Methods to improve RF Localization Accuracy

Transcription

1 Methods to improve RF Localization Accuracy Dr. Eric van Doorn ) Dr. Satya Ponnaluri Dr. Feng Xu Intelligent Automation, Inc. Intelligent Automation, Inc Calhoun Drive, Suite 400 Rockville, MD 20855

2 Indoor localization Solutions for indoor localization are emerging GLANSER, WPI, MSA, others Typically include both inertial and RF sensors Satisfactory accuracy seems feasible in smaller buildings A key question is scalability to larger buildings and more firefighters RF localization component is a bottleneck for scalability Spectrum is crowded, especially in unlicensed bands RF is used for communications, ranging For larger buildings signal attenuation is a major problem To remain scalable, RF ranging solution must operate with reduced bandwidth (improve SNR) Reduced bandwidth means larger errors due to multipath 2

3 RF ranging RF transceivers exchange signals to measure One-way Time Of Flight (OTOF) TDOA Network of synchronized RF receivers collects RF signals emitted by mobile node Potentially use EMS radio as emitter of opportunity Error sources: In weakly scattering environment, synchronization error and baseline between receivers dominate accuracy In strongly scattering environments (indoors), multipath reflections dominate accuracy Same error sources hold for RF ranging Mitigation: Time Difference Of Arrival and RF ranging Better (than GPS) synchronization methods More bandwidth Spatial filtering (averaging) Over distances ~λ multipath de-correlates 3

4 Wireless Synchronization (1) Requirements Accuracy: small fraction of periods for relevant frequencies for throughwall radar, electronic warfare, communications Not achievable with GPS-disciplined clocks Perform under mobility Limited band width (no UWB) Ranges ~hundreds of feet SWAP consistent with portable node Synchronized (loosely) with GPS Approach Master-Slave architecture. Master/slave identical hardware, commissioned at start up TDMA scheduling Exchange of codes, correlating receiver Largely digital implementation Built in tests to eliminate variable hardware delays Provide usable reference signals/interfaces 10MHz USRP2 1PPS radio Ethernet/USB High speed ADC/D/A TDOA receiver Ranging transceiver Comms receiver 4

5 Wireless Synchronization (2) Extensive simulations of algorithms for realistic RF channels (mobile, multi-tap) Hardware/Software completed, ten units built and tested Testing through wired channel with pathloss completed Wireless testing over hundreds of feet range completed Does not require Line of Sight Practical bandwidth limitation RMS synchronization accuracy requirement achieved Currently finalizing work on Further reducing timing bias error Miniaturizing hardware Synchronization error has been eliminated 5

6 Multipath error Range error >0 Constructive Multipath Range error <0 Destructive Multipath Sum LOS LOS Sum Multipath errors can cause both positive and negative range errors Multipath arrive in clusters in time and Angle of Arrival Maximum range = 300 feet (antennas at 3 feet above ground) Ranging resolution = 8.33 feet Carrier = 918MHz PN code length = 64 BW = 15 Mcps Tx power:= +26dBm High Resolution 3-D Angle of Arrival Determination for Indoor UWB Multipath Propagation, Zhang et al, IEEE Trans. on Wireless, Comm, August 2008, Vol: 7 Issue: 8, pp

7 Use of RF synchronization to mitigate multipath Combine RF Round-trip Time Of Flight ranging with other sensors Mobile Receiver equipped with IMU moves to sample a wide aperture IMU provides accurate relative displacement for short time/distance Collect coherent RF data at each location and combine to form beam Low-level fusion not Kalman Filter Receiver beamforming is used to steer the synthetic beam toward transmitter for improved ranging, i.e. suppress the multipath reflection Steering the beam around can also identify multipath scatterers, e.g. walls This may work because multipath arrive in clusters in time and direction 7

8 Receiver Beamforming Receiver moving to different locations at different time IMU records receiver relative position Digital beamforming: Advanced beamforming for accurate Direction Of Arrival: MUSIC Multipath 1 Beams Rx at time 0 Tx LOS Multipath 2 Rx at time 1 Rx at time N 8

9 Simulation: The Corridor Scenario Synchronized Tx, Rx located at the ends of a corridor (10m long, 5m wide) Rx moves across 4m aperture Relative motion measured with IMU Physical Optics (PO) model to calculate multipath scattering from wall RF waveform: 50Mz 350MHz 10dB SNR Wall Tx Rx 9

10 Before beamforming Received Signal after Correlation Location number After beamforming Direction/degree Wall reflection LOS Wall reflection 10

11 Estimation of Direction Of Arrival Use MUSIC to find the directions of different signals Determine the OTOF (range) of each signal Line Of Sight corresponds to direction with the minimum range Beampower Normalized Tapered MUSIC Actual DOA Direction of Arrivial /deg MUSIC improves the DOA accuracy 11

12 Estimation of Range Instantaneous range error limited by bandwidth ~meters Without Beam Forming, averaging peak position (like averaging ranges while moving) With Beam Forming, detect the peak position at the estimated DOA W/O Beamforming actual range estimated range Error=74cm Range /m Beamformed actual range estimated range Error=5cm Range /m 12

13 Extension: Draw Wall Profile Detect range at all directions With estimated position of Tx, we can calculate the position of multipath scatterer, which reflects the wall profile 8 Receiver Transmitter 6 4 Detected Tx Detected Scatterer Walls

14 Conclusions and future work Conclusion: Major sources of TDOA/RF ranging error can be reduced significantly Ongoing and Future work: Experiments Investigation on how odometry/imu error affects accuracy Inverse scattering approach to improve (through-)wall imaging resolution Multi-static measurements 14