OTFS系統中的頻譜預編碼式帶外功率降低法

Abstract

正交時頻空間 (orthogonal time frequency space; OTFS) 系統為一種高效的多載波調變技術，用於具雙重選擇性通道 (doubly selective channels) 上作數據幀之傳輸，此通道有著顯著的都卜勒及延遲擴散效應。透過擴散數據符元至時間-頻率域，OTFS系統相較於正交分頻多工 (orthogonal frequency-division multiplexing; OFDM) 等傳統的多載波調變技術，其能夠利用多路徑分集並對延遲-都卜勒位移有著適應性及穩健性。當矩形脈波被採用為整形脈波時，OTFS可有效地透過由OFDM區塊訊號組成之幀訊號來實現傳輸，並且於都卜勒變化之通道色散環境下其錯誤性能優於OFDM。然而，由於矩形脈波的不連續性，矩形脈波的OTFS受到以f^-2衰減的大功率頻譜旁波瓣影響，從而造成鬆散的功率頻譜。在本篇論文中，具I級冗餘之頻譜預編碼結合矩形脈波的OTFS使得頻譜緊密有著較小的旁波瓣以f^-2I-2或更快的衰減。關於頻譜預編碼式OTFS (spectrally-precoded OTFS; SP-OTFS) 的峰均功率比 (peak-to-average power ratio; PAPR)、可實現的數據速率(achievable data rate; ADR)、頻譜效率 (spectral efficiency) 和等化符元決策等特性亦於本篇論文中作了研究與探討。

Orthogonal time frequency space (OTFS) is a power-efficient multicarrier modulation for data frame transmission over doubly dispersive channels exhibiting large Doppler spread and sparse delay spread. By spreading data symbols in the time-frequency domain, the OTFS system enables the exploitation of diversity in multipath components and is thus resilient and robust to delay-Doppler shifts, when compared with traditional multicarrier modulation schemes like orthogonal frequency-division multiplexing (OFDM). When rectangular pulse is adopted as the shaping pulse, OTFS can be practically and efficiently signaled by a frequency-space precoded frame of OFDM block signals and outperforms OFDM in error performance over Doppler-variant channel dispersion. However, due to the discontinuity in rectangular pulse, the rectangularly-pulsed OFDM-based OFTS suffers large power spectral sidelobes decaying slowly as f^-2 and thus renders loose power spectrum. In this thesis, spectral precoding with a level-I redundancy is integrated with rectangularly-pulsed OFDM-based OTFS to make the compact power spectrum rendering small sidelobes decaying as f^-2I-2 or faster. The characteristics of peak-to-average power ratio, achievable data rate, spectral efficiency, and equalized symbol decision are also investigated for the proposed spectrally-precoded OTFS scheme.