# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 3 Proposed Protocol
### Model-Based Dynamic CST Control
\[P_{1}-P_{2}\geq SNR_{th} \tag{2}\]0  
Since AP2 must be blocked when AP1 is transmitting, AP1 selects \(P^{\prime}\) as the maximum CST that can protect its transmission. Thus, the advertised CST (\(CST_{adv}\)) is calculated as follows. \[P_{1}-P_{2}\geq SNR_{th} \tag{2}\]1  
In the equation, \(P_{M}\) is the _margin_ parameter. The margin parameter is necessary for various reasons. First, it accounts for the mismatch between the model and the real path loss, caused by factors such as fading effects. Also, the calculated CST is based on assumption that there is a single interferer. In practice, there could be multiple nodes transmitting concurrently, which will increase the interference at the receiver. Since it is difficult to predict how many transmissions will take place near the receiver, we use the margin parameter to account for the possibility of multiple interferers. In this paper, we use a fixed margin that is common for all nodes. However, it could be better to select the margin for each node independently, based on the conditions around the node. For example, a larger margin could be used if the node is experiencing large amount of channel variation, and a smaller margin can be used if the channel condition is more stable. Also, a larger margin could be used for a node positioned in a dense environment, compared to a node positioned in a sparse environment. Dynamically selecting margin based on the current environment is a challenging but important issue in terms of achieving high spectral efficiency, and is left as a future work. We return to the example scenario in Figure 1 to continue describing the proposed protocol. Suppose AP1 has calculated the advertised CST and transmitted a packet. When AP2 receives the preamble, it should consider two things before concurrently transmitting its packet with AP1. First, the RSSI of AP1 should be higher than the advertised CST, to protect the on-going transmission. Second, the on-going transmission should not destroy the packet transmitted from AP2 at node C. To find out, AP2 calculates the _required_ CST (\(CST_{req}\)) that will protect its own transmission. The calculation process is identical to how AP1 calculated \(CST_{adv}\). Then, the CST of AP2 at the moment is selected as the minimum of \(CST_{adv}\) and \(CST_{req}\). \[P_{1}-P_{2}\geq SNR_{th} \tag{2}\]2  
If the current interference level is lower than \(CST_{AP2}\), AP2 decides the channel is idle and may begin transmitting a packet to its destination. Please note that both \(CST_{adv}\) and \(CST_{req}\) depends on the _destinations_ of the potential concurrent transmitters. In the scenario, suppose when AP1 transmits a packet, the received signal strength at AP2 is \(-\)80 dBm. In the first case, AP1 sends a packet to A. Using the described model-based method, AP1 calculates \(CST_{adv}\) as \(-\)75 dBm, and includes the information in the preamble of the packet. When AP2 receives the preamble, AP2 was waiting for the channel to send a packet to node C. \(CST_{req}\) calculated at AP2 is \(-60\) dBm. Since \(\min(CST_{adv},CST_{req})\) is higher than \(-80\) dBm, AP2 decides the channels is idle and continues counting down the backoff counter. If the counter reaches zero, AP2 transmits concurrently with the on-going transmission from AP1. In the second case, AP1 sends a packet to B. In this case, the \(CST_{adv}\) is calculated as \(-85\) dBm, because node B is distant from AP1. When AP2 receives the preamble from AP1, its CST becomes \(\min(CST_{adv},CST_{req})=-85\) dBm, which is lower than \(-80\) dBm. Thus, AP2 decides the channel is busy and defers transmission. In summary, the proposed scheme can control CST considering not only the senders but their destinations, to improve spatial reuse while avoiding hidden terminal effects.