# Improving Spatial Reuse of Wireless LAN Uplink Using BSS Color and Proximity Information
## 3 Proposed Method
### Protocol Details
#### 3.2.3 Protocol Operation  
To describe the operation of PSC-UL, we consider the scenario in Figure 5. In the scenario, node A transmits a packet to AP\({}_{1}\), which is also received by node B. We assume that node A and AP\({}_{1}\) are using BSS color 1, while node B and AP\({}_{2}\) are using BSS color 2. By reading the BSS color field in the preamble, node B finds out that the packet belongs to a different BSS. Node B needs to decide whether it should pause the backoff countdown or continue. If it continues to countdown the backoff counter, node B may transmit if packet concurrently with the ongoing transmission. First, node B reads the proximity information field also in the preamble. If the proximity information is unknown (value 0 in the field), then it means node B cannot use the PSC-UL method for parallel transmission. This may be because the ongoing communication is a downlink traffic, or PSC-UL is disallowed by operator decisions. If the proximity information field is some other encoded value, node B decodes the value. We denote the decoded value as \(P_{1A}\). Suppose in our scenario that \(P_{1A}\) is \(-40\) dBm. Now, from node B's neighbor table, we read the average RSSI of the beacon from AP\({}_{1}\) to node B. This value is \(P_{1B}\), and suppose it is \(-76\) dBm in our scenario. We check whether Equation (10) is true. In the equation, \(SNR_{min}\) is the minimum SNR for successfully receiving the packet, and \(m\) is the margin. If Equation (10) is false, node B decides the channel is busy and pauses its backoff. If Equation (10) is true, then we continue. In our example scenario, \(SNR_{min}\) is 23 dB and \(m\) is 5 dB, which makes the equation true. It means that when node B transmits its packet concurrently, AP\({}_{1}\) can still successfully receive the packet sent by node A.  
\[P_{1A}-P_{1B}\geq SNR_{min}+m \tag{10}\]  
Now, we need to check whether the packet sent by node B will be successfully received by AP\({}_{2}\). To do that, we need to check whether Equation (11) is true. In the equation, \(P_{2B}\) can be obtained by reading node B's neighbor table. Suppose \(P_{2B}\) is \(-44\) dBm. \[P_{2B}-P_{2A}\geq SNR_{min}+m \tag{11}\]  
To calculate \(P_{2A}\), we need to known the pathloss between node A and AP\({}_{2}\), and we do this by estimating the distance between node A and AP\({}_{2}\), \(d_{2A}\). We consider two cases: In the first case, we estimate the distance between AP\({}_{1}\) and AP\({}_{2}\) (\(d_{12}\)), as well as the distance between node A and AP\({}_{1}\) (\(d_{1A}\)). As we do not exactly know where node A is located, we consider the worst case and assume that node A is on the line between AP\({}_{1}\) and AP\({}_{2}\), which puts node A in the closest position to AP\({}_{2}\). Then, the distance between node A and AP\({}_{2}\) is \(d_{12}-d_{1A}\). We estimate the distance from RSSI using a pathloss model. Specifically, we use a log-distance model which is frequently used for indoor environment. Equation (12) shows the equation for calculating pathloss from distance using the log-distance model. In the equation, \(PL\) is the pathloss, \(PL_{0}\) is the reference pathloss, \(d\) is the distance between the source and the destination, and \(d_{0}\) is the reference distance. Furthermore, \(\gamma\) is the pathloss exponent and \(X_{g}\) is a random variable that models channel variations such as fast fading. We set \(X_{g}\) to zero, and use the margin parameter to account for channel variations. \[PL=PL_{0}+10\gamma log_{10}\frac{d}{d_{0}}+X_{g} \tag{12}\]  
Using Equation (12), we can calculate the distance if we know the pathloss, as in Equation (13). \[d=d_{0}\times 10^{\frac{PL-PL_{0}}{10\gamma}} \tag{13}\]Using Equation (13), we convert \(P_{12}\) and \(P_{1A}\) to \(d_{12}\) and \(d_{1A}\). We can obtain \(P_{12}\) from the neighbor table of AP\({}_{2}\) which is periodically sent to node B. Furthermore, we can obtain \(P_{1A}\) through proximity information.