在802.11ac媒介存取層中下行多使用者多輸入多輸出的封包排程方法

Abstract

現今的無線區域網路中，MIMO是一個可以增進網路效能的重要技術，而在最新的IEEE 802.11ac標準中，已經可以支援DL MU-MIMO，也就是同時傳輸資料到多個使用者。在multi-user的環境底下，MAC層和PHY層必須做出改變來迎合此環境。MAC層採用了改良式的EDCA，由各個AC去競爭頻道使用權，並且由競爭到頻道使用權的AC來負責分配TXOP的使用權。而在PHY層，結合了多個使用者PPDUs和相同PHY preamble的MU PPDU取代了原先的PPDU。然而我們發現，在每個PPDU中A-MPDU大小會對於整體效能有著明顯的影響。在同一段TXOP的傳輸時間內，當A-MPDU較小時，會有overhead過多影響效能的問題；反之當A-MPDU過大，則會有天線閒置在等待其他天線傳送結束的問題。而A-MPDU的大小，取決於MAC層如何去做frame的排班和相對應的frame aggregation。在此篇論文中，我們提出一套方法將DL MU-MIMO環境底下的frame排程轉換成圖論模型，並且設計一個排程的求解方法，有效的改善整體的效能。

In the WLANs, MIMO has became an important technique to enhance the network throughput. For the latest released IEEE 802.11ac standard, the access point can support DL MU-MIMO, it means that data streams can be transmitted to multiple users simultaneously. In a multi-user environment, the MAC layer and PHY layer must be changed to adapt the environment. For the MAC layer, it adopts the modified EDCA and each AC contends the channel access. After winning the contention of channel access, the owner of channel can decide how to share the TXOP with other ACs. For the PHY layer, the original PPDU becomes MU PPDU which contains multiple user’s PPDUs and same PHY preamble. However, we found that the size of A-MPDU in each PPDUs can influence the network throughput. During the same TXOP, if the size of A-MPDU is small, the overhead will increase large; On the other hand, if the size of A-MPDU is big, the idle time of antennas waiting for another antennas transmission will increase. And the size of the A-MPDU is determined by how to do frame aggregation with scheduling method. In this thesis, we transform the frame scheduling problem into a graph model and propose a method for solving the problem such that the network throughput is enhanced.