- 學位論文
正交時頻間距調變系統之初始時間同步
Initial Time Synchronization for Orthogonal Time Frequency Space Systems
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摘要
正交時頻間距(orthogonal time frequency space; OTFS)調變是一種新型的多載波調變技術，適用於具有稀疏延遲和都卜勒擴散性質的雙重選擇性通道。由於OTFS系統考慮的是線性時變(linear time-varying; LTV)多路徑通道，與以往多載波調變的靜態多路徑通道之性質不盡相同。因此，在OTFS系統中的初始時間同步(initial time synchronization; ITS)，需要針對LTV多路徑通道的特性進行設計和優化，以確保能準確地估計出訊號在傳送過程中的偏移，使後續通道估計與解調等工作能夠順利進行。 本論文採用離散傅立葉轉換序列的梳狀前導波形，透過計算接收訊號和本地離散頻率偏移梳狀前導波形之間的相關性，提出一套適用於離散時間偏移上的基於梳狀ITS (comb-based ITS)方法，以順序識別和選擇最大識別來估計前導幀的起始時間點，進而實現OTFS系統的ITS。Comb-based ITS包含具有寬時間採集範圍(timing acquisition range; TAR)但存在資料自干擾(data self-interference; DSI)的粗略ITS (coarse ITS; CITS)，以及可調整的窄TAR且無DSI的精細ITS (fine ITS; FITS)。 透過對提出的方法進行平均偏差和均方根誤差的性能分析，並檢視在CITS和FITS系統中的模擬結果，發現其估計準確度和穩定性在多種的通道環境中皆表現出色，特別是當通道存在直視路徑與都卜勒偏移較大時，有更為顯著的優勢。
並列摘要
Initial time synchronization (ITS) is investigated for orthogonal time frequency space systems operating over the linear time-varying multipath channels exhibiting real-valued delay and Doppler shifts on the delay-Doppler grid. Particularly, a comb-type preamble waveform carrying discrete-Fourier-transform sequence is designed to meet the constraint of zero discrete-ambiguity-function and thereby facilitate accurate ITS. By taking the cross-ambiguity measures between the received signal and local discrete-frequency-shifted comb-type preamble signals for discrete-time shifts sequentially, the comb-based ITS approaches using sequential identification and select-the-largest identification rules are developed to estimate the start time of a received preamble frame on the leading channel path. Comb-based coarse and fine ITS systems are investigated to operate over a wide timing acquisition range (TAR) under the disturbance of data self-interference (DSI) and over an adjustable narrow DSI-free TAR, respectively. When the channel exhibits a strong leading path and wide ranges of Doppler shifts, the comb-based ITS systems are shown to outperform the conventional ITS systems acquiring linear frequency modulated preamble waveforms and hybrid data/preamble frames, significantly in estimation robustness and in estimation accuracy for sufficiently high preamble signal-to-noise power ratios.
參考文獻
R. Hadani et al., “Orthogonal time frequency space modulation,” in Proc. IEEE Wireless Commun. Netw. Conf., San Francisco, CA, USA, Mar. 2017, pp. 1–6.
R. Hadani et al., “Orthogonal time frequency space modulation,” 2018, arXiv:1808.00519.
P. Raviteja et al., “Interference cancellation and iterative detection for orthogonal time frequency space modulation,” IEEE Trans. Wireless Commun., vol. 17, no. 10, pp. 6501-6515, Oct. 2018.
P. Raviteja, Y. Hong, E. Viterbo, and E. Biglieri, “Practical pulse-shaping waveforms for reduced-cyclic-prefix OTFS,” IEEE Trans. Veh. Tech., vol. 68, no. 1, pp. 957-961, Jan. 2019.
S. Tiwari, S. S. Das and V. R. Rangamgari, “Low complexity LMMSE receiver for OTFS,” IEEE Commun. Lett., vol. 23, no. 12, pp. 2205-2209, Dec. 2019.