Feasibility Studies of Time Synchronization Using GNSS Receivers in Vehicle to Vehicle Communications. Queensland University of Technology

Transcription

1 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

2 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

3 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

4 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 1μs to 100 μs per second which implies a range of the deviation about 5 to 15 seconds per day. 4

5 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

6 2.1 Timing requirements for Vehicular Applications Concept tier that illustrates the requirements of time synchronization accuracy for different applications in VANET. 6

7 Example 1: Scheduling of Channels DSRC Features 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 (a) DSRC Frequency allocation [US], (b) Channel Synchronization SCH Service Msg e.g., IP based, pay to gas etc. 7

8 Example 2: Guard Interval in DSRC Scheduling of Channels and Figure 3: Guard Interval Requirement N i and N j are communicating each others with independent time offsets from a common reference time Δt i and Δt j respectively. While N j send a burst to N i, the observed time offsets (Δt ij ) between them can be estimated as: Δt ij = Δt j Δt i + d ij /c where, d ij is the distance between two nodes and c is the speed of light. Δt ij < T GI 8

9 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 finegrained 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. 9

10 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

11 Timing accuracy and requirements Second level Millisecondlevel Microsecond level Nano second level Turn to turn Navigation LBS Network coordination Relative positioning Channel scheduling Security Cooperative manoeuvre Cooperative sensing 1 sec 1ms 1sec 1us 1ms 1ns 1us Cooperative positioning 11

12 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 p &

13 Timing synchronization function Undefined situation using TA mechanism in pure ad-hoc communication. TSF Synchronization: Adaptive TSF* Multi Hop TSF* nodes nodes 22.4 nodes 39.1 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 Springer. 13

14 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

15 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. Schematic Diagram of the Experimental setup Device: 200MHz Agilent Technology DSO-X 2014 A Oscilloscope. Recording & Analysing: Lab-View software hosted in a Laptop 15

16 Results & Discussion: 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. 16

17 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

18 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. 18

19 Result and Discussion: 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 Eg. Position errors of 300m will affect the clock solution up to 1 us 19

20 Laboratory Test to define Clock Drift in Absence of GNSS signal Experimental Setup Result and Discussion: Figure Schematic Diagram of the experimental set-up between three nodes. The longest tunnel in Australia is 5.25km. 20

21 5. Conclusions Overall GNSS Time Synchronization Solutions Scenarios Condition GNSS Time Synchronization Accuracy Ideal NSAT>=4, GDOP<=6 Full Support 30ns Occasional Loss Blockage (Under Tunnel) NSAT=1~3 GDOP is bad NSAT=0 Good Timing Support Supports up to certain time 300m location error introduce additional 1μs Depend upon the outage time; for 5Km roughly 5~6μs 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

22 For your attention 22