# IEEE 802.11 ax OFDMA Resource Allocation with Frequency-Selective Fading
## 2 OFDMA Transmission  
In 802.11 ax, the whole channel is split into sets of subcarriers (tones) called resource units (RU). An RU may consist of the following numbers of tones: 26, 52, 106, 242, 484, 996, and 2 \(\times\) 996. The available sets of RUs depend on the channel bandwidth. For example, the RU structure for the 40 MHz channel is shown in Figure 1. A 484-tone RU occupies the entire channel and consists of two 242-tone RUs. A 242-tone RUs consists of smaller RUs and so on. Such an RU structure makes OFDMA scheduling in 802.11 ax networks different from scheduling in LTE, where a base station can use any set of resource blocks for downlink transmissions to a user, and any contiguous set of resource blocks for uplink transmissions by a user. According to the 802.11 ax standard, both in the downlink and uplink, OFDMA transmissions occur on a per-frame basis. It means that different frequency parts of a frame are sent to or by various STAs, in the case of a downlink or uplink transmission, respectively. In both cases, the AP determines the duration of the OFDMA frame and allocates RUs to the STAs that will transmit or receive data in this frame. Each STA may be assigned to no more than one RU that may have any available size (26-tone, 52-tone, etc.). The AP also sets the modulation and coding scheme (MCS) that the STA shall use. The amount of transferred data depends on the RU size, frame duration, and MCS. The wider RU and the faster MCS, the more data can be transmitted per time unit. Table 1 shows the data rates for different RU sizes and MCSs. Note that high-speed 1024-QAM MCSs are available only in the wide RUs. To point out the implications of the 802.11 ax RU assignment and its difference from LTE, let us consider a simple scenario when an AP has to schedule resources to two STAs in a 20 MHz channel. The RU structure in 20 MHz channel corresponds to one 242-tone RU in Figure 1. The AP can allocate the entire channel to one STA or divide the channel between two STAs. However, the 242-tone RU cannot be divided into two arbitrary parts. Unlike LTE, 802.11 ax allows either to assign the whole band to one STA or to assign a 106-tone RU (or a smaller one) to each of the STAs. In the latter case, whatever RUs are chosen, at least a 26-tone RU is wasted. As a result, instead of selecting the best RUs for each STA one by one in a greedy way, as it is typically done in cellular systems, the AP has to search for allowed RU configurations and the ways to assign given RUs to the STAs. Figure 1: RU structure in 40 MHz channel. In 802.11 ax, the transmissions within one OFDMA frame shall start and end synchronously. While it can be implemented relatively straightforward in the downlink, uplink OFDMA transmissions require additional control from the AP. As this paper focuses on uplink OFDMA transmissions, let us consider them in detail. Uplink OFDMA transmissions are fully controlled by the AP. To start an uplink OFDMA transmission, the AP sends a trigger frame (TF). It specifies the resource allocation for the STAs: the MCS, power level, RU, frame duration, and other parameters. Short Inter-Frame Spaces (SIFSs) after the STAs send their data in the allocated RUs. Finally, the AP replies with an acknowledgment frame called Multi-STA BlockACK. A general view of the uplink transmission process is shown in Figure 2.  
To schedule resources, the AP needs to know which STAs have data for transmission. For that, the STAs report to the AP that they want to transmit data using the Buffer Status Report (BSR) frames. After receiving the information about the STAs' buffers status, the AP allocates resources and sends the TF. The optimal RU and MCS allocation depends on the signal-to-noise ratio (SNR) at the receiver. Fast MCSs and wide RUs require high SNR for successful transmission. Higher SNR can be achieved by increasing the transmission power, but the maximum transmission power limitation restricts the choice of RU and MCS.