## 2 Related Work  
Many researchers have proposed different MAC protocols in recent years with an aim to reduce the sensor nodes' energy consumption. The most common method is duty cycling [25; 26], which involves turning off the transceiver intermittently to reduce the amount of energy used. Next, a sender-initiated protocol is introduced [27] in which the sender sends a long preamble to seek the receiver's attention. This method has advantages over the synchronous MAC protocol because nodes in sender-initiated protocols do not need clock synchronization to create a schedule. A long preamble does, however, cause additional overhead and introduce a higher chance of collision. Following this, the long preambles have been converted to packets. The most popular MAC protocol that packetizes the preamble is X-MAC [32]. X-MAC has shorter preambles instead of one long one, allowing the receiver to send acknowledgements early between the preambles. However, the shorter preambles still cannot completely overcome the extra overhead. Following sender-initiated protocols, the receiver-initiated protocol has been introduced [28]. In this protocol, the receiver initiates contact and sends a beacon to get the attention of the sender. This introduces significantly less overhead than the preambles sent in the sender-initiated protocols. However, our literature review presented in this section found that even state-of-the-art protocols have not taken link asymmetry into consideration, and therefore may not work properly whenever asymmetric link occurs in real life deployment. In ADP-MAC protocol [33], the receiver uses polling mechanism to ensure the reception of the preamble packet sent by the sender node. However, it does not consider the link asymmetry situation at any side. As a result, receiver fails to know the ongoing preamble packet by the sender in the case of sender to receiver link asymmetry and sender fails to get the early acknowledge by the receiver in case of receiver to sender link asymmetry. PW-MAC [34] works over RI-MAC, and it predicts the wake-up time of the receiver based on previous sending schedule of the intended receiver. It has reduced energy associated with the idle listening of sender node that avoids early wake up. However, if receiver to sender link asymmetry arises, the sender wakes up in time without finding any beacon from the receiver. In the case of link asymmetry, the PW-MAC completely fails to utilize the prediction of wake-up time. A-MAC [35], another receiver initiated asynchronous MAC protocol, works similar to RI-MAC but differs in the way that sender sends an auto acknowledgement message to the receiver to indicate that it has data to send. However, in the case of link asymmetry, receiver cannot receive data even though sender sends data packet after sending an acknowledgement. In EnRI-MAC [36], a notification message is sent by the sender before it receives beacon from the receiver. The goal of sending wake-up notification is to inform other senders within the range of the same intended receiver that it has data to send. Despite getting beacons from the intended receiver, other nodes refrain from sending data in the current cycle. In the case of link asymmetry, other senders cannot get the wake-up notification. In such case, multiple senders transmit their data to the receiver resulting in collision.  
As already stated in Section 1, we explored the benefit of cooperative communication to avoid the asymmetric link problem. This section reviews related works divided into two broad categories: The asymmetric link based protocol followed by cooperative based protocol.