调研-LEO场景

1.相关文献

OTFS Enabled LEO Satellite Communications: A Promising Solution to Severe Doppler Effects

https://ieeexplore.ieee.org/abstract/document/10043628

Published in: IEEE Network ( Volume: 38, Issue: 1, January 2024)

李天时,何睿斯,艾渤,等.星地高速移动场景OTFS性能研究（英文）

[J].Frontiers of Information Technology & Electronic Engineering,2021,22(04):517-528.

OTFS vs OFDM: Which is Superior in Multiuser LEO Satellite Communications

https://arxiv.org/abs/2403.02012

Submitted on 4 Mar 2024

Overview of OTFS Based Channel Estimation Techniques in LEO Satellite Communication

https://ygyc.xidian.edu.cn/next/resource/eds/detail?vid=e_ZWRi%23MTYxMzMzNDQw

Space-Integrated-Ground Information Networks; Dec2022, Vol. 3 Issue 4, p101-106, 6p

https://www.infocomm-journal.com/sigin/article/2022/2096-8930/2096-8930-3-4-00101.shtml

孙宇彤.低轨卫星通信下的OTFS信号检测算法研究[D].哈尔滨工程大学.2023.

赵鑫杰,刘子威,张更新.低轨卫星通信中基于OTFS的信道估计技术综述[J].天地一体化信息网络,2022,3(04):101-106.

OTFS-channel-estimation-scheme

2. 相关仿真

基于STK/Matlab的Starlink星座仿真分析:

https://zhuanlan.zhihu.com/p/68385977

matlab-Calculate Doppler shift at target asset in satellite scenario：

https://ww2.mathworks.cn/help/satcom/ref/matlabshared.satellitescenario.satellite.dopplershift.html

matlab-satelliteScenario

https://ww2.mathworks.cn/help/satcom/ref/satellitescenario.html

Paper-1

OTFS Enabled LEO Satellite Communications: A Promising Solution to Severe Doppler Effects

Abstract

LEO-Sat communications are facing severe Doppler effects,which heavily degrades the link reliability.

existed methods:

• phase locked loop

  mainly carry out the prediction and compensation for Doppler frequency shifts, which demands high implementation complexity , as well as accurate channel estimation

this article

• portrays a new paradigm of reliable LEO-Sat communications with the aid of orthogonal time frequency space(OTFS) modulation scheme, which can average out the channel dynamics through modulating information in the delay-Doppler domain,thereby overcoming severe Doppler effects.

  • Through symplectic Fourier transformation (SFFT), it converts the dual-dispersion channel in the TF domain to an approximately static channel in the DD domain, which can effectively overcome severe Doppler effects for enabling reliable transmissions.

  • the reliability performance of OTFS increases with larger frame length, which motivates the application OTFS to LEO-Sat communications.

    as proved by :J. Shi et al., “Outage probability for OTFS based downlink LEO satellite communication,” IEEE Trans. Veh. Technol., vol. 71, no. 3, pp. 3355–3360, Mar. 2022.

• LEO-Sat channels are featured by sparsity with much fewer scatters, which is also beneficial for low complexity design of OTFS’s receiver.

• the motivations of using OTFS: start with studying the Doppler effect characteristics of LEO-Sat communications

• conceive a case study to show the feasibility of OTFS enabled LEO-Sat communication by evaluating the reliability performance.

• research opportunities and potential challenges

LEO Sat 通信

信道

• General channel characteristics
  • 自由空间大尺度路径衰落
  • 卫星和地面中断间的多径和阴影效应
  • 卫星高速移动带来的多普勒效应
• 常用信道模型：
  • Shadowed Rice(SR) 和 Nakagami-m

借助地心坐标系^[1]，分析不同轨道高度和不同频带，不同仰角的多普勒频率；

Doppler_effect_characteristics_of_LEO-Sat-communications

Orbit altitude: 轨道高度

Tangential veloctiy: 切向速度

Carrier frequency：载波频率

Maximum Doppler frequency shift: 最大多普勒频移

Maximum Doppler frequency shift variation rate：最大多普勒频移变化率

Doppler frequency shift at different elevation angle in Ku band (kHz)：Ku波段不同仰角多普勒频移(kHz)

可以看出：

• 在S,Ku，Ka频率，最大多普勒频移范围为几十kHz到几百kHz之间
• 最大多普勒频移变化率变化范围为几百Hz/s到几千Hz/s

现有相关进展

• Star-link,OneWeb have verified the feasibility of using the millimeter wave bands (Ka-, Ku-band) for LEOSat communications
• the LEO constellation Telesat proposed to use V-band（40~75 GHz)

减轻多普勒的技术

OFDM+LEO的应用

• DVB and IPSTAR satellites launched by SHIN

LEO-Sat 场景下，现有技术对抗多普勒频偏的缺点：

• low spectrum utilization
• demanding massive computing resources
• limited performance of compensating frequency shift

具体技术

1. Proper Setting of Subcarrier Spacing

   e.g. make $\Delta f >10f_{D_{max}}$

2. Frequency Locked Phase Locked Loop Scheme(方案)

   PLL 在实际系统中被用于捕获载频频率和相位偏移，以此可以对多普勒效应进行缓解。

   Point1: large $f_{D_{max}}$ need lagreg Loop bandwidth,which increasing the input nosite and interference

   Point2: PLL is better adapted to single-carrier systems

   Point3： large $f_{D_{max}}$ 及Doppler Variation，使得容易超过PLL的捕获区间

3. Pilots-Based Doppler Frequency Shift Estimation Scheme

   under LEO-Sat scenarios, the wireless channels are generally doubly-selective, which demands:

   • 更多的导频开销以估计更多的信道参数
   • 高动态使得估计使得信道估计的精确度下降
   • 导频估计本身实现的复杂度不低

OTFS +LEO Sat : 优势

Downlink process for OTFS enabled LEO-Sat communication

• Non-Sensitive to Doppler Frequency Shifts

  • the doubly-selective channel in TF domain is converted to an approximately time-invariant channel in DD domain, where the channel responses are relatively sparse and robust.
  • the 2D spreading effect introduced by ISFFT enables each DD symbol spreads onto the entire TF domain, which allows each symbol in a frame to experience almost constant channel gain, regardless of how the Doppler frequency shifts vary.

• Alleviating(缓解) PLL With Doppler Variation

  • PLL only needs to deal with the residual phase offset, OTFS scheme will have much lower requirements on the capture bandwidth for PLL.

  example: when OTFS can support a maximum Doppler frequency shift of 400kHz, the PLL at the receiver only needs to cope with an excess of 100kHz due to the presence of the Doppler variation, whereas that for OFDM will be 500kHz.

• Reducing Channel Estimation Overhead

  • as shown in Fig. 2, the LTV channel is compared with that in DD domain, which shows that the OTFS scheme renders a quasi-static and possibly sparse channel response in DD domain.

  • Based on the pilot estimation method, the number of pilots required for OTFS channel estimation is one-tenth(1/10) of that of OFDM under the similar level of channel conditions ^[2]

  • the estimation accuracy is not obviously affected by the intensification of Doppler effect, as long as the guard intervals of pilots are set up properly.

• Enhanced Spectrum Utilization

  • 在严重多普勒情况下，OTFS子载波间隔可以比OFDM设置的更小^[3]
  • since OTFS signals can be realized in a block-wise fashion, it can significantly reduce the cyclic prefix (CP) overhead through an appropriate pulse shaping filter design (Heisenberg transformation).^[4]

• Low PAPR Characteristics

  • This is due to the spreading operation brought by ISFFT process, and long frame length which allows the maximum energy per bit to be increased under transmitter power constraints.
    • An upper bound on the PAPR of OTFS signals 线性相关与 the frame length
  • the low PAPR feature alleviates the requirements for amplifiers, which is particularly important for the uplink LEO-Sat transmissions.
  • In current LEO-Sat systems, such as OneWeb, the uplink transmissions employ the single-carrier frequency-division multiple access (SC-FDMA) scheme, instead of using OFDM scheme with high PAPR.^[5]

分析：基于OTFS的LEO-Sat下行通信链路


the OTFS based multi-Sat transmission scheme, where the modulator parallely processes the different satellites’ signals.

如何实现这一点？调制器并行处理多路信号？

$R(t)-equal gain combining(EGC)->\mathbf{y}^d$

信道矩阵采用 LS 估计，信道H在DD域中不具有对角结构，导致接收信号$\mathbf{y}^d$存在ISI,进一步进行信号检测,这里仅在DD域上进一步处理。

DD-OAMP（DD domain based orthogonal approximately message passing ）

DD-OAMP-steps

result：

BER performance for OTFS based LEO-Sat

1. The OTFS scheme can effectively overcome the impact of severe Doppler effect.
2. By investing acceptable amount of complexity, our non-linear DD-OAMP, has promising practical meaning for suppressing the ISI effect of the LEO-Sat links.
3. The multiple LEO-Sat transmission can bring the reliability improvement, which can be further enhanced by optimizing the deployment infrastructure of LEO-Sats.

Future Directions and Challenges

• Design of Low-Complexity Detection Scheme

  • LEO-Sats are computingand energy-limited platforms, which limits the application of nonlinear detectors with high-complexity.

  • the Doppler frequency shifts of LEO-Sat channels have the features of high predictability and regularity, which can be captured by machine learning (ML) methods.

  • ML-based detectors can use pre-deployment training or prediction of LEO-Sat channels to reduce the complexity of real-time detection.

  卫星的运行轨迹一般可预知

• Cost-Accuracy Trade-Off for Channel Estimation

  • the OTFS transmissions based on pilot and guard interval, require the sparse feature of the DD domain channels.

    • Nevertheless, in future LEO-Sat communication system, having multi-LEO Sat links, the wireless channels are no longer sparse in the DD domain due the impact of ISI.

    在这种情况下，保持精度的办法即提高导频开销

  • 考虑到LEO-Sat场景的特点，一种可能的解决方案是using ephemeris（星历表） or pre-pilots, and then use intra-frame pilots for accurate estimation.

• Delay-Constrained Multiple Access

  • the reliability performance of OTFS improves as increasing the number of Doppler bins, N, which is benefited from achieving a higher resolution on Doppler domain.
    • 与此同时，将 increase the frame length of OTFS, thereby increasing the delay for user accessing, and even out-dating the communication.
  • 考虑同NOMA等多址技术结合，需要考虑接收机的实现复杂度。

• Energy-Efficient Beamforming Design

  mm-wave frequency band is defined as 30–300 GHz with a wavelength between 1 and 10 mm in the air

  • It is an inevitable trend for LEO-Sat to apply millimeter wave bands (such as Ku-bands for Starlink) to alleviate the increasingly obvious shortage of spectrum resources.
  • MIMO-OTFS

• Air-Space Coordinated Transmission

• Computing-Burden less Security Communication

  • Due to the link features of long-distance and openess, the security of LEO-Sat communication is facing serious challenges.

  • the LEO-Sat platforms are computing deficiency, which thereby longs for burden-less security approaches.

  • Under OTFS modulation, the time-invariant characteristics of the wireless channels in DD domain provide convenience for designing physical layer security (PLS) schemes

  需要物理层的安全设计

参考文献

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1. Ali, N. Al-Dhahir, and J. E. Hershey, “Doppler characterization for LEO satellites,” IEEE Trans. Commun., vol. 46, no. 3, pp. 309–313, Mar. 1998. ↩︎

2. R. Hadani and A. Monk, “OTFS: A new generation of modulation addressing the challenges of 5G,” 2018, arXiv:1802.02623. ↩︎

3. Z. Ding et al., “OTFS-NOMA: An efficient approach for exploiting heterogenous user mobility profiles,” IEEE Trans. Veh. Technol., vol. 67, no. 11, pp. 7950–7965, Nov. 2019. ↩︎

4. P. Raviteja et al., “Practical pulse-shaping waveforms for reduced-cyclic-prefix OTFS,” IEEE Trans. Veh.Technol., vol. 68, no. 1, pp. 957–961, Jan. 2019. ↩︎

5. G. Berardinelli et al., “OFDMA vs. SC-FDMA: Performance comparison in local area IMT-A scenarios,” IEEE Wireless Commun., vol. 15, no. 5, pp. 64–72, Oct. 2008. ↩︎