# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 3 Proposed Protocol
### Measurement-Based Dynamic CST Control  
The model-based CST control scheme has two weaknesses. First, the path loss model may not reflect the real environment. Even with the margin, the error in the model could lead to a CST that makes nodes transmit too conservatively or too aggressively. Second, the model-based CST assumes the worst-case scenario, where the interferer is assumed to exist at the position farthest from the transmitting node. This could make the CST low and limit the spatial reuse. In the measurement-based dynamic CST control, we do not use a path loss model, but use neighbor information exchanged as control messages between the nodes. Let us consider the example scenario shown in Figure 2. In the scenario, AP1 wants to send a packet to node A, and thus it needs to calculate the advertised CST to protect the transmission. To select the advertised CST, AP1 must know the potential interferers of node A. In the measurement-based scheme, node A sends this information to AP1 in a message. To do that, each node collects RSSI of neighboring nodes whenever it overhears their message, and maintains the RSSI information using weighted moving average (WMA) in an \(RSSI\)_table_. For example, node A maintains the average RSSI of AP1, AP2, and node B, as in Figure 2(a). (Assume node C is too far away and its signal does not reach node A.) Periodically, each node broadcasts its RSSI table to its neighbors in a control message, which is transmitted at the lowest data rate. When a node receives an RSSI table from its neighbor, it updates its _neighbor table_ with the information. An example neighbor table of AP1 is described in Figure 2(b). Using the neighbor table, AP1 finds out that when it transmits, the received signal strength at node A will be \(-45\) dBm. If the required SNR threshold \(SNR_{th}\) is \(23\) dB, the interference level at node A  
Figure 3: An RSSI table (**a**) and a neighbor table (**b**) used in measurement-based dynamic CST control. The RSSI table maintains average RSSI values for each of the neighbors. Also, the RSSI tables are periodically exchanged between neighboring nodes. Whenever a node receives an RSSI table from the neighbor, it updates the neighbor table which records the RSSI table of each neighbor. should be less than \(-68\) dBm. Looking at the neighbor table, the expected interference level at node A when AP2 transmits is \(-75\) dBm, which is less than \(-68\) dBm. Thus, AP1 does not need to block AP2. However, if node B transmits, the SNR at node A becomes less than 23 dB and the packet will not be successfully decoded. Therefore, node B becomes the potential interferer for AP1 transmitting to node A. To block node B, AP1 sets the advertised CST as \(-58\) dBm -\(P_{M}\). When AP2 receives the preamble from AP1, it finds out that its transmission will not destroy the on-going transmission. Now, AP2 looks at its neighbor table to find out if when AP2 transmits a packet to node C, it will be successfully decoded at node C. If the packet is expected to survive, AP2 decides the channel is idle. Suppose AP1 wants to transmit its packet to node B. Consulting the neighbor table, AP1 finds that both node A and node AP2 are potential interferers. In this case, the advertised CST is selected based on the farthest interferer, which is AP2 in this case. Thus, the advertised CST becomes \(-80\)dBm - \(P_{M}\), to prevent AP2 from transmitting. When AP2 receives the preamble, it measures the RSSI and reads the advertised CST. Since the RSSI is higher than the advertised CST, AP2 decides the channel is busy and defers its transmission. Please note that the margin parameter \(P_{M}\) is also used in the measurement-based scheme. Although the scheme does not use a path loss model to calculate CST, margin is still needed because of channel variations, and possibility of multiple interferers.