1. RADIO INTERFEROMETRIC GEOLOCATION
Will Hedgecock
EECE 354
11/08/2010
M. Maroti, B. Kusy, G.. Balogh, P. Volgyesi, A. Nadas, K. Molnar, S. Dora, A. Ledeczi. "Radio Interferometric Geolocation". in Proc. ACM 3rd Conference on Embedded Networked Sensor Systems (SenSys'05), November, 2005.

2. Motivation
Many WSN (Wireless Sensor Network) applications require knowledge of the location of individual nodes in the system
Existing localization techniques have limited range and accuracy
Usually acoustic-based
Also true of signal strength methods with accuracies of up to a few meters
Tradeoff between range and accuracy
Difficult to provide stealthy operation modes in traditional approaches
Requires ultrasound
Most existing techniques work only in 2D

3. Radio Interferometric Positioning System (RIPS)

4. Background
Traditional radio interferometry is used in physics, geodesy, and astronomy to measure relative distances to objects
Works by measuring a single signal from two separate directional antennae and performing cross-correlation
Resulting signal interference can be used to determine distance to an object, the precise relative location of the two receiving antennae, or, if the locations of the two receivers are known, the precise location of the radio source
Standard Radio Interferometric systems are quite expensive and not conducive to WSN applications
-In signal processing, cross-correlation is a measure of similarity of two waveforms as a function of a time-lag applied to one of them
-Requires knowledge of system topology – communication between nodes to find the interference signal

5. General RIPS Overview
Directly generate an interference signal using two transmitters at high frequencies
If the transmitters are signaling using slightly different carrier frequencies, the resulting interference signal will have a low-frequency envelope
Can therefore be measured by cheap, low-precision hardware
The phase offset of the interference signal at the receivers corresponds to the relative positions of the four nodes in the system
Thus, with at least 8 nodes, we can calculate the relative location of all of the nodes in 3D
-Low frequency envelope called “low beat frequency”

6. General RIPS Overview

7. RIPS Key Points
The phase offset of a low-frequency signal is measured (cheaply), but it corresponds to the wavelength of a high-frequency carrier signal
This allows low-precision techniques carried out on resource-constrained WSN nodes to produce highly accurate results
Absolute phase offset depends on several factors including exact time instances when signal transmissions were started, BUT relative phases measured by the receivers depends only the distances between the transmitters and receivers and the carrier frequency
By measuring the phase offsets at different carrier frequencies, we can infer the relative positions of the nodes
-Absolute: we’ve talked about transmission jitter, uncertainty, etc.

8. Radio Interferometric Properties
This theorem forms the basis for using phase offsets from a low-frequency envelope to obtain highly accurate results

9. Radio Interferometric Properties

10. Radio Interferometric Properties
-This is a simplification due to the fact that wireless sensors usually have high carrier frequencies relative to the cutoff frequency and a limited signal range

11. Radio Interferometric Properties

12. Radio Interferometric Properties

13. Sources of Error Carrier Frequency Inaccuracy:
Carrier Drift and Phase Noise: All previous theorems operate under the assumption of completely stable signals
Any frequency drift will be directly observable in the relative phase offsets at the receivers
Minimizable by making each phase measurement as short as possible
Phase noise (transients, shocks, etc.) is more serious
Multipath Effects
RSSI measurement delay jitter (in circuitry): not noticeable in measurements
Signal-to-noise ratio: dependent mainly on distances between nodes
Signal processing error: well-studied and there exist good approximations/models to deal with this
Time synchronization error: Assuming 2kHz interference frequency and 2us synch accuracy, this generates a 0.4%*2π phase offset error
-First equation is for 1kHz inaccuracy at ranges less than 1km

14. RIPS Implementation

15. RIPS Implementation Base Station: Motes:
Handles scheduling of transmitting pairs, frequency calibration, calculation of the Dabcd range, and actual localization
Motes:
Handles CC1000 radio chip pure sine wave transmission drivers
Synchronizes participating nodes
Handles transmission/reception of signals
Estimates the frequency and phase offset of the sampled RSSI signal

16. Radio Characteristics
Configured to transmit in the 433MHz frequency band
Capable of transmitting an unmodulated sine wave in a wide frequency band ( MHz)
Necessary for calculating the actual range from the phase offset differences
Ability to tune transmitter with high granularity (65Hz steps)
Necessary to achieve separation of the two transmitters
Short-term frequency stability of transmitter
Precise capture of RSSI with small delay jitter
Capable of transmitting at different power levels
Necessary because transmitter/receiver distances can vary to the point that a closer transmitter completely overwhelms the signal from the more distant one

17. Time Synchronization No network-wide synchronization
Only the nodes participating in the current ranging round are synchronized and only for one measurement
A master node sends a synch message containing its local absolute timestamp and timestamp in the near future indicating when measurements should begin
All receiving nodes convert the measurement time to their local time, set up a start timer, and retransmit the message
In this way, nodes outside the initial receiving range can take part in the ranging round
It was shown that all errors combined in this protocol still allow for microsecond synchronization accuracy of the nodes
To account for circuitry jitter, all received messages are immediately timestamped before being passed to the ADC and other measurement circuitry
Phase offset can then be found relative to the actual receive time without jitter

18. Time Synchronization

19. Radio Tuning and Calibration
Calibration is necessary to take into account temperature and voltage effects on carrier frequency
Can take up to 34ms and should be performed every time the frequency changes by more than 1MHz
Frequency span is separated into calibration channels with channel 0 being MHz, and each separation being MHz apart
Within each channel, fine-grained tuning can be performed in 65Hz intervals without requiring recalibration (very fast)
Nominal tuning frequency, f, obtained from formula:
One limitation is that actual tuning can differ from nominal f by up to 2kHz
Due to measurement time constraints (29ms) and mote sampling rates (9kHz), transmission frequencies must differ in the range Hz

20. Tuning Algorithm
1 transmitter begins transmitting at nominal f, while second transmitter begins transmitting at where i = -15, -14,
A receiver node analyzes the interference signal and determines which i creates an interference frequency closest to 0
It transmits this back to one of the transmitters which adjusts its transmission frequency
It has been noted that these inaccuracies are mainly due to imprecisions in the crystal driving the node
Thus, tuning factors are not constant between different channels on different nodes, but:
Tuning factors are fairly linear across different channels of the same node
Tuning factors can be found for two different channels of a node and interpolated to correct tuning factors for other channels

21. Frequency and Phase Estimation
Performed on each mote then transmitted to the base station along with a quality indicator value
256 samples per measurement
Very resource-constrained (only about 820 CPU cycles per sample for online processing)
Post-processing a bit more relaxed (about CPU cycles available)
No floating point hardware, so computationally expensive solutions, such as Fourier analysis, are not feasible

22. Frequency and Phase Estimation

23. Online Processing Peak detection performed online by the ADC ISR
Raw samples are filtered by a moving average to smooth results and enhance SNR
Min/max values are acquired from the leading 24 samples (must contain at least one full period)
The acquired amplitude value serves as a quality indicator of the measurement
Samples above a threshold of 20% of the max amplitude measured are identified as high amplitude samples
Peaks are defined as center points of 2 consecutive high-threshold crossings (not-high to high, then high to not-high)
Peaks are discarded if the signal has not crossed the low threshold since the last peak to minimize false positives

24. Post Processing
Works exclusively on peaks identified and stored in the online algorithm
Determines the shortest period between subsequent peaks
Accumulates the sum of all periods that are not longer than 130% the length of the shortest period
Frequency is defined as the reciprocal of the average of this sum
Phase is defined as the average phase of accepted peaks
Small frequency errors can result in large phase errors, so phases are computed relative to the center of the sample buffer thereby reducing accumulated phase error
The estimated frequency, phase, and amplitude tuple is sent to the base station

25. Scheduling Two levels:
1) High-level scheduling for selecting the pair of transmitters
Should minimize the required number of interference measurements while producing enough to localize in 3D
2) Low-level scheduling for coordinating the activities of the transmitters and receivers
Includes time synchronization, frequency calibration, and transmission power scheduling
full/full power, full/half power, and half/full power all carried out by each transmission pair
Currently 13 channels 5MHz apart are used between MHz

26. Range Calculation
With enough measurements at different frequencies, we can solve for dABCD
Possibility of multiple solutions differing by small integer multiples of the wavelength
Define error function:
dABCD resulting in smallest error is taken to be the final estimate
The more frequencies used, the better the estimate
RIPS uses 10 frequencies
-Params: Wavelength, integer, and phase offset relative to the wavelength
-Because phase measurements are noisy, this equation isn’t quite valid, must be reformulated as an inequality < epsilon fraction of the wavelength
-di is individual dabcd estimates

27. Localization
RIPS provides ranging estimates between sets of 4 nodes, not pairs of nodes directly
Would require solving a large number of nonlinear equations
Uses a Genetic Algorithm (GA) as a baseline instead
-A solution is defined as a set of physical node positions
-Epsilon is set to the current error value, that way larger errors allow for larger position jumps

28. Tuning Results
Good interference signals measured at double the communication range of the radios (160:80 meters)
Frequency and
Phase Tuning Comparisons:

29. Tuning Results
Mean error in phase measurements using varying amplitude filter thresholds

30. Ranging Results
Algorithm for estimating interference signal frequency and phase offsets determines the average amplitude of the signal which correlates strongly with the error of the estimate
-Thus, we can use a constant amplitude threshold to discard erroneous results with low SNR (12% of maximum ADC range)
-This could be carried out individually on each mote, but is currently done on the base station
-Since nodes with bad frequency measurements will result in even worse phase offset estimates, these need to be discarded
-We do this by identifying a narrow frequency window which includes the largest number of frequency estimates – all other estimates are discarded
-Then, the range for a given set of four is only calculated if the number of “good frequency estimates” is greater than 10
-This results in the following figure: on the next slide

31. Better Ranging Results

32. Localization Results
-When the genetic algorithm ran for 2 minutes, these are the results

33. Limitations
Maximum localizable range between transmitters and receivers is defined by the radio range
Transmitters must be within 2 radio ranges of each other to create a successful interference signal

34. Conclusions
Relies on two nodes transmitting a high-frequency carrier signal at slightly different frequencies
The resulting interference signal has a low beat frequency and can be measured at receiving nodes with cheap hardware
Relative phase offsets measured at two receivers is a function of the distances between the four nodes and the carrier frequency
With at least 8 nodes, it is possible to localize each node in 3D space
Achieves a localization accuracy of 3cm and a range of up to 120 meters

35. Reference
Sha, L. et al. “Priority Inheritance Protocols: An Approach to Real-Time Synchronization.” IEEE Transactions on Computers, Vol. 39, No. 9, Sept 1990.