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MSU-CSE-06-6

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A Study of Radio Signal Behaviors in Complex Environments Lee, Ee Foong Chen Wang Li Xiao Computer Science Department, Michigan State University Michigan, MI Email: {leeee}@msu.edu Abstract—This paper presents a study of how radio signal behaves in complex environments where the effects of reflections and obstructions are taken into consideration. We collect the RSSI data of each environment and use an objective metric to evaluate the experimental result. The base metric that is used for evaluations is the radio signal strength behavior in an open flat area that is free from reflections and obstructions. We reveal that radio signal strength does not work well in all indoor and complex outdoor environments that are selected for experiment purposes, due to the effects of reflections and obstructions. Radio signal strength only correlates with distance in unobstructed outdoor environment. We also demonstrate that the elevation of sensor nodes can reduces the reflections and obstructions effects, both indoors and outdoors. 1 Introduction Many applications of military and robotics use radio signal strength (RSS) for range estimation. To estimate the distance between a pair of sensor nodes, we measure the received signal strength from the sender and find the corresponding distance from a pre-defined RSSI model, in which it has a defined rate of signal strength attenuation over distance. RSSI is defined to be the voltage in the received signal strength indicator (RSSI) pin on our radio signal [1]. Several RSSI models have been developed for range estimation such as the linear and theoretical models. These models do not usually provide accurate distance estimation because signal strength does not correlate linearly with distance [1]. However, this methodology is an attractive alternative because it is costless and easy to implement. The main problem of RSS-based ranging is its high sensitivity to environmental changes. This ranging system has long been known to be difficult to use for range estimation because it is too “unpredictable” to use for ranging [1,14]. The effects of the environment on RSS can be significant, especially in more complex environments where reflections and obstructions occurred continuously. In this paper, we present some of the RSSI models that contradict to the previous defined RSSI models. Radio signal strength does not always fall off linearly or theoretically with distance. In this paper, we collect the RSSI data that can be used for range estimation in more complex environments. The degree of complexity depends on the amount of reflections and obstructions that are present in the environment. Unlike the signal strength behavior in an open space in which there are no reflections and obstructions, the signal strength performs much more unpredictable with increasingly complicated environment. The purpose is to show how great the effects of reflections and obstructions on RSSI characteristics. We demonstrate our methodology in a natural forest with highly dense of tall trees and bushes with only 2 sensor nodes, one is intended for transmitting signals and the other one is for receiving and measuring signal strengths. We then repeat the experiment in an obstructed basketball court and classrooms. We also configure the positions of the sensor nodes in some of the previous environments in order to show how signal strength changes at different elevations. It has been known that small changes in the height of the sensors from the ground can have a large impact on signal strength [1]. We show that the higher the sensor nodes, the stronger the signal strength since there is less obstructions at higher position. Section 2 discusses previous studies that have used RSS for distance estimation. Section 3 describes the implementation of our RSS-based ranging system and our technique to collect the signal strength readings. Section 4 illustrates the differences in signal strength behavior between indoor and outdoor environments. Section 5 focuses on the factors that may influence the signal strength behavior such as the elevation of the sensor nodes, the effect of obstructions, and reflections. 2 Related Work Radio Signal Strength (RSS) has evolved as a common technique used for ranging. Ranging is the process of estimating the distance between two nodes [1]. Many studies on RSS have been carried 1Environment characteristics Height of node Height of grass Transmission power Large room with cluster of chairs Higher position leads to lower attenuation rate and higher range - Higher transmission power yields lower attenuation rate and higher range Open field with low grass (8cm), but with tall trees and buildings Higher position leads to lower attenuation rate and higher range Short grass yields lower attenuation rate and higher range Higher transmission power yields lower attenuation rate and higher range Open field with tall grass (30cm) Higher position leads to lower attenuation rate and higher range Tall grass yields higher attenuation rate and lower range Higher transmission power yields lower attenuation rate and higher range Table A: Summary of the factors that have effects on radio signal strength. on recently to determine its accuracy and consistency for ranging. Most of the studies are majoring into the factors that may influence signal strength behavior in different environments. For example, obstructions, reflections, interference, and sensor nodes variability, can influence greatly the signal strength behavior. Table A summarize how the height of nodes, grass, and the different level of transmission powers affect radio signal strength. Three different environments have been chosen for the experiment purposes. First, the data collections took place in a large room indoor that is filled with chairs and other items. Then, the experiment is moved to a small field with low grass, but with several tall trees and buildings around. Lastly, a slightly different environment is selected, that is in a large open field with tall grass for the same experiment objectives. The results show that the height of nodes, height of grass, and transmission power, all yield the same effects on RSS characteristics. Furthermore, there are other factors that have significant impact on signal strength. Most systems that use RSS for ranging reported that indoor environment is not appropriate for RSS ranging because there is no correlation between signal strength and


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