利用位元域互斥或運算降低OFDM系統之峰均比

Abstract

無線通道本身有許多的效應，包括信號隨路徑迅速衰減、訊號與訊號彼此之間的干擾、雜訊干擾、不同頻道彼此干擾、與多路徑的問題等，導致高速的傳輸不易應用在無線接取網路。正交載波分頻多工調變系統本身是多載波調變技術的一種，它能夠利用增加循環字首與在接收端使用簡單的等化器以抵抗干擾與多路徑的問題。隨著數位電路技術的進步，正交分頻多工調變系統複雜的調變器可以使用快速的反傅立葉轉換法實現。雖然它擁有這些優點，然而，它的訊號波形變化非常大，通常我們可以把這現象量化成峰均比來衡量與判斷系統的優劣。一個傳輸系統如果有太大的峰均比，一般功率放大器的非線性效應會導致訊號失真，而使得不同頻率信號失去彼此間的正交性，進而使得位元錯誤率上升。若要避免這些失真，我們就必須採用高線性度的功率放大器，但是線性度高的功率放大器會使得成本增加。 我們在這篇論文中提出我們的多訊號表示法來降低正交分頻多工調變系統的峰均比，這個方法的原理是利用互斥或閘產生多個不同的位元序列來表示同一個位元序列，並從裡面選出一個峰均比最小的序列來傳送，而我們的方法與其他的方法比較起來則顯得簡單易於實作。這類型的方法是需要額外資訊給接收器來解碼，我將其所增加的額外資訊做一編碼的動作，使其符號錯誤率低，而能拿來當作測試序列。我們將分析我們系統的峰均比，並且評估這個系統在可加性高斯雜訊通道下的位元錯誤率，以及我們的系統在瑞雷通道下的星座圖。我們發現我們的系統若在128個子頻道中使用32個不同的信號表示同一個訊號時，可以減少5 dB的峰均比。此外，我們將我們提出的系統操作在軟限制器 (Soft Limiter)的非線性效應的放大器模型下，來觀測期位元錯誤率與功率密度頻譜的效能，我們發現在輸入退出為8分貝時 (IBO=8 dB)，我們的系統有不錯的效能。

A wireless channel has some bad characteristics, such as large attenuation, interference, multi-path, noise, and so on. Orthogonal frequency division multiplexing (OFDM) is a good solution for the high data rate wireless transmission. With the advancement of digital circuits, the complex modulator can be implemented by inverse fast Fourier transform (IFFT). It also can resist multi-path interference by adding cyclic prefix (CP). Though it has many advantages, the primary drawback is its large envelope fluctuation, which is quantified by the parameters called Peak to Average Power ratio (PAPR) or Crest Factor (CF). The high PAPR of transmission system causes low power efficiency. Thus, it needs a high linearity power amplifier to avoid distortion. In practice, most transmission systems must meet their power constraint. In this thesis, we propose a multiple signal representation (MSR) method to reduce the PAR. We exclusive or the original data bit sequence with different pseudo binary sequences to get different new data bit sequences which represent the same data bit sequence, and then we transmit the lowest PAR data bit sequence among them. This method is easier and simpler than the other methods, and the performance of this method emulates the performance of Tone Injection and Tone Reservation with moderated complexity. For 128-tones 16-QAM OFDM system, our proposed method achieves 5 dB PAR reduction with 32 iterations, while Tone Injection and Tone Reservation can achieve 5-6 dB with moderated complexity. We also evaluate the BER of this system under the Additive White Gaussian Noise (AWGN) channel and utilize the side information as training sequence to implement a comb-type equalizer to decrease BER under Rayleigh fading channel. In our simulation results, we find that for 128-tones OFDM system with 8 iterations applied by our proposed PAR reduction method perform well under the Soft Limiter clipping model when input back-off is 8 dB (IBO=8 dB). Index Terms— Comb-type Equalizer, orthogonal frequency division multiplexing (OFDM), peak to average ratio (PAR or PAPR), Pilot Symbol Assist Modulation (PSAM).