1. Page 1enns@stanfordalumni.org15-04-0335-00-004a The Effect of AWGN on the Accuracy of Time of Arrival Detection Rick Enns IEEE 802.15.4a July 2004

2. Page 2enns@stanfordalumni.org15-04-0335-00-004a 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?

3. Page 3enns@stanfordalumni.org15-04-0335-00-004a 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

4. Page 4enns@stanfordalumni.org15-04-0335-00-004a Basic Time of Arrival Detector Standard receiver and baseband processing Time stamp messages are sent to a triangulation processor

5. Page 5enns@stanfordalumni.org15-04-0335-00-004a 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

6. Page 6enns@stanfordalumni.org15-04-0335-00-004a Correlation Process No noise received signal Target half sine symbol Correlation Function time Vertical scale x 200 Correlated Signal and Target 

7. Page 7enns@stanfordalumni.org15-04-0335-00-004a Correlation Process 8dB SNR received signal Target half sine symbol Correlation Function  Vertical scale x 200 Correlated Signal and Target

8. Page 8enns@stanfordalumni.org15-04-0335-00-004a Correlation Peak Detection Correlation peak for one symbol at 8 dB SNR Based on a 3000 point sliding average Computationally expensive

9. Page 9enns@stanfordalumni.org15-04-0335-00-004a Correlation Peak Detection Correlation peak for one symbol at 8 dB SNR Based on a 100 point sliding average Computationally less expensive

10. Page 10enns@stanfordalumni.org15-04-0335-00-004a Correlation Function Formula:

11. Page 11enns@stanfordalumni.org15-04-0335-00-004a Correlation Function Approximation Signal is the sum of the target and AWG noise: S i = T i + N i where N is AWGN Correlation Approximation

12. Page 12enns@stanfordalumni.org15-04-0335-00-004a Central Limit Theorem When the correlation is done of many sample points the noise averages out

13. Page 13enns@stanfordalumni.org15-04-0335-00-004a 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

14. Page 14enns@stanfordalumni.org15-04-0335-00-004a 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. Page 15enns@stanfordalumni.org15-04-0335-00-004a 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

16. Page 16enns@stanfordalumni.org15-04-0335-00-004a 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