# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 4 Performance Evaluation
### Simulation Results  
#### 4.2.1 Varying Number of Nodes  
In the first experiment, we have placed 100 APs in the area, and deployed varying number of nodes to study the impact of node density. The number of nodes was varied from 20 to 200. The result is shown in Figure 5.  
First of all, the total throughput of all protocols increases with the number of nodes up to a certain point, and then start to decrease. This means that when the number of nodes is small, the space is not fully used and there is room for more concurrent transmissions. However, when the number of nodes becomes large, the space becomes saturated and the throughput degrades due to increased packet collision rate. The increase in packet collisions is reflected in the packet delivery ratio. While the total throughput increases with number of nodes, the bottom 25% throughput rapidly degrades as the number of nodes increase. This means that the channel share becomes less fair, as also can be observed from the fairness graph. With more nodes, the chance of packet collision increases, because the probability that two or more nodes finish the backoff at the same time increases. When two nodes transmit simultaneously, there is higher chance that the packet transmitted from a node closer to the AP survive, compared to the packet transmitted from an edge node. Once an edge node loses packet, it will double its contention window to avoid future collisions, which further reduces the channel share of edge nodes. Thus, more packet collisions lead to lower fairness.  
Figure 5: Performance of protocols varying number of nodes. 100 APs are placed in a grid topology, and 20 to 200 nodes are randomly deployed in the area. (**a**) total throughput, (**b**) bottom 25% throughput, (**c**) [PERSON]’s fairness index, (**d**) packet delivery ratio.  
Comparing the protocols, we can observe that DSC achieves the highest total throughput. However, the bottom 25% throughput of DSC is almost zero for most cases. This means that DSC favors small number of mobile nodes that are closely located to their APs. This behavior of DSC is also reflected in the fairness, where DSC shows the lowest fairness index among all protocols. The delivery ratio of DSC is the highest, but it is because transmission of edge nodes is suppressed by the center nodes.  
The proposed protocol achieves higher total throughput compared to the legacy 802.11 DCF. More importantly, this total throughput is achieved while maintaining the bottom 25% throughput. For nodes that are close to APs, higher throughput is achieved by setting CST high and aggressively transmitting concurrently with other nodes. Meanwhile, transmissions by the edge nodes are protected by advertised CSTs. Thus, the throughput of the bottom 25% nodes is maintained. In terms of fairness, the proposed protocols achieve lower fairness index compared to the legacy protocol. This is because while the throughput is increased for center nodes, the throughput of the edge nodes remains similar when the proposed protocol is used.  
Comparing the model-based and the measurement-based schemes, the measurement-based scheme achieves higher total throughput, and similar bottom 25% throughput compared to the model-based throughput. The model-based chooses the advertised CST by assuming that an interfering node exists at the farthest position from the node, which is a conservative approach. The measurement-based scheme, on the other hand, chooses CST to block the potential interfering node based on RSSI measurement, which could result in higher CST compared to the model-based scheme. Please note that in this experiment and the following experiments, the path loss exponent of the path loss model used in the model-based scheme and the simulation is the same. Later, we study the impact of mismatch in the path loss model.