矩形脈波正交時頻間距調變系統之初始同步

Abstract

正交時頻間距(orthogonal time frequency space; OTFS)調變是一種新型的多載波調變(multicarrier modulation)技術，適用於具有稀疏延遲和都普勒擴散性質的雙重選擇性通道，且不同於以往多載波調變的靜態多路徑通道之性質，OTFS系統考慮的是線性時變(linear time-varying; LTV)多路徑通道。因此，在OTFS系統中的初始同步(initial synchronization; IS)，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時間和頻率偏移，使後續通道估計與解調等工作能夠順利進行。 本論文採用承載離散傅立葉變換(discrete Fourier transform; DFT)序列的梳狀前導幀(comb-type preamble frame; CPF)波形，透過計算接收訊號和本地離散頻率偏移的CPF波形之間的交互模稜測度(cross-ambiguity measure; CAM)，提出以選擇最大識別(select-the-largest identification; SLI)方法為基礎的牛頓法化選擇最大識別(Newtonized select-the-largest identification; NSLI)方法和選擇最大群識別(select-the-largest group identification; SLGI)方法，來估計前導幀在主導路徑上的起始時間點和都普勒偏移，進而實現OTFS系統的初始同步。 透過對提出的方法進行均方根誤差(root mean square error; RMSE)的性能分析，並檢視在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)的概略初始同步(coarse initial synchronization; CIS)和可調整的窄TAR且無DSI的精確初始同步(fine initial synchronization; FIS)系統中的模擬結果，發現在前導信號功率訊雜比(signal-to-noise ratio; SNR)足夠高時，其估計準確度和穩定性在多種的通道環境中皆表現出色，特別是當通道存在強主導(直視)路徑、實數值的延遲偏移與較大的實數值都普勒範圍時，有更為顯著的優勢。此外，在結合CIS和FIS系統的概略結合精確初始同步(coarse and fine initial synchronization; CFIS)系統中，應用所提出的方法將能得到更為精準的估計結果。

Orthogonal time frequency space (OTFS) modulation is a novel multicarrier modulation technique designed for doubly-selective channels with sparse delays and Doppler spread. Unlike traditional multicarrier modulation which deals with static multipath channels, OTFS systems address the characteristics of linear time-varying (LTV) multipath channels. Therefore, in OTFS systems, initial synchronization (IS) needs to be designed and optimized to account for the LTV multipath channel characteristics to accurately estimate the time and frequency shifts of the received signal, ensuring that subsequent channel estimation and demodulation can proceed smoothly. In this paper, we employs a comb-type preamble frame (CPF) waveform carrying the discrete-Fourier-transform (DFT) sequences and calculate the cross-ambiguity measures (CAMs) between the received signal and the local discrete-frequency-shifted CPF waveform to facilitate accurate IS. Based on the select-the-largest identification (SLI) rule, the proposed Newtonized select-the-largest identification (NSLI) rule and the select-the-largest group identification (SLGI) rule are used to estimate the start time and Doppler shift of the preamble frame on the leading path, thus achieving IS for the OTFS systems. By performing root mean square error (RMSE) analysis on the proposed method and examining the simulation results in systems with a wide timing acquisition range (TAR) but with data self-interference (DSI) in coarse initial synchronization (CIS) system and an adjustable narrow TAR without DSI in fine initial synchronization (FIS) system, the estimation accuracy and robustness outperform other synchronization methods across various channel environments if the signal-to-noise ratio (SNR) of the preamble frame is sufficiently high. Particularly, when the channel exhibits a strong leading path, real-valued delay shifts, and wide ranges of real-valued Doppler shifts, the performances demonstrate significant advantages. Additionally, in a combined coarse and fine initial synchronization (CFIS) system, applying the proposed methods yields more precise estimation results.