正交分頻多工架構應用於多輸入多輸出系統之 峰均值分析與序列設計

Abstract

本論文包含兩個部分。在第一個部分，我們考慮了搭配波束成型與正交分頻多工架構之多輸入多輸出系統的峰均值分析與降低峰均值演算法的設計。由文獻中可以得知，高峰均值為正交分頻多工架構所存在的主要問題之一，若加入波束成型技術，則此問題將會更加嚴重，因而增加傳送端功率放大器在設計上的複雜度。在此我們將討論正交分頻多工之多輸入多輸出系統搭配兩種波束成型技術：最大比傳輸與平等增益傳輸。在平等增益傳輸系統下，我們證明了單一傳輸天線之峰均值分布會等於未使用波束成型之正交分頻多工系統。此外，我們使用了排序統計的方法推導出系統之峰均值統計特性。而在最大比傳輸系統下，由於平均能量會隨不同的通道長度有所變化，因而增加了峰均值分析上的複雜度，對此我們考慮了不同通道長度的情況並推導出對應情境下的峰均值分布。模擬結果可以看出我們所推導的結果與實驗是相符合的。根據我們所推導的結果，我們也分別針對這兩種波束成型系統設計出降低峰均值的演算法。此兩種演算法均不需要傳送額外的資訊至接收端。由模擬結果可以看出，若適當調整演算法的設計參數，最大比傳輸的系統能同時改善峰均值與錯誤率；而平等增益傳輸之系統亦能改善峰均值並且維持錯誤率。

在第二個部分，我們考慮了在多基地台環境下採用正交分頻多工架構之同步序列設計。在此環境中，多基地台的干擾會造成同步更加困難相較於在一般單一基地台環境，因此如何減少多基地台的干擾是很重要的。因此，我們設計了一組同步序列，使其不僅具有良好的自相關特性並且更加能抵抗多基地台的干擾。模擬結果可以看出，藉由設計的同步序列，多基地台的干擾能被足夠減緩，以及基地台偵測能被改善。除此之外，由於正交分頻多工系統對於載波頻率偏差很敏感，所以在多基地台環境內，載波頻率偏差的估測和補償也是很重要的。由模擬結果顯示，搭配我們所設計的載波頻率偏差估測方法，我們的同步序列比長期演進規格中的第一同步序列具有更顯著良好載波頻率偏差的估測準確度。

This dissertation consists of two parts. In the first part, we theoretically analyze the PAPR performance of MIMO OFDM systems that adopt either one of the two popular beamforming schemes, i.e. MRT (maximum ratio transmission) and EGT (equal gain transmission). Using beamforming can significantly improve the receive SNR of OFDM systems. However, the combination of transmit signals after beamforming deteriorates the peak-to-average power ratio (PAPR), which has long been considered a major issue of OFDM systems. High PAPR not only complicates the design of the power amplifier, but also increases power consumption. Therefore, the derived results may provide important reference for practical designs when evaluating the required power amplifiers and power consumption. Moreover, the theoretical results show that MRT OFDM systems generally perform much worse than EGT OFDM systems in terms of PAPR. Furthermore, motivated from the derived results, PAPR reduction algorithms are proposed for both MRT OFDM and EGT OFDM systems. It is worthwhile to mention that for MRT OFDM systems, the proposed algorithm can improve both PAPR and bit error rate; for EGT OFDM systems, the proposed algorithm improves PAPR while it only slightly degrades bit error rate.

In the second part, we propose a training sequence that can be used at the handshaking stage for multi-cell networks. The proposed sequence is theoretically proved to enjoy several nice properties including constant amplitude, zero autocorrelation, and orthogonality in multipath channels. Moreover, the analytical results show that the proposed sequence can greatly reduce the multi-cell interference (MCI) induced by carrier frequency offset (CFO) to a negligible level. Therefore, the CFO estimation algorithms designed for single-user or single-cell environments can be slightly modified, and applied in multi-cell environments; an example is given for showing how to modify the estimation algorithms. Consequently, the computational complexity can be dramatically reduced. Simulation results show that the proposed sequences and the CFO estimation algorithms outperform conventional schemes in multi-cell environments.