# A Channel Measurement-Based Listen-Before-Talk Algorithm for LTE-LAA and WLAN Coexistence
## 1 Introduction  
With the proliferation of mobile devices and diverse mobile applications, mobile network operators are experiencing phenomenal mobile data growth worldwide [1]. To meet this challenge, mobile network operators have increased frequency reuse and area spectral efficiency by deploying many small cell base stations [2]. Because the small cells currently operate on licensed bands, the network capacity should be upper-bounded by the availability of these licensed bands. This motivates the operators to utilize the readily available unlicensed spectrum. Thus, 3 rd Generation Partnership Project (3 GPP) has recently initiated a study item on standardized Long Term Evolution (LTE)-License-Assisted Access (LAA) technology to satisfy increasing mobile traffic demands in cellular systems. However, a major concern highlighted by the 3 GPP is the issue of ensuring an efficient coexistence between LTE-LAA and other existing unlicensed systems, especially widely deployed wireless local area networks (WLANs). Without appropriate coexistence considerations, the operation of LTE-LAA systems may incur continuous interference with WLANs because the latter rely on contention-based medium access control and will back off when detecting LTE-LAA transmissions. The European Telecommunications Standards Institute (ETSI) presented a listen-before-talk (LBT) specification for time-frame-based devices to enable the channel to be sharedwith the incumbent WLAN systems [3]. LTE-LAA with the LBT feature was included in 3 GPP Release 13 as a global standard for coexistence in the unlicensed band [4]. In the ETSI specification, two types of LBT schemes are defined: frame-based LBT (FB-LBT) and load-based LBT (LB-LBT). In FB-LBT, the LTE-LAA eNodeB (eNB) is required for listening to the target channel to detect the energy level during a certain interval, the clear channel assessment (CCA) period, before transmission. The LTE-LAA eNB can occupy the channel for a fixed duration, known as the channel occupancy time, only if the interference level is below a predefined CCA threshold. In FB-LBT, the CCA period is fixed to a specified duration, regardless of the channel congestion status. As a result, the unlicensed spectrum is occupied by the WLAN system far more frequently than by the LTE-LAA system, particularly when the WLAN network is congested [5]. This occurs even when the LTE-LAA eNBs have data to transmit. Moreover, when LTE-LAA is used for supplementary downlink purposes, a single LTE-LAA eNB must compete with multiple WLAN nodes for channel access. Consequently, the LTE-LAA eNB often fails to access the channel, as it is frequently sensed as busy with a high probability [6]. To address this issue, LB-LBT mechanisms have been extensively studied [7; 8]. These mechanisms incorporate sensing and backoff functions, similar to the IEEE 802.11 distributed coordination function, in addition to the original LTE medium access control scheduling. [PERSON] et al. [7] demonstrated through simulations that the CCA threshold and contention window size are critical for balancing the performance between LTE-LAA and WLAN systems. The contention window size defines the maximum time the LTE-LAA eNB waits before initiating data transmission, while the CCA threshold specifies the interference level used by the LTE-LAA eNB to determine whether the channel is idle or busy. [PERSON] et al. showed that these parameters must be carefully configured to effectively protect nearby WLAN transmissions while avoiding excessive protection. In the LB-LBT scheme, the contention window size and CCA threshold of the LTE-LAA eNB have a significant impact on its transmission probability and the interference caused by concurrent WLAN transmissions outside the CCA range. However, in most existing LB-LBT schemes, the contention window size and CCA threshold remain fixed. [PERSON] et al. [8] identified the optimal contention window size for maximizing total throughput in the unlicensed band under a saturated traffic assumption.