Study of Carrier Frequency Offset Estimation and Compensation for OFDMA Systems

正交分頻多工接取(OFDMA)為目前無線通訊所常用的實體層技術，由於震盪器不匹配及都普勒效應影響，使得頻率同步為一個必須解決的問題。雖然下行鍊路(downlink)的頻率偏移問題已有很多文獻提出解決方法，但相較於下行鍊路，上行鍊路(uplink)增加了多重使用者干擾的問題，使得效能大幅下降。 本篇論文的想法是要用傳送導航信號的方式來估測信號，並以遞回的方式將信號干擾消除並且重複估測信號，來達到完美的頻率偏移估測，同時，我們嘗試用簡化矩陣的方式來減低原矩陣的計算複雜度，因此，可以得到更低的複雜度，並且對效能不會有太大影響。 在之後的章節，我們考慮到使用者擁有不同功率分配對基地台的影響，不同的使用者會因所擁有的功率不同，產生嚴重的多重使用者干擾，造成頻率估測偏差。因此，我們將第一個方法做修正，將不必要的干擾在估測前消除，使估測效能達到更好。最後，考慮現實將遇到的狀況，環境將會設在超高速移動環境下(高鐵環境)，我們將其嚴重的都普勒效應消除，改善其估測效能，並設法減低演算法的複雜度。Orthogonal Frequency-Division Multiple Access (OFDMA) is a popular physical layer technique for wireless communication nowadays. Due to carrier frequency offset caused by local oscillator mismatch and Doppler Effect, frequency synchronization is a crucial issue for mobile communication. Although many literatures have proposed lots of solutions for OFDMA downlink, there are still lots of problems that degrade the performance in uplink. At the beginning of this thesis, the pilots are inserted in transmitting signals to assist the CFO estimation. With the iterative algorithm, the perfect CFO estimation is achieved at the second iteration. In addition, we also try to exploit the banded matrix to reduce the computational complexity of the full matrix. Therefore, exploiting from such scheme, the low complexity estimation is obtained without sacrificing much performance. Second, in the later chapter, we consider the problem of the unequal power allocation to the base station. Unequal power users would make strong multiuser interference on our estimation and degrade our performance. Therefore, in order to acquire better performance, a modified method is proposed to eliminate the interference before the estimation. Finally, considering practical implementation, we would set the environment to that of Taiwan High Speed Rail. With the proposed algorithm, the estimation performance can achieve the same quality in time-invariant channels