# IEEE 802.11 ax OFDMA Resource Allocation with Frequency-Selective Fading
## 3 Related Work  
OFDMA is widely applied in cable access networks and wireless communication systems and has been studied for a long time. Many different schedulers have been designed for OFDMA: centralized and distributed, optimal and suboptimal, instantaneous and ergodic, cooperative and non-cooperative, and single cell and multicell [6]. In this paper, we focus on centralized single-cell scheduling. A typical scheduler for a general OFDMA system solves an optimization problem [5] to maximize some utility function, e.g., the total rate, by assigning tones to the users and setting the transmission power of the users on these tones. For some utility functions, the scheduling problem can be solved in an optimal way in a time that depends linearly on the number of users, tones, and MCSs [6]. However, the optimization problem for the scheduler becomes more complicated for uplink OFDMA transmissions and for specific wireless communication technologies. For example, in cellular technologies such as LTE, a user cannot be assigned to an arbitrary set of tones for transmission: because of the SC-FDMA scheme, the users can transmit only in contiguous sets for tones. In [7], it is proved that such an additional restriction makes the scheduling problem NP-hard, so the classic solutions to the OFDMA scheduling problem described in [6] become inapplicable. Instead, heuristic solutions are proposed for OFDMA uplink scheduling, e.g., to consider users one by one and to assign them resource blocks with the highest utility function in a greedy way if it fulfills the contiguity constraint. As we can see, the scheduling problem with more restrictions becomes more complex, and in case of 802.11 ax uplink OFDMA new constraints are added: the sets of tones assigned to users can have only specific upper and lower bounds. As a result, we cannot add resource blocks to a user's schedule one by one as it is done in the LTE case, so new solutions should be designed specifically for 802.11 ax. The 802.11 ax standard introduces uplink OFDMA for 802.11 systems that enables simultaneous transmissions by multiple STAs and motivates much research on the resource allocation and scheduling for these mechanisms. In 802.11 ax, uplink OFDMA can be used for deterministic transmissions--when the AP schedules RUs for transmissions by specific STAs--and for random access using the Uplink OFDMA Random Access (UORA) which implements a slotted Aloha-like access scheme with binary exponential backoff. Much research [10; 11; 12; 13; 14; 15; 16; 17] on uplink OFDMA in 802.11 ax is dedicated to the analysis and optimization of UORA while leaving the scheduled part of OFDMA transmissions out of scope or considering a very simplified model of it, e.g., assuming elastic traffic or that the channel is divided into equal RUs. Although random access is useful in some scenarios, e.g., when the traffic is rare or when the transmissions should be made with very low delay [18; 19], still the usage of UORA inevitably leads to collisions and losses of throughput. This paper focuses on the scheduled OFDMA transmissions, which might provide more efficient channel usage. Various schedulers for 802.11 ax have been designed and investigated under different channel and traffic conditions to use the 802.11 ax capabilities fully. In [20], multi-user uplink transmissions are considered. It is shown that 802.11 ax outperforms 802.11 ac in terms of throughput and latency. However, it is assumed that the channel conditions for all STAs are the same, and the scheduler simply divides the entire channel equally for each STA. In [21], the RU allocation problem for users of different service categories (VoIP, video) is investigated. The authors consider such factors as network load, the quality of service parameters, and fairness indicators. They also propose an approach to predict the values of some of these factors using known regression methods (for example, \(k\)-nearest neighbors) if these values are rarely updated.