# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 3 Proposed Protocol  
### Preliminaries and Motivation  
The hidden terminal problem and the exposed terminal problem are well-known problems in a carrier sensing-based medium access control protocol. The occurrence of these problems is closely related to the CST. Consider the scenario illustrated in Figure 1. Nodes A and B are associated with AP1, while node C is associated with AP2. When AP1 transmits a packet to one of its nodes, AP2 overhears the packet. If the received signal strength is higher than its CST, AP2 defers its transmission. If the signal strength is lower than the CST, AP2 will continue the channel contention process and can start transmitting its packet. In this scenario, whether AP2 should actually transmit concurrently with AP1 depends on AP1's destination. Let us assume AP2 wants to transmit a packet to node C. If AP1 is transmitting to node A, AP2 can concurrently transmit its packet, because the SNR at both node A and C are high enough to successfully decode the packet. However, if AP1 is transmitting to node B, AP2 must not transmit, because otherwise the packet transmitted by AP1 cannot be successfully decoded at B due to low SNR. The problem is that AP2 does not know where AP1's packet is heading, nor where the destination node is placed. Thus, AP2 cannot decide whether to transmit or defer accordingly. Instead, AP2 just uses a fixed CST. If CST is set high, AP2 will transmit concurrently with AP1. If AP1 was transmitting to node B, AP2 becomes a hidden terminal causing packet collision at node B. If CST is set low, AP2 will defer its transmission. If AP1 was transmitting to node A, AP1 becomes an exposed terminal to AP2, unnecessarily preventing the transmission and degrading spatial reuse. Figure 1: An example network scenario. AP2 can transmit to node C if AP1 is transmitting to node A, but should defer its transmission if AP1 is transmitting to node B. A proper CST should satisfy two purposes. A node should not transmit if its destination could not successfully receive the packet. Also, even if its transmission should succeed, the node should not transmit if it will cause another transmission to fail. However, the 802.11 DCF uses a fixed CST for all nodes, without considering how a transmission will affect other transmissions. Also, the DSC protocol (described in the previous section) only considers the former purpose and neglects the latter. Thus, DSC creates a lot of hidden terminals. As shown in the performance evaluation section, although DSC achieves high total throughput, it causes starvation to edge nodes and its channel share is significantly unfair. In the above scenario, what is the best action of AP2, when it receives a preamble from AP1 and knows that AP1 is currently transmitting? If node A is the destination, AP2 can regard the channel as idle and continue backoff counting (as in DCF). If the backoff counting is over, AP2 can transmit its packet. If node B is the destination of AP1, AP2 should regard the channel as busy and pause backoff counting. Deciding whether channel is busy or idle should depend on the destination of on-going transmission. Since AP2 does not know where the on-going packet is heading, AP1 should tell AP2 (and other neighboring nodes) which node is the destination node. There are many different ways for AP1 to inform its neighboring nodes which node is the packet destination. It can include the information in a separate message and send the message before the data packet (similar to RTS). Or, AP1 could include the information in the preamble or the MAC header of the packet. Among these design choices, we choose to include the information in the preamble. If the information is included in a separate message, control overhead becomes too large because this message should be transmitted before every data packet. If the information is included in the MAC header, a neighboring node must receive and decode the MAC header before it can cancel the reception and begin concurrent transmission.