# Reliability-Aware Cooperative Node Sleeping and Clustering in Duty-Cycled Sensors Networks
## 1 Introduction
Further, compared to the data transmission and forwarding strategy of the conventional beaconless geographic routing, the RCR-delivery performs better, as shown below:  
* Similar to the time division multiple access (TDMA-like) approach, the time slots of RCR-delivery are allocated by reference nodes (RN) in a centralized scheme before data transmission from one hop to the next hop. Simulation results showed that every time slot should be as short as possible. However, we should guarantee that PFs can receive data packets from other higher priority PFs before their timer expires. Thus, the value of each predetermined time slot should be long enough to accommodate the hop delay at least. Therein, a hop delay denotes the elapsed time from the moment a sensor node sends a data packet (to its farthest neighbor node) to the moment when a neighbor node receives it. Different from the three hand-shaking mechanism of beaconless geo-routing, the RCR-delivery performs with lower delay under the same packet delivery rate in order to avoid collision. * In traditional beaconless geo-routing protocols, the current packet holder (PH) typically broadcasts a probe message to PFs and waits for a reply. After receiving the first reply, which shows that a PF will be the next packet holder, the current PH transmits the data packet to the candidate by unicast and release memory. Different from such a high cost data forwarding scheme, the RCR-delivery does not determine which packet holder will be the next candidate. In addition, due to the centralized forwarding delays allocation of cooperative nodes, the collision ratio of RCR-delivery can be ignored. * In order to achieve a load balancing among in RCR, RN assigns priority to every cooperative node according to its residual energy information. In addition, almost all of the existing traditional beaconless geo-routing schemes cannot effectively guarantee network lifetime [6]. * Due to the dynamical scheduling of duty cycles of cooperative nodes, the RCR-delivery can guarantee on-demand ETE reliability while achieving energy efficiency. By comparison, almost none of the existing traditional beaconless geo-routing schemes can guarantee energy efficiency when transmitting sensory data because all of the PFs must be awake and stay active all the time to attend the election [7]. Generally, wireless sensor networks are powered by energy-limited batteries, which are almost impossible to charge or replace [8]. A common way to extend the lifetime of WSNs is to select one or a few sensors to remain in active status [9] while allowing others to sleep [10]. In RCR-delivery, we try to transmit the sensory data between cooperative nodes with a minimal number of hops for a given requirement for ETE reliability and energy saving. Thus, in this paper, we build an ETE reliability model with reliability-aware cooperative nodes to investigate the following problems: the relationship between the on-demand ETE reliability [11] and the number of cooperative nodes for a certain average node density, link failure ratio and duty cycle [12] at each hop; and the relationship between the on-demand ETE reliability and the duty cycle of cooperative nodes for certain \(k\) numbers of cooperative nodes at each hop. Furthermore, under the condition of a certain duty cycle, we design a sleep scheduling algorithm to achieve load balance and energy saving of cooperative nodes and prolong network lifetime. In our scheme, the cooperative nodes with lower residual energy have more chance to enter sleeping status. Table 1 gives the expression of the main notations.