## 2 Background
### Wireless Network Fairness  
In wireless networks, fairness primarily concerns the equitable sharing and allocation of resources based on fair and reasonable strategies. Unfair resource allocation can lead to resource scarcity, waste, or redundancy. The definition of fairness in Wi-Fi is broad and can vary depending on specific requirements [6], including fair energy consumption [7], power control [8], flow scheduling [9], channel allocation [10], rate control [11], and routing selection [12]. In this paper, we focus on site-based fairness and airtime fairness in flow scheduling.  
Wi-Fi networks can cover extensive indoor areas. To ensure stable connectivity for devices at various locations and to reduce wireless packet loss due to signal attenuation, devices closer to the access point typically maintain higher wireless rates, while devices farther from the AP maintain lower wireless rates. We can implement a site-based fairness algorithm at the AP. For example, consider two devices, A and B, connected to the AP, where device A is farther from the AP, and device B is closer. The channel occupancy process is illustrated in Figure 1.  
The per-station fairness algorithm ensures that devices A and B experience equitable throughput when transmitting data by alternately sending frames of the same size to both devices. However, because device A is farther from the AP and has a slower transmission speed, it requires more channel time to transmit the same size frame. Consequently, even though device B has a higher transmission rate, its actual transport layer rate is limited by device A's slower rate. This means that as the number of connected devices increases, the overall performance of the wireless network is determined by the rate of the slowest device.  
To address this issue, researchers proposed airtime fairness, also known as time-based fairness. Airtime fairness is widely recognized as a solution to the performance anomaly in Wi-Fi networks with varying speeds [13] and has been deployed in many router devices [14, 15, 16]. Its goal is to provide each user with an equal amount of airtime. In the scenario described, when the AP's scheduling algorithm is replaced with airtime fairness, the channel occupancy process for the two devices is illustrated in Figure 2.  
Figure 1: Per-station fairness in Wi-Fi transmission, where P refers to packet.  
Figure 2: Airtime fairness in Wi-Fi transmission, where P refers to packet.  
Airtime fairness shifts the focus from providing each station with equal throughput to allocating transmission time equally among devices. This ensures that all devices have equal access to the wireless channel, preventing slower devices from reducing overall network performance. For example, a node with a faster wireless rate, such as node B, will receive more transmission opportunities from the AP. This allows node B to utilize its higher rate advantage, transmitting more data and thus increasing the overall network throughput. Conversely, a node with a slower transmission rate, such as node A, will have reduced transmission opportunities to prevent it from consuming excessive airtime and affecting the performance of other devices.  
In IEEE 802.11 ac, transmission rates can vary significantly between nodes. For instance, with a guard interval (GI) of 400 ns, an mcs0 node has a wireless rate of 32.5 Mbps, while an mcs9 node has a rate of 433.3 Mbps. When transmitting the same amount of data, the airtime ratio between these nodes is 13.3 to 1. Under traditional fairness, both nodes would transmit the same amount of data, resulting in an average access point bandwidth of approximately 60 Mbps. However, with airtime fairness, both nodes occupy the same amount of airtime, leading to an average access point bandwidth of about 233 Mbps. This represents a 388% increase in total throughput compared to traditional fairness.