# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 1 Introduction  
Recent years have seen a rapid growth of using wireless sensor networks to monitor the physical world [1; 2; 3; 4; 5; 6]. A wireless sensor network consists of many tiny sensors deployed in the field; these sensors can detect temporal-spatial physical signals and transmit their measurements to end users through wireless (radio) links. The wireless links are capable of interconnecting the sensors into a communication network and then allow end users both to remotely observe the sites that interest them and to obtain a great deal of sensory data that contribute to more accurate decision making. As a promising instrument for collecting data about the physical world, therefore, wireless sensor networks have been adopted in a variety of monitoring applications, including the natural environment, the military field, urban traffic, building structure health, and so on. Wireless communication greatly facilitates the monitoring system deployment and potentially saves human resources in data collection, yet it has to be carefully considered in reality [7; 8; 9; 10; 11]. In practical wireless sensor network systems, the energy efficiency is a key performance metric, because it dominates the longevity by which the system can serve end users; the radio chip or the transceiver equipped in wireless sensors is the most energy hungry component. To date, in both industry and academia, researchers and engineers en masse have moved to the low-power wirelesscommunication, aimed to fundamentally reduce the energy consumption of sensors in communications. The IEEE802.15.4 [12], for instance, is a typical low-power wireless standard, which usually works at the 2.4-GHz or the 868/915-MHz ISM band, and supports a 250-kbps data rate with the energy budget of only at most a few milliwatts. To be compatible with the IEEE 802.15.4 standard, more and more radio chips, such as TR1000, CC1000, CC2420, etc., are fostered and widely adopted in practice. From the past engineering experiences, the involvement of low-power wireless links is an inevitable choice in wireless sensor networks aimed at long-term monitoring. Such low-power radio links, however, often lead to unreliable communication or data transportation in wireless sensor networks; and sometimes, they are rendered with high dynamics, especially when they are deployed in harsh environments with more obstructions or interference sources. Inherently, the radio signal propagation will be reflected, scattered and diffracted by the surrounding objects [13; 14]; these effects are more significant and unpredictable for low-power wireless links [15; 16]. In particular, the signals emitted by the low-power, low-cost transceivers of wireless sensors are easily distorted by the overspreading internal and ambient noise. Though wireless sensor networks and traditional wireless networks share several link characteristics, low-power links are more lossy and time-varying in coverage [17]. Therefore, understanding the low-power radio link well is very critical and indispensable to design effective and efficient wireless sensor networking protocols, such as medium-accessing control (MAC), routing and network topology control. This paper attempts to investigate the performance of the low-power wireless link in forest environments. Our study is motivated mainly by two facts. First, large-scale and long-term forest monitoring has emerged in recent years as an effective way by which domain scientists can comprehensively investigate the forestry resources, as well as the in-forest environmental parameters and then accurately model the natural evolution related to forests [6; 18; 19; 20]. For instance, combining the forest data and the weather data, environment scientists can study the relationship between climate change and the growth of trees or they can evaluate the function of trees in conserving soil and water. Wireless sensor networks, however, are facing new challenges in forest monitoring due to the complex forest environment.