# Improving Spatial Reuse of Wireless LAN Uplink Using BSS Color and Proximity Information
## 2 Related Work
Specifically, a node close to its AP uses a high CST so that it transmits aggressively, while a node far away from its AP uses a low CST so that it transmits conservatively. Evaluation of DSC shows that while it can increase throughput, DSC can causes severe unfairness among nodes; nodes closer to their APs get much larger share of the channel bandwidth, while nodes far away from their APs can be starved [16; 17; 18; 19; 20]. Many proposals were proposed aiming to address the problem of DSC. For example, [PERSON] et al. [21] proposed a method where edge nodes that experience high FER (Frame Error Rate) use RTS/CTS to protect their packets from hidden terminals. [PERSON] et al. [22] propose a technique where both transmit power and CST are adjusted based on ETX (Expected Transmission Count). A node with high ETX increases transmit power and CST to increase SNR as well as transmission opportunity. (1)  
In order to distinguish inter-BSS communications from intra-BSS communications, each BSS chooses a BSS color (an integer), and a node includes the BSS color in the preamble of the packet. OBSS/PD is shown to achieve higher throughput compared to the legacy scheme with fixed CST [23]. However, as OBSS/PD still relies on channel status measured at the transmitter, it suffers from hidden and exposed terminals similar to the legacy scheme. PSR is another method for spatial reuse. It can be used when uplink communication is scheduled by a trigger frame sent from the AP. In the trigger frame, the AP includes a PSR value which is calculated as in Equation (2). In Equation (2), \(I_{AP}^{max}\) is the acceptable interference level calculated as in Equation (3). In Equation (3), \(UL\_Target\_RSSI\) is the expected receive power of the UL packet, set by the AP and included in the trigger frame. Furthermore, \(min\_SNR\_MCS\) is the minimum SNR required for receiving the highest MCS (Modulation and Coding Scheme) among the scheduled UL traffic with less than 10% PER (Packet Error Rate). \[PSR=TX\_PWR_{AP}+I_{AP}^{max} \tag{2}\]  
\[I_{AP}^{max}=UL\_Target\_RSSI-min\_SNR\_MCS-margin \tag{3}\]  
When a node receives a trigger frame from an AP that the node is not associated with, it looks at the PSR value and checks whether it can transmit concurrently with the scheduled UL traffic of the other BSS. Specifically, the node estimates the interference it will cause at the other AP and determines whether the UL packet will be successfully received at the AP even if the node transmits concurrently. If the estimated interference is less than the acceptable interference level at the AP, the node ignores the UL traffic that follows the trigger frame and transmits its packet after counting the backoff counter to zero. [PERSON] et al. [24] show that PSR has advantages over the legacy scheme, especially in reducing packet latency of uplink traffic [24]. However, as PSR only protects the UL traffic by advertising the PSR value, the node transmitting concurrently with the UL traffic may fail to send the packet if its receiver is close to the node sending uplink traffic [25]. In our proposed method, a node considers the SNR at both receivers when deciding to transmit a packet concurrently or defer its transmission. There are other schemes that try to improve spatial reuse under various assumptions. For example, [PERSON] et al. [26] proposed a CST control scheme based on node locations, assuming nodes know their locations through devices like GPS. Furthermore, [PERSON] et al. [27] proposed a scheme using directional antennas, while [PERSON] et al. [28] proposed a method that requires devices with full-duplex capability. Figure 1: CST selection with transmit power control in the OBSS/PD method. With the recent advance of artificial intelligence, machine learning techniques were applied to the problem of spatial reuse. [PERSON] et al. [29] proposed a centralized approach where a central coordinator collects information and runs a neural network to find the best transmit power and CST to use. [PERSON] et al.