以編碼降低正交分頻多工訊號之功率峰均比

PAPR Reduction for OFDM Signals with Coding Techniques

程國祥(Guo Xiang Cheng)

指導教授 ： 李穎

元智大學/工程學院/電機工程學系/碩士(2003年)

摘要

OFDM訊號具有高的功率峰均比(Peak-to-Average power ratio，簡稱PAPR)。容易因傳輸端放大器非線性扭曲失真而受損。在本研究中我們著重在使用編碼來降低OFDM訊號的PAPR，只傳送PAPR低的訊號序列，可降低非線性效應對OFDM訊號產生的性能損失。由Reed-Muller碼來生成格雷序列(Golay sequence)，是既有文獻中性能最好的編碼方法，能有效的將OFDM訊號峰均比限制在3 dB以下。但Reed-Muller碼並無法生成所有峰均比不超過3 dB的頻域訊號序列。在本研究中，擷取頻域向量的特徵，搭配窮盡搜尋所有頻域序列。結果顯示，子載波長度N=8時，Reed-Muller碼所生成的QPSK格雷序列只佔了所有峰均比不超過3 dB頻域序列數目的3/4。若能使用其餘1/4的序列，就可以在最大峰均比不變的情況下，來提昇碼率。
討論將編碼法應用在802.11a的規格中，來觀察降低峰均比的效能。結果顯示，因為pilot tone的位置將使得編碼子區塊變小，在經過分區編碼後，降低PAPR的效能會受限。雖然PAPR降低的不明顯，但因為編碼法具有錯誤更正能力，會使訊號品質提昇。

關鍵字

OFDM ； PAPR ； Golay

並列摘要

OFDM siganl exhibits high peak-to-average power ration(PAPR), and thus is vulnerable towards transmitter nonlinearity. In this thesis, we use coding techniques to reduce PAPR by transmitting only low PAPR signal sequences to reduce the performance loss of nonlinearity. First we take double difference of the phase multiple sequences, those sequences after operating called dd patterns. Using dd pattern can help us observe the relationship of the signal sequences, PAPR and aperiodic autocorrelation function. And use dd pattern can reduce the sequence number that we need to observe. Exhaustive search the QPSK OFDM signal sequences whose length is 8 using dd patterns can reduce the sequence numbers to one sixteenth. Observing the PAPRs, all we found that the number of sequences whose PAPR are not larger than 3 dB is twenty five percent more than the number of Golay sequences generated by Reed-Muller code. Employing all low PAPR sequences can increase the code rate without increasing PAPR. Another way to increase code rate is through subblock coding. But when the number of subblocks is too large, the PAPR reduction performance will be limited. Finally, applying the coding techniques to 802.11a OFDM signals through simulations, we found that the PAPR reduction performance is limited by pilot tone. The performance can be improved by changing the pilot tone positions.

並列關鍵字

HASH(0x9190394)

參考文獻

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