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)

CRS

Cell-specific Reference Signal

OTDOA

Observed Time Difference Of Arrival

PCI

Physical Cell Identity

PRB

Physical Resource Block

PRS

Positioning Reference Signal

RE

Resource Element

RSTD

Reference Signal Time Difference Measurement

SF

Subframe

SMLC

Serving Mobile Location Center

UE

User Equipment

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

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

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
  earfcnRef ARFCN-ValueEUTRA 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

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

I_PRS index = 12, N_PRS 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