無保護間隔的頻域預編碼式正交分頻多工

Abstract

正交分頻多工(frequency-division multiplexing, OFDM)通常會跟保護間隔做連結，以抵抗符元間干擾（inter-symbol interference, ISI）和載波間干擾（inter-carrier interference, ICI）。但由於在傳送訊號中加了保護間隔，會降低傳送訊號的頻寬使用效益。在本篇論文中，我們考慮以無保護間隔的正交分頻多工(nonguarded OFDM)系統來節省頻寬，並把頻域預編碼技術與此結合，以達到更高的頻譜緊密程度。藉由頻域預編碼的技術，無保護間隔的頻域預編碼式正交分頻多工有比較緊密的頻譜並保有低峰均功率比（peak-to-average power ratio, PAPR）的特性。另外為了消除符元間干擾，在接收端以較小的時間區間來做接收，進而形成了有較強載波間干擾的非正交接收器。結果顯示此無保護間隔的頻域預編碼式正交分頻多工系統在通道長度夠小時可以保有一樣的通道容量(channel capacity)但有較佳的頻譜效益(spectral efficiency)。我們也提出保護間隔刪除的線性最小均方差等化器（linear minimum mean square error, LMMSE）和混合干擾消除(hybrid interference cancellation, HIC) 來消除較強的載波間干擾。值得一提的是在嚴重延遲擴展多路徑衰減通道下，編碼可以提供通道的多樣性，所以低峰均功率比且正交式編碼的無保護間隔正交分頻多工會有比起沒有編碼的正交分頻多工和無保護間隔的正交分頻多工更好的位元錯誤率。

Orthogonal frequency-division multiplexing (OFDM) signaling format is commonly associated with guard subintervals in order to reject intersymbol interference (ISI) and enable simple one-tap frequency-domain equalization at the receiver when received over dispersive channels. Despite the advantage, the guard subintevals expand the width of spectrum main-lobe, which is proportional to the ratio of guard subinterval length to useful data subinterval length, and thus trade off the spectral efficiency significantly. In this paper, the nonguarded OFDM (NG-OFDM) signaling format which is exactly OFDM without inserting guard subintervals is considered in order to save mainlobe width, and combined with spectral precoding to further suppress spectrum sidelobes. In particular, spectral precoders are designed for NG-OFDM to provide extremely compact signal spectrum while maintaining low peak-to-average power ratio (PAPR) in the transmitted waveform. In order to reject ISI among different NG-OFDM blocks, guard subintervals are removed at the conventional OFDM receiver and this turns the resultant receiver to be nonorthogonal over subcarrier subchannels, thus resulting in artificial intercarrier interference (ICI). Based on our analysis, the capacity achieved by this NG-OFDM system can remain unchanged but with higher spectral efficiency when channel length is small enough. LMMSE-FDE and hybrid interference cancellation (HIC) with guard removal receiver architectures are also proposed for elminate the severe interference. Particularly, in heavy delay spread multipath fading channel, reduced-PAPR orthogonally coded NG-OFDM with guard removal and LMMSE-FDE can outperform nonprecoded CP-OFDM and nonprecoded NG-OFDM.