By harnessing the delay-Doppler (DD) resource domain, orthogonal time-frequency space (OTFS) substantially improves the communication performance under high-mobility scenarios by maintaining quasi-time-invariant channel characteristics. However, conventional multiple access (MA) techniques fail to efficiently support OTFS in the face of diverse communication requirements. Recently, multi-dimensional MA (MDMA) has emerged as a flexible channel access technique by elastically exploiting multi-domain resources for tailored service provision. Therefore, we conceive an elastic multi-domain resource utilization mechanism for a novel multi-user OTFS-MDMA system by leveraging user-specific channel characteristics across the DD, power, and spatial resource domains. Specifically, we divide all DD resource bins into separate subregions called DD resource slots (RSs), each of which supports a fraction of users, thus reducing the multi-user interference. Then, the most suitable MA, including orthogonal, non-orthogonal, or spatial division MA (OMA/ NOMA/ SDMA), will be selected with each RS based on the interference levels in the power and spatial domains, thus enhancing the spectrum efficiency. Then, we jointly optimize the user assignment, access scheme selection, and power allocation in all DD RSs to maximize the weighted sum-rate subject to their minimum rate and various practical constraints. Since this results in a non-convex problem, we develop a dynamic programming and monotonic optimization (DPMO) method to find the globally optimal solution in the special case of disregarding rate constraints. Subsequently, we apply a low-complexity algorithm to find sub-optimal solutions in general cases.

翻译：通过利用时延-多普勒（DD）资源域，正交时频空间（OTFS）通过保持准时不变信道特性，显著提升了高移动性场景下的通信性能。然而，面对多样化的通信需求，传统的多址接入（MA）技术无法有效支持OTFS。近年来，多维多址接入（MDMA）作为一种灵活的信道接入技术出现，它通过弹性利用多域资源来提供定制化服务。因此，我们利用用户在DD、功率和空间资源域中的特定信道特性，为一种新型多用户OTFS-MDMA系统构思了一种弹性多域资源利用机制。具体而言，我们将所有DD资源单元划分为称为DD资源时隙（RS）的独立子区域，每个时隙支持一部分用户，从而减少多用户干扰。然后，根据功率域和空间域的干扰水平，在每个RS中选择最合适的MA技术，包括正交、非正交或空分多址（OMA/NOMA/SDMA），从而提高频谱效率。接着，我们联合优化所有DD RS中的用户分配、接入方案选择和功率分配，以在满足用户最低速率和各种实际约束的条件下最大化加权和速率。由于这导致了一个非凸问题，我们开发了一种动态规划和单调优化（DPMO）方法，在忽略速率约束的特殊情况下寻找全局最优解。随后，我们应用一种低复杂度算法在一般情况下寻找次优解。