OFDM系統中利用Reed-Solomon編碼之低複雜度選擇性映射式峰值對平均功率比降低技術

A Low-Complexity SLM-Based Peak-to-Average Power Ratio Reduction Technique Using Reed-Solomon Coding for OFDM Systems

林新智(Xin-Zhi Lin)

指導教授 ： 王晉良

國立清華大學/電機資訊學院/通訊工程研究所/碩士(2007年)

摘要

正交分頻多工(orthogonal frequency-division multiplexing，簡稱OFDM)是一種非常有效率的多載波系統，這個系統的傳輸效率非常高，並且也能有效的對抗多重路徑通道效應。但是OFDM系統的輸出信號會產生峰值對平均功率比(peak-to-average power ratio，簡稱PAPR) 的問題。為了解決這個問題，因而發展出許多的峰值對平均功率比降低技術。如選擇性映射式(selective mapping，簡稱SLM)是一種非常重要的峰值對平均功率比降低技術，但是，這方法卻有著需要傳送額外訊息 (side information)，和高計算複雜度的缺點。在這篇論文中，我們提出了利用Reed-Solomon編碼之低複雜度選擇性映射式峰值對平均功率比降低技術。這個方法不需要傳送額外訊息。主要作法是在完成通道編碼之後，犧牲了一部分錯誤更正能力使得OFDM訊號在不同載波上的傳送訊號產生錯誤，產生不同的候選訊號，並在這些後選訊號中，選擇一個峰值對平均功率比最低的候選訊號來進行傳送，藉此來降低高鋒值對平均功率的機率。為了降低整體的複雜度，我們先產生隨機的訊號，並將這些隨機的訊號存在唯讀記憶體(ROM)中。如此一來，在資料序列的部份經過處理之後，只要加上唯讀記憶體中不同的隨機訊號所產生不同的結果，便可以選擇其中一個峰值對平均功率比最低的序列傳送。藉由這個方法，我們就可以有效的降低峰值對平均功率比。而所需要的條件，只要一個唯讀記憶體、一些加法器，和一個快速傅利葉反轉換(inverse fast Fourier transform，簡稱IFFT)模組即可。經由模擬和比較結果，我們也可看出所提出的低複雜度技術，卻實能維持不錯的峰值對平均功率比之表現。

關鍵字

正交分頻多工 ； 峰值對平均功率比 ； 選擇性映射式

並列摘要

Orthogonal frequency division multiplexing (OFDM) is an efficient transmission scheme for multicarrier systems. Its major advantages include high data rate and the immunity to multipath fading channels. One of the major problems associated with OFDM signals is the high peak-to-average power ratio (PAPR), which may cause distortion when the OFDM signals pass through a nonlinear amplifier. A number of techniques have been proposed for PAPR reduction. These techniques can be roughly classified as the signal distortion methods, the coding methods, and the multiple signal representation (MSR) methods. All these methods have their own drawbacks, such as signal distortion, limitation of the number of subcarriers, requirement of side information, and high computational complexity.In this thesis, we propose a new selective mapping (SLM)-based method for PAPR reduction. The proposed method does not suffer the drawbacks mentioned above. By sacrificing a little part of error correcting capacity of channel coding, the proposed SLM-based method generates a set of candidate signals by adding the symbol block after channel coding with a set of different random error patterns. Then the candidate signal with the lowest PAPR is chosen for transmission. To further reduce the complexity of the proposed method, the set of random error patterns are generated in advance and the corresponding IFFT output sequences are prestored in a ROM. Then, the candidate signals of the proposed method can be produced by only one IFFT computation and some extra additions. It is shown that the computational complexity of the proposed SLM-based method is much lower than the ordinary SLM method that needs a set of U IFFT computations, where U is the number of candidate signals. Computer simulation results demonstrate that the proposed method has a reasonable and acceptable performance loss in terms of PAPR reduction and symbol error rate, as compared to the ordinary SLM scheme.

參考文獻

[2] J. Terry and J. Heiskala, OFDM Wireless LANs: A Theoretical and Practical Guide. Sams, 2002.

[3] ETSI, “Radio broadcasting systems: Digital Audio Broadcasting (DAB) to mobile, portable and fixed receivers,” WTS 300 401 v1.3.2, Sept. 2000.

[5] IEEE, “part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” IEEE Std. 802.11, Aug. 1999.

[7] ETSI, “High Performance Radio Local Area Networks (HIPERLAN): Requirements and architectures for wireless ATM access and interconnection,” TR 101 031 v1.1.1, July 1997.

[8] P. S. Chow, J. C. Tu, and J. M. Cioffi, “Performance evaluation of a multichannel transceiver system for ADSL and VHDSL services,” IEEE J. Selected Areas Commun., vol. 9, pp. 909-919, Aug. 1991.