Precoding Based PAPR Reduction Techniques and CFO Estimation for OFDM Systems

[[abstract]]正交分頻多工技術已經被廣泛地運用在有線或無線通訊系統中，例如：數位用戶迴路，數位音訊廣播，數位電視廣播等等。正交分頻多工系統有兩個主要缺點：一是傳送端的高峰均功率比值，二是接收端的載波頻率偏移引起的載波互相干擾。本篇論文針對高峰均功率比值暨載波頻率偏移的問題提出下列幾個解決的方法。首先，本論文提出可調式圓形限制技術去克服高峰均功率比值的問題。可調式圓形限制技術利用調整訊號的星座圖去降低峰均功率比值，此演算法藉由設定峰值訊號的壓縮率與調整訊號在星座圖的位置，系統可具備有效的峰均功率比值降低與可容許的位元錯誤率。通常，有效的峰均功率比值降低常導致位元錯誤率的上升，本論文提出最佳的預編碼技術能同時具有優秀的峰均功率比值降低與最小的位元錯誤率。最佳的預編碼矩陣可藉由我們提出的方法而產生。論文已證明：當預編碼矩陣的奇異值皆為1時，是最佳的預編碼矩陣其中之一。藉由數值模擬證明我們提出的方法能有效地解決高峰均功率比值的問題。接著，為了消除載波頻率偏移引起的載波互相干擾，本論文提出多階的載波互相干擾自我消除法與具有預編碼技術的訓練基礎的載波頻率偏移估測法。前者可藉由多階式執行的方法而得到良好的載波互相干擾的消除且不需改變訊號的峰值與傳輸功率。具有預編碼技術的訓練基礎載波頻率偏移估測法具備低的峰均功率比值與良好的載波頻率偏移估測性能。最後，藉由數值模擬證明我們提出的方法能有效地解決載波互相干擾與載波頻率偏移的問題。[[abstract]]Orthogonal frequency division multiplexing (OFDM) has been chosen in many wired and wireless applications, such as digital subscriber lines (DSL), digital audio broadcasting (DAB), digital video broadcasting (DVB), etc. The major drawbacks of OFDM systems are the high peak-to-average power ratio (PAPR) of transmitted OFDM signals at the transmitter end and carrier frequency offset (CFO) which causes inter-carrier interference (ICI) at the receiver end. In this dissertation, some algorithms were proposed to resolve the high PAPR and CFO problems. First, the Adjustable Circle Constraint (ACC) method is proposed to combat the problem of the high PAPR. The ACC scheme can reduce the PAPR with modifying the signal constellation. By setting the circle constraints with various circle centers, radii and compression ratios of the signal peak value on the constellation, excellent PAPR reduction and acceptable BER can be attained. In general, it is a tradeoff between the BER performance and PAPR performance in OFDM systems. Secondly, the optimal precoding matrix approach which can provide excellent performance for PAPR reduction and maintains the system error probability at the minimum concurrently is proposed. A systematic procedure in designing such an optimal precoding matrix is provided. It has been proven that the precoding matrix with all the singular values equal to 1 is one of the optimal matrices. Numerical results to verify the effects of the proposed algorithms in PAPR reduction are illustrated. To combat the effects of ICI caused by CFO, the multi-step ICI self-cancellation algorithm and the training-based CFO estimation algorithm are proposed. The former can be executed recursively to reach the required performance without amplifying the data symbols, which maintains the same transmitted power in any step of ICI self-canceling modulation. Then the training-based CFO estimation scheme for OFDM systems with the precoding scheme is proposed. The PAPR reduction and CFO estimation scheme are performed concurrently for the OFDM system. Finally, the numerical simulations are carried out for verifying the excellent effects in ICI cancellation and CFO compensations