快速OFDM訊框時間差估計方法之設計與其在無線中繼網路降低換手頻率之研究

Abstract

本論文係研究在一個廣泛佈建OFDM無線中繼網路的都會區域中，快速移動的用戶端設備遭遇換手次數頻繁之問題，並提出相關的解決方案。在OFDM無線中繼網路中，引入一個新的網路元件－中繼站(Relay Station)，用以改善因建築物遮蔽所產生的訊號品質衰落問題，並藉以提高訊號涵蓋範圍。中繼站具備完整訊框結構，對基站(Base Station)而言，中繼站被視為一個用戶端(Mobile Station)；對用戶端而言，中繼站扮演基站的角色。隨著中繼站佈建數量的增加，快速移動的用戶端將面臨換手次數提高的問題。 在一個以OFDM為基礎的通信系統中，訊框同步是個重要的議題。對於一個快速移動的用戶端而言，此問題尤其嚴重。但接收機在處理訊框同步過程中所估算到的訊框長度差，卻恰好可作為反應基站(含中繼站)與用戶端之間相對運動的參數。基於此參數，我們設計一個新的換手目標選擇指標，以有效的抑制換手次數的提升。 本論文的第一部分提出一個基於實體層的快速訊框到達時間估算方法。OFDM無線中繼網路的訊框結構中，不論是基站或中繼站，均擁有獨立的廣播標頭(Preamble)。此標頭以BPSK調變，在通過IFFT區塊之後，會呈現獨特的對稱特性。藉此特性，我們可以完成訊框到達事件偵測，並得到FFT長度與循環字首(Cyclic Prefix)的長度，而不須預先經過MAC層控制信令傳送給接收機端。藉由上述實體層資訊，我們可以估測訊框到達時間以及訊框長度，並與系統預設之訊框長度作比較後可以得到訊框時間差。 本論文第二部份設計一個新的換手目標選擇指標，並實作一個模擬都會區域廣泛佈建中繼站的類曼哈頓(Manhattan-like)模擬器。此模擬器實做IEEE 802.16j相關實體層參數、信號衰落模型，以驗證新指標的性能。模擬結果顯示此新指標能有效抑制換手頻率，對於降低換手次數有顯著的效果。

In this thesis, we investigate how to efficiently using the relative movement information of MS to reduce handoff event and control signaling overhead for OFDM-Based wireless relay networks. In wireless relay networks, the relay station (RS) is introduced to improve the link quality and coverage. However, RS may cause the increase handoffs when an MS moves across the boundary betweens RS and BS, thereby result in signaling overhead for handoff. A handoff algorithm in wireless relay networks should intelligently reduce unnecessary handoffs. Firstly, we develop a frame arrival event detection and frame arrival time estimation algorithm based on fast PHY-layer Schemes for obtaining cyclic prefix (CP) Lengths and fast Fourier transform (FFT) Sizes. Traditionally, CP length and FFT size are broadcasted via media access control (MAC) layer to pre-handoff MS. The algorithm we proposed is based on the conjugated symmetry characteristics of binary phase shift keying (BPSK) modulated preamble. Secondly, the difference of the estimated consequent frame inter-arrival time could be used to judge that MS moves toward a RS or leaving away a RS. This is called the relative movement information, which is considered to develop a time-of-arrival based target selection algorithm for handoff procedure. In simulated Manhattan-like urban model, physical layer parameters, path-loss model and handoff procedure of IEEE 802.16j are implemented. We evaluate the performance of the proposed algorithm. Our result shows that proposed algorithm can reduce the handoff frequency significantly compared to traditional signal-based target selection.