| 摘要: | 在寬頻分碼多工系統(WCDMA)下，使用正交可變展頻係數碼(OVSF code)可 以選擇不同的傳送速度，因此可以更有彈性的提供使用者不同速度需求的服務。 不過使用正交可變展頻係數碼有一些限制，例如同時使用的碼，彼此必須互相正 交，這個限制就可能會造成碼切斷(code blocking)問題。碼切斷的定義是雖然系 統仍有足夠的頻寬，但因為必須互相正交的限制，造成使用者仍會被拒絕服務， 這種碼切斷的情形也可看成是外部碎裂(external fragmentation)的問題。另外一個 限制是每個碼所提供的速度是呈倍數方式增加，這個限制則會產生內部碎裂 (internal fragmentation)的問題，即系統用過多的資源去滿足使用者的需求。 要解決外部碎裂的問題，其實就是要解決碼的分配及重分配的問題。碼的分 配問題即是討論要如何分配不同的碼給使用者，才能使碼切斷發生的情形能盡量 減少；碼的重分配問題，則是考慮如何才能使重分配的成本(重分配時所引發的 搬移次數)最少。在這兩個問題上，先前的研究，不是沒有考慮正交可變展頻係 數碼樹的結構，就是不能很有效率的使用這些碼。本篇論文即提出數種碼分配及 重分配的方法，如最左優先法(leftmost)，最擁擠優先法(crowded-first)等。這些方 法的基本觀念是讓分配後的正交可變展頻係數碼樹盡量緊密，以減少碼切斷的機 率。透過實驗的驗證，我們發現這些方法的確能大幅提高系統的效率。 為解決內部碎裂的問題，我們建議讓使用者的每個連線都能使用一個以上的 碼。文中將說明如何利用多個碼來降低內部碎裂。同時，我們也分析了使用多個 碼的硬體成本與系統效率之問的取捨；分析結果說明使用2 或3 個碼較能符合成 本效益。針對多個碼的分配與重分配，需考慮多個面向，針對各個面向，本論文 亦提出了多種策略。 為了更進一步提高寬頻分碼多工系統的效能，允許一個正交可變展頻係數碼 由多個使用者分時共享是一個很好的觀念。允許不同的使用者分時共享同一個 碼，則必須要有一個排程的演算法，決定『那一個時間分配那一個碼給那一個使 用者』。本論文採用一個使用者可以同時使用多個碼的環境，在此環境下，我們 提出兩個排程演算法，目標是讓每一個使用者都能公平地達到其個別的傳送速度 需求。提出的演算法考慮了多重的連線品質，因此更符合實際的系統運作。在不 同的品質下，這些方法調整每個碼的展頻係數以達使用者的連線速度需求。透過 理論分析與實驗測試，這些方法優越性都得到了驗證。 The use of OVSF codes in the WCDMA system can provide variable data rates to flexibly support applications with different bandwidth requirements. However, there are some constraints when using the OVSF codes, such as code blocking and exponentially quantized data rates. Code blocking, which is defined as the condition that a new call is rejected even though the system has enough bandwidth, induces external fragmentation of an OVSF code tree. Exponentially quantized data rates, resulting from the exponentially decreased spreading factors, induces internal fragmentations for requests. Both external and internal fragmentations waste the precious wireless bandwidth. In this dissertation, the effects of these constraints are investigated and several strategies are provided to eliminate such limitations. Two important issues on such an environment are the code assignment problem and code reassignment problem. The former may have significant impact on code utilization and thus code blocking probability, while the latter may affect the code reassignment cost if dynamic code assignment is to be conducted. The general objective is to make the OVSF code tree as compact as possible so as to support more new calls by incurring less blocking probability and less reassignment costs. Earlier studies about these two problems either do not consider the structure of the OVSF code tree or cannot utilize the OVSF codes efficiently. Two code assignment and reassignment strategies, leftmost and crowded-first, are proposed in this dissertation to solve these problems. Simulation results show that the crowded-first scheme increases the OVSF code tree utilization significantly. To reduce internal fragmentation, it is suggested to use multiple codes to support a call.We show how using multiple codes can reduce internal fragmentation of a OVSF code tree. The tradeoff between bandwidth utilization and hardware complexity of a multi-code system is analyzed. The result shows that using 2 or 3 codes will be quite cost-effective. Several multi-code assignment and reassignement strategies, namely random, leftmost, crowded- first-space, and crowded-first-code, are also proposed based on such environment. In order to further increase the bandwidth utilization, strategies that utilize time-shared OVSF codes are proposed to enhance statistical multiplexing. In particular, we propose to allow a user to simultaneously use multiple OVSF codes in a time-sharing manner, which we call a multi-code, shared model. Using multiple codes allows us to compensate those users suffering from communication interferences or even errors. The proposed schemes can tolerate a multi-state link condition (compared to the typically assumed two-state, or good-or-bad, link condition) by adjusting the spreading factors of OVSF codes. Through theoretical analyses and computer simulations, the proposed strategies are verified to be efficient and cost-effective. It is expected that the capacity of WCDMA systems can be effectively utilized when the strategies proposed in this dissertation are applied. |
