第五代（5G）蜂巢式網路中的多播與廣播服務（Multicast and Broadcast Services, MBS）可透過共享無線電資源，將相同的多媒體內容傳送給多個車載用戶，相較於重複使用單播傳輸，可有效提升無線電資源的使用效率。在傳統多播傳輸中，所採用的調變與編碼機制（Modulation and Coding Scheme, MCS）通常受限於通道品質指標（Channel Quality Indicator, CQI）最差的車載用戶，因而可能降低通道條件較佳車載用戶所能接收的視訊品質。雖然以子群組為基礎的多播傳輸可依據不同的 gNB CQI 值，將車載用戶劃分至不同子群組，並分別配置資源區塊（Resource Blocks, RBs），以緩解上述問題，但在僅採用 gNB 直接多播傳輸的情況下，gNB 接收條件較差的車載用戶仍可能需要大量 RB，或僅能接收較低品質的視訊內容。為解決上述問題，本論文提出一種車對車（Vehicle-to-Vehicle, V2V）輔助多媒體 MBS 之階層式子群組與資源配置方法，稱為 V2V-MBS-HSRA。所提方法結合 gNB 多播傳輸與 V2V 群播傳輸，以提升基於可調式視訊編碼（Scalable Video Coding, SVC）之多媒體 MBS 傳輸效能。在所提架構中，訂閱相同多媒體 MBS 的車載用戶會形成一個階層式子群組架構，其包含內部子群組、數個中繼輔助子群組，以及邊緣子群組。內部子群組中的車載用戶由 gNB 透過車對基礎設施（Vehicle-to-Infrastructure, V2I）多播傳輸直接提供服務，且其中部分車載用戶亦可作為中繼車載用戶。各中繼輔助子群組中的車載用戶則由內部子群組中的中繼車載用戶透過 V2V 群播傳輸提供服務。邊緣子群組則由無法與任何內部子群組車載用戶建立可行 V2V 連線的車載用戶所組成，因此仍須依賴 gNB 直接 V2I 多播傳輸。所提 V2V-MBS-HSRA 方法主要包含兩個階段。第一階段為決定各多媒體 MBS 所採用的子群組架構。首先，根據所有車載用戶排序後的不重複 gNB V2I-CQI 值，考量不同的內部子群組車載用戶組成方式，以產生多個候選子群組架構。針對每一個候選子群組架構，每一個不屬於內部子群組的剩餘車載用戶，會選擇能提供最強且可行 V2V-CQI 的內部子群組車載用戶作為其中繼車載用戶。因此，選擇相同中繼車載用戶的車載用戶會形成一個中繼輔助子群組，而無法找到適合中繼車載用戶的車載用戶則形成邊緣子群組。接著，每一個由內部子群組、數個中繼輔助子群組與邊緣子群組所組成的候選子群組架構，皆會透過 MBS 內資源配置程序進行評估，並選擇效用值最高的候選子群組架構作為對應多媒體 MBS 最終採用的子群組架構。第二階段則是在所有多媒體 MBS 的子群組架構皆決定後，對所有子群組執行 MBS 間 RB 配置。資源配置過程共同考量受益車載用戶數量、視訊層級提升所帶來的服務品質體驗（Quality of Experience, QoE）增益、不同視訊層級間的公平性，以及所需的 RB 成本。效能評估結果顯示，所提 V2V-MBS-HSRA 方法可在高分散與中分散情境下有效提升多媒體 MBS 的傳輸效能。相較於其他比較方法，所提方法可達到較高的平均資料速率（Average Data Rate, ADR）、最低的用戶不滿意度指標（User Dissatisfaction Index, UDI），以及較佳的 V2V 輔助服務涵蓋率。

By transmitting the same multimedia content to multiple vehicular users through shared radio resources, Multicast and Broadcast Services (MBS) over the 5G cellular network can improve radio resource utilization compared with repeated unicast transmissions. In conventional multicast transmission, the adopted Modulation and Coding Scheme (MCS) is usually limited by the vehicular user with the poorest Channel Quality Indicator (CQI), which may reduce the received video quality of vehicular users with better channel conditions. Although subgrouping-based multicast can mitigate this problem by splitting vehicular users with different gNB CQI values into different subgroups, for which each subgroup can have its own allocated Resource Blocks (RBs), vehicular users with poor gNB reception conditions may still require a large number of RBs or receive lower video quality when only direct gNB multicast transmission is used. To address this issue, this thesis proposes a Vehicle-to-Vehicle (V2V)-assisted multimedia MBS Hierarchical Subgrouping and Resource Allocation method, called V2V-MBS-HSRA. The proposed method jointly utilizes gNB multicast transmission and V2V groupcasting to improve SVC-based multimedia MBS delivery. In the proposed framework, vehicular users subscribing the same multimedia MBS are organized into a hierarchical subgrouping pattern that consists of an inner subgroup, some relay-assisted subgroups, and an edge subgroup. Vehicular users in the inner subgroup is directly served by gNB’s Vehicle-to-Infrastructure’s (V2I’s) multicast transmission and contains the vehicular users that may also serve as relay vehicular users. Vehicular users in each relay-assisted subgroup is served by a relay vehicular user in the inner subgroup through V2V groupcast transmission, while the edge subgroup consists of the vehicular users that cannot establish feasible V2V links with any inner-subgroup’s vehicular user and therefore still relies on direct gNB V2I multicast transmission. The proposed V2V-MBS-HSRA method consists of two main stages. In the first stage, the proposed method determines the adopted subgrouping pattern for each multimedia MBS. Multiple candidate subgrouping patterns are generated by considering different choices of the inner subgroup’s vehicular users based on the sorted distinct gNB V2I-CQI values of all vehicular users. For each candidate subgrouping pattern, each one of the remaining vehicular users that do not belong to the inner subgroup is associated with the inner-subgroup’s vehicular user that provides the strongest feasible V2V-CQI as its relay vehicular user. Consequently, the vehicular users that are associated with the same relay vehicular user become a relay-assisted subgroup. Thereafter, those vehicular users that cannot find their respective relay vehicular users form the edge subgroup. Each candidate subgrouping pattern that consists of an inner subgroup, some relay-assisted subgroups and an edge subgroup is then evaluated through an intra-MBS resource-allocation process, and the pattern with the highest utility is selected as the adopted subgrouping pattern of the corresponding multimedia MBS. In the second stage, after the adopted subgrouping patterns of all multimedia MBSs are determined, the proposed method performs inter-MBS RB allocation among all subgroups. The allocation process jointly considers the number of benefited vehicular users, the QoE gain of video-layer upgrade, fairness among video layers, and the required RB cost. The performance evaluation results show that the proposed V2V-MBS-HSRA method can effectively improve multimedia MBS delivery performance in high-dispersed and mid-dispersed scenarios, where the proposed method achieves higher Average Data Rate (ADR), the lowest User Dissatisfaction Index (UDI), and better V2V-assisted service coverage than the compared methods.

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