行動全球互通微波接取 (Mobile WiMAX, Worldwide Interoperability for Microwave Access) 是一種多用戶正交分頻多工系統，也是一種無線寬頻技術，其可以用共同的寬頻無線接取技術來同時接取行動用戶和非行動用戶。在發展行動全球互通微波接取系統時，其中一個主要的挑戰就是要設計出一個有效率、可以處理系統中所有使用者的排程系統。一個實際針對行動WiMAX設計的排程系統，需要有下列條件：1. 可以同時滿足傳送即時封包 (real-time packets) 的使用者和傳送非即時封包 (non-real-time packets) 的使用者其網路服務品質 (QoS, Quality of Service)。2. 排程系統在做資源分配的時候可以同時考慮到行動使用者和非行動使用者的通道 (channel) 特性。為了這些目標，我們發展出利用跨層的概念來設計多使用者的排程機制，此跨層涵蓋了實體層和資料連結層。每位使用者再搭配我們提出的適應性調變/編碼 (AMC, Adaptive Modulation/ Coding) 演算法，此適應性調變演算法是針對所需要的封包錯誤率（PER, packet error rate）來設計的。在本論文中，我們將會介紹所提出的排程機制和適應性調變/編碼演算法。
對於適應性調變/編碼演算法，我們推導出行動全球互通微波接取系統中在AWGN通道和Nakagami-m通道下封包錯誤率和訊雜比的近似數學模型。藉由此模型和所要求的封包錯誤率，可以得到各個調變模式的邊界值，進而可以由收到訊號的訊雜比和此邊界值結果來決定要用什麼調變模式。而此屬於實體層的結果會利用跨層來交換給資料連結層的排程器來提升系統的傳輸速率。
對於排程器，會利用到標準中傳輸訊號框架 (frame) 長度是固定的特點，來預測每個訊號框架的傳輸起始時間為何時，如此就可以根據即時性的封包其可容忍延遲時間 (deadline) 以及子通道的頻率響應來預先安排屬於哪個框架和分配到哪些子通道。剩餘的資源就可分配給非即時性封包來傳輸。由電腦模擬指出，這樣的設計和傳統的排程器比較，的確可以讓即時性封包滿足網路服務品質中的可容忍時間延遲，也可以對非即時性封包提升傳輸速率。

Mobile WiMAX (Worldwide Interoperability for Microwave Access), namely IEEE 802.16-2005, is a kind of multiuser OFDM systems that can support both mobile and fixed broadband wireless services. The main challenge in Mobile WiMAX is to design an efficient traffic scheduler for resource assignment to the entire users in the system. A practical scheduler for Mobile WiMAX should have the following essentials: 1) it can provide guaranteed quality-of-service (QoS) for real-time and non-real-time packets without violating fairness; 2) it should take the channel properties of both fixed and mobile broadband networks into account when allocating resource. To meet these two requirements, we purpose a cross-layer design for the Mobile WiMAX scheduler based on joint considerations of the physical and medium access control (MAC) layers, with each user employing adaptive modulation/coding (AMC) based on the desired packet error rate.
For the AMC algorithm, we derive new closed-form relations between the packet error rate (PER) and the received signal-to-noise ration (SNR) in the Mobile WiMAX system over AWGN and Nakagami-m fading channels. With the derived PER formulas and the target PER requirement, we can obtain the boundary SNR values for changing modulation/coding modes. The mode changing information is then cross-layer utilized in the design of the purposed QoS-guaranteed scheduler to maximize the system throughput.
The proposed scheduler, referred to as Pre-Processing Scheduler, aims at providing the most appropriate treatments to data connections based on their QoS requirements as well as reducing processing needs at the beginning of transmission of each time frame. By exploiting the feature of the Mobile WiMAX standard that the duration of each TDD frame is fixed, the proposed scheduler can predict the starting instant of each frame and then pre-assign appropriate frames and subchannels for transmitting real-time packets based on their transmission deadlines and the corresponding channel responses. After this, the remaining resources are allocated to non-real-time packets. Computer simulation results show that the proposed scheduler can grantee QoS for real-time packets and attain a higher throughput for non-real-time packets, as compared to the conventional scheduling approach.

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