OFDM系統中使用改良式星座擴張之功率峰均值降低技術

Improved Constellation Extension Schemes for Peak-to-Average Power Ratio Reduction in OFDM Systems

黃頤清(Yi-Ching Huang)

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(2010年)

摘要

摘 要
正交分頻多工技術受到注目的原因在於擁有較高的資料傳輸以及克服頻率選擇衰減通道(frequency selective fading channel)，因其調變方式是將原本需要傳輸的資料串分成多個子資料串(sub-streams)，並分別以互相正交的子載波(subcarriers)傳送出去，再加上適當長度的保護區間(guard interval)，如此就可以對抗通道多路徑衰減(multipath fading)。但是因為OFDM調變技術為多個子載波訊號的總和，所以調變訊號的振幅有可能會出現相當大的峰值(peak)，造成其動態範圍太大，也就是功率峰均值 (peak-to-average power ratio，簡稱PAPR)太大。過高的功率峰均值會造成數位類比轉換器(DAC)和類比數位轉換器(ADC)的複雜度增加，為維持系統的性能，其解析度必須隨著高峰值均值比而提高；另外，由於高的功率峰均值會使其工作在非線性區而因此降低射頻系統中的功率放大器的使用效率，所以PAPR的降低技術是不可或缺的。為了解決這個問題，目前已經提出許多的PAPR降低技術，如選擇性映射(selective mapping，簡稱SLM)和部分傳輸序列(partial transmit sequence，簡稱PTS)，其觀念是利用不同序列擾動，產生多組不同序列，輸出訊號選擇具最小的功率峰均值的序列傳送。此外還有在星座圖上做一些變化使其可提供較好的PAPR效能，此種方法稱作CE (constellation extension)。
在本篇論文中，我們提出不同於以往的幾個星座圖表示法，來解決如何在降低PAPR的同時，仍可以保持其低訊號錯誤率(bit error rate，簡稱BER)，經由模擬圖的結果中我們可以發現，相較於傳統的星座擴張CE技術，所提出的這幾個星座圖可以同時達到不錯的PAPR以及BER的效能，而且，此技術不需像SLM和PTS需要傳送額外的訊息(side information，簡稱SI)。

關鍵字

正交分頻多工系統 ； 峰值均值比 ； 星座擴張 ； 錯誤率

並列摘要

Abstract
Orthogonal frequency division multiplexing (OFDM) is a promising technique for high-bit-rate transmission in radio environments. Because the total bandwidth is divided into many narrowband subcarriers, the effects of multipath delay spread can be diminished. One major disadvantage associated with an OFDM system is the high peak-to-average power ratio (PAPR) of the transmitter’s output signal, where the value is proportional to the number of subcarriers used in the system. Due to the high PAPR feature, an OFDM signal may suffer from significant intermodulation distortion and undesired out-of-band radiation when it is passed through a nonlinear device, such as a transmit power amplifier. These drawbacks have motivated the search for PAPR reduction techniques. There are many schemes available for PAPR reduction, such as clipping, selective mapping, partial transmit sequence, constellation extension (CE), and so on; however, most of them require transmitting side information and thus reduce the bandwidth efficiency. The existing CE schemes do not need to transmit side information, but it usually increases the transmit power and causes bit-error-rate (BER) degradation.

In this thesis, we propose three new CE schemes for PAPR reduction of OFDM signals with 16-QAM modulation, referred to as schemes A, B, and C. Each of the proposed CE schemes has four extendable constellation points, and each extendable constellation point has two possible extended constellation points. Scheme A adopts the four corners in the 16-QAM constellation as the extendable constellation points. Scheme B takes four specific points forming a square from the exterior constellation points (excluding the four corners) in the 16-QAM constellation as the extendable constellation points. Scheme C could be regarded as a hybrid from schemes A and B, where the extendable constellation points are the same as those of scheme A but the extended constellation points are the same as those of scheme B. As compared to previous related CE schemes, the proposed ones have fewer extendable constellation points with more extended constellation points each, resulting in better BER performance with almost the same PAPR reduction. Computer simulations are given to demonstrate the effectiveness of the proposed approaches.

並列關鍵字

orthogonal frequency division multiplexing (OFDM) ； peak-to-average power ratio (PAPR) ； constellation extension (CE) ； bit-error-rate (BER)

參考文獻

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

[3] C. Tellambura, “Computation of the continuous-time PAR of an OFDM signal with BPSK subcarriers,” IEEE Commun. Lett., vol. 5, pp. 185–187, May 2001.

[4] R. Gross and D. Veeneman, “Clipping distortion in DMT ADSL systems,” IEEE Electron Lett., vol. 29, pp. 2080-2081, Nov. 1993.

[5] R. W. Bauml, R. F. H. Fischer, and J. B. Huber, “Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping,” IEE Electron. Lett., vol. 32, pp. 2056-2057, Oct. 1996.

[6] C.-L. Wang, M.-Y. Hsu, and Y. Ouyang, “A low-complexity peak-to-average power ratio reduction technique for OFDM systems,” in Proc. 2003 Global Telecommun. Conf. (GLOBECOM 2003), San Francisco, CA, Dec. 2003, pp. 2357-2379.