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
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 2
1.1 Background: Vehicular Networks Cellular ‐ V2X DSRC-V2X V2V and V2I Scenario. *The radio interface between the UE and the Node B is called Uu
1.2 Why Timing is needed ‘ 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 1 μ s to 100 μ s per second which implies a range of the deviation about 5 to 15 seconds per day. 4
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 5
2.1 Timing requirements for Vehicular Applications Concept tier that illustrates the requirements of time synchronization accuracy for different applications in VANET. 6
Example 1: Scheduling of Channels DSRC Features 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. (a) DSRC Frequency allocation [US], (b) Channel Synchronization 7
Example 2: Guard Interval in DSRC N i and N j are communicating each others with independent time offsets from a common reference time Δ t i and Δ t j respectively. Scheduling of Channels and While N j send a burst to N i , the observed time offsets ( Δ t ij ) between them can be estimated as: Figure 3: Guard Interval Requirement Δ t ij = Δ t j – Δ t i + d ij /c where, d ij is the distance between two nodes and c is the speed of light. Δ t ij < T GI 8
Example 3: Security 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 8 μ s 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 . 9
2.3 Summary of Timing Requirements Applications DSRC ‐ specific Timing class Accuracy requirements Essential Network coordination No Coarse ~ms Channel scheduling Non ‐ slotted Coarse <1ms (DSRC ‐ related) 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 Non ‐ slotted Coarse 11% (DSRC ‐ related) Slotted Fine <10ns 10
Timing accuracy and requirements 1 sec 1ms 1us 1ns ‐ 1sec ‐ 1ms ‐ 1us Second ‐ level Turn ‐ to ‐ turn Navigation LBS Millisecond ‐ Network level coordination Relative positioning Microsecond Channel ‐ level scheduling Security Cooperative manoeuvre Nano ‐ second Cooperative ‐ level sensing Cooperative positioning 11
3.1 Existing Time Synchronization Recommendation with DSRC 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. IEEE 802.11p & 1609.4 12
Timing synchronization function Undefined situation using TA mechanism in pure ad-hoc communication. TSF Synchronization: 124.5 μ s @20 nodes Adaptive TSF* 500.2 μ s @60 nodes 22.4 μ s @20 nodes Multi ‐ Hop TSF* 39.1 μ s @60 nodes *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. 13
3.2 GNSS time synchronization 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 14
4.1 Synchronization Accuracy of 1PPS Signal of Consumer Grade GNSS receivers . 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: Schematic Diagram of Lab-View software hosted in a Laptop the Experimental setup 15
Results & Discussion: Figure: Time offset distribution of 5 mins data. Figure: Time offset between receivers of different Figure: Time offset between receivers of the same models over a long period. model over a long period. 16
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. 17
A VAILABILITY OF GNSS T IME S OLUTIONS 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. 18
Result and Discussion: The number of satellites under the signal Vehicle tracks of GPS, BDS and GPS+BDS on high coverage of BDS and GPS. rising roads. Table: No. of Satellites available with different Table: GDOP with Different GNSS services GNSS services Constellation Eg. Position errors of 300m will affect the clock solution up to 1 us 19
Laboratory Test to define Clock Drift in Result and Discussion: Absence of GNSS signal Experimental Setup Figure Schematic Diagram of the experimental set-up between three nodes. The longest tunnel in Australia is 5.25km. 20
5. Conclusions Overall GNSS Time Synchronization Solutions Scenarios Condition GNSS Time Accuracy Synchronization Full Support Ideal NSAT>=4, GDOP<=6 30ns Good Timing Occasional Loss NSAT=1~3 300m location error introduce additional 1 μ s GDOP is bad Support Supports up to Blockage NSAT=0 Depend upon the (Under Tunnel) certain time outage time; for 5 Km roughly 5~6 μ s • 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 21
For your attention 22
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