[2021] [paper notes] 6G technology vision - otfs modulation technology

Preface

type
$Terahertz+signal\; communication$
Periodical
$Journal\; of\; Physics$
author
$Feng\; Wei,Wei\; Shuting,Cao\; Juncheng$
Time
$2021$

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research objective

In order to achieve 6G, Now countries all over the world have put forward various plans , Include THz technology 、 New beam technology 、 multiple access techniques 、 Channel coding technology 、 On a large scale MIMO technology 、 Spectrum management etc.

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THz wave

THz The bridge Infrared and microwave bands

1. Compared with infrared communication , THz Waves can penetrate smoke and dust , Strong adaptability to different environments , Not susceptible to weather factors , therefore THz Communication makes it easier to track and calibrate the beam

2. THz Waves can also penetrate like wood 、 ceramics 、 plastic 、 Fat and other non-metallic and non-polar devices , therefore THz Communication is not easily interrupted by obstructions , Adapt to a wider range of situations .

3. And X ray （ The frequency is higher than ultraviolet ） comparison , THz The energy of waves is small , It is not easy to cause harm to human body , Therefore, its safety performance is higher .

4. Compared with millimeter wave band , THz Good directional performance , It is difficult for unauthorized users to choose from narrow THz Eavesdropping in the beam , This ensures the confidentiality of the information

5. THz Communication can also be very good Use spread spectrum 、 Frequency hopping technology to combat jamming attacks , Make it more difficult for attackers to steal information

THz Ground communication is mostly applicable to Local short-range communication , It has the characteristics of high data rate and low latency

THz Communication can achieve hundreds Gbit/s To dozens of Tbit/s High data transmission rate , It can meet people's demand for exponential growth of data traffic . use 100 Gbit/s Wireless connection for , You can download about in a second 25GB Blu ray movie

THz Waves can not only travel in the air , It can also propagate in plasma medium

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6G key technology

New beam technology

power amplifier (PA) The nonlinear characteristic of the system limits the design of the system

New beam design is needed

When higher-order modulation is not applicable , Non orthogonal waveform (NOW) Discrete Fourier transform can be improved , Spread spectrum , Orthogonal frequency division multiplexing (DFT-s-OFDM) The spectral efficiency of

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Orthogonal time-frequency space (OTFS) Modulation technology It is an integrated code division multiple access (CDMA) And orthogonal frequency division multiplexing (OFDM) New modulation scheme of performance . It is found that the modulation technology is used in high Doppler channel 、 Large antenna array or in high-frequency channels , Each transmission symbol can obtain a nearly constant channel gain , And in Delay - Doppler channel Within the scope of , OTFS Show significantly lower than OFDM Error rate

Time delay - Doppler channel ：
In Fourier transform ,
$$\begin{gathered} Time\; domaintDoppler\nu \\ frequency\; domainfTime\; delay\tau \end{gathered}$$

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multiple access techniques

Using large phased array beam , take Millimeter wave 、 Non orthogonal multiple access （NOMA） Combine

NOMA On the uplink and downlink , Provide services for multiple users at the same time , Greatly improve the spectrum utilization .NOMA It also improves the optimal throughput ？

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Channel coding

5G Adopted Polarity code As a way of control channel coding
—— high reliability 、 High frequency spectral efficiency

In a digital communication system , Use positive level 、 Negative level or zero level indicates binary information “1” and “0” The code type of is called polarity code

The code construction of polarity code involves specific channel characteristics , It directly affects the performance and complexity of decoding algorithm

Polarity codes have poor neutral performance in fading channels , But by using Dynamic freeze symbol The obtained code can provide 0.8 dB The gain of

5G Mobile communication standard , The data channel currently adopts Block code - Low density parity code (BC-LDPC), But the decoding convergence speed of this scheme is slow , High decoding complexity , Long decoding delay , The encoding length is short , Low bit rate

Convolutional code LDPC (CC-LDPC) It has obvious advantages in these aspects , some time 6G High adaptability in mobile communication

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On a large scale MIMO technology

Energy saving 、 High security 、 Low delay 、 Efficient use of spectrum energy

$$Number\; of\; antennas\uparrow +SNR\uparrow *Capacity\uparrow$$
in addition , On a large scale MIMO Technology uses spatial multiplexing SM technology , Increase the capacity by more than ten times

Add more remote antenna units （RAUs） And the layout density of users , Make the overall spectral efficiency higher !

Graphene antenna
stay $THz$ Under the frequency band , Due to the development of graphene plasma nano antenna array technology , Occupy only $1m{m}^2$ Array can be achieved $1THz$ Of $1024\times 1024$ Very large scale MIMO System

Honeycomb free large scale MIMO System

Honeycomb free large scale MIMO The system is in throughput aspect , Significantly better than small cellular systems , Large scale without honeycomb MIMO Every user of 95% The possible throughput of is almost that of a small cellular system 20 times

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Spectrum management

5G Time , Regulators introduced A variety of spectrum management methods , Allow the deployment of new cellular networks

because 6G The frequency spectrum is diverse , Different propagation characteristics , Therefore, there are various methods of spectrum management

6G We should pursue new frequency bands , And protect the existing spectrum users .6G The demand for shared spectrum is higher than ever , take 6G And sensing 、 signal communication 、 AI convergence

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5G Phased array beam and large-scale are used in the development of technology MIMO Technology to improve communication capacity and compensate serious path loss , But the manufacturing cost is high 、 High power consumption

In order to further expand the communication capacity and improve the data transmission rate , 6G The frequency range for mobile communication will be expanded

2019 FCC Decided to open up $95GHz—3THz$ As 6G Test frequency band , At present, researchers have achieved nearly 100 Gbit/s Terahertz communication systems with transmission rate mostly focus on $300—500GHz$ Within the frequency band of .

Visible light communication is a kind of communication with a frequency range of $400—800THz$ High speed communication technology , This spectrum does not require authorization , Operators and equipment manufacturers are free to apply
Visible light communication will also be realized in the future 6G One of the key spectrum resources of Technology

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6G Technology trends

6G Wireless communication technology hopes to achieve better results than 5G high $10\sim 100$ Times the data transmission rate

1. By improving the spectrum efficiency
2. Use a larger frequency band bandwidth Or spectrum resources

according to $C=Blo{g}_2(1+S/N)$ You know , To achieve $1Tb/s$ The transmission rate of , The minimum bandwidth is $0.2THz$

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$*$THz channel

THz The frequency band is more serious than the millimeter wave band Path loss 、 Atmospheric absorption 、 Diffuse reflection

Now? THz Channel measurements are concentrated on $100\sim 300GHz$ near , The scenes are mostly indoors

LOS Line of sight propagation ： Point to point linear signal propagation
NLOS Non line of sight propagation ： Covered 、 Reflection 、 attenuation 、 Diffraction 、 Path loss signal propagation

If LOS The path is the main communication channel , NLOS Path and LOS The path is very weak , Can be ignored （ The loss in line of sight propagation is relatively small ）

NLOS The path loss index mainly depends on the influence of the material on the main communication channel Reflection loss

ceramic tile 、 in / High density fiberboard and other materials , The reflected energy is mainly concentrated in the mirror direction , The reflected energy in the non specular direction is at least $40dB$ The attenuation of , And no matter how big the incident angle is , Large attenuation will occur in the non specular direction

When the ground is a solid concrete surface , NLOS Path interference pair BER Greater impact , And because of the grass surface THz Wave absorption is higher , So for BER The impact is less , And that leads to this LOS The path is subject to The mirror NLOS route （ Related to ground reflection ） Interference of , And this interference signal Sensitive to ground characteristics

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Conclusion

THz Communication has the advantages of high data transmission rate and wide bandwidth , High safety performance in the process of information transmission , Expected to introduce 6G In the system .

Explore THz New methods of production 、 Develop new antenna technology to improve THz The signal gain , Optimize system resource allocation （ Save volume 、 Power waste 、 cost ） , To realize miniaturization 、 Low power consumption and low cost Of THz communication system , Increase communication coverage , Improve data transmission rate and transmission distance

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problem

Differences between optical fiber communication and visible light communication ？

1. The wavelengths of light waves used in optical fibers are ：850nm、1310nm、1550nm These three
2. Part of the wavelength range of visible light is ：390nm~760nm, Which is greater than 760nm The part of is infrared light , Less than 390nm Part of it is ultraviolet light

Because the optical fiber material itself has material dispersion 、 Mode dispersion 、 Waveguide dispersion and other factors , And the dispersion is 850nm、1310nm and 1550nm The loss at these three wavelengths is a minimum , These three wavelengths of invisible light are used

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LOS What is the Fresnel region in line of sight propagation ？

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