# A Survey of Wi-Fi 6: Technologies, Advances, and Challenges
## 5 SR in Wi-Fi 6
### Related Works
#### 5.4.1 Threshold Manipulation  
The efficiency of SR increases by increasing the carrier-sensing threshold value. However, decreasing the value causes the hidden node problem, which may be symmetric or asymmetric. In the symmetrical hidden node problem, station A does not trigger station B's carrier sense threshold and accordingly, B cannot hear from A. This causes repetitive collisions among these two stations. The asymmetric hidden node problem means that station A is located out of the carrier sense range of station B but station A can hear station B. Thus, B causes collisions for the packets of A while A lets some packets from B be received. Then, the backoff value will increase more frequently for A than for B. The asymmetric hidden node problem can happen in both inter- and intra-BSSs causing severe unfairnessamong stations [114]. The problem is handled by different parameter values (transmission power, etc.). The following papers focused on resolving the hidden node problem. Controlling the threshold or applying RTS/CTS help to reduce the hidden terminal problem [114]. The impact of hidden nodes on the uplink transmissions in the presence of legacy Wi-Fi and Wi-Fi 6 stations was investigated in [115; 116]. They proposed three possible solutions: (1) increasing the threshold of all stations in the association phase with an HE access point; (2) specifying different channels for adjacent BSSs; (3) the NAV of a station could only be set if an intra-BSS station is transmitting data. [PERSON] et al. [117] proposed a trigger-based approach for uplink multi-user transmissions. The access point sends a multi-user RTS. If some stations do not respond because they have a non-zero NAV (i.e., they are hidden nodes), other stations not in the trigger list can respond with a CTS if they have a zero NAV. Some other works focused on optimal CCA threshold tuning. Dynamic Sensitivity Control (DSC) [118; 119] tunes the carrier sense threshold of the access point according to the RSSI of the farest station to transmit more data concurrently. This means that the access point tunes the value based on the worst case. The performance of DSC was examined in [120] where DSC with a well-tuned margin for threshold outperformed legacy mechanisms in throughput. Nonetheless, it also depends on the node density and topology. The fairness in medium access also depends on the margin. Since DSC causes a hidden node problem, [121] combined it with RTS/CTS. The authors of [122] leveraged DSC and TDMA mechanisms jointly to solve the hidden node problem. They proposed to reserve multiple stations or one BSS within a time interval to have timely orthogonal BSSs. [PERSON] et al. [123] bound the CCA threshold value in the PSR mechanism, which manages parallel link interference, to improve QoS. The authors in [124; 125] leveraged stochastic geometry in carrier-sensing threshold manipulation. Every access point tunes the carrier-sensing threshold and transmission power based on its received power. They concluded that a linear increase in the carrier-sensing threshold with respect to the received power maximizes the throughput. [PERSON] et al. [126] extended [127] to reduce memory usage and converge faster, which applied a branch-and-bound method to optimize the transmission power and scheduling. The utility function is a function of the thermal noise, transmission power, and channel gain, i.e., a factor of sent and received signals and noises that includes the effects of path loss, shadowing, and fading. They tried to find the right value for the carrier-sense threshold. A dynamic resource scheduling regularly runs the mechanism to consider prior radio resources per user for optimal resource allocation. [PERSON] et al. [128] aimed to reduce interference and improve SR with a dynamic channel allocation and by controlling the transmission power, respectively. They applied multi-armed bandits to find the optimal combination of transmission power and channel to enhance throughput. [PERSON] et al.