# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 2 Related Work  
With the increase of node density and user traffic, improving system spectral efficiency has become a critical factor in servicing user needs. However, the problem of IEEE 802.11 WLAN in which the user performance significantly degrades when the network becomes dense has become well known, and the research community has proposed many different proposals to improve the system spectral efficiency of the WLANs. One line of research has focused on deriving analytical models to find the optimal CST taking account of environmental factors such as node density [3; 4; 5]. However, as we will discuss in the next section, a common CST cannot reuse space efficiently, as the proper CST depends on the location of the receivers as well as the senders. Although adjusting CST can improve spatial reuse, there is a limit to how much we can achieve by using a common CST. Another approach for improving spatial reuse is to explicitly identify exposed terminals and allow them to transmit concurrently with the on-going transmission. [PERSON] et al., [6] proposed a protocol where each node maintains a Conflict Map (CMAP). The CMAP is a table that records information on which nodes should not transmit together. This table is constructed by observing packet collisions, and exchanging the information with direct neighbors. When a node overhears a packet, it decides whether it should concurrently transmit or defer transmission by looking up the CMAP. [PERSON] et al., [7] proposed CSMA/OCA, where hidden and exposed terminals are identified using Request-To-Send (RTS) and Clear-To-Send (CTS) messages. Suppose a node overhears an RTS. The node checks whether the CTS for that particular RTS is also overheard, or if the destination of the RTS is one of its neighboring nodes. If neither is true, the node thinks the RTS sender is an exposed terminal, and transmits its packet concurrently with the on-going transmission. [PERSON] et al., [8] proposed Concurrent-MAC, which uses a centralized approach to identify exposed terminals. In Concurrent-MAC, all nodes measure signal strengths of neighboring nodes and report the information to a central coordinator, and the coordinator computes which nodes can transmit concurrently. Once computed, the information is propagated to all nodes in the network. Whenever a node transmits a packet by winning the channel, it selects one of its nodes as a \"privileged node\" and includes its address in the header. When the privileged node overhears the message, it begins transmitting its packet concurrently with the other packet. The approach that is most related to our proposed protocol is to independently select CST based on neighborhood conditions. The DSC technique proposed for IEEE 802.11 ax selects CST based on proximity between the sender and the receiver, estimated using received signal strength. Specifically, the CST of a mobile node is calculated using Equation (1). \[C_{i}=\max(C_{min},\min(C_{max},RSSI_{i}-Margin)) \tag{1}\]  
In the equation, \(C_{i}\) is the CST of node \(i\), \(RSSI_{i}\) is the RSSI of the associated AP measured at node \(i\), \(C_{max}\) and \(C_{min}\) are the maximum and the minimum CST values selected by the operator, and \(Margin\) is the margin which defines the relation between RSSI and the selected CST. According to Equation (1), nodes close to their APs will select a high CST and transmit aggressively, whereas nodes at the edge will select a low CST and transmit conservatively. Several papers report that DSC can improve system and per-user throughput compared to using a fixed CST [2; 9; 10; 11]. However, some reports have shown that DSC can cause severe unfairness among nodes [12; 13; 14]: By transmitting packets aggressively, nodes close to APs get larger share of channel resource compared to cell-edge nodes. Also, aggressive nodes become hidden terminals to edge nodes, causing packet collisions and further starving the nodes.