# Smart Preliminary Channel Access to Support Real-Time Traffic in Wi-Fi Networks
## 3 Related Works
The authors study a method of reducing channel access delay in which STAs access several channels simultaneously and transmit in the first accessed one. The authors of [44] propose an adaptation of this method for MLO. When a multi-link device generates a packet, it duplicates it and contends for access in multiple channels. If the device receives an acknowledgment on at least one channel, it removes the copies of the packet from the queues of other channels. This approach reduces the channel access delay because the delay is determined by its minimal value among multiple channels. In [9; 10; 11; 45; 46], the authors also explore the potential of MLO to reduce delays. Note that all the methods studied in these works require the PHY complication of RTA devices, in particular, to have at least one extra interface for wireless medium access, which significantly increases the cost of devices. Some works consider simple ways to reduce delays. For example, the authors of [13] introduce a new AC for delay-sensitive traffic. The parameters of this AC will guarantee that RTA STAs win the contention with non-RTA STAs. However, this method cannot reduce the delay associated with waiting for the end of non-RTA transmission. Other approaches consider predictable RTA traffic. In this case, RTA STAs expect the arrival of data and can start accessing the channel in advance. Thus, the authors of [47] propose to start the backoff countdown in advance before the RTA frame appears at the STA to reduce the channel access time. However, with this solution, the STA loses channel access if the data frame has not arrived by the moment when the backoff countdown is over. To address this drawback, in [15] we propose the preliminary channel access (PCA) method, according to which the STA reserves the channel by sending an RTS frame. As a result, the STA that successfully transmits an RTS frame but has no data frame does not lose channel access until an RTA data frame is delivered. We also consider reducing the TXOP Limit for delay-insensitive traffic as an alternative method to reduce the delay associated with waiting for the end of a non-RTA transmission. The paper shows that in the case of rare RTA traffic, the PCA method is more efficient than the reduction of the TXOP limit. However, the study is carried out in the scenario with only one RTA STA in the network. Increasing RTA STAs numbers can cause a significant increase in delay due to contention between STAs and waiting for the reservation time of another RTA STA. To sum up, many of the existing methods for RTA traffic service in Wi-Fi networks either require STAs to have a more complex PHY or cannot guarantee reliability in the presence of legacy STAs. At the same time, approaches without these disadvantages are compared only in scenarios limited to one RTA STA in the network. In this paper, we state a problem to develop an improvement of the PCA method, which makes it possible to increase the channel usage efficiency of non-RTA STAs providing a given delay quantile limit for RTA STAs. The improved method should not complicate the PHY of RTA STAs and should work if other STAs in the network do not support the new WiFi standard. We also state a problem to compare the improved method with the previously proposed ones and to determine the traffic parameters for which the improvement is more efficient to use.