基於矩形波形之循環前綴正交時頻空間調變系統之低複雜度兩階段等化器設計

Low Complexity Two-Stage Equalization for Cyclic Prefix Orthogonal Time Frequency Space Modulation with Rectangular Waveforms

陳曦鈞(Xi-Jun Chen)

指導教授 ： 蘇柏青

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201034

摘要

正交時頻空間(Orthogonal Time Frequency Space, OTFS)調變系統是近期被提出用來解決高都卜勒敏感度問題的技術，其概念是將一個衰落、時變的通道轉換至另一個與時間獨立的二維通道，讓通道增益具有接近常數的效果，此二維平面被稱作延遲都卜勒域(Delay-Doppler Domain)。因此，相較於傳統的OFDM，OTFS調變更適合用於具有龐大都卜勒頻率偏移的高移動性通訊場景中。然而，現有針對採用矩形波形(Rectangular Waveforms)之循環前綴(Cyclic Prefix, CP)正交時頻空間調變系統的通道等化技術所需運算複雜度都非常高。在本論文中，透過遵循兩階段等化器(Two-Stage Equalization)架構，我們在兩個階段都提出了新方法。在第一階段，提出了一種基於理想波形假設的最小均方誤差(Minimum Mean Square Error, MMSE)等化器來初始估計資訊符號(Information Symbols)；在第二階段，開發了一種迭代最大比值合併(Maximal Ratio Combining, MRC)檢測器以進一步消除殘餘干擾。
此外，藉由將第一階段等化器替換成現有存在的方法，還提出了另外兩種新的兩階段等化器組合來提高性能。我們所提出的接收器能以較低的複雜度被實現，從模擬結果也驗證了在傳播路徑數量足夠多的場景之下，與傳統的MMSE等化器、訊息傳遞演算法(Message Passing Algorithm, MPA)檢測器、以及現有的兩階段等化器相比，它們在高訊號雜訊比(Signal-to-Noise Ratio, SNR)區域會有更好的位元錯誤率(Bit Error Rate, BER)性能。

關鍵字

正交時頻空間 ； 兩階段等化器 ； 時變通道 ； 最大比值合併 ； 低複雜度

並列摘要

Orthogonal time frequency space (OTFS) modulation has been recently proposed to tackle the high Doppler sensitivity problem, which converts the fading, time-varying channel into a two-dimensional time-independent channel with near-constant channel gain in delay-Doppler domain. Hence, compared to the conventional OFDM, OTFS modulation is more compatible with high mobility communication scenarios with large Doppler frequency shifts. However, the computational complexity of the existing equalization techniques for cyclic prefix (CP) OTFS modulation using rectangular waveforms is very high. In this thesis, by following a two-stage equalization architecture, we proposed new methods in both stages. In first stage, an ideal-waveform-assumption-based minimum mean squared error (MMSE) equalizer is proposed to initial estimate information symbols. In second stage, an iterative maximal ratio combining (MRC) detector is developed to further eliminate the residual interference. In addition, another two new combinations of two-stage equalization are also proposed by substituting the first stage equalizer with the existing methods to improve the performance. Our proposed receivers can be implemented with low complexity, and simulation results validate that they have better bit error rate (BER) performance in the high signal-to-noise ratio (SNR) region under the scenario when the number of propagation paths is large enough compared to conventional MMSE equalizer, message passing algorithm (MPA) detector, and existing two-stage equalizers.

並列關鍵字

Orthogonal Time Frequency Space (OTFS) ； two-stage equalization ； time-varying channel ； maximal ratio combining ； low complexity

參考文獻

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[3] R. Hadani, S. Rakib, M. Tsatsanis, A. Monk, A. J. Goldsmith, A. F. Molisch and R. Calderbank, "Orthogonal Time Frequency Space Modulation," in Proc. IEEE Wireless Communications and Networking Conference (WCNC), pp. 1-6, Mar. 2017.

[4] R. Hadani, S. Rakib, S. Kons, M. Tsatsanis, A. Monk, C. Ibars, J. Delfeld, Y. Hebron, A. J. Goldsmith, A.F. Molisch and R. Calderbank, "Orthogonal Time Frequency Space Modulation," 2018, arXiv:1808.00519.

[5] A. Monk, R. Hadani, M. Tsatsanis, and S. Rakib., "OTFS - Orthogonal Time Frequency Space," 2016, arXiv:1608.02993.