在以寬頻分碼多工存取為基礎之高速下行鏈路封包存取系統中利用快速重傳來避免封包壅塞之方法進行速率調整及封包排程之研究

Abstract

隨著高速率多樣化資料傳輸上的需求與日劇增，在無線網路上引發了新的挑戰。為了克服這些挑戰，在這篇論文中提出了一個以成本函數為基礎的速率調適機制針對有著高速率下行鏈路封包存取概念的寬頻分碼多工存取系統進行速率調適以及排程技術的研究。在以寬頻分碼多工存取為基礎之高速下行鏈路封包存取系統中利用快速重傳來避免封包壅塞之方法進行速率調整及封包排程之研究 為了在寬頻分碼多工存取系統(WCDMA)上支援多種類的資料流型態，此機制綜合考量了實體層以及較高層的一些重要參數來增進效能。所提出來的速率調適機制是利用了在媒體存取控制(MAC)層裡面的運輸格式(transport format)選擇程序來實現的。因此，藉由結合實體層資訊(也就是信號雜訊比(SNR))以及媒體存取控制層的資訊(也就是暫存器使用量以及服務優先權)到一個統一化的成本函數，我們發展了一套運輸格式選擇演算法。透過此跨層級的成本函數，此運輸格式選擇程序可以在每個傳送時間區間(transmission time interval)內動態的選出合適的展頻因子，此區間通常是2毫秒到160毫秒。因此，此程序可以達到流通量之增進、功率消耗之節省以及在多個服務之間提供公平性效能改善的目的。 寬頻分碼多工存取系統中之高速率下行鏈路封包存取採用了適應性調變技術、高效率的排程技術以及混合式的自動重送請求技術以達到10 Mbps的高速率傳輸。為了增進系統效能，排程演算法則扮演著重要的角色。一個好的排程演算法目標在於從眾多使用著中，考量到通道的影響、延遲時間以及公平性後進行使用者之傳輸排程。在這篇論文中，我們檢驗現有適合用在此高速率下行鏈路封包存取概念之排程演算法的公平性效能，這些演算法包括最大信號干擾比 (maximum C/I) 排程法、知更鳥式循環(round robin)排程法、比例式公平(proportional fair)排程法以及指數型法則(exponential rule)排程法。我們發現現有的排程演算法在此公平性指標上的表現並不是那麼公平。因此，我們提出一個新的排程演算法，稱為避免封包壅塞之方法(stall avoidance + queue-based exponential rule)排程法，來提供比比例式received goodput in upper layer排程法以及指數型法則排程法還要好的效能，並且保持高流通量以及低延遲時間。

The objective of this thesis is to propose a stall avoidance scheduler for an N-channel Stop ¡ And ¡ Wait (SAW) Hybrid Automatic Request or Retransmission Query (HARQ) mechanism to improve the end-to-end throughput and reduce total system delay in the WCDMA systems with High Speed Downlink Packet Access (HSDPA). An increasing demand for both larger system capacity and higher data rates has launched the evolution of the mobile communication market. HSDPA appears as an umbrella of features using key technologies such as Fast Link Adaptation, Fast Scheduling, Adaptive Modulation and Coding (AMC) and Fast Retransmission Strategy (HARQ) whose combination improves the network apacity, increases the peak data rates up to 10 Mbps for downlink packet tra±c and decreases the system throughput delay. HSDPA can allow users to enable high peak data rate services with a lower cost per delivered data bit, improve the QoS of already existing services. In order to satisfy the growing aggressive demand for the wireless packet data services, scheduling algorithms are widely used in WCDMA wireless multimedia networks to provide the guaranteed quality of service. The design of such such algorithms is a challenge due to the mobility and connectivity problems that encountered in Wireless Networks. Among many protocol enhancements designed to maximize downlink throughput, the N-Channel SAW-HARQ is one of the required schemes that can boost the user throughput. The proposed Stall Avoidance (SA+QER) scheduler is to be implemented in MAC layer and its role is to improve the end-to-end throughput and reduce the overall end-to-end delay in the presence of packet stall (or gap) in the receiver reordering bu®er in the WCDMA systems with HSDPA. The overall performance comparison of conventional cheduling policies with timer and window mechanisms is presented through simulations. Simulations results show that in terms of received goodput in upper layer and head-of-line delay, the proposed stall avoidance scheduler outperforms all the other existing schedulers, including maximum C/I, round robin, proportional fair, queue-based exponential rule. Furthermore, the overall system delay is signi‾cantly reduced by the stall avoidance scheduler.