## 1 Introduction  
Nowadays, increasing wireless nodes utilize the IEEE 802.11 mechanism to regulate the wireless channel utilization [1]. The wireless nodes often adjust dynamically data transmission rates according to the signal strength and interference in the environment to achieve better transmission performance. [PERSON] et al. [2] shows that performance anomaly occurs when different transmission rates simultaneously coexist in the network inevitably, the DCF mechanism in legacy IEEE 802.11 provides an equal long-term transmission opportunity for each node. Thus, if each node has the same bitrate and uses the same frame size, the throughput fairness can be achieved. Multiple bitrates are defined in the legacy 802.11 standard. However, the throughput fairness might sometimes severely degrade the network performance in multi-rate Wireless Local Area Networks (WLANs). The fundamental reason for the degradation is that the lower rate nodes occupy more channel time to transmit data frame, rather than the higher rate nodes. Many algorithms have been proposed to target different types of fairness, such as throughput fairness, time fairness, max-min fairness, and proportional fairness, to remedy this problem. However, in multi-rate WLANs, the ideal channel allocation fairness mechanism is that the channel occupation time is equal to all stations. It confirmsthat achieving time fairness among stations can tackle the performance anomaly, maximize the system throughput, and improve system performance in [3]. Besides, another serious problem is that as the network density increases; DCF also causes more collisions and heavily deteriorates the network performance. The legacy DCF adopts the binary exponential back-off (BEB) mechanism, when the node has data to send, it randomly selects a backoff counter from zero to contention window (CW). When the backoff counter reaches zero, the node can begin its transmission. However, as the nodes density increases, the probability of nodes choosing the same backoff counter is greater, which caused more collisions and degraded the overall performance. Fundamentally, the bigger CW can result in the wasting of a lot of idle time and the smaller CW can lead to more data collision in the heavy traffic scenario. Therefore, there is a pervasive viewpoint that channel access mechanism should be designed more sophisticated to overcome the highly challenging in dense network scenarios. Furthermore, various mechanisms for tuning the CW sizes were proposed in [4; 5; 6; 7; 8; 9] to mitigate collision and improve throughput. The issue that the network capacity will be severely underutilized when the legacy DCF operates as core part of WLANs has already been recognized. Recently, the standardization of IEEE 802.11 ax [10] has been initiated and one of its prime goals is to increase the capacity of Medium Access Control (MAC) in dense WLANs. According to [6] and [11], the orthogonal frequency division multiple access (OFDMA) technology has been introduced into 802.11 ax to improve the MAC layer. In 802.11 ax, a full channel is divided into multiple orthogonal sub-channels based on OFDMA technology, so as to support multi-user parallel access, and the AP is in charge of scheduling both downlink (DL) and uplink (UL) transmissions. In addition, in the UL transmission phase, a hybrid channel allocation mechanism, including random access and scheduled access, is also proposed [12]. In this paper, from another perspective, we focus on how to avoid the intensive channel contention and solve the performance anomaly in dense multi-rate WLANs. We introduce a decoupling MAC mechanism, namely decoupling MAC mechanism (DMAC), to achieve these goals. Motivated by the idea of hybrid channel mechanism in 802.11 ax, DMAC employs a hybrid channel allocation mechanism that integrates a distributed channel contention processing and centralized data transmission scheduling. In DMAC, the channel time is partitioned into the contention process (CP) and data transmission process (TP) by a decoupling channel access mechanism.