可壓抑正交分頻多工領航訊號旁脈頻譜之消除符元設計

Abstract

在現行的正交分頻多工(OFDM)系統和標準，估測訊號多用來作為通道的估測。這些估測訊號波形都來自於設計良好的估測訊號序列如Zadoff-chu sequence，這些序列都具有是由相同振幅的元素所組成以確保估測最佳化，同時也能達到低峰均功率比(PAPR)。然而，這些波形具有較大的旁脈，對於鄰近的頻段產生干擾。本篇論文，利用設計消除符元(Cancellation symbols) 插入技術達到壓抑正交分頻多工領航訊號旁脈頻譜，同時在半平穩多重路徑(quasi-static multipath)通道下，估測訊號可達到最佳的通道估測。過去所提出的消除符元同時壓抑資料與估測訊號，接收端無法知道消除符號。此篇論文提出的消除符元僅和估測訊號相關，因此，接收端可利用放在保護頻帶上的已知消除符元協助通道估測抑或者將消除符元與資料符元使用同一載波傳送可進一步減少消除符元所需要的能量。 相較於直接使用改變估測訊號序列的方法僅能應用到特定長度的估測訊號，插入消除符元可以應用到任意的長度的估測訊號，同時，維持較低的低峰均功率比。

In this thesis, the cancellation symbol insertion (CSI) technique is adopted to suppress the sidelobe power of the OFDM pilot waveform while maintaining the channel estimation (CE) optimality on the quasi-static multipath channel. Different from the conventional CSI technique where the cancellation symbols (CSs) are designed to suppress the sidelobe power of composite OFDM waveforms conveying data and pilot symbols, the proposed CSs depend on pilot symbols only and can be known to the receiver. Hence, the CSs can be used to assist channel estimation when they are conveyed on guard subcarriers or reduce the cancellation symbol power without causing interference to data demodulation when they are superimposed on data subcarriers. The composite OFDM waveforms conveying spectrally precoded data symbols, optimum pilot symbols, and the proposed CSs are shown to exhibit highly compact spectrums, while providing enhanced CE and requiring reduced cancellation symbol power.