Abstract
Orthogonal time frequency space (OTFS) modulation has been widely considered for high-mobility scenarios. Satellite-to-ground communications have recently received much attention as a typical high-mobility scenario and face great challenges due to the high Doppler shift. To enable reliable communications and high spectral efficiency in satellite mobile communications, we evaluate OTFS modulation performance for geostationary Earth orbit and low Earth orbit satellite-to-ground channels at sub-6-GHz and millimeter-wave bands in both line-of-sight and non-line-of-sight cases. The minimum mean squared error with successive detection (MMSE-SD) is used to improve the bit error rate performance. The adaptability of OTFS and the signal detection technologies in satellite-to-ground channels are analyzed. Simulation results confirm the feasibility of applying OTFS modulation to satellite-to-ground communications with high mobility. Because full diversity in the delay-Doppler domain can be explored, different terminal movement velocities do not have a significant impact on the performance of OTFS modulation, and OTFS modulation can achieve better performance compared with classical orthogonal frequency division multiplexing in satellite-to-ground channels. It is found that MMSE-SD can improve the performance of OTFS modulation compared with an MMSE equalizer.
概要
第六代移动通信系统(sixth generation,6G)相比5G(fifth generation)移动通信系统在应用场景、传输性能等方面需满足更高指标。为应对卫星、无人机和高速铁路等高移动性场景,6G面临的挑战之一是满足高达1000km/h超高速移动场景的通信需求。空天地一体化通信系统以及高速率通信业务发展对下一代卫星通信系统的空口波形提出更高要求。正交时频空(orthogonaltime frequency space,OTFS)调制技术利用时延-多普勒域信道变化缓慢特性,将信息符号变换到时延-多普勒域传输,可充分利用时间、频率分集。与正交频分复用(orthogonal frequency division multiplexing, OFDM)技术相比,在高速移动场景下传输性能提升明显,且具有更高频谱利用率,更低峰均比(peak to average power ratio, PAPR)。相比OFDM技术,OTFS调制更加适合于星地高移动性通信场景。本文主要研究OTFS技术应用于星地通信的性能及信号检测算法,利用3GPP最新的5G非地面网络信道模型(5G NTN),在典型场景下开展深入的仿真分析与验证,对相关技术的发展和系统设计具有指导意义。
OTFS发送端首先在时延-多普勒域进行数据符号复用得到Xdd,之后将逆有限傅里叶变换(ISFFT)应用于Xdd,再进行海森堡(Heisenberg)变换可得到OTFS系统发送端时域信号s(t),在接收端进行以上变换的逆过程(Wigner变换和有限辛傅里叶变换),即可得到OTFS系统的输出信号Ydd。OTFS系统的信道估计通常可采用在时延-多普勒域中插入导频,利用导频信号进行信道估计,可获得用于信号检测的信道状态信息。OTFS系统的性能与采用的信号检测方法密切相关,在高移动速度性场景下,由于多径传播和多普勒效应,时延-多普勒域中数据符号都会受到相邻符号干扰,OTFS系统的性能会因干扰而损失,线性均衡方法由于缺乏干扰消除,通常性能表现欠佳,因此采用基于连续性干扰消除的最小均方误差算法(minimum mean square error with successive dection, MMSE-SD)进行信号检测。
利用3GPPTR 38.811中的星-地移动信道模型,开展了OTFS调制技术在星地高速移动场景下的性能研究,仿真结果表明,OTFS技术与OFDM技术相比BER性能更优,尤其在高移动速度场景下,在相同信噪比下,OTFS调制的误码率几乎不受多普勒频移增加的影响。而对于OFDM调制,误码率随着多普勒频移增加而显著增加。
比较了LoS和NLoS条件下4种不同场景(密集城区、城市、郊区、农村)下OTFS和OFDM调制的误码率性能。结果表明,在LoS条件下,在郊区场景下的OTFS系统的误码率性能最优,在密集城区、城区和乡村场景下的OTFS系统性能接近,而在NLoS条件下,场景对误码率的影响相对较小。
对比了低轨道卫星-地面通信中,OTFS和OFDM调制在密集城区、城区、郊区和乡村场景下的误比特率性能。结果表明,在NLoS条件下的OTFS与OFDM的性能差距比在LoS条件下更大,在NLoS条件下,不同场景对OTFS和OFDM性能影响不大。
比较了不同速度和载频下MMSE-SD和MMSE检测算法的误比特率性能,采用NTN-TDL-B信道模型,考虑密集城区场景。2.2 GHz和20GHz对应的子载波间隔分别为15 kHz和60 kHz。结果表明,利用MMSE-SD算法可以提升OTFS调制在LEO星地通信中的性能,对于不同的终端速度和载频,当误码率为10−4时,MMSE-SD算法与MMSE相比可获得约2.3dB的信噪比增益。
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Tianshi LI, Ruisi HE, and Haoxiang ZHANG designed the research. Tianshi LI, Bo AI, and Mi YANG designed the signal processing algorithm. Tianshi LI, Ruisi HE, and Haoxiang ZHANG conducted the simulations and drafted the manuscript. Zhangdui ZHONG helped organize the manuscript. Zhangdui ZHONG and Haoxiang ZHANG improved the simulations. Tianshi LI and Ruisi HE revised and finalized the paper.
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Tianshi LI, Ruisi HE, Bo AI, Mi YANG, Zhangdui ZHONG, and Haoxiang ZHANG declare that they have no conflict of interest.
Project supported by the National Key R&D Program of China (No. 2020YFB1806903), the National Natural Science Foundation of China (Nos. 61922012, 62001519, 52042201, U1834210, and 61961130391), the State Key Laboratory of Rail Traffic Control and Safety, China (Nos. RCS2020ZT008, RCS2019ZZ007, and RCS2020ZT010), the Teaching Reform Project, China (No. 134811522), and the Fundamental Research Funds for the Central Universities, China (Nos. 2020JBZD005 and I20JB0200030)
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Li, T., He, R., Ai, B. et al. OTFS modulation performance in a satellite-to-ground channel at sub-6-GHz and millimeter-wave bands with high mobility. Front Inform Technol Electron Eng 22, 517–526 (2021). https://doi.org/10.1631/FITEE.2000468
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DOI: https://doi.org/10.1631/FITEE.2000468
Key words
- Delay-Doppler channel
- High-mobility communications
- Minimum mean squared error with successive detection (MMSE-SD)
- Orthogonal time frequency space (OTFS)
- Satellite-to-ground communications
- Millimeter-wave communications