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Energy-Efficient Time Synchronization Achieving Nanosecond Accuracy in Wireless Networks

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Invited talk, 2016 International Conference on Internet of Things and 5G Mobile Technologies (2016 ICIOT-5GMT), Guangzhou University, Guangzhou, Nov. 27-28, 2016.

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Energy-Efficient Time Synchronization Achieving Nanosecond Accuracy in Wireless Networks

  1. 1. Energy-Efficient Time Synchronization Achieving Nanosecond Accuracy in Wireless Networks Kyeong Soo (Joseph) Kim (With S. Lee and E. G. Lim@XJTLU) Department of Electrical and Electronic Engineering Xi’an Jiaotong-Liverpool University 2016 ICIOT-5GMT Guangzhou University 27-28 November, 2016 1 / 49
  2. 2. Outline Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Simulation Results Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 3 / 49
  3. 3. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 4 / 49
  4. 4. An Asymmetric Wireless Network HeadeNode Internet RemoteeUser SensoreNodes 5 / 49
  5. 5. Head Node A base station that serves as a gateway between wired and wireless networks. A center for fusion of data from distributed sensors. Equipped with a powerful processor and supplied power from outlet. 6 / 49
  6. 6. Head Node A base station that serves as a gateway between wired and wireless networks. A center for fusion of data from distributed sensors. Equipped with a powerful processor and supplied power from outlet. 6 / 49
  7. 7. Head Node A base station that serves as a gateway between wired and wireless networks. A center for fusion of data from distributed sensors. Equipped with a powerful processor and supplied power from outlet. 6 / 49
  8. 8. Sensor Node Measuring data and/or detect events with sensors and connected to a WSN only through wireless channels. Limited in processing and battery-powered. 7 / 49
  9. 9. Sensor Node Measuring data and/or detect events with sensors and connected to a WSN only through wireless channels. Limited in processing and battery-powered. 7 / 49
  10. 10. Design Goals Achieving sub-microsecond time synchronization accuracy Through propagation delay compensation. With higher energy efficiency at battery-powered sensor nodes Minimize the number of packet transmissions and the amount of computation at sensor nodes. 8 / 49
  11. 11. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 9 / 49
  12. 12. Effects of Time and Space The effects of time and space are so closely related that they cannot be easily separated from each other as in the following examples: Synchronization and localization accuracies. In time-based localization. e.g. Time of arrival (TOA). Clock offset and propagation delay. In one-way synchronization. e.g. Flooding time synchronization protocol (FTSP). 10 / 49
  13. 13. Synchronization and Localization Accuracies Accuracies 1 ms ↔ 300 km 1 µs ↔ 300 m 1 ns ↔ 30 cm 1 ps ↔ 0.3 mm Time-based localization schemes Time of arrival (TOA) Time difference of arrival (TDOA) A special variation of TDOA with virtual anchors does not require synchronization among devices. ⇒ See the next slide. 11 / 49
  14. 14. TDOA with Virtual Anchors 1 Anchor Agent Virtual Anchors 1 E. Leitinger et al., IEEE J. Sel. Areas Commun., vol. 33, no. 11, pp. 2313–2328, Nov. 2015. 12 / 49
  15. 15. Clock Offset and Propagation Delay Can the receiver distinguish between the following two cases if θ = d? Packet with Timestamp T vs. Packet with Timestamp T TX RX TX RX • : Clock offset • : Propagation delay Answer is “No”. Two-way message exchanges needed for delay compensation. 13 / 49
  16. 16. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 14 / 49
  17. 17. Conventional Two-Way Message Exchanges I Master sHead Node) Slave sSensor Node)Measurement Interval of Time Sync. si.e., 2-Way Message Exchange) …… Report Request Response Report Measurement T1 T2 T4 T3 Sensor nodes transmit “Request” messages for synchronization. In addition to measurement data packets. 15 / 49
  18. 18. Conventional Two-Way Message Exchanges II The sensor node can estimate its clock offset w.r.t. the head node and synchronize its clock to that of the head node: Clock offset: ˆθ = (T2 − T1) − (T4 − T3) 2 . Propagation delay: ˆd = (T2 − T1) + (T4 − T3) 2 . 16 / 49
  19. 19. Reverse Two-Way Message Exchanges I Master sHead Node) Slave sSensor Node) Beacon/ Request sMeasurement) Report/ Response T1 T4 T3T2 d tm Sensor nodes do not transmit any other messages except “Request/Response” messages. If there are no measurement data, sensor nodes just receive messages. 17 / 49
  20. 20. Reverse Two-Way Message Exchanges II The head node can estimate the clock offset of the sensor node, but the sensor node cannot. As a result, the information of all sensor node clocks is centrally managed at the head node. “Response” (synchronization) and “Report” (measurement data) messages can be combined to save the number of message transmissions from the sensor node. Optionally measurement data and corresponding timestamps can be bundled together in a “Report/Response” message when there are no strict timing requirements. 18 / 49
  21. 21. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Next Steps: Extension to Multi-Hop Time Synchronization 19 / 49
  22. 22. Hardware Clock Model Time Ti of the hardware clock of the ith sensor node at the reference time t is modeled as a first-order affine function: Ti(t) = (1 + i)t + θi, where (1 + i) ∈ R+: Clock frequency ratio.2 θi ∈ R: Clock offset. 2 i is called a clock skew in the literature. 20 / 49
  23. 23. Logical Clock Model Time Ti of the logical clock of the ith sensor node at hardware clock time Ti(t) is modeled as a piecewise linear function: For tk<t≤tk+1 (k=0, 1, . . .), Ti Ti(t) = Ti Ti(tk) + Ti(t) − Ti(tk) 1 + ˆi,k − ˆθi,k, where tk: Reference time when a kth synchronization occurs. ˆi,k: Estimated clock skew from the kth synchronization. ˆθi,k: Estimated clock offset from the kth synchronization. 21 / 49
  24. 24. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Next Steps: Extension to Multi-Hop Time Synchronization 22 / 49
  25. 25. Measurement Time Estimation Error: Conventional Two-Way Message Exchanges Master sHead Node) Slave sSensor Node) Measurement Request Response Report s1 s2≈s3 s4 d tm When Tm d, ∆ˆtConv. m ∼ Tm × ∆ˆi, where ∆ˆi is the clock skew estimation error. 23 / 49
  26. 26. Measurement Time Estimation Error: Reverse Two-Way Message Exchanges Master sHead Node) Slave sSensor Node) Beacon/ Request sMeasurement) Report/ Response T1 T4 T3T2 d tm When Tm d, ∆ˆtRev. m ∼ Tm 2 × ∆ˆi. 24 / 49
  27. 27. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Next Steps: Extension to Multi-Hop Time Synchronization 25 / 49
  28. 28. Message Departure and Arrival Times Let td(k) (k=0, 1, . . .) be the reference time for the kth message’s departure from the head node. td(k) also denotes the value of the timestamp carried by the kth message. Then the arrival time of the kth message with respect to the ith sensor node’s hardware clock is given by ta,i(k) = Ti (td(k)) + d(k) = (1 + i)td(k) + θi + d(k), where d(k): One-way propagation delay in terms of the ith sensor node’s hardware clock. 26 / 49
  29. 29. Joint Maximum Likelihood Estimators For a white Gaussian delay d(k) with known mean d and variance σ2 , ˆθML i (k) = t2 d · ta,i − td · tdta,i t2 d − td 2 − d, ˆRML i (k) = tdta,i − td · ta,i t2 d − td 2 , where x k j=0 x(j) k , xy k j=0 x(j)y(j) k . 27 / 49
  30. 30. Regression through The Origin (RTO) Model The problem of asynchronous source clock frequency recovery (SCFR) can be formulated as a linear RTO model as follows: For k = 1, 2, . . ., ˜ta,i(k) = (1 + i)˜td(k) + ˜d(k), where ˜ta,i(k) ta,i(k)−ta,i(0), ˜td(k) td(k)−td(0), ˜d(k) d(k)−d(0). 28 / 49
  31. 31. Cumulative Ratio (CR) Estimator ˆRCR i (k) = ˜ta,i(k) ˜td(k) = Ri + ˜d(k) ˜ts(k) , where Ri: Ratio of the ith sensor node hardware clock frequency to that of the reference clock (i.e., 1+ i). 29 / 49
  32. 32. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 30 / 49
  33. 33. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization 31 / 49
  34. 34. Estimated Clock Skews with Gaussian Delays: σ=1 ns 5 10 15 20 25 30 35 40 45 50 Number of Messages 10−21 10−20 10−19 10−18 10−17 10−16 10−15 MSE RLS CR Joint MLE GMLLE (Two-Way) LB for CR CRLB for Joint MLE LB for GMLLE 32 / 49
  35. 35. Estimated Clock Skews with Gaussian Delays: σ=1 µs 5 10 15 20 25 30 35 40 45 50 Number of Messages 10−15 10−14 10−13 10−12 10−11 10−10 10−9 MSE RLS CR Joint MLE GMLLE (Two-Way) LB for CR CRLB for Joint MLE LB for GMLLE 33 / 49
  36. 36. Estimated Clock Skews with AR(1) Delays3 : σ=1 µs 5 10 15 20 25 30 35 40 45 50 Number of Messages 10−14 10−13 10−12 10−11 10−10 MSE RLS CR Joint MLE GMLLE (Two-Way) 3 ρ = 0.6. 34 / 49
  37. 37. Estimated Clock Skews with AR(1) Delays: σ=1 ms 5 10 15 20 25 30 35 40 45 50 Number of Messages 10−8 10−7 10−6 10−5 10−4 MSE RLS CR Joint MLE GMLLE (Two-Way) 35 / 49
  38. 38. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization 36 / 49
  39. 39. Estimated Frequency Ratio (Sensor Node) and Measurement Time (Head Node): SI=100 s -4E-11 -2E-11 0E+00 2E-11 4E-11 FrequencyDifference[ppm] Proposed (w/ CR) Two-Way (w/ GMLLE) 0 500 1000 1500 2000 2500 3000 3500 Time [s] -1E-02 -8E-03 -6E-03 -4E-03 -2E-03 0E+00 2E-03 4E-03 MeasurementTimeError[s] Proposed (w/ CR) Two-Way (w/ GMLLE) Two-Way 37 / 49
  40. 40. Estimated Frequency Ratio (Sensor Node) and Measurement Time (Head Node): SI=1 s -4E-11 -2E-11 0E+00 2E-11 4E-11 FrequencyDifference[ppm] Proposed (w/ CR) Two-Way (w/ GMLLE) 0 500 1000 1500 2000 2500 3000 3500 Time [s] -1E-04 -8E-05 -6E-05 -4E-05 -2E-05 0E+00 2E-05 4E-05 MeasurementTimeError[s] Proposed (w/ CR) Two-Way (w/ GMLLE) Two-Way 38 / 49
  41. 41. Estimated Frequency Ratio (Sensor Node) and Measurement Time (Head Node): SI=1 ms -4E-11 -2E-11 0E+00 2E-11 4E-11 FrequencyDifference[ppm] Proposed (w/ CR) Two-Way (w/ GMLLE) 0 500 1000 1500 2000 2500 3000 3500 Time [s] -1E-06 -8E-07 -6E-07 -4E-07 -2E-07 0E+00 2E-07 4E-07 MeasurementTimeError[s] Proposed (w/ CR) Two-Way (w/ GMLLE) Two-Way 39 / 49
  42. 42. Effect of SI on Time Synchronization and Energy Consumption4 Synchronization Skew Estimation Measurement Time NTX NRX Scheme MSE Estimation MSE Proposed SI=100 s 8.8811E-25 5.8990E-19 100 36 SI=1 s 9.1748E-25 5.4210E-19 100 3600 SI=10 ms 1.0887E-24 4.7684E-19 100 360100 Two-Way with GMLLE SI=100 s 1.9021E-24 4.7784E-19 136 36 SI=1 s 1.7034E-24 6.1452E-19 3700 3600 SI=10 ms 9.0992E-25 4.0485E-19 360100 360000 Two-Way SI=100 s N/A 3.4900E-05 136 36 SI=1 s 3.4564E-09 3700 3600 SI=10 ms 3.3638E-13 360100 360000 4 Estimations are for the samples taken after 360 s (i.e., one tenth of the observation period) to avoid the effect of a transient period. 40 / 49
  43. 43. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization 41 / 49
  44. 44. Effect of Bundling on Measurement Time Estimation5 0 500 1000 1500 2000 2500 3000 3500 Time [s] -2.0E-09 -1.0E-09 0.0E+00 1.0E-09 2.0E-09 MeasurementTimeError[s] NBM=1 NBM=2 NBM=5 NBM=10 5 SI = 1 s. 42 / 49
  45. 45. Effect of Bundling on Time Synchronization and Energy Consumption Synchronization Scheme Measurement Time NTX NRX Estimation MSE Proposed NBM = 1 5.4210E-19 100 3600 NBM = 2 5.1116E-19 50 3600 NBM = 5 3.7504E-19 20 3600 NBM = 10 2.6468E-19 10 3600 In interpreting the results, the following should be taken into account: The bundling increases the length of message payload. The increased message payload also can affect the frame errors and the number of retransmissions. 43 / 49
  46. 46. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 44 / 49
  47. 47. Multi-Hop Extension through Gateways 45 / 49
  48. 48. Challenges and Opportunities Tradeoff between time-translating and packet-relaying gateways.. The multi-hop extension should be implemented together with a routing protocol. As in LEACH protocol6 and its many variations, the energy efficiency is also critical in the formation of a hierarchy and the selection of cluster heads (i.e., the gateway nodes in the multi-hop extension of the proposed scheme). 6 W. R. Heinzelman et al., Proc. HICSS’00, Jan. 2000, pp. 1–10. 46 / 49
  49. 49. Challenges and Opportunities Tradeoff between time-translating and packet-relaying gateways.. The multi-hop extension should be implemented together with a routing protocol. As in LEACH protocol6 and its many variations, the energy efficiency is also critical in the formation of a hierarchy and the selection of cluster heads (i.e., the gateway nodes in the multi-hop extension of the proposed scheme). 6 W. R. Heinzelman et al., Proc. HICSS’00, Jan. 2000, pp. 1–10. 46 / 49
  50. 50. Challenges and Opportunities Tradeoff between time-translating and packet-relaying gateways.. The multi-hop extension should be implemented together with a routing protocol. As in LEACH protocol6 and its many variations, the energy efficiency is also critical in the formation of a hierarchy and the selection of cluster heads (i.e., the gateway nodes in the multi-hop extension of the proposed scheme). 6 W. R. Heinzelman et al., Proc. HICSS’00, Jan. 2000, pp. 1–10. 46 / 49
  51. 51. Preliminary Results 47 / 49
  52. 52. Next . . . Introduction Time and Space in Synchronization Energy-Efficient Time Synchronization for Asymmetric Wireless Networks Hardware and Logical Clock Models Effect of Clock Skew on Measurement Time Estimation Asynchronous Source Clock Frequency Recovery at Sensor Nodes: One-Way Clock Skew Estimation Simulation Results Performance of One-Way Clock Skew Estimation Performance of Measurement Time Estimation and Energy Efficiency Effect of Bundling of Measurement Data Next Steps: Extension to Multi-Hop Time Synchronization Conclusions 48 / 49
  53. 53. Conclusions Propose an energy-efficient time synchronization scheme for asymmetric wireless networks achieving sub-microsecond time synchronization accuracy. Also, discuss the optional bundling of measurement data in a “Report/Response” message. Topics for further study include Extension to multi-hop synchronization through packet-relaying or time-translating gateway nodes; Energy-delay tradeoff and the effect of frame errors and retransmissions in bundling of measurement data. 49 / 49
  54. 54. Conclusions Propose an energy-efficient time synchronization scheme for asymmetric wireless networks achieving sub-microsecond time synchronization accuracy. Also, discuss the optional bundling of measurement data in a “Report/Response” message. Topics for further study include Extension to multi-hop synchronization through packet-relaying or time-translating gateway nodes; Energy-delay tradeoff and the effect of frame errors and retransmissions in bundling of measurement data. 49 / 49
  55. 55. Conclusions Propose an energy-efficient time synchronization scheme for asymmetric wireless networks achieving sub-microsecond time synchronization accuracy. Also, discuss the optional bundling of measurement data in a “Report/Response” message. Topics for further study include Extension to multi-hop synchronization through packet-relaying or time-translating gateway nodes; Energy-delay tradeoff and the effect of frame errors and retransmissions in bundling of measurement data. 49 / 49
  56. 56. Conclusions Propose an energy-efficient time synchronization scheme for asymmetric wireless networks achieving sub-microsecond time synchronization accuracy. Also, discuss the optional bundling of measurement data in a “Report/Response” message. Topics for further study include Extension to multi-hop synchronization through packet-relaying or time-translating gateway nodes; Energy-delay tradeoff and the effect of frame errors and retransmissions in bundling of measurement data. 49 / 49
  57. 57. Conclusions Propose an energy-efficient time synchronization scheme for asymmetric wireless networks achieving sub-microsecond time synchronization accuracy. Also, discuss the optional bundling of measurement data in a “Report/Response” message. Topics for further study include Extension to multi-hop synchronization through packet-relaying or time-translating gateway nodes; Energy-delay tradeoff and the effect of frame errors and retransmissions in bundling of measurement data. 49 / 49

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