A Study of Radio Signal Behaviors in Complex Environments

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 1 zxzxy 高亮 zxzxy 高亮 zxzxy 高亮 zxzxy 高亮 Environment 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 distance [3,12,13]. This shows that the reflections and obstructions can make a major impact on signal strength behavior. A study has reported that radio signals can take multiple paths while they transmit and their signal strength changes when they hit an obstacle [7]. The hitting on obstruction will results in two different signals, called the transmitted and reflected signal, respectively. Their report also shows that the strength of the transmitted and reflected signals depends on the angle at which they hit the obstructions [7]. However, there seem to have too many variances in the experiment. One of the problems is that no one can guarantee at which point, the original signal is going to hit on the obstacle even though the degree at which the signal is facing the obstacle is fixed. There is no ways to determine a signal transmission path because a signal can travel in multiple ways to reach the receiver. Therefore, we will only demonstrate how signal strength behaves in both indoor and outdoor environments with the present of reflections and obstructions. The effect of obstructions on RSS is certainly easier to be tested as compared to reflections because ones can manually place an obstacle between the sender and the receiver to see how that obstacle affects signal strength. However, it is difficult to test the reflections effects since reflections can cause by the floors, ceilings, walls, and other materials in the surroundings. In this paper, we have majored the effects of both obstructions and reflections in a more complicated indoor and outdoor environments in order to show that RSS is even worse for range estimation in those environments. 3 An RSS Ranging System Radio Signal Strength (RSS) ranging system works by measuring the received signal strength. The value of the signal strength can be obtained from the RSSI pin on the radio signal, and the RSSI value is inversely proportional to the signal strength. Greater RSSI value implies weaker signal strength, and vice versa. Throughout this paper, we will use the RSSI value as an indicator for the signal strength. The system is consists of a sender and a receiver. The 2 Maximum measurable distance vs Transmission power 0 20 40 60 80 100 120 140 -20dBm -13dBm 0dBm 5dBm 10dBm Transmission power M ax im um d is ta nc e Maximum distance Figure 1: The transmission power is linearly proportional to distance. Higher transmission power can be used to measure longer distance between a pair of sensor nodes. Figure 2: Outdoor Data was collected in an open flat parking lot with no obstructions. sender sets its transmission power to the highest value, 10dBm. The sender sends out messages continuously, and the receiver collects the RSSI value of each message at certain distance from the sender. For each 100 message received, the receiver computes the mean RSSI and output the mean value on the PC through an I/O port. This step is repeated by varying the sender at different locations. By comparing the RSSI value with some pre-defined RSSI models such as the linear RSSI model and the theoretical RSSI model, the distance between the sender and the receiver can be estimated. Before any RSSI values can be received, we need to know how far that a signal can transmits at different transmission powers. In order to test how transmission power relates to distance, we performed an experiment in which the sender is fixed and programmed with different level of transmission power. For each transmission power level, we measured the maximum distance in which the receiver can receive the signal that is transmitted from the sender. The experiment took place at the hallway on the third floor of the Engineering Building. The result of the experiment is shown in Figure 1. By increasing the transmission power, the sender can transmits the radio signal for a longer distance. When the transmission power increases from -20dBm to 10dBm, the maximum measurable distance also increases by up to 99.5ft. This corresponds well with the predicted result [1]. Each increment in the transmission power will increase the strength of the signal, and therefore, the signal can travel in a longer distance. 4 Signal Strength Behaviors in both Indoor and Outdoor Environments Different environments cause signal strength to act differently. We perform some experiments in both indoor and outdoor environments to illustrate this point. In these experiments, we fixed the receiver in the middle of the selected location and varied the sender at the distances of 10ft, 20ft, 30ft, 40ft, 50ft, 60ft, 70ft, and 80ft from the receiver. For each range, we measured the received signal strength. 4.1 Comparing Indoor with Outdoor Environments In this section, we compare the signal strength behavior in indoor and outdoor environments. The experiment took place at the third floor of the Engineering Building (indoor) and at the parking lot on Service Road (outdoor). Figure 2 shows the signal strength measurement in the open parking lot. The result of the experiment is shown in Figure 3. The result shows that signal strength is correlated with distance in the outdoor environment, but not in the indoor environment. In the open outdoor field, as the distance between the sender and the receiver increases, the strength of the signal becomes weaker. However, this is not the case in the open hallway. Signal strength does not correlate with distance. The signal strength fluctuates in an unknown pattern over distance. As shown in Figure 2, the signal 3 0 50 100 150 200 250 300 350 10ft 20ft 30ft 40ft 50ft 60ft 70ft 80ft Distance R SS I Open hallway (mean) Open field (mean) 0 50 100 150 200 250 300 350 10ft 20ft 30ft 40ft 50ft 60ft 70ft 80ft Distance R SS I Grassy f ield (mean) Open f ield (mean) Figure 3: Signal strength behaves differently in indoor and outdoor environments. Signal strength does not correlate with distance in the open indoor hallway. Figure 4: Signal strength behaves almost similarly in slightly different outdoor environments. The grassy field is a soccer field with short grasses. strength at the range of 30ft is even stronger than at the range of 20ft. This makes RSS-based ranging difficult in an indoor environment, even though the environmental factors are often held constantly indoor. However, the signal strengths appeared to be weaker (indicated by higher RSSI) in an outdoor environment than an indoor environment. Another disadvantage of using RSS ranging system in an outdoor environment is that the maximum range is smaller. As shown in Figure 3, the maximum range is only 60ft in the open field, which is smaller than in the open hallway. As a result, both the indoor and outdoor environments have their own disadvantages in using RSS- based ranging. 4.2 Comparing Different Outdoor Environments In this section, we compare the signal strength behavior in different outdoor environments. The experiment took place at a small soccer field next to Demonstration Hall and at the parking lot on Service Road as shown Figure 2. The result of the experiment is shown in Figure 4. No obvious distinction can be obtained from Figure 4. Again, the result shows that signal strength is correlated with distance in different outdoor environments. It shows that the greater the range, the smaller the signal strength. Those short grasses in the soccer field do not have a great impact on the signal strength. Therefore, the effects of short grass on RSS can be ignored. However, tall grass can have large effects on signal strength. The taller grass yields weaker signal strength and affects the range estimation accuracy [1]. Such effects can be minimized as long as the height of the grass is less than the height of the sensors from the ground. On the other hand, there are some other factors that may have a great influence on signal strength. These factors will be analyzed in the following section. 5 Environmental Effects on RSS In Section 5.1, we will demonstrate the effects of different elevations of sensor nodes on radio signal strength. Then, in Section 5.2 and 5.3, we will show the effect of obstructions and reflections on signal strength, respectively. 5.1 Height of the Sensor Nodes from the Ground In this section, we will demonstrate how the position of sensor nodes from the ground affects the radio signal strength. In order to show this, we performed the experiment in which both the sender and the receiver are placed on the ground at first, and then, we lifted them up to 2.5ft, 3ft, 3.5ft, and 4ft from the ground using two tripods for elevation, one for each sensor. The receiver is fixed and the sender is varied at the distances of 10ft, 20ft, 30ft, 40ft, 50ft, 60ft, 70ft, and 80ft from the receiver. The experiment took place at the open parking lot on Service Road as shown in Figure 2. The experiment setting is shown in Figure 5. Figure 6 shows the result of this. For the same experimental objective, we performed another experiment at two classrooms on 4 0 50 100 150 200 250 300 350 10ft 20ft 30ft 40ft 50ft 60ft 70ft 80ft Distance R S SI Open field (on the ground) Open field (2.5ft from the ground) Open field (3ft from the ground) Open field (3.5ft from the ground) Open field (4ft from the ground) Figure 5: Outdoor Data was collected in an open flat parking lot with two tripods to raise the sensor nodes. Figure 6: The effects of elevation of sensor nodes on signal strength in the outdoor environment. We use the tripods in order to raise both the sensor nodes. the first floor of the Engineering Building. Similarly, both the sender and the receiver are placed on the ground at first, and then, we lifted both sensors from the ground using desks. We placed the sender in one classroom and the receiver in the other classroom. We then varied both the sender and the receiver at the same distance from the wall that separated the two classrooms. The setting is shown in Figure 7. Figure 8 shows the result of this experiment. Both the sensor nodes in the previous two experiments are positioned on the same height. However, we are interested in determining how likely signal strength behavior changes if both the sender and the receiver are of different height from the ground. To show that, we performed an experiment in which we placed the sender on the ground and varied the receiver at the heights of 2ft, 2.5ft, 3ft, 3.5ft, and 4ft from the ground. In the experiment, we fixed the distance of 20ft between the sender and the receiver. We then repeated the experiment by changing the distance between the sender and the receiver to 40ft and 60ft. The experiment took place in the same location as shown in Figure 2. Figure 9 displays the experimental result. We later repeat the same experiment with slightly different settings, in which the receiver is placed on the ground and the sender is varied at different heights from the ground. Figure 10 shows the result of this experiment. Both Figure 6 and 8 showed that the height of the sensors from the ground can have a great effect on signal strength. It shows that the higher the position of the sensor from the ground, the stronger the signal strength. This conclusion holds for both outdoor and indoor environments. This corresponds well with the predicted result. The idea is that a signal can possibly get rid of most of the obstructions (eg. pedestrian, moving vehicle, and etc) if the sensors are positioned higher from the ground. How obstructions affect the signal strength behavior will be discussed in the Section 5.2. However, there is a difference between indoor and outdoor environment. There is a linear correlation between signal strength and distance in outdoor field, but not in indoor field. The results for indoor environment seem to be unpredictable due to the effects of obstructions and reflections. In the indoor environment, there are more obstructions such as walls. When a signal path is blocked, it can either passes through the obstacle and has weaker signal strength after passing it, or reflected from the obstructions. The effects of obstructions and reflections will be discussed in the following sections. On the other hand, there are less obstructions and reflection effects in the outdoor environment. The signals are not easily reflected by obstructions because the area of an outdoor environment is larger than an indoor environment. As a result, the RSS range estimation is believed to be more accurate if the sensor nodes are positioned higher from the ground. However, this result is not necessarily true if both the sensor nodes are positioned at different elevations. This can be seen from Figure 9 and 10 in which only one of the sensors is placed on the ground and the other one varied at various heights. Both figures show that signal strength does not rise linearly with height. One possible reason is