- 學位論文
在寬頻多重分碼存取環境中使用單一碼排除正交可變展頻因子碼的內部碎裂
Eliminating internal fragmentations of OVSF codes in WCDMA systems with single code
摘要
在寬頻多重分碼存取環境中(Wideband Code Division Multiple Access；WCDMA)，為了提供使用者不同速率頻寬的服務，使用正交可變展頻因子(Orthogonal Variable Spreading Factor；OVSF)碼。正交可變展頻因子碼在分配的過程中可能會產生碎裂問題，包含內部碎裂(Internal fragmentation)與外部碎裂(External fragmentation)；本篇文章處理內部碎裂的問題。當分配出去的傳輸率比要求的速率還要高時，會產生內部碎裂，為降低內部碎裂，一般是以多重碼(Multi-Code)的方式解決，即同一時間用許多較小的碼來支援進來的要求，本篇在解決內部碎裂問題時使用的是單一碼(Single-Code)的方式，將要求的速率分割成數個二的指數次方之速率，這些分割好的速率分別在不同的時間裡被服務，並且因此達到降低內部碎裂的效果。本論文提出最短切割與動態切割兩種方式；最短切割將每個呼叫以最少的切割數分好；動態切割則視碼樹上可被分配碼的情況來使用。模擬結果顯示，比較各種解決內部碎裂的方法，最短切割和動態切割在內部碎裂指數與平均完成時間上的表現較佳。
並列摘要
The use of OVSF codes in WCDMA systems can provide variable data rates to flexibly support applications with different bandwidth requirements. The data rates provided by the OVSF codes are exponentially quantized since the spreading factors are exponentially decreased. One important issue when using the OVSF codes is the internal fragmentation problem which happens when the provided data rate is larger than what is requested. The precious wireless bandwidth is wasted if there is internal fragmentation. In this paper, we try to solve the internal fragmentation problem using one single OVSF code per connection. If the amount of the requested data rate is known, we can partition the request into several subrequests which can be exactly served by OVSF codes. This eliminates the internal fragmentation completely. We also consider the time issue when solving the internal fragmentation problem. To guarantee that every user can meet his/her deadline, a scheduling mechanism is needed. We proposed several partition, code assignment and scheduling schemes in this paper. The goal is to remove internal fragmentation for delay-sensitive services. Simulation results verify that the proposed schemes avoid internal fragmentation and reduce user’s waiting time concurrently.