# Time and Energy Efficient Relay Transmission for Multi-Hop Wireless Sensor Networks
## 1 Introduction  
A wireless sensor network is a kind of new wireless network that transmits sensing data to a destination device. Devices in the wireless sensor network operate on battery power and at low data rate since it is generally impossible to connect them to a power wire.  
The IEEE 802.15.4 standard is a new standard protocol for low-power and low-rate WPANs (wireless personal area networks) that defines the physical (PHY) and media access control (MAC) layers [1; 2]. The IEEE 802.15.4 standard can configure the wireless network using PHY and MAC layers and can provide a network service such as ZigBee and 6 LoWPAN to a higher layer [3; 4; 5]. The upper layer standard of the IEEE 802.15.4 standard defines a communication scheme between source devices and destination devices and can be applied to various applications. In particular, the IEEE 802.15.4 andZigBee standards are suitable for wireless sensor networks that require low power and lightweight communication solutions.  
The IEEE 802.15.4 standard configures the WPAN by connecting each device around the PAN coordinator, and the WPAN can include various sensor devices or coordinators. When a link quality between the PAN coordinator and an end device is good enough, the PAN coordinator can communicate with the end device directly. However, because sensor nodes are designed to operate on low power, they should communicate with each other using the limited transmission power and have problems such as a limited communication coverage. If the sensor device is out of communication range of the PAN coordinator, an auxiliary device needs to relay data frames between the PAN coordinator and the end device. In general, a routing protocol serves this multi-hop transmission. When the number of hop between the PAN coordinator and the end device increases, the transmission delay increases, and the network performance degrades. Also, because the existing routing protocols require more processing for multi-hop transmission in the network layer, they consume more energy. To address this problem, various approaches have been developed to maintain the energy efficiency at every layer of the protocol stack by offering new algorithms and protocols [6; 7; 8; 9]. However, the separation of layers has become an obstacle to improving the network performance. Also, when the IEEE 802.15.4 standard is extended to a multi-hop wireless network, all nodes in the network have to operate on the same duty cycle, and the number of nodes belonging to the same WPAN is limited.  
Also, in the cluster-tree topology of the legacy IEEE 802.15.4 standard, the coordinators must compete against their children devices to relay data frames. Therefore, when the traffic load or the number of devices increases, the network performance also decreases. Also, the IEEE 802.15.4 cluster-tree topology may not be able to provide sufficient bandwidth for the increased traffic load or the number of devices and may not deliver the additional information to manage the network successfully. The common drawback of the existing approaches is that they do not address the poor bandwidth utilization problem in IEEE 802.15.4 cluster-tree networks, so it is hard to increase the network performance. Therefore, to solve this problem, we propose new relay transmission scheme to reduce the transmission delay time and the energy consumption in the IEEE 802.15.4 cluster-tree networks. Also, we propose the enhanced beacon scheduling scheme to extend the coverage of wireless sensor networks and to optimize the wireless resource allocation.