# A Channel Measurement-Based Listen-Before-Talk Algorithm for LTE-LAA and WLAN Coexistence
## 3 Proposed CM-LBT Scheme
### Clear Channel Assessment at LTE-LAA eNB  
The CCA threshold is an important component in the LBT-based coexistence system because it is the level of sensitivity required for detecting the existence of ongoing transmissions. In particular, controlling the CCA threshold at LTE-LAA eNB plays a key role in the performance of LTE-LAA and WLAN systems [7]. Lowering the CCA threshold makes the LTE-LAA eNB more sensitive to ongoing WLAN transmissions. Therefore, it refrains from transmitting more frequently. Conversely, a high CCA threshold increases interference from concurrent WLAN transmissions. In this situation, we develop an analytical model to investigate the LTE-LAA eNB transmission rate according to the CCA threshold. In Section 3.3, we then estimate the throughput of LTE-LAA and WLAN systems based on this model. We consider a scenario where LTE-LAA and WLAN systems coexist, as illustrated in Figure 3. It is assumed that LTE-LAA eNB uses an omnidirectional antenna and transmits with fixed power \(P_{e}\). Let \(R_{I}\) denote the radius of the interfering range. We then assume that a WLAN APs or nodes in the interfering region can cause interference with the LTE-LAA device. With \(R_{cca}\) and \(P_{cca}\) denoting the radius of the CCA range and CCA threshold, respectively, we employ a propagation model to obtain \(R_{cca}\) as follows [17]:  
\[R_{cca}=\bigg{(}\frac{P_{cca}}{KP_{e}}\bigg{)}^{-1/\eta}. \tag{11}\]  
where \(K\) is a constant and \(\eta\) is the propagation loss exponent. Figure 3: Interference from WLAN systems when LTE-LAA device \(u\) is located (**a**) inside and (**b**) outside the CCA range of the LTE-LAA eNB (the shaded area is the interfering area). Let us consider LTE-LAA device \(u\) at distance \(r_{u}\) from its serving LTE-LAA eNB as shown in Figure 3. We use the signal-to-interference plus noise ratio (SINR) as the criteria for radio quality. Let \(P_{rx,u}\) and \(P_{I}(R_{cca})\) denote the received signal power at the LTE-LAA device from the LTE-LAA eNB and the cumulative interference from multiple concurrent WLAN transmitters when the CCA threshold is \(P_{cca}\), respectively. The SINR of the LTE-LAA device \(u\) is as follows:  
\[\gamma(R_{cca})=\frac{P_{rx,u}}{P_{I}(R_{cca})+P_{N}}. \tag{12}\]  
where \(P_{rx,u}=Kr_{u}^{-\eta}P_{e}\), and \(P_{N}\) is the noise. To estimate \(P_{I}(R_{cca})\) at LTE-LAA eNB, we consider the interference from a fixed finite number of interfering WLANs as a continuum field throughout the network [17; 18]. In this context, the network is characterized by the densities of active APs and WLAN nodes, denoted by \(\lambda_{a}\) and \(\lambda_{s}\), respectively. Let \(A_{I}\), \(I_{a}\), \(\rho_{i}\), and \(\sigma_{i}\) denote the interfering area, the set of interfering APs, the ratio of downlink transmissions to all transmissions in the basic service set managed by AP \(i\), and the channel utilization of the basic service set managed by AP \(i\), respectively. In the case of the intra-operator scenario, LTE-LAA eNB can obtain this information from the APs. Otherwise, they are measured by the channel monitoring of LTE-LAA eNB. Then, \(\lambda_{a}=\sum_{i\in L_{a}}\sigma_{i}\rho_{i}/A_{I}\) and \(\lambda_{s}=\sum_{i\in L_{a}}\sigma_{i}(1-\rho_{i})/A_{I}\), where \(A_{I}=\pi(R_{I}^{2}-R_{cca}^{2})\)[17]. Each area element \(vdvd\theta\) at distance \(v\) from LTE-LAA device \(u\) contains interfering APs and WLAN nodes, where the total number is \((\lambda_{a}+\lambda_{s})vdvd\theta\). Their contribution to \(P_{I}(R_{cca})\) is \((\lambda_{a}P_{a}+\lambda_{s}P_{s})vdvd\theta Kv^{-\eta}\), where \(P_{a}\) and \(P_{s}\) are the fixed transmit power values of an AP and a WLAN node, respectively. Using the sine law, distance \(v\) can be approximated according to the scaling factor \(\alpha\) and angle \(\theta\), as shown in Figure 3a, as follows:  
\[v(R_{cca},\alpha,\theta)=