## 1 Introduction  
Wireless communication has made huge contributions to modern technology and information communication. Since the development of this technology, numerous protocols have been proposed to improve communication performances, and several other protocols continue to be introduced. To make new improvements, it is necessary to gain an in-depth understanding of previous and current issues and trends. In this paper, we have highlighted the significant improvements made in the area of wireless sensor networks (WSNs) regarding the routing mechanisms underlying energy-constrained sensor nodes. Our main focus is the ability of WSNs to balance energy consumption loads across battery-operated sensor nodes in order to extend network lifetime. The duty cycle [1] mechanism, which was introduced to reduce the energy consumption of sensor nodes, is arguably one of the greatest milestones in the development of WSNs. When one node performs packet transmission, other nodes in its neighborhood hold themselves from transmittingpackets that are unnecessary for them, and silently listen to the ongoing transmission. This idle listening results in a significant waste of energy. Although the duty cycle technique can reduce unnecessary energy consumption, it also introduces additional issues [2]. A node needs to know when the neighbor node wakes up in order to start its transmission, and for that the network falls into a time domain. Nodes using duty-cycling must use a synchronization mechanism that coordinates neighboring sensor nodes for smooth transmission. Nonetheless, multi-hop network time synchronization leads to a high control packet overhead, and border nodes have to match their duty cycle according to their neighbor nodes, which extends the duration of active time. Moreover, the energy consumption during synchronous duty-cycling increases sharply with one or multiple communications and parallel traffic flows, and as a result, synchronization does not scale well in large and condensed networks. On the contrary, the asynchronous duty cycle mechanism does not require any previous synchronization. During a less congested traffic load, a large number of available asynchronous MAC protocols minimizes energy consumption. Hence, using an asynchronous duty cycle over a synchronous mechanism in a WSN is more energy-efficient. Because our concern is in minimizing the energy consumption of sensor nodes, we consider traditional unicast routing to be too expensive for duty-cycled WSNs. In conventional unicast routing, nodes must wait for the destined node until it wakes up, which costs additional energy and time. On the other hand, opportunistic routing chooses its receiver opportunistically and provides dynamic routing progress, which is both energy and time efficient. In this study, we focused on advancements in the area of opportunistic routing with asynchronous duty-cycled WSNs. First, we prefer opportunistic routing over unicasting in asynchronous duty-cycled WSNs in order to decrease the energy consumption load on the sender node while it waits for the predefined forwarder node to wake up and receive the packet. Using other available routes for sending packets instead of waiting longer times for one specific node is more energy-efficient. Moreover, using different routes for different transmissions can also balance the load distribution of the energy-constrained WSNs. However, two major issues arise with this method. First, multiple forwarders wake up at the same time, and as a result, there is a high probability of collisions occurring among them while attempting to forward the same packet to the next hop. Second, the duplicate packet problem will result in unnecessary energy consumption. Many studies have been conducted to resolve these issues. The main objective of these protocols is to minimize the size of the forwarder set so that only the best forwarder node can forward the packet effectively and help the transmission run smoothly.