## 1 Introduction  
Wireless Sensor Networks (WSNs) have been extensively investigated for various fields, e.g., agriculture and environmental sensing [1, 2]. In wild environments, sensor nodes, despite their limited battery capacity, are not attended to (no recharge, no battery exchange) any more once deployed. Therefore, the question of how to design energy-efficient protocols to extend network lifetime is a big challenge. Because of severe attenuation in wireless signal propagation, multi-hop communication is generally used to collect sensor data from sensor nodes to a sink node in large-scale WSNs. However, in this way, sensor nodes close to the sink node consume energy more quickly than other nodes due to packet forwarding, which may lead to network disconnection. Therefore, the energy efficiency of relay nodes is an important factor for extending the lifetime of a WSN. In previous research, a mobile sink [3] has been proposed to mitigate the above problem. In this method, a sink node can move and accordingly routes to the sink node and traffic flows change, distributing loads among sensor nodes. Nevertheless, a mobile sink has two problems. One is the overhead generated when nodes update routes to the sink node, and the other is the increase of a node's active time in monitoring aperiodic queries from the sink node. The first problem can be partly solved, e.g., by using asymmetric communication [4]. However, nodes must be active at least when receiving the routing information from the sink node. The second problem can be partially solved, e.g., by using wake-up control [5]. In the case of multi-hop communication, each sensor node needs to send a wake-up message to activate its relay node on the communication route. Because an ordinary communication module is often used for transmitting the wake-up message, this leads to large power consumption. This paper proposes an energy-efficient data collection method where sink-based centralized transmission scheduling (SC-Sched) is realized by integrating asymmetric communication and Wake-up Radio (WuR). This is based on the fact that the sink node and other sensor nodes are not the same. A sink node, typically mounted on a vehicle, has a stable power supply (using a rechargeable vehicle battery with a much larger capacity) unlike sensor nodes. Therefore, the sink node can use a large amount of power to transmit messages, directly reaching sensor nodes distributed in a wide area without relaying. Together with the wake-up control, it enables the sink node to directly control each sensor node. In this way, the proposed SC-Sched method reduces node active time by activating a sensor node only for data collection. In addition, it solves the problems of a mobile sink and improves network lifetime by exploiting sink mobility. Specifically, in the proposed method, sensor nodes do not update routing information according to the position of the sink node. Instead, the sink node collects node position information, and on this basis determines a tree-based routing and a transmission order. In addition, each sensor node is equipped with a wake-up receiver (WuRx). At the time that a packet is to be exchanged between two sensor nodes, the sink node transmits a wake-up message using a transmission power larger than that used for data transmission, directly activating both transmitting and receiving sensor nodes simultaneously to start the communication. Here, the communication path between the sink node and sensor nodes is asymmetric, one-hop in the downlink (from the sink node to sensor nodes), and multi-hop in the uplink (from sensor nodes to the sink node). In the initial experiments, it is noticed that severe packet loss occurs in the presence of multipath fading, and the data collection time is long due to delay occurring at the time of activating the transmitting and receiving sensor nodes by a wake-up message per hop.