# Dynamic Carrier-Sense Threshold Selection for Improving Spatial Reuse in Dense Wireless LANs
## 1 Introduction  
IEEE 802.11-based wireless local area networks (WLANs) are widely deployed in indoor environments, providing convenient Internet connection to users. The recent trend is its densification: number of access points (APs), number of mobile devices, and amount of traffic are all rapidly increasing. In office buildings, it is easy to find tens of APs in the vicinity of a user. With increased density, the system spectral efficiency becomes a significant issue. It is important to maximize user throughput in a given space where many users are sharing channel resource. To increase user throughput, spatial reuse (SR) must be improved: more users should be able to concurrently send packets in a given area. The problem is that concurrent transmissions could result in a packet loss, because one signal becomes interference to another signal which degrades SNR at its receiver. Therefore, current wireless LAN uses carrier sensing-based medium access control (CSMA/CA) to prevent packet collision. Before transmitting a packet, a node checks whether the channel is busy or idle. The node can transmit a packet only when it thinks the channel is idle, and defers its transmission when the channel is busy. Deciding whether the channel is busy or idle depends on the carrier-sense threshold (CST): if the node receives another signal that has a signal strength higher than CST, the node decides the channel is busy; otherwise the channel is idle. CST is a tunable parameter, which can be independently set by each node. A node transmits conservatively when CST is low, and aggressively when CST is high. Finding optimal CST is not trivial in the systems perspective, because CST of a node not only affects throughput of the node itself, but also affects throughput of its neighboring nodes. The well-known hidden terminal and exposed terminal problems depend on CST. A high CST creates hidden terminals, whereas a low CST createsexposed terminals. Both problems are harmful for system spectral efficiency, causing packet collisions and waste of channel resource. Until recently, a common practice was to use a fixed CST such as \(-82\) dBm. This threshold is quite low and leads to conservative packet transmissions. When the network is sparse, avoiding packet collisions from hidden terminals is more important than promoting concurrent transmissions. However, when the network becomes dense, a low CST causes significant waste of channel resource by unnecessarily blocking nodes who can successfully send packets to their receivers. IEEE 802.11 ax, the newest WLAN standard, includes the Basic Service Set (BSS) coloring scheme to improve spatial reuse [1]. Each AP is assigned a unique ID, which is shared with its associated mobile nodes. Whenever a node transmits, the BSS ID is included in the preamble. When a node detects a preamble, it reads the BSS ID in the preamble and checks whether the incoming packet is from the same BSS or another BSS. If the packet is coming from another BSS, the node aborts receiving the packet, since it is not the destination of the packet. Moreover, the node can apply different CSTs depending on whether the packet is coming from the same BSS or a different BSS. The idea is to use a higher CST when the on-going packet is from another BSS, to transmit more aggressively and thus increase spatial reuse. Many techniques were proposed where nodes use different CSTs depending on their positions and neighborhood conditions, instead of all nodes using a common CST. In particular, Dynamic Sensitivity Control (DSC) [2] was proposed to be included in IEEE 802.11 ax, although not incorporated in the standard. The idea of DSC is to select CST based on the proximity between a mobile node and the AP. If a mobile node is placed close to the AP, it can transmit more aggressively because the SNR at the receiver can still be enough even when there is interference from other nodes. (DSC is described in more detail in the next section).