- 學位論文
多重輸入輸出正交分頻多工系統搭載有限回授子頻帶波束成形技術
MIMO-OFDM Subband Beamforming with Limited Feedback
摘要
由於通訊資料量日益增長，多重輸入輸出(Multiple-Input Multiple-Output)的多天線技術更加重要。它不僅能夠提供可靠的資訊傳輸方式還能增加通訊通道的容量，以符合未來所需的傳輸量。除此之外，正交分頻多工(Orthogonal Frequency-Division Multiplexing)的技術也廣受歡迎，因為正交分頻多工技術可以將資訊分為不同頻帶傳送而且可將多重路徑的通道衰減轉化為平坦性衰減，使得訊號接收的後處理能夠較為簡單而順利。因此，將多重輸入輸出與正交分頻多工技術結合將成為未來通訊系統的重要方法之一。 此外，波束成形技術也時常與多輸入輸出和正交分頻多工系統作結合。波束成形技術是於訊號傳送前將複數的係數加載在傳輸訊號上，使得目標位置的接收訊號為建設性干涉，其餘地方為破壞性干涉，達到目標訊號最佳化的目的。正交分頻多工領域的波束成形運用類型包括子載波波束成形、內插波束成形和子符號波束成形等等。子載波波束成形是針對每個子載波作對應的波束成形計算，效果為最佳的，但其複雜度與回授量也最大；內插波束成形是將部分波束成形係數作內插得出其餘的係數；子符號波束成形則是全頻用相同的係數，相比效果最差，但其複雜度也最低。 本篇論文中考慮多重輸入輸出正交分頻多工系統，其中運用了子載波波束成形與子頻帶波束成形混合式方法計算係數。為了減少回授資訊量，將所有的子載波分群，每群使用相同的波束成形係數。特別的是，所有的係數皆於接收端進行MRC計算，之後則將屬於傳送端的波束成形係數藉由無損回授通道傳回給傳送端。眾多參考文獻中，在計算傳送端波束成形係數時會有多種不同面向的考量，而本篇論文則使用最大化接收端訊號訊雜比的方法計算係數，以求接收端能得到最佳的訊雜比。從最後的模擬結果圖可以看出，本篇論文提出的混合式方法雖然效能較子載波波束成形差一點，但訊號回授量大幅降低，計算複雜度也下降許多，為一種節省通道回授量兼具波束成形效果的方法。
並列摘要
Due to the growing demand in data, the Multiple-Input Multiple-Output (MIMO) technique is getting more and more important. It can not only provide a reliable ransmission but also increase the channel capacity to fit in the ransmission quantity in the future. Besides, Orthogonal Frequency-Division Multiplexing (OFDM) is also a popular technique because it can allocate the data streams to different frequency bands and transmit through their own band without bothering neighbors. Moreover, OFDM can turn the multipath fading channel into flat fading one to ease the difficulty in data receiving. Therefore, combining the technique of MIMO and OFDM will be one of big issues in the future communication system. In addition, the beamforming technique is always utilized both with MIMO and OFDM. Beamforming is a transmitting optimization method that carries the complex coefficients on data before transmitting and causes constructive interference in the target position and other position with destructive interference. OFDM beamforming schemes include subcarrier-wise beamforming, interpolation beamforming, symbol-wise beamforming and so on. Subcarrierwise scheme calculates the beamforming vectors according to each subcarriers and has the best performance but has the greatest complexity and feedback requirement. Interpolation scheme calculates the beamforming vectors by interpolating with other vectors. Symbolwise scheme uses the same beamforming vector on all the subcarriers, apparently, it has the worst performance but has the lowest complexity. In this thesis, we consider the MIMO-OFDM system with subband beamforming technique to calculate beamforming coefficients. To reduce complexity, the subcarriers are split into many subbands and the subcarriers in the same subband share the same beamforming vectors. Specifically, all the beamforming vetors are calculated at the receiver end by using maximum ratio combining (MRC) technique and the beamforming vectors corresponding to transmitter end are sent back through error-free feedback channel. In the bunch of papers, there are many objective functions to be considered in beamforming calculation process, however, we use the maximum subcarrier SNR one to make receiver have maximum SNR. In the simulation result, the scheme we proposed has a little bit performance worse than the subcarrier-wise scheme but has a lot of decreasing in feedback requirement and complexity quantity.
參考文獻
延伸閱讀
- 李孟穎（2014）。基於波束成型技術下之空間預編碼頻分雙工大規模多輸入多輸出系統〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU.2014.01325
- 連瑞屏（2005）。應用於感知無線電系統之多重輸入輸出正交分頻多工基頻收發機設計〔碩士論文，國立臺灣大學〕。華藝線上圖書館。https://doi.org/10.6342/NTU.2005.02559
- 蘇建忠（2008）。多輸入多輸出正交分頻多工系統之適應性混合式的自動重傳機制〔碩士論文，國立臺北科技大學〕。華藝線上圖書館。https://doi.org/10.6841/NTUT.2008.00154
- Hsu, C. N. (2009). 正交多工分頻存取兩跳系統下結合子載波搭配與功率分配 [master's thesis, National Taiwan University]. Airiti Library. https://doi.org/10.6342/NTU.2009.03270
- Yang, Y. H. (2007). Multiuser MIMO Downlink Beamforming with SINR Constraints [master's thesis, National Taiwan University]. Airiti Library. https://doi.org/10.6342/NTU.2007.00823