# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 4 Performance Evaluation
### Simulation Results
#### 4.2.4 Varying Path Loss Exponent  
Until now, the path loss model used in the simulations is identical to the model used in the model-based scheme. The weakness of the model-based scheme is that performance degrades as the gap between model and the real path loss increases. To study its effect, we have varied the path loss exponent in the log-distance model from 2 to 4. The exponent used in the model-based scheme is fixed to 3. Please note that the path loss exponent reflects the propagation characteristics of an environment. For example, empirically obtained path loss exponent for free space, office with soft partition and office with hard partition are 2, 2.6, and 3.0 [25]. Similar to the previous experiment, 100 APs and 100 mobile nodes were placed in the simulation area. The result is in Figure 8.  
As the path loss exponent becomes higher, the total throughput of both model-based and measurement-based protocols increase. With high path loss exponent, the signal strength quickly drops down as the distance is increased. Thus, if the signal source and the interference source is at the same position, the receiver receives the packet with higher SNR if the path loss exp  
Figure 8: Performance of the model-based scheme and the measurement-based scheme varying path loss exponent \(\gamma\). For the model-based scheme, the mismatch between the model and the real environment can affect the performance. (**a**) total throughput, (**b**) bottom 25% throughput, (**c**) [PERSON]’s fairness index, (**d**) packet delivery ratio.  
look at the bottom 25% throughput, the throughput of model-based scheme is degraded when the path loss exponent becomes high, whereas the throughput of measurement-based scheme continuously increases. This means that the model-based scheme is creating hidden terminals. When the path loss exponent of the environment is higher than the model, the CST estimated from the model is higher than the necessary CST to avoid the hidden terminal problem. In other words, the carrier-sense range is set smaller due to the mismatch between the model and the real environment. On the other hand, the measurement-based scheme does not use path loss models to calculate the CST. Thus, it could be a better solution when the path loss condition cannot be precisely modeled.  
In summary, the proposed protocol increases system throughput by improving spatial reuse, while not starving the edge nodes. While the model-based scheme is simpler and does not require exchanging neighbor information, it might create hidden terminals if the real path loss is far from the model used to calculate the CST. On the other hand, the measurement-based scheme does not rely on path loss models but uses neighbor information to calculate the CST. As shown in the simulation results, the measurement-based scheme outperforms the model-based scheme especially when the model does not reflect the real environment well. While the measurement-based scheme can be preferred due to its performance and robustness, the model-based scheme could also be practical due to its simplicity such as less requirements for exchange of control messages and data management.