IEEE 802.16e OFDMA實體層測距技術與數位訊號處理器實現之探討

Abstract

本篇論文介紹IEEE 802.16e正交分頻多工存取(OFDMA)裡，測距(ranging)的問題、演算法、分析、以及實作方面的議題。 在WIMAX的規格裡，測距是一個很重要的程序。初始和週期測距是保證所有動態使用者的訊號能夠同步到達基地台的2個重要上行程序，如此上行正交分頻多工存取系統中子載波(subcarrier)的正交性得以維持。其中，週期測距讓行動台能調整傳輸參數以維持和基地台之間的上行通訊。在這篇論文中，我們只討論週期測距的細節及其演算法。然而，其他測距程序，如初始測距、頻寬要求及換手(handover)測距，將不會在此論文中詳談。 在週期測距程序裡，基地台需要偵測出每一個有傳送週期測距碼的使用者之測距碼，並作時間、功率偏移及也可能包括頻率偏移之估計。然後，基地台以廣播的方式回傳測距回報訊息(ranging response message)，其中包含需要做的校正及狀態通知。 我們使用頻域上的方法來完成測距碼的偵測以及時間偏移的估計。我們發現落在-0.1到0.1之間的頻率偏移並不會對效能造成明顯的影響，因此忽略頻率偏移的估計。我們對演算法作一些簡單的分析，並在可加性白色高斯雜訊通道(AWGN)和多路徑Rayleigh衰減通道下做模擬，並觀察其效能。 最後，我們把程式修改成定點運算的版本，將其實作到數位訊號處理器平台上，並盡可能利用一些最佳化技巧來加速測距函式的速度。我們提供了時脈次數(clock cycle)的模擬結果來說明我們的測距作業可以達到即時處理的要求。

In this thesis, we introduce the ranging problems, algorithms, analyses and implementation issues for IEEE 802.16e OFDMA PHY system. Ranging is one of the significant processes in the mobile WiMAX standard. Initial and periodic ranging are two important uplink processes to ensure that the signals from all active users arrive at the BS synchronously so that the orthogonality among the subcarriers in the uplink of OFDMA systems is maintained. Periodic ranging allows the MS to adjust transmission parameters so that the MS can maintain uplink communication with the BS. In this thesis, we discuss about the details of periodic ranging and algorithms. In fact, there still exist other types of ranging process such as: initial ranging, bandwidth request ranging and handover ranging. However, periodic ranging is the only ranging process being discussed in this thesis. In the periodic ranging process, the BS is required to detect different received ranging codes and estimate the timing, power and possibly frequency offset for each user that transmits a periodic ranging code. The BS then broadcasts a ranging response message (RNG-RSP) with needed adjustments and a status notification. We employ a frequency domain method to accomplish the ranging code detection and timing offset estimation. We find that the frequency offset which was within the range of [-0.1,0.1] of subcarrier spacing did not cause an effect on the performance in evidence. Thus, we ignore the estimation of frequency offset. We perform some simple analyses of the ranging algorithm, simulate our ranging system in both AWGN and multipath Rayleigh fading channel and see the performance. In the end, we modified the program to fixed-point version, implement them on the digital signal processor (DSP) platform and employ some optimization techniques to accelerate functions of ranging as fast as we can. Finally, some clock cycles simulation results were provided to show that the ranging task can achieve real-time requirements.