新穎正交分頻多重存取系統之測距方式

Abstract

正交分頻多重存取系統可根據使用者的多樣性以及時間軸等自由度並搭配合適資源以及能量的分配來有效率的提高頻譜使用效率。在本論文中提出單一正交分頻多重存取系統信號長度之測距信號結構和信號碼設計，並搭配信號提出所對應的信號偵測以及測距方式。首先，在頻譜上利用線性相角旋轉方式可以產生多組測距信號。而這種設計可以改善效能、測量距離和能量外也可提供較好的對抗多使用者信號干擾以及多路徑通道之能力。電腦模擬可以證實設計的信號比現存的架構有以上等優點。 在介紹完利用線性相角旋轉方式產生多組測距信號這種方式後，將介紹測距信號碼之設計。當在設計測距信號碼時，減少錯誤判斷的機率是重要優先目的之一。根據 Bose, Chaudhuri, and Hocquengham (BCH) 碼所衍生之新測距信號，可以提供較多的信號組數。峰值平均功率比也是重要的設計條件之一，而所設計出的信號除了有較低的峰值平均功率比外也有較低的信號相關性，以供接收機偵測信號以及估計所對應的時間誤差。設計的信號碼提供較好的改善效能以及較準確的距離估計。模擬結果顯示設計的信號比隨機二位元信號碼有以上等優點。 兩層式網路由傳統的蜂巢式網路在加上微型基地台所組成，可以提供較好的網路容量以及覆蓋範圍。在全頻譜重複使用的情況下，兩層式網路互相干擾將導致資料之接收錯誤以及測距信號偵測錯誤的機率大幅上升，而這種情況在測距程序上益發明顯。兩階段之微型基地台以及使用者之間上傳信號同步方式因此而生。在第一階段，微型基地台必須透過廣播頻道進行上傳時間誤差信號的廣播。移動使用者必須接收此信號並根據該信號指令將測距信號和基準信號等傳送時間上進行調整。在第二階段，即可採用正常的測距方式進行。舉例而言，上傳時間誤差信表示微型基地台到傳統蜂巢式之間的傳送延遲。利用兩階段之微型基地台以及使用者之間上傳信號同步方式，大幅改善測距信號的效能。

The OFDM-based multiple access (OFDMA) scheme has been shown to be capable of achieving very high spectral efficiency. A key advantage of an OFDMA system is that it can exploit the diversities offered by the time-varying and user(location)-dependent nature of the subchannels via proper scheduling and power/subchannel allocation. In this thesis, we first present a new single-symbol based initial ranging code structure. We also address issues related to the associated signal detection, timing synchronization methods and novel ranging sequences. These sequences are generated by a prototype sequence with proper linear phase rotations in frequency domain. The proposed detection method is capable of detecting single and multiple ranging codes, estimating the corresponding timing offsets and power strengths. Our approach offers improved performance and enhanced robustness against multi-user interference and multipath fading. Computer simulations are provided to assess the effectiveness of the proposed method and its advantages over existing alternatives. We proceed to present a novel ranging sequence design method in the second of the thesis. When designing ranging sequences, reducing the probability of collision is one of top priorities. Our ranging sequences design is based on the well known Bose, Chaudhuri, and Hocquengham (BCH) codes. The design generate more ranging sequences to increase the ranging opportunities. Peak to Average Power ration (PAPR) is taken into our design consideration as well. Our sequences have the desired properties of rendering low peak to average power ratio (PAPR) and cross-correlation values, which enable a receiver to effectively detect multiple ranging codes and estimate the corresponding time offsets. It therefore gives improved performance and enhanced robustness against multi-user interference and multi-path fading. Numerical results verify the effectiveness of the proposed method and its advantages over that based on pseudo-random binary sequences (PRBS). The last part of this thesis considers a two-tier pico/femtocell network comprising a conventional cellular network plus embedded pico/femtocell stations. Such network architecture offers an economically viable solution to achieving high cellular capacity and improved coverage. For an OFDMA network using universal frequency reuse and timedivision duplex (TDD), however, the resulting cross-tier interference causes unacceptable high outage probability and ranging signal detection error. The problem becomes more critical when performing a ranging process. A two-step uplink synchronization method is provided for uplink synchronization between a mobile station (MS) and a pico/femto base station (BS) that overlays upon a macro/micro cellular network. By using the proposed uplink synchronization method, a unified synchronous ranging channel may be used for ranging and UL access in pico/femrocells with reduced interference.