# Reliability-Aware Cooperative Node Sleeping and Clustering in Duty-Cycled Sensors Networks
## 1 Introduction  
Due to the limitation of sensor nodes in wireless sensor networks (WSNs), we have to pay more attention to the reduction of node energy consumption and prolonging of network life. Meanwhile, due to the high dynamics of wireless links, reliable data transmission is also indispensable. The duty-cycled sensor networks provide a new perspective for improving the performances of multi-hop cooperative sensor networks, especially energy efficiency and reliability assurance, which rely on optimal relay assignment, relaying candidate selection and cooperative communication [1; 2]. In this paper, we first propose a novel routing scheme with high energy efficiency and high fault tolerance, named the reliability-aware cooperative routing (RCR), which is based on RCR-selection and RCR-delivery. We consider energy-efficient cooperative node sleeping and clustering problems in cooperative sensor networks where clusters of relay nodes jointly transmit sensory data to the next hop [3]. In addition, the key idea for guaranteeing reliability is to exploit the on-demand number of cooperative nodes (RCR-selection), facilitating the prediction of personalized end-to-end (ETE)reliability. Thus, a probabilistic ETE reliability model is built to compute an optimal duty cycle for RCR in the online manner. Besides the sensory data transmission strategy (RCR-delivery) between cooperative nodes with the minimal number under the condition of the given requirement of ETE reliability, it also controls the energy consumption. In RCR-selection, a source node initializes the network by a broadcast probe (PROB) message. Then, it transmits the probe message to find a first-hop reference node (RN). The RN selects \(k\)-cooperative nodes according to the residual energy and distance from its neighbors and calculates the location of the next-hop RN. The sensory data are transmitted by the cooperative node with the highest residual energy in every hop using a probe message, until the sink node receives the data packet. After the network has run for a certain period, the source node will resend a probe message to find a new set of cooperative nodes since the energy of nodes is reduced. In RCR-delivery, RNs will allocate the same length back-off delay to each of their cooperative nodes. A cooperative node with more residual energy will be assigned a time slot with higher priority. After predetermining the time slots, the sensory data will be broadcast by a packet holder during the data delivery phase. When receiving a data packet, each cooperative node fires its timer through the predetermined back-off delay. The one whose timer expires first, which is usually a cooperative node with the highest priority, will continuously broadcast the data packet. At the same time, the other cooperative nodes will notice that another node has forwarded the data packet by snooping the data message and quit the contention process immediately. Compared to the relaying candidate selection [4] and potential forwarders (PFs) selection of conventional beaconless geographic routing [5], the RCR-selection has the following advantages:  
* The PFs' selection strategy of traditional beaconless geo-routing relies on the shape and size of a certain sensory data forwarding region. However, the number of PFs in the forwarding region for each relay node (RN) is uncertain, and it fluctuates from hop to hop. Although some nodes have a relatively large number of PFs, the ETE reliability still cannot be guaranteed because some \"bottleneck\" hop has very few PFs. Compared to the beaconless geo-routing, the RCR-selection always ensure \(k\) cooperative nodes at each hop, to improve personalized ETE reliability.