We introduce a method by which to locate sensors in a 2.4 GHz wireless network. We tested our system in a rough welding hall, where plasma cutters and welding machines commonly cause radio interference. Indoor positioning is a challenge because standardized GPStype global-positioning technologies are not available. There is therefore a need for extra infrastructure, such as wireless sensors, but moreover there is no standardized communication between sensors. Therefore, it is reasonable to investigate alternative technologies. We found that a reliable positioning system could be created by using Chirp Spread Spectrum (CSS) modulation. Also, we studied which CSS parameters affect the transfer data rate in order to determine the most reliable speed for both top and average transfer

Joni Jämsä

Mika Luimula

Ossi Saukko

Sakari Pieskä

English

Journal of Vibroengineering

  VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
27 
713. Indoor positioning using symmetric double-sided 
two-way ranging in a welding hall 
 
Joni Jämsä, Mika Luimula, Sakari Pieskä, Ossi Saukko 
CENTRIA Research and Development, 
Central Ostrobothnia University of Applied Sciences, Ylivieska, Finland 
E-mail: {joni.jamsa, mika.luimula, sakari.pieska, ossi.saukko}@centria.fi 
 
 (Received 6 September 2011; accepted 14 February 2012) 
   
Abstract. We introduce a method by which to locate sensors in a 2.4 GHz wireless network. We 
tested our system in a rough welding hall, where plasma cutters and welding machines 
commonly cause radio interference. Indoor positioning is a challenge because standardized GPS-
type global-positioning technologies are not available. There is therefore a need for extra 
infrastructure, such as wireless sensors, but moreover there is no standardized communication 
between sensors. Therefore, it is reasonable to investigate alternative technologies. We found 
that a reliable positioning system could be created by using Chirp Spread Spectrum (CSS) 
modulation. Also, we studied which CSS parameters affect the transfer data rate in order to 
determine the most reliable speed for both top and average transfer. 
 
Keywords: indoor positioning, wireless sensor network. 
 
Introduction  
 
Data throughput speed is a compromise between the frequency bandwidth and the channel 
use time. Determining a reliable combination of these helps to prevent collisions on wireless 
channels and increases the total data-transfer capacity of the machines. A couple of different 
methods can be used with indoor positioning methods. For example, Alippi et al. [1] used RFID 
tag readers to cover the area of tag movement with an accuracy of about 0.6 m when the readers 
are less than 3 m apart. Radio positioning systems are used in many ways. The Chirp Spread 
Spectrum (CSS) signal was used successfully by Huang et al. [2] to compensate for positioning 
errors in low-noise conditions (signal to noise ratio, SNR, below –20 dB), resulting in a 
resolution of better than 1 m. Still, there is a need to synchronize the clock between the 
transmitter and the receiver. Yoon et al. [3] generated an anchor-free positioning system by 
using CSS radios, showing that there is no need to provide an anchor. This is because the 
networks locate themselves and a mobile phone is used as a gateway to collect nearby location 
information. In this way the system builds up time and costs will decrease, increasing its 
potential for use. Robust outdoor localization methods have been presented by Stoleru et al. [4] 
and Murty et al. [5]. Some of these principles are also valid for the coarse indoor localization of 
mobile machines. 
Our studies on the remote navigation of mobile robots focused on improving the accuracy of 
indoor positioning. In earlier studies performed by us [6], we found that WLAN- and ZigBee-
based positioning systems are unsuitable for remote navigation. Based on a literature review, we 
believe that UWB positioning is a promising technology, although penetrating metal surfaces is 
still challenging [7]. We used CSS radios, which use a given bandwidth and a progressively 
increasing frequency, as an “up-chirp” in this study. Based on our previous results this seems to 
be more suitable and reliable for indoor positioning in the metal industry than is the Received 
Signal Strength Indication (RSSI) method [15]. Because of a relatively large bandwidth (80 
MHz), CSS pulses are insensitive to electromagnetic distortion and do not experience multipath 
fading. In addition, CSS is able to utilize RF echoes when calculating distances. Distance 
calculation could proceed during normal data transfer, or the radio could be programmed with a 
special ranging mode, when the whole bandwidth is used for positioning purposes. Both radios 
send two messages: a data question and a response. As a result, by using signal propagation and 
processing delays the range between the radios could be calculated. 
 713. INDOOR POSITIONING USING SYMMETRIC DOUBLE-SIDED TWO-WAY RANGING IN A WELDING HALL. 
JONI JÄMSÄ, MIKA LUIMULA, SAKARI PIESKÄ, OSSI SAUKKO  
 
 
 VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
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Transfer speed here means the average data rate on the channel. We also recorded the 
maximum data rate during our measurements. Message length affects transfer speed, and when 
more data are included, such as one frame and one header, more data are transferred. The 
bandwidth could be set at 22 or 80 MHz, but larger bandwidth causes interference with other 
ISM-transmitters, such as WLANs in the same area, although this should provide higher data 
rates. This method might not be able to be used in all environments. The radio used provides a 
special high data rate mode, where the positioning capabilities are reduced and more time is 
required for data transfer. 
 
Experiments 
 
In indoor conditions, where many reflecting and conducting materials are nearby, multipath 
propagation causes errors in estimating the distance between radios. This is because radio waves 
reach the receiver along different pathways and the measurable distance varies when the 
measurements are repeated. The rough conditions inside a metal factory present additional 
challenges for radio-frequency positioning systems. Steel walls, plates in storage areas, and 
sheet metal stacks cause reflections and prevent line-of-sight propagation of the signal. Welding 
machines and plasma cutters cause noise within and interference between radio waves. Most of 
the received signals are reflected and a straight route near the floor cannot be used.  
These conditions cause radio wave fading and reflections. Also, local, stronger values could 
be collected when reflected waves in the same phase reach the receiver, causing the estimates of 
distance to be too small. This seems to be common in our measurements inside a factory. We 
conducted our positioning measurements in a metal factory welding hall during the normal 
activities of the business. There were eight operating welding machines in the hall. Our test 
environment is illustrated in Figure 1. As shown, there were obstacles along the routes between 
node AP1–TAG (lift truck) and node TAG–AP2 (shelf).    
 
 
 
Fig. 1. Radios and obstacles in our welding hall test environment  
 
Positioning measurements 
 
We installed five radio nodes, denoted as AP1–AP4 and TAG. After installation, we 
confirmed that all of the radios were able to communicate and measured their actual distances by 
using a steel meter stick. The distances are shown in Table 1.  
 
Table 1. Actual distances (m) between nodes 
AP1 to 
AP2 
AP1 to 
AP3 
AP1 to 
AP4 
AP1 to 
TAG 
AP2 to 
AP3 
AP2 to 
AP4 
AP2 to 
TAG 
AP3 to 
AP4 
AP3 to 
TAG 
AP4 to 
TAG 
12.0 17.5 27.6 18.7 18.5 15.0 9.2 28.1 15.3 12.5 
 713. INDOOR POSITIONING USING SYMMETRIC DOUBLE-SIDED TWO-WAY RANGING IN A WELDING HALL. 
JONI JÄMSÄ, MIKA LUIMULA, SAKARI PIESKÄ, OSSI SAUKKO  
 
 
 VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
29 
We used several parameters of our user interface to determine how distances should be 
measured. The first distance could be the average, minimum, or maximum values of the last 
measurement. We recognized that some spans make the nodes appear to be farther apart while 
other spans make the nodes appear closer together. We gave both kinds of spans different 
multipliers for the change in their influence on the calculated distances. Table 2 provides the 
distance measurements based on the average of the last 50 values. 
 
Table 2. Average values of the calculated distances (m) between nodes 
AP1 to 
AP2 
AP1 to 
AP3 
AP1 to 
AP4 
AP1 to 
TAG 
AP2 to 
AP3 
AP2 to 
AP4 
AP2 to 
TAG 
AP3 to 
AP4 
AP3 to 
TAG 
AP4 to 
TAG 
13.6 19.7 29.5 23.2 23.4 17.5 29.4 30.1 18.9 14.2 
13.8 19.6 29.5 21.9 22.9 17.6 32.1 32.3 25.4 14.2 
 
All of the calculated values are larger than the actual ones. Barriers between nodes 
completely eliminate line-of-sight waves. This means that even the fastest waves were reflected 
around surfaces, leading to a larger than actual calculated distance. Even worse results were 
observed when a larger effect was given to pushing spans. For this reason we decided not to use 
the maximum measured values. We calculated the error percentage between the calculated and 
measured values (Table 3). 
 
Table 3. Error percentage of the average values versus actual distances 
AP1 to 
AP2 
AP1 to 
AP3 
AP1 to 
AP4 
AP1 to 
TAG 
AP2 to 
AP3 
AP2 to 
AP4 
AP2 to 
TAG 
AP3 to 
AP4 
AP3 to 
TAG 
AP4 to 
TAG 
15.0 12.0 6.9 17.1 23.8 17.3 248.9 14.9 66.0 13.6 
13.3 12.6 6.9 24.1 26.5 16.7 219.6 7.1 23.5 13.6 
 
The shelf between TAG and AP2 blocked the waves, and there was no room to propagate the 
signal under the shelf. The connection between nodes was established through the ceiling and 
adds 20 m to the distance. We chose the minimum distance of the last measurement for each 
span, again using multipliers. In Table 4, we increased the shortened multiplier. 
 
Table 4. Calculated distances based on minimum values 
AP1 to 
AP2 
AP1 to 
AP3 
AP1 to 
AP4 
AP1 to 
TAG 
AP2 to 
AP3 
AP2 to 
AP4 
AP2 to 
TAG 
AP3 to 
AP4 
AP3 to 
TAG 
AP4 to 
TAG 
12.8 19.2 28.9 20.7 21.8 16.9 31.3 29.6 29.6 13.5 
13.2 18.6 28.9 20.5 20.2 16.9 12.8 29.7 23.2 13.7 
12.4 19.2 28.4 20.5 19.5 16.9 16.9 29.5 26.8 13.7 
12.9 19.1 28.8 20.8 21.8 16.9 31.0 29.5 26.8 13.6 
 
The calculated distances varied for span AP2–TAG. Sometimes waves could penetrate 
parcels on the shelf and in those cases the distance is nearly correct. When this happens is not 
easy to predict because all of the nodes were stable and untouched during all measurements. 
Perhaps the nearby plasma cutter reflected or weakened the waves enough to prevent penetration 
through the shelf. The cutter was active continuously during the measurements, but there were 
small breaks between the moving head and the steel plate. 
 
Data Rate Measurements 
 
We collected the data throughput measurements under different conditions. At the larger 
scale, we adjusted bandwidth to between 22 and 80 MHz. We then adjusted the bitrate to 
between 1000 and 16000 ns. These measurements were conducted with different message 
 713. INDOOR POSITIONING USING SYMMETRIC DOUBLE-SIDED TWO-WAY RANGING IN A WELDING HALL. 
JONI JÄMSÄ, MIKA LUIMULA, SAKARI PIESKÄ, OSSI SAUKKO  
 
 
 VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
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lengths of between 1 and 128 bytes. We measured the average data throughput speed between 
two nodes and the line-of-sight span. 
 We started with the 22-MHz bandwidth, and the data rates (in bytes per second) are shown 
in Graph 1.  
 
 
Graph 1. Data rates at 22 MHz bandwidth 
 
Shorter bitrates provide higher data speed. However, if the rate is too short (1000 ns here) the 
transfer speed drops dramatically. As expected, longer messages give a better overall data rate. 
The relative parts of the frame headers stay small and more of the entire bandwidth is used for 
the actual data. The exception here was with 16000 ns, where the 128-byte data were too long to 
reach the receiver. Based on these results, we tested a higher bandwidth with a relatively short 
bitrate. This time we started as low as 500 ns and proceeded to 4000 ns to see if we could get 
even higher speeds. Graph 2 provides the data rates (in bytes per second) when the entire 80-
MHz frequency range was used. 
 
 
Graph 2. Data rates at 80MHz bandwidth 
  
As expected, shortening the bit rate and lengthening the message gives a larger data rate. 
Saturation of the rate was encountered with the shortest bitrate (500 ns). A comparison between 
the different bandwidths could be made with bitrates from 1000 to 4000 ns. At 2000 ns the 
larger bandwidth did not help any, and we obtained almost the same data rates at both 22 and 80 
MHz. High bandwidth provides better results and we succeeded in obtaining over 30000 ns.  
 713. INDOOR POSITIONING USING SYMMETRIC DOUBLE-SIDED TWO-WAY RANGING IN A WELDING HALL. 
JONI JÄMSÄ, MIKA LUIMULA, SAKARI PIESKÄ, OSSI SAUKKO  
 
 
 VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
31 
We used 22 MHz to test how the presence of one wall between two nodes affects the data 
rate. As shown in Graph 3, transmission failures with longer message lengths dramatically 
reduce the average speed. Packets reattempts are easily seen here at average speeds. When there 
are obstacles along the transmission route, the packet length should be limited to 32 bytes in 
order to optimize performance.  
 
 
Graph 3. Data rates at 22MHz bandwidth through a wall 
 
Conclusions  
 
Positioning was quite reliable by using this method. Low impact of pushing longer multiplier 
and higher impact for shorter multipliers give better results. Barrels blocks usually waves totally. 
Sometimes occasionally, waves could access without reason barrel. We never estimated 
distances that were lower than the actual distances.  
The use of a wider bandwidth or a shorter symbol increases the throughput data rate. The 
data rate seems to remain stable and there were only minor variations during the measurement 
period. Walls and obstacles along the transmission route cause transmission reattempts and 
significantly decrease the data rate if message lengths over 32 bits are used. Narrowband 
communication has more distortion than does wideband when obstacles are located along the 
line of sight. By choosing the wideband transmission mode and a short symbol length, the effect 
of obstacles along the route could be minimized.  
By comparing results, this positioning method provided longer than actual distances. We 
suppose this occurs because of delays in the processor when processing times are very short. 
Very good accuracy will be observed when several measurements are minimized. In this way we 
observed distances about 12% longer than the actual distances, and this is easy to compensate 
for by using software. The longest connection inside the welding process hall was 56 m. One 
sheet metal wall was within a shorter range of 43 meters. We were unable to estimate distances 
through the outer wall. 
 
Acknowledgements 
 
The authors would like to thank everyone who has been involved in this research, and 
everybody who has contributed to the development of the Locawe platform and the mobile robot 
system. This work was supported by the European Regional Development Fund, the Council of 
the Oulu Region, the Finnish Funding Agency for Technology and Innovation (Tekes), the TE 
Centre of Northern Ostrobothnia, the Ylivieska Region, the Nivala-Haapajärvi Region, the 
Haapavesi-Siikalatva Region, and several companies.  
 
 713. INDOOR POSITIONING USING SYMMETRIC DOUBLE-SIDED TWO-WAY RANGING IN A WELDING HALL. 
JONI JÄMSÄ, MIKA LUIMULA, SAKARI PIESKÄ, OSSI SAUKKO  
 
 
 VIBROENGINEERING. JOURNAL OF VIBROENGINEERING. MARCH 2012. VOLUME 14, ISSUE 1. ISSN 1392-8716    
32 
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Indoor positioning using symmetric double-sided two-way ranging in a welding hall

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Indoor positioning using symmetric double-sided two-way ranging in a welding hall

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