
    earfcnNeighbour ARFCN-ValueEUTRA OPTIONAL
    rstd INTEGER (0..12711)
    rstd-Quality OTDOA-MeasQuality

PRS subframe configuration

PRS configuration index
PRS periodicity
Tprs (subframes)
PRS subframe offset
Δprs (subframes)
0 – 159 160 Iprs
160 – 479 320 Iprs - 160
480 – 1119 640 Iprs - 480
1120 – 2399 1280 Iprs - 1120
2400-4095 reserved

Graphical view of PRS in one SFN period (1024 frames of 10ms)

IPRS index = 12, NPRS subframes = 4, PRS muting sequence = 0011
If a bit in the PRS muting sequence is set to "0", then the PRS is muted in the corresponding PRS positioning occasion

PRS period