上行多使用者多天線系統之存取協定設計

Abstract

隨著移動裝置普及及應用程式的多元，由移動裝置產生的資料量 逐漸增加，對於高速無線網路資料上傳的需求日趨迫切，多使用者多 天線系統在提升整體效能扮演關鍵性的角色，多使用者多天線系統主 要是利用網路節點所配置的多天線，讓多個使用者可以同時傳送資料 。有別於傳統單一使用者無線網路，多使用者無線網路面臨了下列數 個多天線系統衍生的難題：1) 每位使用者的最佳傳送速率會與其共同 傳送者有關，傳統單一使用者網路往往利用個人歷史傳輸品質記錄來 推測最佳傳送速率，此方式將不再適用於多使用者多天線系統。2) 上 行的多使用者網路比起傳統無線網路更容易受到隱藏節點問題或碰撞 干擾，任何一個封包錯誤會使得所有同同時傳輸的封包無法被正確解 碼，若只是單純重傳所有封包將會大幅降低多天線系統之效能。3) 為 了更進一步擴充多天線系統的頻寬，可利用後端有線網路將數個實體 路由器連結成多天線虛擬路由器，建構出``網路式多天線系統'。然而 將任意天線組成虛擬路由器，會增加路由器之間的干擾並降低頻寬使 用率，因此，如何將天線組成適當的網路式上行多天線系統，將成為 決定效能的關鍵。 本論文試著解決以上的問題，我們首先將目光著眼於單一路由器之 多天線系統，並設計一套讓每個使用者可以根據同時傳送者的通道來 來調變最佳傳送速率的通訊協定，使得整體無線網路的效能可以被妥 善運用；由於多使用者無線網路允許多個使用者同時傳送資料，當同 時傳送使用者逐漸增加，因隱藏節點問題或者無線傳輸碰撞所造成的 錯誤率也隨之上升而大幅降低系統效能，我們進而提出一套多使用者 多天線系統之封包修復通訊協定，讓系統迅速地修復碰撞的封包。最 後，本論文提出一套動態式網路式多天線系統，讓虛擬路由器架構可 以根據使用者的地理位置分布與需求資料量做及時的架構調整，以降 低跨網路之干擾並提升整體效能。我們已經將以上設計實作於軟體定 義平台上，不論是在實際測試或者模擬分析上，本論文所提出之系統 設計皆較傳統單一使者網路及現有的多使用者多天線系統有更卓越的 效能提升。

With the popularity of mobile devices and data-intensive applications, the amount of traffic generated by existing wireless networks has been tremendously increasing. This phenomenon introduces the urgent requirement for high speed wireless transmissions. One predominate approach is Multiuser MIMO (MU-MIMO) systems, which exploit multiple antennas equipped at a phyiscal access point or a virtual access point connected by backhaul networks to enable several users to communicate simultaneously. These strategies introduce several new challenges: 1) The optimal ransmission bitrate of a user will change with the concurrent transmitters on a per packet basis.Therefore, traditional historical-based bitrate selection algorithms cannot work in MU-MIMO systems. 2) In a uplink MU-MIMO system, every single error caused by hidden terminals or collisions will corrupt the whole packets.Therefore, MU-MIMO systems are especially more vulnerable to errors than single-user networks. Simply retransmitting collided packets will take away the performance gains provided by multiple concurrent transmissions. 3) A system can further combine multiple access points as a multi-antenna virtual access point. However, in such a ``network MU-MIMO system', arbitrarily forming several antennas as an access point could increase the probability of inter-cell interference and further decrease channel utilization. How to form practically-sized virtual access points becomes a critical problem in network MIMO systems. This dissertation tries to solve the above problems. First, this dissertation focuses on a single access point scenario, and proposes a MAC protocol for each user to properly select their bitrates based on the channels of concurrent transmitters such that the performance gain provided by MU-MIMO can be fully utilized. To address the error vulnerability problem of MU-MIMO,this dissertation proposes a packet recovery strategy for MU-MIMO systems to repair collided packets without significant overhead. Finally, this dissertation introduces a new structure for network MIMO systems, which adaptively forms practically-size virtual access points based on dynamic client distributions and traffic demands to reduce inter-cell interference and enhance antenna utilization. The above designs have been implemented on software defined radio platforms and analyzed through simulations. In both testbed experiments and simulations, the proposed systems can achieve a significant performance gain, as compared to the traditional wireless network and existing MU-MIMO systems.