於正交分頻多工系統中雙選擇性通道之研究

A Study on Doubly Selective Channels in OFDM Systems

陳柏翰(Po-Han Chen)

指導教授 ： 馮世邁

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202103484

摘要

正交分頻多工系統已被廣泛地應用在許多通訊系統中。在接收端使用單通道等化器即可良好的消除多路徑效應造成的符碼干擾。然而，當考慮了裝置的高速移動，通道將具有雙選擇性，此時傳統的接收機將不足以應付。因此，近來許多文獻著重在正交分頻多工系統在雙選擇性通道下的問題。
本文中，考慮了兩種雙選擇性通道模型。當時變效應不嚴重的情況下，通道可以用第一種模型—複指數基底擴展模型來表示。針對此模型的特殊結構，我們的目標是設計一個低複雜度的等化器。我們提出一個低複雜度的近似方法來簡化強制歸零等化器。並且，提出一種新型的混合等化器，以及它的低複雜度近似。實驗結果顯示，我們的方法能以較少的實現成本達到滿意的錯誤率表現。
另外，我們基於分接延遲線模型，提出一個新的時變通道模型。此模型以少量的都卜勒頻率偏移參數，即能有效的描繪都卜勒效應。對於此模型，我們聚焦在都卜勒頻率偏移的估測及接收器設計。我們推導出等效的離散時間上行傳輸雙選擇性通道模型。接著，我們利用天線陣列來接收信號，提出一個盲蔽估測都卜勒頻率偏移的方法。我們將考慮均勻線性陣列及均勻平面陣列。除此之外，我們也提出了兩種對應的等化器。在我們的實驗結果中，使用均勻線性陣列的估測器與接收機有良好的均方誤差及錯誤率表現。此外，使用了均勻平面陣列之後的效能，有更進一步的提升。

關鍵字

雙選擇性通道 ； 正交分頻多工系統 ； 通道等化 ； 督卜勒頻率估測 ； 接收器設計

並列摘要

Orthogonality frequency division multiplexing (OFDM) is widely used in many communication systems. Inter-symbol interference (ISI) caused by the multipath effect can be removed by one-tap equalizers at the receiver. However, the movement of the devices will result in double selectivity to the channel, in which the conventional receiver is not sufficient. Thus, there has been a lot of interest in the study for the doubly selective channel in OFDM systems.
In this thesis, we consider two models of doubly selective channels. When the time variation is not severe, the channel can be represented by the first model — the complex-exponential basis expansion model. Based on the special structure of this channel model, we focus on the low-cost equalizers design. We first propose a low-cost approximation of the zero-forcing (ZF) equalizer, and then introduce a new hybrid equalizer with its low-cost approximation. The hybrid equalizer consists of a MMSE diagonal matrix and a ZF inverse matrix. Simulation results show that the proposed methods give satisfactory BER performances at much lower costs.
In addition, we propose a new model of time-varying channels based on the tapped delay line (TDL). The proposed TDL model can efficiently capture the Doppler effect by using a few parameters called Doppler frequency offsets (DFO). For this model, we focus on the DFO estimation and receiver design. We first derive an equivalent discrete-time uplink channel model of the doubly selective channel in OFDM. Then, we propose a blind DFO estimator when the receiver is equipped with an antenna array. Both uniform linear array (ULA) and uniform planar array (UPA) configurations will be considered. Moreover, two equalizers are proposed for this model. We carry out numerical simulations to show the performance of the proposed methods. Our results show that both of the mean squared error (MSE) and the bit error rate (BER) performance are good for ULA configuration. Also, the use of UPA can further improve the performance.

並列關鍵字

doubly selective channel ； OFDM systems ； channel equalization ； DFO estimation ； receiver design

參考文獻

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