# Time and Energy Efficient Relay Transmission for Multi-Hop Wireless Sensor Networks
## 2 Background
### Related Works  
In recent years, there has been an increasing interest in wireless networks. Since wireless nodes are usually energy constrained, the energy efficiency of a wireless network is an essential requirement to maximize the lifetime of the entire network. In [10], the authors have classified the major sources of energy waste in wireless networks such as collisions, overhearing and idle listening. In this paper, we propose new relay transmission scheme to eliminate the sources of the energy waste by using time division cluster scheduling and GTS mechanism. Network scalability is an important requirement in a wireless sensor network. Thus, there have been several research works aimed at improving network scalability. A real-time routing algorithm minimizing energy consumption was proposed in [11], where the authors have assumed a collision-free MAC protocol, and they have used a multicommodity flow model to schedule the optimal flows' paths in terms of energy consumption while not exceeding links' bandwidths. The routing algorithm ensures polynomial-time complexity but no scheduling is considered. In this paper, we present a cluster-tree routing protocol where the routing paths are unique, and the routing decisions are simple and time efficient. Our work also utilizes short addresses for routing decisions of relay nodes. The advantage of using short addresses for relay nodes is that they do not need to maintain routing tables to forward incoming data, since by simply looking at the destination address, they can decide to forward the packet upwards (to a parent) or downwards (to a child) [12]. [PERSON] et al. [13] proposed an algorithm for collision-free beacon scheduling in IEEE 802.15.4 cluster-tree networks, using the time division approach. In [13], nodes implement a distributed scheduling algorithm so that interfering coordinators maintain non-overlapping superframes. While a pure time-division approach solves the beacon frame collision problem, it limits the network scalability regarding the number of clusters. In [14], the authors proposed an algorithm to adjust the resource allocation for the cluster tree topology on the fly. The authors proposed this algorithm to apply to applications which deliver data to the root of the tree. However, this algorithm is not useful in the case of simultaneous flows in opposite directions. One of the most significant current topics of discussion regarding wireless networks is cooperative communication. The idea of cooperative communication is to improve the performance and coverage of the wireless network with the support of a relay node. Recently, various studies have been carried out with cooperative communication. Notably, early research on cooperative communication mainly focused on physical layer [6; 7; 8; 9; 15; 16; 17; 18; 19; 20; 21]. However, a lot of research is still needed for a practical high layer protocol to realize effective cooperative communications. From a physical layer's point of view, the source simply broadcasts its data and does not need to know about the relay node. However, from a higher layer's point of view, a link between the source node and the destination node should be established for non-broadcast services. Also, to select the relay node, the higher layer needs the information for neighbor devices. Different MAC protocol designs supporting cooperative communication among nodes are discussed and compared in [22; 23], where the corresponding authors proposed cooperative communication schemes to mitigate the throughput bottleneck caused by low-data-rate nodes. To address the throughput bottleneck, in [22; 23], a high-data-rate node is enabled to help a low-data-rate node through a two-hop transmission. However, two protocols based on the IEEE 802.11 DCF are not suitable for the IEEE 802.15.4 standard which is managed by the PAN coordinator. Also, two cooperative MAC protocols usually focus on a single-hop network, but the MACdesign for the wireless sensor network needs to extend to address a multi-hop link.