Feasibility Studies of Time Synchronization Using GNSS Receivers in - - PowerPoint PPT Presentation

Feasibility Studies of Time Synchronization Using GNSS Receivers in Vehicle‐to‐Vehicle Communications

Khondokar Fida Hasan

Professor Yanming Feng Professor Glen Tian

Queensland University of Technology

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Agendas

• 1. Background: V2X and motivation of research
• 2. Requirement analysis of Time Synchronization in

Vehicular Networks

• 3. Non-GNSS Vs GNSS Time Synchronization
• 4. Feasibility analysis of GNSS Time

Synchronization: accuracy, availability

• 5. Conclusions

1.1 Background: Vehicular Networks

DSRC-V2X V2V and V2I Scenario.

Cellular ‐V2X

*The radio interface between the UE and the Node B is called Uu

1.2 Why Timing is needed

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‘Time’ is one of the important and fundamental parameters for successful

communication in a wireless network and its accuracy is highly responsible for many applications to be effective like active safety applications

Clock: 1. Atomic Clock 2. Quartz Clock (Commonplace)

In general, every physical clock drifts away from the actual day time by 1μs to 100 μs per second which implies a range of the deviation about 5 to 15 seconds per day.

1.3 Motivation of Research

• Time Synchronization in other networks

– In computer networks: NTP – In industry control: PTP – In WLAN: Time Advertisement (TA)

• In vehicular networks:

– Dynamics and mobility – Various applications, various requirements – In DSRC standards: GPS provides UTC time and TA – Less studied, least understood

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Concept tier that illustrates the requirements of time synchronization accuracy for different applications in VANET.

2.1 Timing requirements for Vehicular Applications

Example 1: Scheduling of Channels

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(a) DSRC Frequency allocation [US], (b) Channel Synchronization

75 MHz @ 5.9 GHz

70 MHz is slotted into 7 Channels 10 MHz Guard Band

1 Control Channel 6 Service Channels

70 MHz is slotted into 7 Channels CCH Safety Msg SCH Service Msg e.g., IP based, pay to gas etc.

DSRC Features

Example 2: Guard Interval in DSRC

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Figure 3: Guard Interval Requirement Ni and Nj are communicating each

• thers with independent time
• ffsets from a common reference

time Δti and Δtj respectively. While Nj send a burst to Ni, the

• bserved time offsets (Δtij) between

them can be estimated as:

Δtij = Δtj – Δti + dij/c

where, dij is the distance between two nodes and c is the speed of light.

Δtij < TGI

Scheduling of Channels and

Example 3: Security

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Precise time synchronization is a key tool for development of traceable and reliable communications. This allows reconstruction of the packet sequence on the channel, and thus effectively helps overcome the threats. It is indicated that a fine- grained analysis of channel activity between concurrent transmissions requires stringent timing guarantees of 8μs

Example of Security issue. (a) Cyber Forensic, (b) Cyber Attack (Security). In both of the cases time synchronization is important to log the events accurately .

2.3 Summary of Timing Requirements

Applications DSRC‐specific Timing class Accuracy requirements Essential Network coordination No Coarse ~ms Channel scheduling (DSRC‐related) Non‐slotted Coarse <1ms Slotted fine <1µms Relative positioning No <3ms Security No fine <8µs Desirable Cooperative positioning No fine <1ns (ToA) Cooperative manoeuvre No fine <100ns Guard interval (DSRC‐related) Non‐slotted Coarse 11% Slotted Fine <10ns

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Timing accuracy and requirements

1 sec 1ms ‐1sec 1us ‐1ms 1ns ‐1us Second‐level Turn‐to‐turn Navigation LBS Millisecond‐ level Network coordination Relative positioning Microsecond ‐level Channel scheduling Security Cooperative manoeuvre Nano‐second ‐level Cooperative sensing Cooperative positioning

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GNSS offers UTC time solutions at the application layer Time Advertisement based Time Synchronization (at PHY layer)

(a) BSS Communication, Road Side Unit (RSU) sending beacon containing TA frame to synchronize. (b) TA frame is transmitting from RSU to OBU. (c) Time development (transfer) in TA process.

3.1 Existing Time Synchronization Recommendation with DSRC

IEEE 802.11p & 1609.4

Timing synchronization function

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Undefined situation using TA mechanism in pure ad-hoc communication.

Adaptive TSF* 124.5 μs @20 nodes 500.2 μs @60 nodes Multi‐Hop TSF* 22.4 μs @20 nodes 39.1 μs @60 nodes

TSF Synchronization:

*Cheng, X., Li, W., and Znati, T. (2006). Wireless Algorithms, Systems, and Applications: First International Conference, WASA 2006, Xi’an, China, August 15-17, 2006, Proceedings, volume 4138. Springer.

3.2 GNSS time synchronization

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This approach offers five advantages:

• It does not need inter‐vehicle signalling.
• It increases synchronization accuracy.
• Independent of the no. of nodes
• Unaffected with node speed.
• Modern vehicles are already integrated

with GPS .

(a) In-band, Decentralized TS (b) Out-of-Band Centralized TS

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End-to-End time offset between two GNSS receivers through 1PPS output signal:

• Time offset between receivers of the dame model
• Time offset between receivers of different models

Experimental Setup Details: Receiver : Ublox and Furuno Antenna: Active GPS patch Antenna with same length. Device: 200MHz Agilent Technology DSO-X 2014 A Oscilloscope. Recording & Analysing: Lab-View software hosted in a Laptop

4.1 Synchronization Accuracy of 1PPS Signal of Consumer Grade GNSS receivers.

Schematic Diagram of the Experimental setup

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Figure: Time offset distribution of 5 mins data.

Figure: Time offset between receivers of the same model over a long period. Figure: Time offset between receivers of different models over a long period.

Results & Discussion:

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• 4. 2 GNSS time solutions in challenging

environments

• 1. Signal blockages such as unavailability in high-rise Urban

areas

• 2. Signal outage, like under the tunnel or locally failure due to

the GPS jammer or certain other kind of attacks.

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AVAILABILITY OF GNSS TIME SOLUTIONS in

Challenging Environments

• The testing area are selected considering to include different types of environment such as dense

urban canyon surrounding skyscraper, trees, crossing overhead pedestrian ways etc., in Brisbane downtown.

• 19 minutes of 10 Hz data collected.
• Trimble Net R9 used as the reference station, R10 as the rover.

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Vehicle tracks of GPS, BDS and GPS+BDS on high rising roads. The number of satellites under the signal coverage of BDS and GPS. Table: No. of Satellites available with different GNSS services Constellation Table: GDOP with Different GNSS services

Result and Discussion:

• Eg. Position errors of 300m will affect the clock solution up to 1 us

20 Figure Schematic Diagram of the experimental set-up between three nodes.

Laboratory Test to define Clock Drift in Absence of GNSS signal Experimental Setup Result and Discussion: The longest tunnel in Australia is 5.25km.

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• 5. Conclusions
• Consumer grade GPS receiver can serve tens of ns timing

accuracy.

• With multi-GNSS receiver, the availability of time solutions is

much higher than validated position solutions (most 100% vs 80% in Brisbane CBD)

• In general, GNSS can meet essential V2X timing applications

and most of desirable applications

Scenarios Condition GNSS Time Synchronization Accuracy

Ideal NSAT>=4, GDOP<=6 Full Support 30ns Occasional Loss NSAT=1~3 GDOP is bad Good Timing Support 300m location error introduce additional 1μs Blockage (Under Tunnel) NSAT=0 Supports up to certain time Depend upon the

• utage time; for 5 Km

roughly 5~6μs

Overall GNSS Time Synchronization Solutions

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For your attention