###### Abstract
It is equipped with an algorithm to dynamically turn on/off links and switch between them to minimize energy consumption with little performance degradation. [PERSON] et al. [12] have developed a constraint-aware aligned downlink ending protocol for simultaneous downlink transmissions over multiple channels for devices with no ability to simultaneously transmit and receive. Listen-before-talk schemes (LBT) for multicarrier aggregation of Long-Term Evolution with License-Assisted Access (LTE-LAA; LTE for unlicensed spectrum) [13; 14] have been studied, and enhanced schemes for use in the presence of RF power leakage were proposed in [15; 16; 17]. As proposed by the above-mentioned works, multi-link operation requires a new channel access scheme to better exploit multi-link transmission opportunities. However, none of these prior research works focused on the issue of coexistence with legacy single-link Wi-Fi devices for multi-link channel access schemes. Exploring the coexistence problem and designing solutions within the framework of EHT will be essential to facilitate the wide and fast penetration of the new approach. This paper aims to illustrate enhanced multi-link channel access schemes for IEEE 802.11 be EHT, identify the associated coexistence challenge, and propose solutions applicable to cases both with and without the capability of simultaneous transmission and reception. First, we describe the concept, architecture, and asynchronous and synchronous channel access schemes of multi-link operation, along with the RF power leakage problem. Next, we describe the design variants of the synchronous channel access scheme and demonstrate the coexistence challenge of the enhanced scheme. Subsequently, we propose four features to mitigate this challenge by assigning penalties to multi-link devices: repicking a backoff count, doubling the contention window size, switching to another contention window set, and compensating the backoff count. Then, five coexistence solutions are derived from combinations of these features. Comparative evaluation results are provided and analyzed for dense single-spot and indoor random deployment scenarios, demonstrating that the throughput and latency gains of multi-link operation differ between schemes. We also investigate the coexistence performance of multi-link operation with and without the capability of simultaneous transmission and reception and demonstrate that the proposed solutions mitigate the coexistence problem to different degrees. In particular, compensating the backoff count achieves the highest coexistence performance among the proposed solutions, with a marginal throughput decrease of multi-link devices. A metric for evaluating both the throughput and latency gains and the coexistence performance of a multi-link channel access scheme using a single value is also proposed. In summary, the main contributions of our work are listed as follows:  
* Identification of the issue concerning the coexistence of an enhanced synchronous multi-link channel access scheme with legacy devices, which has not been explored yet in the literature. * Design of features to assign a penalty to a backoff process and coexistence solutions derived from various feature combinations. * Comprehensive simulation to illustrate the throughput and latency gains and coexistence performance of various multi-link channel access schemes including the proposed solutions when multi-link devices coexist with legacy devices. * Design of a new metric for evaluating both the throughput and latency gains and the coexistence performance of multi-link operation using a single value. The rest of this paper is organized as follows. Section 2 explains the details of multi-link operation. Section 3 describes the variants of the synchronous multi-link channel access scheme and the coexistence issue.