# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 4 Performance Evaluation  
### Simulation Setup  
Performance of the proposed protocol was evaluated using the ns-3 simulator [23]. In a 100 m \(\times\) 100 m area, APs are placed in a grid topology, and mobile nodes are randomly deployed (as shown in Figure 4). Once deployed, each mobile node selects and associates with an AP after scanning the area for beacons. In the simulations, we assume that every mobile node has a downlink traffic to receive from the AP. We use UDP traffic with 1472 bytes of packet size, and the MCS is set so that the link speed is 54 Mbps. For the path loss model, we use the log-distance model. We use the default values used in ns-3 for the parameters in the path loss model; the reference distance (\(d_{0}\)) is 1 m, the reference path loss (\(PL_{0}\)) is 46.67 dB, and the path loss exponent (\(\gamma\)) is 3. Transmit power and noise floor are set to 20 dBm and \(-93.97\) dBm, respectively. We compare four different techniques. The first one is denoted as \"Legacy\", which is the conventional IEEE 802.11 DCF with fixed CST, which is set to \(-82\) dBm. The second one is \"DSC\", which selects CST considering the proximity between pair of communicating nodes. For the parameters of DSC (shown in Equation (1)), \(C_{min}\) and \(C_{max}\) are set to \(-99\) dBm and \(-39\) dBm, and \(Margin\) was set to 25 dBm [2]. The third and the fourth schemes are the ones proposed in the paper. The third one, \"Model-based\", uses the path loss model to calculate the CST which is advertised in the preamble. The model-based scheme uses the log-distance path loss model, with \(\gamma=3\). The fourth one, called \"Measurement-based\" uses neighbor information exchanged through control messages to decide the advertised CST. For both schemes of the proposed protocol, the margin parameter is set to 6 dB. To evaluate performance, we use the following metrics. First, we measure the total throughput, which is the number of data bits successfully transferred in the network per second. With the same number of nodes and the same amount of traffic, higher spatial reuse will lead to more bits transferred in the same time period. Second, we measure the bottom 25% throughput. We sort the node throughput in the ascending order, and select 25% of the nodes with the least throughput. The bottom 25% throughput is the aggregate throughput of these nodes. As discussed earlier, a system can increase total throughput by letting a small subset of node dominate the channel, which leads to starvation for most of the other nodes. If protocol A achieves higher total throughput than protocol B but gets lower bottom 25% throughput, we cannot say that protocol A outperforms protocol B. On the other hand, if protocol A achieves higher number for both the total throughput and the bottom 25% throughput compared to protocol B, it is more probable that protocol A is reusing the space more effectively than protocol B. Third, we measure [PERSON]'s fairness index [24], which is a more direct metric used to measure fairness of a system. The equation for [PERSON]'s fairness index is shown in Equation (12). In the equation, \(x_{i}\) is the throughput of node \(i\), and \(n\) is the total number of nodes. \[J(x_{1},x_{2},...,x_{n})=\frac{(\sum_{i=1}^{n}x_{i})^{2}}{n\cdot\sum_{i=1}^{n} x_{i}^{2}} \tag{12}\]  
Fourth, we measure the delivery ratio of the system. Delivery ratio is calculated as the number of successfully received packets divided by number of transmitted packets. Packet losses can occur because of packet collisions, which are caused from two major reasons. First, since the wireless LAN uses random backoff for channel contention, it is possible that multiple nodes could finish counting down their backoff counters and start transmissions simultaneously. Other than that, packet collisions are caused by hidden terminals, which is related to CST. If CSTs are not properly set, it could create hidden terminals which increases packet collisions and decrease the delivery ratio.