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

Communications

Khondokar Fida HasanProfessor Yanming FengProfessor 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-V2XV2V 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 slottedinto 7 Channels

10 MHz Guard Band

1 Control Channel6 Service Channels

70 MHz is slottedinto 7 Channels

CCH Safety Msg

SCH Service Msge.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 others with independent time offsets from a common reference time Δti and Δtj respectively.

While Nj send a burst to Ni, the observed 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 RequirementsApplications 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 requirements1 sec 1ms

‐1sec1us‐1ms

1ns‐1us

Second‐level Turn‐to‐turnNavigation

LBS

Millisecond‐level

Networkcoordination

Relativepositioning

Microsecond‐level

Channelscheduling

Security

Cooperativemanoeuvre

Nano‐second‐level

Cooperative sensing

Cooperative positioning 11

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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 nodes500.2 μs @60 nodes

Multi‐Hop TSF* 22.4 μs @20 nodes39.1 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 FurunoAntenna: 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 inChallenging 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

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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 outage time; for 5 Km 

roughly 5~6μs

Overall GNSS Time Synchronization Solutions

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For your attention