# Novel Reliable and Energy-Efficient Routing Protocols for Wireless Sensor Networks
## 1 Introduction  
A wireless sensor network (WSN) is a distributed system consisting of several nodes, and one or more base stations (BS). The nodes are typically small and cheap. BS's have higher processing capabilities and a wired electrical power source. The sensor nodes are equipped with application-specific sensors and a mini central processing unit. There is also a wireless radio transceiver attached to each sensor node; it provides the node with the capability to communicate with each other and with the BS. These nodes often operate on their battery power and use unreliable radio channels corrupted by noise and fading. As a result, the primary concern, when designing and operating such a network, is to maximize the life span and the data throughput of the nodes [1]. Packets are sent to the BS either in a single-hop manner or in a multi-hop manner. In case of single hop, the node sends the generated packet directly to the base station, wherein a multi-hop manner, source nodes send packets to BS via multi-node path, each node of the path forwards the received (or, in the case of the source node, the generated) packet to another node until the BS receives the packet [2]. Packets are transmitted to the BS via unreliable radio channels corrupted by noise and fading [3]. Many techniques have been developed to increase the energy efficiency of WSNs, covering almost all layers of communication protocols ranging from the physical layer to the application layer [4, 5]. Routing is instrumental when designing and operating WSNs; the researchers in [6] describe a group of routing metrics and constraints; they provide a guideline for the composition of these metrics to achieve efficient routing schemes. Load balancing and reliable information transfer are contributory factors in energy-efficient communications. Lack of load balancing causes the death of some nodes too early, which breaks theconnectivity of WSN and reduces the life span of the WSN [7]. The lifespan is defined as the length of time interval until the first node goes flat [8]. The reliability in WSN categorized into three classes: packet reliability, event reliability, and packet-event reliability. In this paper, we focus on packet reliability. It is classified to up-stream packets, (which mostly represents data packet), and down-stream packets, (which generally represent control packets) [3]. Low reliability leads to high packet loss ratio in the case of low energy consumption. Most protocols achieve energy efficiency by enforcing uniform energy distribution and maximum longevity, but they do not provide reliable links where lost data means wasted energy. Also, they don't use distinct measure for the uniformity of residual energy distribution. In this paper, we try to strike a good balance between reliability and energy consumption (longevity). Our concern in the first part of this paper is to develop novel routing algorithms that maximize the minimum residual energy subject to a predefined reliability target. To achieve this, we propose the Maximum of the Minimum Residual Energy Protocol (MMREP). According to the proposed method, we select a path by which a packet is sent to the BS over the nodes, of which the minimum remaining energy is the maximum subject to the reliability constraint. The reliability constraint means that the packet will reach the BS with a given probability. This strategy can significantly extend the life span of WSNs. In the second part of this paper, we improve the performance of the basic MMREP in terms of energy efficiency and complexity. We propose an Optimal Residual Energy Based Protocol (OREBP). In OREBP, we use the entropy-like function to measure the uniformity of the residual energy distribution. The packets are sent to the BS in a multi-hop manner. The protocol guarantees that the distribution of the residual energy will remain as close to uniform as possible. We will demonstrate that this method achieves optimal load balancing. In this paper, we introduce two novel routing algorithms.