正交分頻多工系統與濾波器多載波系統複雜度降低與效能提升之研究

Abstract

四代行動通訊系統採用正交分頻多工(Orthogonal Frequency Division Multiplexing)技術，該調變方式的頻譜有過高的旁波瓣，相對的現行研究中第五代行動通訊通訊技術對於旁波瓣有極嚴格的限制，另一方面，降低OFDM運算的複雜度，一直是必要的研究議題。 目前文獻已有提出實數基底轉換之OFDM，有低複雜度的好處，本論文將針對此議題討論。首先透過直觀的物理觀念與數學推導，詳細分析離散餘弦轉換(Discrete Cosine Transform)為基底的OFDM的限制。另外對於離散哈特利轉換(Discrete Hartley Transform)為基底的OFDM，亦詳細推導驗證其互補頻率的干擾，並證明經過處理解決干擾後，結果與一般OFDM等效，故DHT為基底的OFDM不存在低複雜度的優勢。 另外我們針對OFDM旁瓣較大的劣勢提出針對頻域子載波傳送資料先進行Sigma-modulation運算，再做IDFT運算產生時域輸出訊號，如此可有效的將低旁瓣，此外若輸入子載波訊號為BPSK或QPSK時，其經過此差分運算後可產生約一半為零的子載波資料值，進而降低IDFT的運算複雜度。我們將討論其架構的運作原理與討論效能，錯誤率及偵測演算法等議題。 5G候選波形濾波器組多載波(Filter Bank Multicarrier)系統濾波器頻譜特性雖然較佳，但是卻有高複雜度的問題，因此我們嘗試利用其OQAM訊號特性，使用實數FFT化簡，約可以省一半的複雜度。本論文亦討論如何改用DHT為基底以降低OFDM系統的複雜度，詳細分析系統的正交性與推導如何用PPN化簡，以及討論未來的展望。

OFDM modulation has the disadvantage of high side lobes, which is undesirable for the future fifth generation mobile communication technology which demands stringent spectrum specifications. Moreover, reducing the complexity of a baseband transceiver is always a critical topic for mobile communication which require low-power comsumption and long battery operating time. Specifically, we will try to reduce the complexity of FFT operations involved in OFDM and 5th generation FBMC (filter bank multicarrier) transmission techniques. DCT-based and DHT-based OFDM have been proposed which have lower complexities than conventional DFT-based OFDM, since their transformation bases are real numbers instead of complex numbers. Although it has low complexity, there are some limitations in the DCT-based OFDM, according to our analysis by means of both physical and mathematical derivations. Regarding DHT-based OFDM, we also derived the interference due to complementary subcarriers in more detail in the DHT, and proved that the system after removing interference is the same as traditional OFDM without additional benefit. In addition, we design an OFDM with Sigma-modulated input signal to reduce the side lobes in the traditional OFDM system. Since roughly about one half of the input signals will become zero after the sigma modulation, the IDFT complexity in the sigma-modulated transmitter will be lower than the traditional OFDM. In this work, theoretical analysis discuss the theorem, performance, BER and detection algorithm of the modified OFDM are also discussed. The future 5G hopeful FBMC system has much better spectrum characteristics than OFDM system, but has the drawback of high complexity. This thesis proposes a technique that reduces the associated FFT complexity to half of orignal by utilizing the OQAM characteristics. Besides, this thesis also discusses how to reduce the complexity of OFDM system by applying DHT as basis. Finally, this thesis also states possible further issues to be resolved.