# A Channel Measurement-Based Listen-Before-Talk Algorithm for LTE-LAA and WLAN Coexistence
## 4 Simulation Results  
In this section, we present comparative simulation results of CM-LBT, F-LBT, and the conventional LB-LBT with a fixed contention window size and fixed CCA threshold, in terms of fairness, and reward value as described in Section 3.3. We developed a simulation program in C++ to model the CM-LBT, F-LBT, and conventional LB-LBT schemes, focusing primarily on their medium access control protocols. The CM-LBT scheme was implemented based on the operations described in Section 3, the F-LBT scheme followed the operations outlined in [5], and the conventional LB-LBT scheme was implemented according to the operations detailed in [7, 8]. We consider the coexistence network scenario presented in Figure 3. In this scenario, one LTE-LAA small cell coexists with multiple WLAN APs at a 5.8-GHz carrier frequency over a 20-MHz bandwidth. For the LTE-LAA system, \(n_{u}\) LTE-LAA devices are uniformly located within the cell with a radius of \(R_{l}\) for downlink transmissions. We assume \(n_{u}\) equals 10. For the WLAN system, \(n_{t}\) APs are uniformly distributed within the LTE-LAA cell, where \(n_{t}\) is set to 5 or 15. The number of WLAN nodes associated with each AP equals 5. The urban micro (UMi) non-line-of-sight (NLOS) for a hexagonal cell layout model is used to predict the path loss (PL) [25]. With \(r\) and \(f_{c}\) denoting the distance in meters, and the carrier frequency in GHz, \(PL=36.7\log_{10}{(r)}+22.7+26\log_{10}{(f_{c})}\) (dB). We can then obtain the corresponding values of \(K=10^{-7.254}\) and \(\eta=3.67\) in Equation (11) [19]. The WLAN transmission rate, \(R_{W}\), is set to 72 Mbps, and that of LTE-LAA can be calculated from Equation (16). It is assumed that \(S_{w}^{E}=R_{W}\). The medium access control and physical layer (PHY) header sizes are 272 bits and 128 bits, respectively. Moreover, the acknowledgment packet and payload sizes are set to 336 bits and 12000 bits, respectively. The values of \(T_{u,s}\), \(T_{u,c}\), \(T_{w,s}\), and \(T_{w,c}\) depend on \(R_{W}\), \(R_{U}\); the sizes of the headers, acknowledgment, and payload; and the lengths of DIFS, SIFS, and the propagation delay [10]. The propagation delay is set to 2 \(\upmu\)s. We assume every WLAN node has the probability of \(q_{w}\) that a packet will become available to the medium access control layer in a slot, i.e., \(\forall q_{i,j}=q_{w}\) for \(1\leq i\leq n_{t}\) and \(1\leq j\leq n_{i}\). We furthermore assume that packets arrive at the medium access controls of LTE-LAA eNB and WLAN nodes in a Poisson manner with rates \(\lambda_{p}^{(u)}\) and \(\lambda_{p}^{(w)}\), respectively, which can be obtained from Equation (22). In the F-LBT scheme, \(T_{SP}\) and \(T_{thr}\) equal 200 \(\upmu\)s and 20 \(\upmu\)s, respectively [26]. Unless otherwise specified, the following results were obtained assuming the parameters reported in Table 3, in particular, \(q=0.2\), \(q_{w}=0.01\), \(W=8\), and \(P_{cca}=-82\) dBm [5, 6]. Figures 4 and 5 illustrate the reward and fairness values versus \(W\) and \(P_{cca}\), respectively, in CM-LBT. Tables 4 and 5 show the average throughput and fairness of our proposed CM-LBT scheme for contention window size and CCA threshold, respectively. The throughput of each system in Tables 4 and 5 is presented with 95% confidence intervals. It can be observed that the reward value strongly depends on the fairness value. This is because small \(\epsilon=0.3\) in Equation (26) means that fairness is more important than the total system throughput to determine the optimal \(W\) and \(P_{cca}\). This is also due to the fact that the normalized total system throughput is relatively comparable under different \(W\) and \(P_{cca}\). Meanwhile, Figure 4 shows that, in the case of \(n_{t}=5\), the reward value increases to a peak value at \(W=12\) and thereafter decreases as \(W\) increases, whereas the reward value for \(n_{t}=15\) continuously decreases as \(W\) increases. Table 4 shows the throughput and fairness for \(n_{t}=5\) and \(n_{t}=15\).