## 1 Introduction
The higher is the value, the better is the link quality and vice versa. Packet Reception Ratio (PRR) is a software-based technique that measures link quality by calculating the proportion of packets that were successfully received to the number of packets transmitted in total. The absolute difference between PRR of sender to receiver and PRR of receiver to sender is used to identify the link asymmetry [20]. Several researchers analyze this difference to classify the wireless link. If this difference is greater than 1%, the link is asymmetric [21]. According to [PERSON] et al. [22], when the uplink and downlink PRR difference is greater than 40%, then the wireless link is considered asymmetric. [PERSON] et al. transmitted 100 packets at different levels of power to analyze asymmetric  
Figure 1: Asymmetric link situation. links [13]. Table 1 shows that, at high power levels, links are mostly symmetric, but, as power level decreases, link asymmetry sees an increase. Link asymmetry has significant impact on the quality of connectivity [23]. It degrades the overall network performance. In [24], [PERSON] et al. showed that protocols that do not consider link asymmetry may work fine in simulation but fail completely in real life deployments. Hence, link asymmetry is one major but challenging issue in designing a MAC protocol for WSNs. A significant portion of radio links, observed in recent studies, are asymmetric in typical WSN deployment. It is also observed from test-bed results that asymmetric links mostly occur due to low transmission power. In such cases, nodes fail to transmit data which eventually make the network almost non-functioning. Therefore, considering link asymmetry while designing a MAC protocol is indispensable. Battery power limitation is another key challenging issue in WSNs. \"Synchronous duty cycling\" is a preliminary approach to reduce energy consumption [25; 26]. However, it needs clock synchronization, which increases the system overhead. To overcome this problem, \"asynchronous duty cycling\" approach is introduced that saves more energy compared to the synchronous approach. Asynchronous protocols work in two modes: sender initiated mode and receiver initiated mode. In sender initiated mode, sender sends preamble [27] and in receiver initiated mode, receiver sends beacon to seek attention of the transmitting node [28]. Previous finding shows that receiver initiated mode is more energy efficient than sender initiated mode [29]. Asynchronous MAC protocol in receiver initiated mode is therefore drawing much attention from the researcher community for its inherent capability of saving more energy. However, link asymmetry is a key factor that adversely affect receiver initiated asynchronous MAC protocols to a great extent where nodes mostly remain in sleep mode to save more energy. An extensive literature review presented in \"Section 2: Related Work\" shows that ASYM-MAC [30] works on an asynchronous mechanism that sufficiently takes the link asymmetry into consideration. Nodes can keep a routing table that stores the link status of surrounding nodes. They can exchange a beacon message to find asymmetric links and update periodically. However, wireless sensor nodes are memory constrained. Moreover, extra periodic beacon exchange increases the possibility of collisions as well as energy consumption. To avoid the asymmetric link problem in low power asynchronous WSNs, we have been motivated to exploit cooperative communication without an extra control message exchange. One of the apparent solutions to avoid the link asymmetry is maintaining and periodically updating the neighborhood table. Unlike our previous work reported in [31], here we consider a tree-based protocol. None of the existing work has used cooperative communication with a tree-topology with consideration to the asymmetric link situation to the best of our knowledge. In the preliminary version of this work, we avoid link asymmetry through cooperative communication, but we do not select an efficient cooperative node.