適用於正交分頻多工系統之低複雜度多工偵測技術研究

Abstract

正交分頻多工(OFDM)系統利用頻域上之多工技術以達到高資料傳輸率的目的，然而多工偵測部分通常會有相當高之複雜度，因此本論文對於無線通訊中之內接收機的多工偵測部分(包含晶胞辨別碼偵測以及資料偵測兩大區塊)提出新穎的低複雜度演算法設計。 對於晶胞辨別碼偵測(Cell ID Detection)部分，本論文針對正交分頻多工系統提出了高效率的非同調晶胞辨別碼偵測(Non-coherent Cell ID Detection)技術以同時達成高效能以及低複雜度之目的。除此之外，本論文更基於此演算法延伸出了兩個晶胞辨別碼偵測演算法。實驗結果顯示此三種演算法相對於傳統之方法在多路徑雷利通道下將會具有更佳的效能；除此之外，本論文亦針對所提出的方法之統計特性做進一步的分析，而分析的結果與實驗所得之結果相當接近。接著本論文將進一步將先前針對傳送BPSK調變之參考訊號之系統延伸至傳送Zadoff-Chu參考訊號之系統，結果顯示所有方法在上述兩種系統中皆具有較佳之效能。特別是所提出的第三種方法不僅僅比傳統方法具有更佳之效能同時亦具有更低之複雜度。 另一方面，針對正交分頻多工系統中之資料偵測(Data Detection)而言，時變之多路徑通道將會造成子載波間干擾(ICI)，此干擾將會造成系統效能之下降，而目前之漸進式資料偵測(Successive Data Detection)演算法將會耗費相當高之運算複雜度以處理此問題。以現存的方法來說，考慮了信號與干擾能量比(SINR)之最小均方差漸進式偵測(MMSE Successive Detection)演算法具有最佳之效能，而本論文藉著整合牛頓循環式矩陣倒置法(Newton’s Iterative Matrix Inversion Method)以及最小均方差漸進式偵測演算法提出了一個低複雜度之資料偵測演算法。為了有效地整合牛頓漸進式矩陣倒置法以及最小均方差漸進式偵測演算法，本論文亦提出了一個低複雜度之方法以產生牛頓法所需之初始矩陣。基於所提出之初始矩陣，本論文得以降低計算信號與干擾能量比之複雜度。因此，所提出的論文將可將複雜度自原始之最小均方差漸進式偵測演算法的O(N3)降低至O(N2)，其中N表示所有的子載波數量。除此之外，在不同通道情況下之模擬結果亦顯示所提出之低複雜度偵測演算法之效能將會相當逼近於原始最小均方差漸進式偵測演算法。 然而，當系統自單天線(SISO)系統延伸至多天線(MIMO)且多中繼站(Multi-hop)系統時，減低單一區塊之耗能對整體系統帶來的影響有限，以系統的觀點來看整個系統時可發現低耗能以及高頻寬使用效率兩者之間有個取捨問題(Tradeoff)。換句話說，多天線系統目的是提高頻譜使用效率(Bandwidth Usage Efficiency)，但同時亦會提高系統整體耗能(Energy Consumption)。本論文從最根本的問題切入，在選擇天線數量時，同時考慮頻寬使用效率以及系統耗能，針對上述取捨問題做分析，得到了天線數量與兩者之間的相對關係，進而提供了一個完整的依據以供系統設計上對於耗能以及頻譜效率間取捨的參考，使得整體系統運作上將可更有效率。

Orthogonal frequency division multiplexing (OFDM) systems utilize the multiplexing technique over the frequencies for high transmission rates. However, in order to obtaining the transmitted data at receivers, multiplexing detections generally cost lots of computational complexities. As such, this work proposes new algorithms regarding cell identification and data detections for reducing the complexities of multiplexing detections. To achieve high-performance cell identification (ID) detection with low complexity, this work proposes an efficient non-coherent cell identification (ID) detection technique based on a new optimization metric for OFDM systems. Furthermore, the metric is simplified to two lower-complexity metrics. As such, two more modified cell ID detection methods extended from the first one are also proposed. Experiments show that all the three new cell ID methods achieve better performances than the conventional methods in multipath Rayleigh fading channels. This work also conducts the statistical analysis to characterize the proposed techniques completely. It is shown that the results of the theoretical analysis are close to the simulation results. Besides, the discussions also consider the system performances of the BPSK-modulated reference sequences and Zadoff-Chu reference sequences. Among the new techniques, specifically, the third proposed method using the second simplified new metric has much lower complexity and higher performance than the conventional methods. On the other hand, for data detection in OFDM systems, the time variation of a multipath channel results in inter-carrier interference (ICI). It leads to performance degradations. In reducing the problem, current successive detection methods cost very high computational complexities. Among them, the minimum-mean-square-error successive detection (MMSE-SD) method has the best performance. This work proposes an improved data detection method with low complexity by integrating the techniques of Newton's iterative matrix inversion method and the MMSE-SD method which considers the effects of signal-to-interference-plus-noise ratio (SINR). In order to efficiently integrate Newton's and MMSE-SD method, we develop an effective scheme with low complexity for generating the initial values required by Newton's method. Based on the new initial value scheme, we are able to simplify the criterion of maximum SINR determination to an equivalent one with lower complexity. As a result, the proposed algorithm has a much lower complexity of O(N2) than O(N3) of the MMSE-SD algorithm, where N is the number of subcarriers. Moreover, simulation results in different channel conditions show that performances of the proposed MMSE-SD method are very close to the MMSE-SD method. Furthermore, multiple-input and multiple-output (MIMO) and multi-hop communication systems are also an application of multiplexing techniques for increasing the transmission data rate and bandwidth usage efficiency. This work further considers a MIMO multi-hop network and analyzes the relationship between energy consumption and bandwidth. The minimum energy consumption is formulated as an optimization problem. By taking both transmit antennas (TAs) and receive antennas (RAs) into consideration, the energy-bandwidth tradeoff in MIMO multi-hop wireless networks is investigated. Moreover, the minimum energy of an equal-spaced relaying strategy is investigated for various numbers of antennas. In addition, the minimum energy over all possible antenna pairs is derived. Finally, the effect of the number of hops used to relay the information on the energy-bandwidth tradeoff is considered.