Page The Effect of AWGN on the Accuracy of Time of Arrival Detection Rick Enns IEEE a July 2004
Page 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?
Page 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
Page Basic Time of Arrival Detector Standard receiver and baseband processing Time stamp messages are sent to a triangulation processor
Page 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 in 2.5GHz band
Page Correlation Process No noise received signal Target half sine symbol Correlation Function time Vertical scale x 200 Correlated Signal and Target
Page Correlation Process 8dB SNR received signal Target half sine symbol Correlation Function Vertical scale x 200 Correlated Signal and Target
Page Correlation Peak Detection Correlation peak for one symbol at 8 dB SNR Based on a 3000 point sliding average Computationally expensive
Page Correlation Peak Detection Correlation peak for one symbol at 8 dB SNR Based on a 100 point sliding average Computationally less expensive
Page Correlation Function Formula:
Page 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
Page Central Limit Theorem When the correlation is done of many sample points the noise averages out
Page 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
Page 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
Page 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
Page Recommendations Consider MAC extensions that allow detection nodes to track existing 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