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Antenna Finding and Interpolation/Extrapolation of Signal Strength

Werner Van Belle1* - werner@yellowcouch.org, werner.van.belle@gmail.com

1- Internet & Communication Technology Norut IT; Research Park; 9294 Tromsø; Norway

Abstract :  This article investigates the possibility to determine the position of a WiFi antenna by sampling the environment using a standard wireless card. The method is based on a search algorithm (simulated annealing) in a renormalized antenna model. The article further documents a sample acquisition phase in which a static wireless card measured the local environment. This might allow the integration of various oscillatory phenomena into the model.

Keywords:  antenna finding, transmitter finding, WiFi, wireless networks
Reference:  Werner Van Belle; Antenna Finding and Interpolation/Extrapolation of Signal Strength; November 2006


1 Introduction

Figure 1: WiFi signal strength map as used in an experiment at the USC Robotics Research Lab [1, 2]. Picture used without asking permission. Each color represents the signal strength of a specific antenna. A mobile robot used the three signal strengths to determine its position in the building.
Image usc-sal200-rss-5cm

This article investigates the possibility to determine the position of a WiFi base station using irregular sampling. Based on various sample points we try to interpolate the expected signal strength. Sample points are acquired using 'samplers', which are mobile transceivers. The position of a sampler is however not under our control. Each sampler records position, time and signal strength of all available base stations. The signal strengths at the same position can vary greatly (mist, outliers). We want to integrate those irregular sampled points into a coherent signal map, which is half symbolic and half numerical. The symbolic part indicates the base station identifier, the numerical part reports on the strength of that specific base station identifier.

The general goal of such a map is to be able to predict the signal strength of yet unknown areas. Figure 1 gives an example of a WiFi signal strength map with 3 base stations in a building. The topic on signal strength interpolation itself has received some attention due to the possibility to determine the position based on the signal strengths of various antennas [3].

The first section of the article handles the problem of interpolating signal strength, or differently stated: given a small number of samples, that might contain errors both in position and signal strength, how can we assess the signal strength of nearby position. This problem also includes the problem of local variations and blind spots in antenna coverage due to buildings, land mass, reflections and so on. The second section investigates the possibility to find the position of an transmitter using simulated annealing on a renormalized antenna model. This is typically achieve by integrating expected physics (inverse square) into a framework of sample points. Up-sampling in general is done by means of expanding the signal and then getting the signal through a low-pass filter of appropriate design. The third part focuses on some practical experiments. This will give us more insight in the nature of WiFi communication. This includes understanding the variance one might expect, whether the time of day and date are related to signal strength, how cross correlations between wireless stations can be handled and whether there are signal strength oscillations that we need to take into account.

2 Local Interpolation using Thin Plate Splines

In this part we investigate the problem of extrapolating and interpolating signal strength of one WiFi base station (isotropic transmitter) based on a number of sample points that might have errors both in the measured signal strength as well as the current position.

2.1 Thin Plate Splines

Figure 2: Demonstration of thin plate spline interpolation of 15 points. This image is made with [4]
Image tpsdemo

The interpolation between various 2 dimensional points appears similar to the interpolation problem of 3 dimensional points. To understand this one must realize that the signal strength can be set out as the $z$-axis while the coordinate $x$ and $y$ represent the geographical position. Splines are well defined in 2 and 3 dimensions and are based on preserving the smoothness of a curve as good as possible. Splines can have either a fixed (static first order differential) or non-fixed end (non-static first order differential). If they have a fixed end then the slope in those points must be zero in all directions. Which brings us to the interesting question: how well developed are thin plate splines to represent sampled magnetic fields without knowing the actual source of the transmission. Research regarding splines can be found in [5, 4, 6, 7, 8, 9, 10]. Extensions exist that are better suited to handle discontinuities (such as walls) [11, 12]. Figure 2 shows how such splines look like. We tested a number of simple problems to see how TPS performs.

2.2 Sample Scenarios

In what follows we tested the interpolation/extrapolation capabilities based on a number of simulated sample points. These were taken from a map of calculated transmission at specific points.

  1. The first scenario simulates a sampler that goes linearly through a coverage area and irregularly samples the signal strength. The question now how good TPS will be able to predict other points ?
  2. The second scenario samples two samplers walking orthogonal to each other.
  3. This scenario samples a number of points at random around the antenna
  4. This scenario samples a number of points before the antenna. Will our predictor be able to extrapolate the other side of the antenna ?
Those sample maps are visualized in the first column of figure 3.

2.3 TPS Interpolation/Extrapolation

Figure 3: Thin plate spline estimates of a WiFi zone. The first column shows the simulated signal strength, with red maximal signal and blue the lowest signal. A logarithmic scale is used and linearyly mapped to the hue. The sampling points are shown as black circles. The base station is positioned at the red circle. The second column shows the estimated signal strength solely based on the samplepoints. The third column shows the error (the absolute difference between the estimated and the original signal).
Image lin-tps-signal Image lin-tps-estimate Image lin-tps-error
Image ort-tps-signal Image ort-tps-estimate Image ort-tps-error
Image box800x600-tps-signal Image box800x600-tps-estimate Image box800x600-tps-error
Image box400x300-tps-signal Image box400x300-tps-estimate Image box400x300-tps-error

Based on these sample points we interpolated a new signal strength map (middle column of figure 3), after which we measured the error between the proper map and the interpolated map (third column in figure 3). Our overall assessment is that the prediction is locally correct, but that it is unable to properly extrapolate, nor interpolate properly near the exponentially increasing antenna.

  1. In case a sampler always follows the same route, then an estimation will be very accurate for that zone.
  2. If many different points are uniformly sampled then the estimate is quite correct, except for positions close to the base station there we find the the TPS estimation always underestimates the slope.
  3. If only one side of a base station is sampled then the other side cannot be extrapolated properly.
  4. Since the TPS formalism will fit the spline through every point we end up with problems when measurement errors occur. E.g: one sampler measuring no signal (mist or so, or bad hardware). Then the spline interpolation will go through it entirely.
  5. This model is accurate for local estimates but it fails to take into account the quickly changing center and the symmetry around the center.
The TPS fitting seems appropriate for local estimates but it fails to account for transmitter specific physics. As such it is necessary to introduce extra information in the model by trying to fit a decaying signal model $\left(\frac{1}{d^{2}}\right)$ to the sample set and assess where the center is. Once that information is available it can be used to subtract a baseline from the TPS estimation. The mismatch between the model and the local environment can then be modeled through TPS interpolation.

3 Transmitter Finding

3.1 The Transmission Model

This section concerns itself with the problem of estimating the position of the antenna. This will make it possible to model the energy landscape based on a very simple formula. To find the transmitter-center, we assume that we have an exponential decaying signal such as $\frac{1}{r^{2}}$. In practice electromagnetic waves decay based on the permeability of the ether ($\epsilon_{0}$) and the strength of the original signal. It is often measured in decibels, related to some unit power (e.g. dBm). We can model the signal-strength as

Log\left(\frac{1}{\alpha r^{2}}\right)
\end{displaymath} (1)

in which $\alpha $ is a constant based on the current weather conditions $\epsilon_{0}$. $r$ is the distance to the transmitter. To determine the transmitter position $\left(c_{x},c_{y}\right)$ based on a set of sample points $x_{i}$, $y_{i}$ and $z_{i}=Log(\frac{1}{\alpha r^{2}})$we need to solve the following set of equations:

...x_{i}+c_{x}^{2}+y_{i}²-2c_{y}y_{i}+y_{i}²\right) & = & e^{-z_{i}}\end{eqnarray*}

3.2 Direct use of expected signal strength

Such a system of equations is difficult to solve using Gauss-Jordan and standard linear equations. We first resorted to a brute force numerical algorithm that for every pixel in the area determined how big the error between the estimated signal strength and the measured signal strength was. With the proper scaling factor $\alpha $ the results were determined as follows.

\mbox{err}_{1}\left(x,y\right)=\sum_{i}\left\vert Log\left(\...
\end{displaymath} (2)

Figure: An antenna is positioned in the center of the picture and all sample points are assigned their modeled strength. Based on this we estimate the error $\mbox{err}_{1}$ if we would place the antenna at every position in the image. Red areas are those where the mismatch between the samples and the model are largest. Blue are the areas where the mismatch is lowest. If the $\alpha $ parameter is choosen badly, the blue areas will drift away from the real position of the antenna. See figure 5.
Image box400x300-centerhere Image box800x600-centerhere
Image lin-centerhere Image ort-centerhere

Calculating $err_{1}$ for one position requires $\mathcal{O}\left(n\right)$ operations ($n$ being the number of samples). So, for the entire map we have $\mathcal{O}\left(s_{x}.s_{y}.n\right)$ calculations. Figure 4 shows the result of such a map. Blue are the areas where the mismatch would be minimal if the transmitter was located at that position. Red are areas that do not match at all. By experimenting with values of $\alpha $ it became clear that the found center would actually shift if $\alpha $ was chosen badly. As such, the presented method is useful for educative purposes it will not live up to the test in real environments, since there we will need to estimate $\alpha $ as well.

To resolve this problem we investigated a more suitable algorithm for our purpose: simulated annealing combined with a simplex minimization algorithm [VF02a]. By using 2 we tried to find $\alpha $, $cx$ and $cy$ automatically. Here it turned out that there are many local minimal solutions outside the box. Specifically when $\alpha $ is very large, or when $\left(x,y\right)$ is far outside the sampling area, then we find that $err_{1}$ mainly measures $\sum_{i}z_{i}$, which itself is a much smaller error than position nearby the sampling area. This explains why the simulated annealing procedures combined with equation 2 can suddenly fall outside the area and not enter it again. To illustrate this point further, we created a much larger map that illustrates how the error function reacts on large distances or large $\alpha $values (Fig. 5).

3.3 Renormalization

Figure 5: A field-plot of the logerr and disterr functions with different values of $\alpha $. As can be observed, small values for $\alpha $make it difficult to find the center using the logerr measure. Even when $\alpha $ is correct we find that the logerr leads to a discontinuous behavior near the center. When $\alpha $ becomes larger than the actual decay rate, we find that the logerr can find the center but that the disterr will place the best antenna position in a circle outside the center. Nevertheless, the minimal error with large $\alpha $ values grows as well, so a minimization algorithm will always find it beneficial to lower $\alpha $ in such situations. Furthermore, since the minimal circle becomes smaller, it will drag along the minimum to the center.
$\alpha $ logerr disterr
$0.01$ Image box100x75-a001-logerr Image box100x75-a001-disterr
$1.00$ Image box100x75-a1-logerr Image box100x75-a1-disterr
$5.00$ Image box100x75-a5-logerr Image box100x75-a5-disterr
$50.0$ Image box100x75-a50-logerr Image box100x75-a50-disterr

From the experiment in section directUse, we find that the error function $err_{1}$ might not be a proper choice. Therefore we went back to the basic questions: a) what do we want to minimize and b) can we renormalize $err_{1}$ to avoid low signals at improbably positions ? The answer to a is: the error in the position estimation. However, since $err_{1}$ goes through an inverse function and a logarithm we have a very inconsistent energy landscape in which to find the minimal mismatch position.

As such we tried to get rid of the inverse and log functions by, instead of using the dB scale, using a distance scale as input in the error function. Instead of measuring the signal strength in dBm we convert it to its actual distance by taking the inverse and sqrt-ing it. So if $v$ is the measurement at a specific position, of which the distance to the antenna is not know then we can simply take $\sqrt{\frac{1}{v}}$as a linear correlate to the distance function. So , to realize b), we must replace $err_{1}$ with

\end{displaymath} (3)

Figure 6: The color of every position in a plot determines the absolute error between the estimated distance and the sampled distance for all samples, if at this position our point charge was located. This kind of smooth landscape is very useful for simulated annealing methods.
Image box400x300-distance-err-here Image box100x75-distance-err-here
Image lin-distance-err-here Image ort-distance-err-here

This error function will become larger for all values far away from the sampling area as well as it will become large for positions where we barely find a signal. What one can do know is simply walk this novel reorganized landscape and find the minimum. A quick test on this method shows that it is a very good solution to the presented problem. It makes it possible to estimate $\alpha $, $cx$ and $cy$ fairly accurate. Figure 6 shows the various outputs. Furthermore since the measurement scale becomes linear, it also becomes a good set of input functions for thin plate splines.

3.4 Error sensitivity

Further tests on equation 3 revealed that is is not sensitive to outliers and that jitter slightly influences the outcome. It will actually be able to estimate the transmitter position better than the underlying accuracy of the position measurement. However, in a practical setting these are not expected to form a problem.

4 Sampling WiFi Signals

The description in previous part is important from a modeling point of view. Now that the theoretical side has been approached, we need to conduct a number of test to get a 'feeling' for WiFi zones. This include verifying whether there is a relation between the time of day/day of week/day of month and signal availability as well as measuring some upper/lower-bounds on the signal strength variance and signal quality variance.

4.1 Test Procedure & Units

Figure 7: An abstract plot showing the decay of the signal strength in (dBm) with increasing distance.
Image decayplot

Figure 8: Signal strength plot by recording the beacons as they are transmitted by various senders. The receiver was stationary.
Image signal-sampled Image signal-quality

As a small experiment we have been sampling our direct environment from a static WiFi card. The information was taken with iwlist scan and parsed using various scripts. The reports include both signal strength (expressed in dBm) and signal quality (expressed in %). The unit of the signal strength is measured by the WiFi card as dBm, which does not stand for dB meter but rather dB miliWatt. Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making 3 dBm equal to about 0.5 milliwatt [13]. The formula to obtain dBm from power is given as


If we take into account the distance and define $\alpha $ as $\frac{mW}{P}$, then we can directly relate dBm to our model (eq. eq:w)


Figure 7 illustrates how such a decaying signal will look like, if we assume that this model is correct.

The quality of a signal is dependent on the background noise. If it is high the signal quality lowers. The sampling period started Mon Oct 23 16:50:20 CEST 2006 and ended Tue Oct 31 22:07:10 CET 2006. Every 10 seconds a sample was acquired. Figure 8 presents the results for signal level and quality. The different base station hardware addresses are listed in different colors.

4.2 Quality Variance

Figure 9: Signal variance over time. The window size was 10 minutes. Every hardware address has a different added baseline, which should be subtracted before interpreting the data.
Image svar Image qvar

To assess upper and lower limits to the quality over a specific time we measured the absolute variance over time. These are set out in figure 9. Interestingly we find that the signal variance is relatively low, but there seems to be some cross correlation between the different base stations both regarding the signal strength and the signal quality.

4.3 Cross-linked Channels

Correlating the different data sets gives the following measures. All the values where we do not have a measurement for one of the two sets have been removed. Only dataset with more than 30 overlapping samples have been retained. Others report nan.

Correlation between signal strength of different senders
        hw2     hw3     hw4     hw5     hw6     hw7    
hw1     -0.03   0.2     0.1     0.06    -0.003  -0.2    
hw2             0.1     -0.03   -0.09   -0.07   nan     
hw3                     0.07    0.3     -0.04   nan     
hw4                             0.008   0.01    -0.5     
hw5                                     -0.08   nan     
hw6                                             nan     

Correlation between signal quality   

        hw2     hw3     hw4     hw5     hw6     hw7    
hw1     0.1     -0.09   -0.2    -0.05   -0.05   -0.6        
hw2             0.1     -0.01   -0.09   -0.07   nan        
hw3                     -0.1    0.3     -0.05   nan     
hw4                             -0.09   0.06    -0.5    
hw5                                     -0.08   nan     
hw6                                             nan    

Correlation between signal variance   

        hw2     hw3     hw4     hw5     hw6     hw7       
hw1     -0.2    -0.004  0.003   -0.5    -0.08   -0.2      
hw2             -0.03   0.05    -0.2    -0.06   0.4       
hw3                     0.1     0.7     0.02    0.4       
hw4                             0.05    0.02    -0.2      
hw5                                     0.06    0.2       
hw6                                             0.8       

Correlation between quality variance

        hw2     hw3     hw4     hw5     hw6     hw7   
hw1     nan     nan     nan     nan     nan     nan             
hw2             0.7     -0.1    -0.5    nan     nan     
hw3                     0.9     nan     nan     nan      
hw4                             nan     nan     nan      
hw5                                     nan     nan     

4.4 Oscillations

Figure 10: Oscillations in signal strength. Left) Oscillation strength on hourly basis. Middle) Oscillation on minute basis. Right) Oscillation in Hz.
Image hourly-oscilation Image minute-oscilation Image freqhw1

Our analysis shows multiple oscillations of the signal strength. Figure 8 and 9 already hints that the signal quality and strength oscillates over time. Irregular sampling can provide us with more detailed information (above the Nyquist frequency) [14, 15]. Using the Lomb-periodogram, we analyzed the oscillation of hardware address 1. Figure 10 plots the results. As can be seen, we have a strong signal strength oscillation every $\pm12$ hours, which is not so unexpected. The 3daily oscillation is due to our sampling over the weekend. Looking at the oscillation on an hourly basis we find that we have a strong signal every 20 minutes, which coincides with the DHCP leases. At the Hz level we indeed find that we have an oscillation around 0.2 Hz (every 5 seconds) and 0.8 Hz (every 1.25 seconds). Where these come from is unclear. The conclusion from these small experiments is that we need to take into account the date at which moment a signal is sampled since it seems to relate to the signal strength.

5 Conclusion

In this article we investigated the possibility to model signal strength based on finding the antenna position and modeling local variations. We found out that thin plate splines are well suited to model local variations, which might occur due to specific local conditions (walls, reflections). They were however unsuited to find the position of the antenna and extrapolate outside their local environment. To resolve this we estimated the antenna position based on simulated annealing in combination with a simplex search method. This technique worked very well after renormalization of the energy landscape. The combination of both techniques yields the required model for one base station. The antenna position, specified as longitude, latitude and permeability provides an excellent tool to model the expected strength at any position, while the thin plate splines approach can interpolate a local error function.

We did not investigated the relaxation of the thin plate spline interpolation to have better estimation power under the presence of oscillations and measurement errors. We further assumed that an exponentially decaying signal (figure 7) represents a correct model. We found information documenting the possibility of a linear decaying signal [16] (last page). We also did not study anisotropic antennas. An experiment in measuring signal strength over a week long period revealed important, yet unmodeled, phenomena: crosstalk and signal strength oscillations at various periods.


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