## 1 Introduction  
Long-range directional wake-up radios (LDWuR) can be an important technology for reducing the energy consumption of nodes and improving the communication range in wireless mobile networks. For nodes that must keep their radios on for accepting incoming commands or packet relaying, idle listening can dominate the total energy consumption of the sensor nodes. Although low-power listening MAC protocols have been proposed to reduce idle-listening energy by duty cycling the receiver, they incur long wake-up latency, while the energy consumption is still lower-bounded by the duty cycle. WuRs represent a promising solution to completely eliminating the idle listening cost and significantly extending their lifetime. We illustrate LDWuR through three examples. Clemson's Intelligent River project Clemson (1999) deploys sensor nodes along the 312-mile Savannah River from the headwaters in North Carolina to the port in Savannah in a large scale. The sensor network provides real-time data on water quality and quantity. The sensor data are important for improving water resources management regarding drinking water and industrial pollutants. The other example is the IBM smart water project Clemson (1999), which distributes a sensor network along the 315 miles of the Hudson River to collect and analyze data continuously and in real time. Another example is the flying fox tracking project Savannah (1999), where a resourceful base node needs to communicate with resource-constrained collar nodes on demand with the maximum possible range. These applications exemplify long-range sensor network deployments, which motivate us to take the long communication range into account. To achieve a long communication range of the WuR, we adopt directional antennas to extend communication distance by increasing the antenna gain. As existing low-power WuRs provide short-range communication, they adopt modulation and coding schemes to improve the distance by reducing the packet error rate (PER). However, they provide a limited improvement on the communication range. A directional antenna represents a promising solution that can bring an order of magnitude improvement to communication distance. A directional antenna focuses the radiation beam in one direction to bring more antenna gain. We take advantage of direction antennas to make long-range communication practical. The challenge with adopting directional antennas is to search the direction in which the transmitter locates. As the directional antenna focuses the radiation beam in one direction, the transmitter might not lie in the same direction. This incurs the overhead of a motor to rotate the antenna in all directions [PERSON] (2000). To address this, our proposed LDWuR adopts static and dynamic strategies. A static strategy means that the LDWuR uses multiple directional antennas in different directions. A dynamic strategy means that the LDWuR uses multiple omnidirectional antennas that cooperate to form beams in different directions. To evaluate our LDWuR and strategies, we design a MAC protocol and system model to investigate salient characteristics in terms of total power consumption and latency. We design an active LDWuR. Our design is a battery-powered radio. It employs the same battery with the node's data radio. For a long-range communication application in wireless mobile networks, the requirements of WuR aretwo-fold. First, we adopt high-gain directional antennas to enhance the communication range with the same power consumption. The characteristic of the directional antenna supports less interferences from other directions, which improves the wake-up signal and noise ratio. From the relationship of the power consumption and packet rate in Figure 7a, our design consumes about 0.0276 W with short data packets at a packet rate of one packet per second. Its power consumption is close to the existing work [5]. Our WuR design still needs to be optimized to reduce its power consumption further to compare with existing WuRs. That will be our future work.