## 2 A Primer on Multi-Link Aggregation in IEEE 802.11 be
### Multi-Link Architecture: STR and Non-STR Cases  
EHT topic interest group/study group (TIG/SG), which is inherited by IEEE 802.11 be, as its name would suggest, was initiated to aim for extremely high throughput supporting a maximum throughput of at least 30 Gbps. One of the significant features of IEEE 802.11 be is multi-link operation. Nowadays, since many access points (APs) are already operating with more than dual radios concurrently, this multi-link operation has been a significant issue to enhance Wi-Fi peak throughput with minimum modification of the existing Wi-Fi system. In Figures 1a,b, infrastructure models for conventional IEEE 802.11 and for IEEE 802.11 be with multiple links are illustrated, respectively. In the conventional model, each station is an addressable unit and can typically be regarded as a physical device. The station can transmit and receive data frames after associating with an AP. Compared to the conventional IEEE 802.11 infrastructure model, a newly defined entity, named MLD, can aggregate multiple stations into one physical device for a multi-link infrastructure model in IEEE 802.11 be. An MLD consists of multiple stations that may operate in the same band or a different band. Taking the actual example as an example, a laptop with two radios supporting 2.4 GHz and 5 GHz, respectively, is regarded as two separate stations in conventional IEEE 802.11, whereas a laptop with two radios supporting multi-link operation can be regarded as one MLD with dual radios in IEEE 802.11 be. In the latter case, each radio within the laptop operates as an independent station, and the laptop aggregates and orchestrates transmissions and receptions over those radios. Transmission and reception operation over multi-link can be categorized into two modes [4]: simultaneously transmit and receive (STR) and non-STR. If two stations within the same non-AP MLD are in STR, as shown in Figure 1b, one non-AP station can transmit a frame to the AP MLD, and the other non-AP station can receive a frame from the AP MLD simultaneously without any interruption. That is to say, they can be considered as exactly two independent stations. Hence, any modification of DCF would not be needed. On the other hand, there may be the case where it is impossible to receive on one station while transmitting on another station, and this case is called a non-STR case. This non-STR case can occur when the distance between center frequencies of stations within the same MLD is too close. Specifically, this STR/non-STR capability can vary according to both the distance between two links of the radios and the device's internal chip design. If the device's radio transceivers inside are well-designed enough to prevent leakiness of signal from other links, the device can withstand them. However, in the opposite case, the device will not overcome the signal leakiness so that the device will become a non-STR case. Additionally, these two transmission modes would be set up in the process of association in the stations' or AP's judgment. To sum up, an estimated throughput with STR-enabled dual-radio can perform nearly doubled compared to a single-link device in saturated buffer conditions. If some stations within an MLD are on a non-STR condition, some modifications of DCF should be considered to fully utilize the stations attached to the MLD. Specifically, in designing multi-link DCF for non-STR, two things should be considered in particular, which are depicted in Figure 2. The first problem is the transmission hindrance  
Figure 1: Infrastructure models for conventional IEEE 802.11 and 802.11 be. (**a**) Conventional IEEE 802.11 infrastructure model and (**b**) Expected IEEE 802.11 be multi-link infrastructure model with multiple links. problem, which is shown in Figure 2a. In conventional 802.11, a station detects a radio signal whether the signal's power exceeds a certain threshold or not, and in consequence, the stations can distinguish whether the link is in an idle state or in a busy state.