First Responder 

Indoor Location Using 

LTE Direct Mode Operations

Howard Fan & Julian Wang

University of Cincinnati

PSIAP July 2019
2

DISCLAIMER

This presentation was produced by guest speaker(s) 

and presented at the National Institute of Standards 

and Technology’s 2019 Public Safety Broadband 

Stakeholder Meeting. The contents of this 

presentation do not necessarily reflect the views or 

policies of the National Institute of Standards and 

Technology or the U.S. Government.

Posted with permission
OVERVIEW

• Indoor location is important to first responders

• GPS signals are not available indoors

• LTE Direct Mode Proximity Service (ProSe) for voice 

communication – mission critical

• We piggy-back location service on LTE ProSe voice 

communication links

• Incorporate Building Information Modeling (BIM) onto location 

& display – critical building info (doors, windows, fire escapes)

OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information 

Modeling (BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information 

Modeling (BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
Form a D2D Wireless Network

• Need a protocol to establish D2D communication

• No such existing protocol

• A simplified multiuser protocol for D2D (MUP-D2D)

§ Assume 6 UEs, each given an ID

§ All start manually at (approximately) the same time

§ A pre-assigned timing diagram is followed

§ During each stage, one UE Tx, all other UEs measure TOAs

§ The Tx signals include 1) synch signal for TOA measurement

2) measured TOA to processing center

Stage UE

1 UE2 UE3 UE4 UE5 UE6

Stage1 Tx UE1-

UE2

UE1-

UE3

UE1-

UE4

UE1-

UE5

UE1-

UE6

Stage2x Tx UE2-

UE3

UE2-

UE4

UE2-

UE5

UE2-

UE6

Stage3x UE3-

UE2

Tx UE3-

UE4

UE3-

UE5

UE3-

UE6

Stage4x UE4-

UE2

x Tx UE4-

UE5

UE4-

UE6

Stage5x UE5-

UE2

xx Tx UE5-

UE6
• UE6 acts as a processing center

Transmit

ter

TOAs calculated

at UE6

TOAs received by UE6 from

other UEs

UE1 UE1-UE6 None

UE2 UE2-UE6 UE1-UE2

UE3 UE3-UE6 UE1-UE3, UE2-UE3

UE4 UE4-UE6 UE1-UE4, UE2-UE4, UE3-UE4

UE5 UE5-UE6 UE1-UE5, UE2-UE5, UE3-UE5,

UE4-UE5

UE2 UE3-UE2, UE4-UE2, UE5-UE2

Stage

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Stage 6
OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information 

Modeling (BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
Measuring TOAs

• LTE has synch signals for comm. purpose (PSS/SSS)
LTE BW

• Simulation performed in the baseband

• USRP test performed in the ISM band (900 MHz)
LTE BW

LTE BW Mode 

(MHz)

Effective Signal BW (MHz)

PSS/SSS PSBCH

1.4 1.08 1.08

3 1.08 2.7

5 1.08 4.5

10 1.08 9

15 1.08 13.5

20 1.08 18

• PSBCH is used 

for demod. 

reference, is a 

known comm. 

overhead signal

• Use PSBCH for 

max BW and 

location accuracy
• This is 5 MHz BW case

• PSBCH is used for 

demodulation reference, 

and is a known comm 

overhead signal

• So can be used for our 

TOA by correlation

• No payload resource is 

used for TOA measurement
Multipath Mitigation

• Multipath is prevalent in indoor propagation

• LOS may also be blocked by walls etc.

• Receive multiple and delayed copies of signal

• Must find the first arrival time for location

• Correlation will give multiple peaks (large delay)

or distorted peaks (small delay)

Method Accuracy Computation Cons

POCS Suboptimum 

solution

Light to medium Costs about 50% more computation

MCC Suboptimum 

solution

Light Setting a threshold is a challenge

NN1: 

SON

Suboptimum 

solution

Light once 

trained

Needs training to cover all possible 

settings

NN2: 

2SSN

Suboptimum 

solution

Light once 

trained

Needs training to cover all possible 

settings

NN3: 

1HCO

Suboptimum 

solution

Light once 

trained

Needs training to cover all possible 

settings

• We investigated many mitigation methods, distilled down to 5:
Simulation 

with an indoor 

channel model

SNR = 5 dB

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

5 10 20

Average TOA Estimate Error (m)

LTE Bandwidth

Average TOA Error vs. LTE Bandwidth

Reference Signal: Custom PRN | SNR: 5 dB

NN1: SON Conventional Correlation MCC

NN2: 2SSN SD POCS

NN3: 1HCO
Simulation 

with an indoor 

channel model

SNR = 15 dB

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

5 10 20

Average TOA Estimate Error (m)

LTE Bandwidth

Average TOA Error vs. LTE Bandwidth

Reference Signal: Custom PRN | SNR: 15 dB

NN1: SON Conventional Correlation MCC

NN2: 2SSN SD POCS

NN3: 1HCO
OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information 

Modeling (BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
The dTDOA Algorithm

Sensor Nodes with 

unknown locations 

have different clockoffsets and drifts

Problem: One way (TOA) and (TDOA) 

by RF ranging signals requires nanosecondlevel clock synchronization to reach meterlevel positioning precision
The dTDOA 

Algorithm

• It takes two Tx and 

two Rx to get a dTDOA 

equation

• Tx do not need to be 

synchronized

• TOAs are measured 

with local 

unsynchronized clocks
The dTDOA Algorithm

Transmitted signal from the mth sensor and received at the 

ith sensor:

Unknown transmission start time

Unknown propagation delay

Unknown clock offset of the ith sensor

Delay at sensor i

s t m m mi i ( ) - - -D tt t

t m

tmi mi = / d c

Dti

mtt t i m mi i = + / +D d c
The dTDOA Algorithm

• Two Tx and two Rx:

• Form the dTDOA (double difference):

A A AC AD

C A CD A D

B B BC BD

C B CD B D

d d

c c

d d

c c

t t tt t t

t t tt t t

= + +D = + +D

= + +D = + +D

( )( )

( )/

AB A B A A B B

CD CD CD C D C D

d d d dc AC AD BC BD

t t t tt tt = - =- - -

= --+
The dTDOA Algorithm

• With N sensors, N(N-3)/2 Independent equations

• In 2-D, 5 sensors à 5 independent equations

6 sensors à 9 independent equations

7 sensors à 14 independent equations

• Can only find relative locations

• Some sensors need to be “anchored”

( )( )

( )/

AB A B A A B B

CD CD CD C D C D

d d d dc AC AD BC BD

t t t tt tt = - =- - -

= --+
Implementation of dTDOA Using LTE ProSe

• Only one Tx can transmit at any given time to avoid collision
• This may cause a receiver to mix its clock offset with an 

unknown receive time from another Tx 
Solution

• Each Rx must set an arbitrary 

time reference point (e.g. time 

the first sample is received)

• Each Rx clock keeps running

without stop

• All TOAs are measured w.r.t. 

this time reference

• So that its unknown clock 

offset is kept unchanged Dti
dTDOA Simulation

• Simulated 6 nodes in a 100mx100m 2D space

• Two nodes are fixed, with known locations

• Four nodes are at unknown locations

• Simulated noisy and multipath TOA measurements to 

come up with 9 independent dTDOA equations

• Solved 8 unknowns using iterative Levenberg-Marquardt 

algorithm 

( )/ AB tCD AC AD BC BD = --+ d d d dc
dTDOA 

Simulation

SNR = 10 ~ 25 dB 

varies among UEs

ITU indoor 

channel model

5 MHz BW
dTDOA 

Simulation

SNR = 10 ~ 25 dB 

varies among UEs

ITU indoor 

channel model

20 MHz BW
OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information 

Modeling (BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
BIM-related Research Work

Objectives:

• Investigate extraction of BIM (Building Information 

Modeling) data including building geometric and semantic

information, and incorporate into the proposed location 

service.

• Develop a BIM– based interactive 3D indoor model 

viewing method for first responder use.
BIM-related Research Work

• 38% Architecture, Engineering and Construction users use 

BIM now, and it is expected to increase to 54% in the next 3-

5 years. 

• The unique feature of a BIM-compliant database is that it 

contains geometric and semantic information of all 

building elements and fixtures, which would provide 3D 

mapping and visualization and for indoor emergency 

decision support. 

Context:
Approaches and Workflow

BIM data structure 

investigation

Emergency-related data 

extraction algorithms

Geometric data Semantic data

Location 

measurement and 

computation

Simplified 3D model Data Visualization and Scale

Interactive 3D model viewer Location information 

and coordinates 

Location information 

integration and display
BIM Data Structure Investigation

Add a screenshot in Revit 

interface showing where the 

data - wall fire rating hours is 

structured

• Took the International Fire Code, National Fire Protection Association Fire Code, and International 

Building Code 

• Distilled a clear sense of how building egress-related elements and information are set and defined. 
BIM Data Structure Investigation

Fire rating hours is 

structured in wall-typeproperties/identity data

How is emergency-related data structure in BIMcompliance dataset
Emergency-Related Data Extraction

1

2

4

5

1. The Categories node is used to select the Wall (element) whose data has to be extracted.

2. All Elements Of Category node selects all the elements of the selected categories that are present in the current Revit file.

3. Element Get parameter value by name uses a string input (wall type and element ID) and extracts all data corresponding to 

the selected element.

4. A list of all the elements’ data is created using the List Create node.

5. File Path node is used for the data file type selection and output. 

Workflow of extracting semantic data

3

Example of building wall extraction Extracted building wall list
Workflow of extracting semantic data

Example of building wall’s fire rating extraction Extracted fire rating hour list

1. The Categories node is used to select the Wall (element) whose data has to be extracted.

2. All Elements Of Category node selects all the elements of the selected categories that are present in the current Revit file.

3. Element Get parameter value by name uses a string input (Fire Rating) and extracts all data corresponding to the selected 

element.

4. A list of all the elements’ data is created using the List Create node.

5. File Path node is used for the data file type selection and output.

1

2

3 5

4

Emergency-Related Data Extraction
Workflow of extracting geometric data

1. Geometry Walls, Windows and Stairs can be extracted by selecting the elements using the Categories, All Elements of 

Category and then running it after connecting it to the Element Geometry node.

2. Generate Simplified Selected 3D Model.

3. Element Parameter gives details such as volume, height, type of the selected elements.

1

2

3

Examples of wall, windows, and stairs extraction 

1

2

3

1

2

3

Emergency-Related Data Extraction
BIM-Indoor

(BIMIL) 

Location User 

Interface 

Portal

With user 

locations 

integrated
BIM-Indoor Location User Interface

• To select a building BIM model 

to display, click the “Select 

Building Model” button.

• To select an emergency-related 

semantic data file, click the “Select 

Building Info File” button.

• When ready, press the “Start 

Simulation” button to start viewing 

the building file.
BIM-Indoor Location User Interface
BIM-Indoor Location User Interface
BIM-Indoor Location User Interface
OBJECTIVES

• Form an ad hoc ProSe network that establishes D2D direct 

communication among UEs

• Measure ranging signal TOA among UEs w. comm. signals

• Use the dTDOA algorithm to find locations of all users

• Extract critical information from Building Information Modeling 

(BIM) data base

• Display all user locations on building layout with critical 

information (doors, windows, etc.)

• Test on USRP software defined radios & display with BIM-IL
USRP Implementation
OTA Transmitted Signal
OTA Received Signal
Implementation Details

• 6 USRPs in 2D, 2 anchored

• Implemented in MATLAB for simplicity

• BW = 5 MHz

• Used the MCC alg. for multipath mitigation for 

simplicity 

• Displayed on the conference demo site layout, 

no BIM data available for this building
Conf. 

Demo 

Site 

Layout
The team