# Improving Spatial Reuse of Wireless LAN Uplink Using BSS Color and Proximity Information
## 4 Performance Evaluation
### Simulation Results  
#### 4.2.1 Varying Number of Nodes  
In the first experiment, we have varied the number of user stations in the network to study the impact of node density on the protocol performance. The number of stations was varied from 20 to 200. The results are shown in Figure 8.  
Figure 8: _Cont._  
In Figure 8a, we can see that the PSR achieves the highest total throughput, followed by PSC-UL. PSC-UL achieves higher throughput compared to Legacy, OBSS/PD, and Dual-CST. As the number of stations increases, the difference of throughput between PSR, PSC-UL, and the other three methods increases, meaning that PSR and PSC-UL find more opportunities for parallel transmission compared to other methods. Although PSR achieves higher total throughput compared to PSC-UL, PSR suffers from significant unfairness, as can be observed from the other graphs. Figure 8b,c shows bottom 50% throughput and bottom 25% throughput. As the number of stations increase, these values decrease for all methods. Especially for PSR, almost no throughput is gained for bottom 50% nodes. This is also reflected in Figure 8d,e, which shows the Jain's fairness index and non-starvation ratio. While the fairness index of PSC-UL is similar to Dual-CST, a much lower fairness index is achieved by PSR and OBSS/PD.  
This occurs because the fairness index is low for SNR, as shown in Figure 8e; too many nodes are starved. PSR is designed so that the spatial reuse opportunity is taken only when the on-going communication can be protected. It does not guarantee that the parallel communication will succeed. Naturally, nodes that are far away from their APs tend to lose packets in parallel transmission, while nodes close to their APs have their packets  
Figure 8: Performance of methods varying number of nodes. (**a**) Total throughput; (**b**) Bottom 50% throughput; (**c**) Bottom 25% throughput; (**d**) Fairness; (**e**) Non-starvation Ratio; (**f**) Delivery Ratio.  
successfully received. This is because OBSS/PD has low fairness index as can be seen in Figure 8f, which shows the delivery ratio. With OBSS/PD, nodes attempt to transmit their packets too aggressively, which results in a lot of packet losses due to collision. PSC-UL shows lower fairness index compared to Legacy. This is due to the fact that in PSC-UL, stations that are in proximity to their APs gain more throughput compared to Legacy, while the throughput of stations that are far from their APs is comparable to Legacy. It is natural that stations that have higher SNR margin at their receivers have higher chance of initiating a parallel transmission.  
To take a deeper look, we have plotted the throughput of each node, sorted in the ascending order of throughput. Figure 9a,b shows the result of simulations using the default configuration. They are the same graph, except that the range of the y-axis was adjusted in Figure 9b to get a better look at nodes with lower throughput. We can see that the throughput of all methods except PSR starts to grow at similar times. Furthermore, the throughput of PSC-UL grows faster than other protocols. On the other hand, the throughput of PSR stays near zero until index 60, and then starts to grow. This is consistent with the earlier observation that a lot of stations are starved in PSR.  
Figure 9: Sorted node throughput. Panel **(a)** is the full graph whereas panel **(b)** is a partial graph where the y-axis is from 0 to 3 Mbps.