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The Effect of AWGN on the Accuracy of Time of Arrival Detection

The Effect of AWGN on the Accuracy of Time of Arrival Detection. Rick Enns IEEE 802.15.4a July 2004. Issues. What effect does AWGN have on the accuracy of measuring a packet’s time-of-arrive? Can narrow band modulation be used in the presence of AWGN?

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The Effect of AWGN on the Accuracy of Time of Arrival Detection

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  1. The Effect of AWGN on the Accuracy of Time of Arrival Detection Rick Enns IEEE 802.15.4a July 2004 15-04-0335-00-004a

  2. Issues • What effect does AWGN have on the accuracy of measuring a packet’s time-of-arrive? • Can narrow band modulation be used in the presence of AWGN? • What might be some of the issues that differentiate UWB and narrow band time detection? 15-04-0335-00-004a

  3. Position Sensing: Time Detection and Triangulation • Triangulation: n messages for one event detects position in n-1 dimensions • Time stamp data is based on local clocks- clock synchronization is not part of this paper 15-04-0335-00-004a

  4. Basic Time of Arrival Detector • Standard receiver and baseband processing • Time stamp messages are sent to a triangulation processor 15-04-0335-00-004a

  5. Simulation Parameters • Half-sine signal lke a O-QPSK symbol • Zeros allow easy noise level verification • Repeats 1000 times for statistics • Simulation signal with 8dB SNR AWGN • Time unit = 0.5 nsec  15 cm  6” for 802.15.4 in 2.5GHz band 15-04-0335-00-004a

  6. No noise received signal Target half sine symbol Correlation Function Correlation Process time  • Correlated Signal and Target Vertical scale x 200 15-04-0335-00-004a

  7. 8dB SNR received signal Target half sine symbol Correlation Function Correlation Process  • Correlated Signal and Target Vertical scale x 200 15-04-0335-00-004a

  8. Correlation peak for one symbol at 8 dB SNR Based on a 3000 point sliding average Computationally expensive Correlation Peak Detection 15-04-0335-00-004a

  9. Correlation peak for one symbol at 8 dB SNR Based on a 100 point sliding average Computationally less expensive Correlation Peak Detection 15-04-0335-00-004a

  10. Correlation Function • Formula: 15-04-0335-00-004a

  11. Correlation Function Approximation • Signal is the sum of the target and AWG noise: Si = Ti + Ni where N is AWGN • Correlation • Approximation 15-04-0335-00-004a

  12. Central Limit Theorem • When the correlation is done of many sample points the noise averages out 15-04-0335-00-004a

  13. Timing Accuracy – Error Histograms • 8dB SNR, STD = 3.9 time units averaged over one symbol • 8dB SNR, STD = 0.7 time units averaged over 64 symbols • 14dB SNR, STD = 1.9 time units averaged over one symbol 15-04-0335-00-004a

  14. Averaging and Mobility • Increased accuracy can be achieved through averaging • Averaging can be done at the cost of more circuits of and higher operational power within the wireless detector node • Averaging can also be done in the triangulation processor for free but it will lessen support for mobility 15-04-0335-00-004a

  15. Summary • One variable of of timing accuracy is AWGN SNR • Other important variables are: $$ + power and mobility • Existing narrow band systems can support accurate time detection under AWGN under $$ and power constraints • Multipath noise may better differentiate UWB approaches than does AWGN 15-04-0335-00-004a

  16. Recommendations • Consider MAC extensions that allow detection nodes to track existing 802.15.4 targets • Construct the standard to allow wireless detector nodes to be designed with different detection accuracies so power, $$ and accuracy can be optimized to an application • a node’s time stamp accuracy needs to be a reported value 15-04-0335-00-004a

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