Performance Analysis of Synchronous Multi-Radio Multi-Link MAC Protocols in IEEE 802.11 be Extremely High Throughput WLANs  
[PERSON]  
1  
[PERSON]  
2  
Footnote 1: [[https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci)]([https://www.mdpi.com/journal/applsci](https://www.mdpi.com/journal/applsci))  
Footnote 2: [[https://www.mdpi.com/journals/article/pdf/1009/1](https://www.mdpi.com/journals/article/pdf/1009/1)]([https://www.mdpi.com/journals/article/pdf/1009/1](https://www.mdpi.com/journals/article/pdf/1009/1))  
dates. Features incorporated in the Release 1 will be prioritized in the standardization process of 11 be are wideband spectrum utilization, EHT PHY layer protocol data units (PPDUs) on the PHY layer, and multi-link operation on the MAC layer. On the other hand, features that are likely to be included in the Release 2 will mature later [2]. Among new features newly introduced in IEEE 802.11 be Release 1, multi-link operation, which enables a single physical device to establish multiple links and operate them simultaneously, is the most representative feature of the IEEE 802.11 be in that the multi-link operation can provide efficient use of the available spectrum. Therefore, it can achieve higher data rates than the conventional single-link operation and can implement load balancing.  
When it comes to multi-link operation in terms of IEEE 802.11 be standardization, a new entity called multi-link device (MLD) has been defined [3]. This MLD is a physical device with more than one affiliated station and has one interface toward the upper layer. Consequently, such MLDs can transmit and receive simultaneously in different bands or links, such as 2.4 GHz, 5 GHz, and 6 GHz, and thus they can increase the peak throughput by enabling simultaneous operations.  
However, it is not always possible to freely use these multiple links. For example, a transmission from one of the MLD links can affect receptions over other links within the same MLD, and thus the reception may fail due to in-device interference from transmission power leakage inside the MLD. For example, this phenomenon may happen if the center frequencies of the MLD links are too close. Therefore, a modified multi-link operation is required to deal with the in-device interference mentioned above. For this purpose, a synchronous transmission method in which the end times of transmissions from the same MLD must be aligned in the same MLD has been claimed to avoid in-device interference. As synchronous multi-link transmission behavior has not been considered in the conventional 802.11, a modified DCF suitable for synchronous multi-link must be devised.  
Although a detailed DCF procedure for multi-link has not been standardized in 802.11 be yet, several modified DCF methods to support synchronous multi-link operation have been proposed during TGbe, and their performance evaluations are being made. However, to the best of our knowledge, none of the researches has introduced an analytical model for the synchronous multi-link DCFs.  
The main contributions of this work are three-fold: (1) We investigate multi-link DCFs applicable for synchronous multi-link transmission and devise a Markov Chain model for them. (2) We analyze average throughputs and fairness between MLDs and single-link devices with extensive mathematical analysis when two links are established. (3) We discuss the advantages and disadvantages of the analyzed multi-link DCFs.  
The rest of this article is organized as follows. In Section 2, we introduce a detailed multi-link architecture and its operation discussed in IEEE 802.11 be. In Sections 3 and 4, we classify the proposed multi-link DCFs suitable for synchronous transmission into four types and construct Markov Chain models for each, respectively. Next, performances for the multi-link DCF methods are numerically analyzed from various perspectives in Section 5. Finally, we conclude our work in Section 6.