# Improving Spatial Reuse of Wireless LAN Uplink Using BSS Color and Proximity Information
## 2 Related Work  
From the early days of IEEE 802.11 WLAN, it was well known that the performance of WLAN degrades significantly in dense scenarios, mainly due to the fact that its medium access control method, DCF, is based on CSMA/CA (Carrier-Sense Multiple Access/Collision Avoidance). As collision avoidance is based on random backoff, the probability of collision increases with node density, leading to packet loss and frequent retransmissions. Furthermore, the fact that carrier sensing is done by the transmitter leads to the hidden and the exposed problems, resulting in further performance loss and unequal share of channel bandwidth between the nodes [6]. Naturally, a significant research effort was made to improve the spectral efficiency of DCF. One of the goals was to find an optimal CST for the nodes, because using a fixed CST is the problem that leads to hidden and exposed terminal problems [7]. For example, [PERSON] et al. [8] studied the conditions for setting the CST to eliminate hidden terminals. However, such CST can increase exposed terminal that result in wasted channel bandwidth. [PERSON] et al. [9] proposed a centralized method for controlling CST. The proposed algorithm measures SINR at the stations when two neighboring APs transmit in parallel and adjusts CST based on the information in order to increase number of parallel transmissions. [PERSON] et al. [10] proposed a method where hidden and exposed terminals are identified through RTS (Request-To-Send) and CTS (Clear-To-Send) packets. Specifically, if a node overhears and RTS but not the corresponding CTS, the node thinks that the RTS sender is an exposed terminal and transmits its packet concurrently. [PERSON] et al. [11] proposed a method where each node maintains a CMAP (Conflict Map) which records which nodes should not transmit together. The CMAP is constructed by observing packet collisions and exchanging messages with neighbor nodes and is used along with the carrier sensing to avoid hidden and exposed terminal problems. [PERSON] et al. [12] proposed a method for dynamically adapting its CST. If a node cannot transmit for a long time, it increases CST to mitigate the exposed terminal problem. On the other hand, if the node fails to receive ACK (Acknowledgement) multiple times, it decreases CST to mitigate hidden terminal problem. [PERSON] et al. [13] proposed Concurrent-MAC, where a transmitting node can select one of its neighboring nodes as a \"privileged node\" which can transmit a packet concurrently. In order to do so, information on who can transmit in parallel is collected by a central coordinator and disseminated throughout the nodes. [PERSON] et al. [14] proposed G-DCF, where nodes form groups in a distributed manner if they can successfully transmit in parallel. Once one of the nodes transmits a packet, it triggers other nodes in the group to transmit their packets together. [PERSON] et al. [5] proposed a method where two CSTs are used instead of one. When a node transmits a packet, it includes an \"advertised CST\" in the preamble, which should be obeyed by the neighbors to protect the ongoing transmission. Nodes hearing the preamble use their own CST as well as the advertised CST to determine whether to transmit or not. The CSTs are dynamically chosen to protect the transmissions by estimating the SNR (Signal-to-Noise Ratio) at the receivers. Although this method uses dynamic CST to improve spatial reuse, the method is still conservative in choosing CSTs because it assumes that the transmitters are in the closest position to the other receivers. The CST are chosen so that the packets can be received even in the worst case. When developing the most recent standard, IEEE 802.11 ax, spatial reuse has gained significant attention, due to its potential of considerably improving perceived quality of service [1]. One of the earlier proposals was DSC (Dynamic Sensitivity Control) [15], which selects CST based on the proximity between the sender and the receiver.