有近似週期性導引之TDD-OFDMA信號之頻率同步

Abstract

本文旨在研究OFDMA系統的頻率同步相關課題。跟一般OFDM系統不同的是來從事頻率同步的導引信號只具備近似的週期性。缺乏理想週期性的原因是由於離散傅立葉轉換大小為2的冪次方，無法被奇數整除。我們若仍然運用傳統基於週期性導引信號的相關性來做頻率估計的方法，其性能將可能很不理想。

我們利用內插法從接收的信號樣本中重建具完整週期性的導引信號，使得應用時域相關性的頻率偏移估測方法可以適當的使用而不至造成性能損失。我們會比較線性和基於sinc函數（即理想低通）的內插濾波器。如同我們所預期的，電腦模擬的結果顯示我們所提出的解決方法在AWGN和複徑衰退的通道下皆有性能的改善，尤以後者更為顯著。我們也觀察到性能增益與OFDM符號(或者FFT )長度的有很大的關係。這個長度決定了非週期性的程度也直接影響了內插（或性能）增益的多寡。

This thesis deals with the frequency synchronization issue of an OFDMA system. We consider the situation that the preamble used for synchronization and channel estimation has an almost-periodic structure. The lack of ideal periodicity is due to the fact that the discrete Fourier transform size, being a power of two, is not divisible by three. This non-periodicity causes the frequency synchronizer that assumes a periodic pilot structure to suffer from serious performance loss.

We propose an interpolation approach to reconstruct the received preamble samples in a periodic format so that conventional time-domain correlation-based frequency offset estimation algorithms become applicable. We examine both linear and sinc-based interpolation filters. Simulation results indicate that, as expected, the proposed solution brings about performance improvement in both AWGN and multipath fading channels. Significant performance enhancement is obtained in the latter case. We also observe that the performance gain is a function of the OFDM symbol (or FFT) size which determines the extent of non-periodicity and therefore the interpolation gain.