# Improving Spatial Reuse of Wireless LAN Uplink Using BSS Color and Proximity Information
## 3 Proposed Method
### Idea Overview
If node A \"tells\" node B that the received power from AP\({}_{1}\) to A is \(-50\) dBm, node B knows that the SNR at AP\({}_{1}\) will be approximately 20 dB if both node A and node B transmits. If that SNR is not enough to receive [PERSON]'s packet, node B must defer its transmission. Next, we consider the exposed terminal scenario described in Figure 3. Similar to the previous scenario, node A is associated to AP\({}_{1}\) and node B is associated to AP\({}_{2}\). Node A begins to transmit its packet to AP\({}_{1}\), and its preamble is heard by node B. As node B is close to node A, the received power at node B can be above the CST. In legacy DCF, node B unnecessarily defers its transmission, although both packets can be successfully received at their respected receivers. Figure 3: An exposed terminal scenario. Node A is exposed to node B, so node B unnecessarily defers its packet even both transmissions can take place in parallel. Figure 2: A hidden terminal scenario. Node B is hidden to node A, and the packet at AP\({}_{1}\) is lost when both node A and B transmit together. In PSR, if AP\({}_{1}\) sends a trigger frame to schedule UL transmission, node B can find out the acceptable interference level at AP\({}_{1}\) using the PSR value in the preamble. Thus, node B can transmit its packet if node B's interference at AP\({}_{1}\) is expected to be lower than the acceptable interference level. However, PSR cannot guarantee that node B's transmission will be successfully received at AP\({}_{2}\). Suppose AP\({}_{2}\) was in a location close to node A, as in Figure 4. In this scenario, node B should actually defer its transmission, but transmits aggressively according to PSR and fails. When node B transmits its packet in parallel with node A, node B should be confident that its transmission is successfully received at AP\({}_{2}\). The signal level at AP\({}_{2}\) can be predicted because B is receiving beacons from AP\({}_{2}\). Predicting the interference level at AP\({}_{2}\) is not trivial, because we do not know how far node A is from AP\({}_{2}\). Although node A may know the proximity between itself and AP\({}_{2}\) through beacons, it is impractical for node A to include proximity information with non-associated APs in the preamble. We need to estimate the interference level with other information. As APs are mostly stationary, an AP can keep track of the proximity between itself and other neighboring APs. For example, in Figures 3 or 4, AP\({}_{2}\) maintains a neighbor table, where it records the average RSSI for its neighboring APs. The RSSI can be obtained by listening to the beacons. Furthermore, AP\({}_{2}\) sends the table to node B, so that it knows the proximity between AP\({}_{2}\) and AP\({}_{1}\). When node B receives preamble from node A, it can estimate the distance between node A and AP\({}_{2}\) using the proximity between AP\({}_{2}\) and AP\({}_{1}\), as well as the proximity between node A and AP\({}_{1}\). Let us denote the distance between AP\({}_{1}\) and AP\({}_{2}\) as \(d_{12}\), and the distance between node A and AP\({}_{1}\) as \(d_{1A}\). Then, the distance between node A and AP\({}_{2}\), \(d_{2A}\), is less than or equal to \(d_{12}-d_{1A}\). The placement of nodes in Figure 3 is the worst case, where \(d_{2A}=d_{12}-d_{1A}\). Using this information, we estimate the interference level at AP\({}_{2}\) and determine whether node B can transmit concurrently with node A. As we do not know exactly where node A is located in vicinity of AP\({}_{1}\), we consider the worst case and assume that node A is in the position closest to AP\({}_{2}\), in order to avoid collisions caused by the hidden terminal problem. We summarize the idea of the proposed method with the scenario shown in Figure 5. In the scenario, node A is transmitting its packet to AP\({}_{1}\), and its preamble is received by node B. Node B needs to decide whether it can transmit its packet concurrently with node A. In order to do so, node B needs to find out the following values.