基於速率拆分多工的無蜂巢系統之下行傳輸跨層優化研究

Abstract

當前的無線通信系統基礎設施主要依賴TCP/IP所定義的多層網路協議模型進行運作，並通過部署在目標區域內的基站 (BS) 為多個使用者 (UE) 提供服務。然而，隨著通信應用的快速發展，這種嚴格的層次結構和傳統通信技術逐漸難以滿足多樣化應用的需求。因此， 5G/6G技術在近年來越來越受到學術界和業界的關注，雙方正致力於突破傳統限制，構建更靈活且具彈性的無線通信架構，以滿足更高的傳輸速度和服務品質標準。 為了緩解層與層之間嚴格劃分可能帶來的不利影響，跨層設計（Cross-Layer Design）透過適度的層間訊息交換，實現通訊網路整體性能的提升。然而，靈活的通信協定需要一個靈活的通信系統來充分發揮其潛力。與傳統蜂巢系統 (cellular system) 相比，近年來常在5G/6G相關研究中提到的無蜂巢系統 (cell-free system) 提供了更靈活的服務模式。通過移除蜂巢架構，並使用多個小型基地台 (AP) 協作服務使用者，無蜂巢系統可以解決使用者在細胞邊緣 (cell edge) 通訊品質低落的問題。此外，由於無蜂巢系統將傳統蜂巢系統中的基站 (BS) 功能分離為負責大量計算的中央處理單元 (CPU) 和多個負責發射或是接收訊號的AP，整體通訊系統的覆蓋率也可以有所提升。 儘管無蜂巢系統具備上述優勢，它仍面臨諸多挑戰，其中最棘手的問題之一是AP對非目標使用者之間的干擾。速率拆分多工 (Rate-Splitting Multiple Access, RSMA) 正是為了應對這種複雜且多變的干擾環境而誕生。作為將干擾完全當作雜訊的空分多工 (Space-Division Multiple Access, SDMA) 和完全解碼干擾的非正交多工 (Non-Orthogonal Multiple Access, NOMA) 之間的銜接者，速率拆分多工能夠通過適當的功率和訊息分配，選擇性地將部分干擾作為雜訊處理，而其他干擾則解碼並利用連續干擾消除 (Successive Interference Cancellation, SIC) 將其扣除。綜合以上敘述，本研究旨在針對速率拆分多工輔助的無蜂巢系統，設計跨層優化架構，以提升通訊系統的可達傳輸速率總和 (achievable sum rate) 及最小可達傳輸速率 (achievable minimum rate)。

The current wireless communication system infrastructure primarily operates based on the multi-layer network protocol model defined by TCP/IP, providing services to multiple users through base stations (BS) deployed in target areas. However, with the rapid development of communication applications, this rigid layered structure and traditional communication technologies are increasingly insufficient to meet the demands of diverse applications. Consequently, 5G and 6G technologies have garnered significant attention from academia and industry in recent years, with efforts focused on overcoming traditional limitations to build a more flexible and resilient wireless communication architecture that meets higher standards for transmission speed and quality of service. To mitigate the adverse effects of strict layer separation, cross-layer design enhances overall network performance by allowing appropriate information exchange between layers. However, fully achieving the potential of a flexible communication protocol also requires a flexible communication system. Compared to traditional cellular systems, cell-free systems offer a more adaptable service model. By eliminating conventional cellular architecture and using multiple access points (APs) to collaboratively serve each user, cell-free systems effectively address the issue of poor communication quality often experienced by cell-edge users. Additionally, by decoupling the functions of the base station (BS) into a central processing unit (CPU) for complex computations and multiple APs for signal transmission and reception, cell-free systems can further improve the overall coverage of the communication network. Despite these advantages, cell-free systems still face significant challenges, with one of the most critical being interference between APs and unintended users. Rate-splitting multiple access (RSMA) has emerged as a promising solution to address these complex and dynamic interference environments. RSMA bridges the gap between space division multiple access (SDMA), which treats interference as noise, and non-orthogonal multiple access (NOMA), which fully decodes interference. By selectively managing interference, RSMA treats some as noise while decoding and canceling others using successive interference cancellation (SIC). This research focuses on designing a cross-layer optimization framework for RSMA-aided cell-free systems to improve both the achievable sum rate and the achievable minimum rate.