# A Self-Scrutinized Backoff Mechanism for IEEE 802.11 ax in 5G Unlicensed Networks
## 1 Introduction  
Fifth-generation (5G) wireless systems are becoming a priority for telecom operators, as 5G comes with the promise of unseen services and a broad range of new use cases and business models ranging from smart transport systems to smart agriculture and factories. 5G is expected to push the digitization of the economy further due to its ability to handle large volumes of data with low latency in real time. The evolution of the 5G era has promised an age of boundless connectivity and intelligent automation. 5G wireless networks will support 1000-fold gains in capacity, connections for at least 100 billion devices, and 10 Gb/s individual user experiences capable of extremely low latency [1; 2]. To support massive capacity and connectivity, the IEEE 802.11 ax high-efficiency wireless local area network (HEW) is promising as the foundation to evolve the 5G radio access systems to unlicensed band (namely described as, 5G-U) [3]. However, wireless local area networks (WLAN) will face huge challenges to access this unlicensed band, especially for highly dense user deployments. WLAN medium access control (MAC) protocol mainly focuses on maximizing the communication radio utilization using fair MAC layer resource allocation (MAC-RA) [4; 5] employing a carrier sense multiple access withcollision avoidance (CSMA/CA) scheme of distributed coordination function (DCF) for the Wi-Fi user equipment's (WEs) competition to access the medium. To achieve the maximum communication radio utilization through fair MAC-RA in the WLANs with the ever-increasing density of contending WEs, the CSMA/CA scheme of the current DCF is of great importance as a part of 5G-U [5]. The binary exponential backoff (BEB) scheme is the typical and traditional CSMA/CA mechanism, which was introduced in IEEE 802.11 DCF [6]. A randomly generated backoff value for the contention procedure is used. At the first transmission attempt, the WE generates a uniform random backoff value \(B\), from the contention window interval \([0,\ W_{cur}]\), where \(W_{cur}\) is initially set to the minimum value \(W_{min}\). After each unsuccessful transmission, \(W_{cur}\) is doubled until it reaches the maximum value \(W_{max}=2^{m}W_{min}\), for \(m\) maximum number of backoff stages (\(b\)), that is, \(b\in(0,m)\). Once a WE successfully transmits its data frame, \(W_{cur}\) is reset to the minimum value \(W_{min}\). For a network with a heavy load, resetting \(b\) to zero and contention window (W) to its \(W_{min}\) value after successful transmission will result in more collisions and poor network performance due to an increase in probability to select similar backoff value \(B\). Similarly, for fewer contending WEs, the blind exponential increase of \(W_{cur}\) for collision avoidance causes an unnecessarily long delay due to the wider range for selecting \(B\). Besides, this blind increase/decrease of the backoff window is more inefficient in the highly dense networks proposed for IEEE 802.11 ax enabling for 5G-U, because the probability of contention collision increases with the increasing number of WEs. Thus, the current MAC-RA protocol does not allow WLANs to achieve high efficiency in highly dense environments and become a part of future 5G-U, whereas the upcoming HEW will suffer from such unresolved issues as they will be required to achieve four times higher network efficiency, even in highly dense network deployments [3]. Hence, to withstand this challenge, WLAN needs a more efficient and self-scrutinized backoff mechanism to promise enhanced user quality of experience (QoE). This paper describes some of the use cases of HEW deployments for 5G-U networks. To solve the critical medium collision problem incurred by a large number of densely deployed contending WEs, a practical channel observation-based scaled backoff (COSB) mechanism [4] is presented in this paper.