# Analysis of Human Body Shadowing Effect on Wireless Sensor Networks Operating in the 2.4 GHz Band
## 1 Introduction  
Wireless sensor networks have been the subject of intensive research in recent years. The development of highly miniaturized electronic integrated circuits has made possible the design of modern sensors that can perform very complex functions, such as measuring the physical parameters of an environment or the physiological parameters of people, and transmit this measurement data via a wireless link to the receiver. Wireless sensor networks have found applications in many fields, including agriculture [1], marine environment monitoring [2], ambient assisted living [3], industry [4], and healthcare [5]. Wireless sensor networks often operate without a dedicated licensed frequency band, utilizing open bands in the existing radio environment (e.g., 2.4 GHz Industrial, Scientific, Medical--ISM) that do not require licenses [6]. Because the transmit power of open bands is limited, they are suitable when the operating range of a single node is not very large (usually tens to hundreds of meters). On the other hand, whole networks composed of numerous nodes can cover an entire building or even a group of buildings [7]. Use of the 2.4 GHz band for wireless sensor networks is greatly facilitated by the large numbers of miniature transceivers available. There are also many transmission protocols suitable for wireless sensor networks, such as IEEE 802.15.4, Bluetooth, and ANT [8]. The capacity of miniature batteries still places an important limitation on the performance of wireless sensor networks with very small nodes [9; 10]. This limitation also affects the performance of the wireless links that are used to transmit data from the sensor nodes. Due to the constraints on the available power resources, designers need to minimize the transmit power and lower the fading margin in the wireless link power budget. One of the important factors to consider in wireless sensor network design is the path loss that occurs, depending on the physical environment, between the transmitter and the receiver. In high frequency bands (with frequency measured in GHz), path loss can be increased by the presence of additional obstacles in the transmission path, in particular by people in proximity to the transmitter. This effect is called body shadowing, and can have an important impact on the performance of wireless systems. Figure 1 shows two simplified scenarios in which wireless transmission between sensors occurs with and without human body shadowing. In the first case (Figure 1a), there is no obstacle between the transmitting node 1 and receiving node 2. Assuming signal attenuation in an open environment there will be successful transmission. In the second case (Figure 1b), the presence of a human body in the transmission path between the nodes causes additional attenuation of the wireless link and body shadowing occurs. If the wireless sensor has a low power transmitter and low fading margin, this may lead to a loss of packets or even break the wireless link. Therefore, it is important for designers of wireless sensor networks to consider the additional path loss introduced by human bodies. The body-shadowing effect has been investigated by researchers working on wireless systems as well as on body area networks in a very wide range of frequencies [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. Statistical analysis is often used to model this effect, including the time variation caused by people in motion and the properties of the indoor environment, which can influence the propagation of the signal [13; 14; 15]. Other research has applied the experimental approach, taking numerous measurements of path loss due to human bodies in typical propagation environments [16; 17; 18]. Based on the results, propagation models have then been constructed with specific parameters that can be modified for different operating scenarios [19; 20]. The 2.4 GHz band is widely used for wireless transmission systems because in most countries, this is an unlicensed band.