6G - The Sixth Generation Mobile Network

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Dec 8, 2022 11:20 PM

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Jaekuryu

What is 6G ?

Simple answer is 'We don't know yet' -:). At least, as for now, when I was writing the first note on this topic.

But from various presentations and documents that I've gone through, it seems that we may identify a few pillars as we did at the early stages of 5G as shown below. I tried to set the three pillars as in 5G. It seems that the first two pillars Terahertz and AI/ML seems to the ones that are most commonly mentioned in the early discussion but I am not sure whether the third pillar High Data Rate in eMTC/URLLC can be a clear target for 6G or not.

Teraherz/Tera bps : In many of the documents, this goal is described as Terahertz. The literal meaning of Teraherzh implify that the carrier frequency of the signal is in the range of TeraHz(1000 Ghz). But I would interpret this as a Tera bps rather than Tera Hertz since the carrier frequencies being proposed in various documents are not always in the range of Tera Hz. The frequency range being discussed in those documents are in the range of several hundreds of Ghz or in Tera Hz (For the details of 6G the frequency range being actively discussed, refer to 6G Spectrum page). However, it seems to be clear that the final target is to achieve Tera bps range of data transfer (Refer to 6G KPI page for the details of 6G data rate requirement).

Now you may have some fundamental questions and stick to it for several years. What kind of electronics would be used to achieve this goal ? Would we call this mmWave ? or Optical wave ? (For the details of the electronics for 6G implementation, refer to 6G Electronics page)

AI(Artifical Intelegence)/ML(Machine Learning) : In 6G, it seems that AI/ML would be a feature that is integrated into the functionality of the radio and core network. It would implie that most of the network component should be virtualizied to apply the flexibility of AI/ML to the network functionality.

There are many documents that defines 6G in various different aspects. None of them defines better than others. You would not know of the best definition for a while even after 6G realization. The only thing we can do as of now (May 2021) would be to collect as much diverse opinions as possible and try to get another picture of your own.

Source : The shift to 6G communications: vision and requirements (Dec 2020)

Following illustrations shows a 6G definition by Huawei. As you see, eMBB, mMTC, URLLC would take evolutionary path into 6G and Network Sensing will become a new pillar in 6G. All the components will be coordinated / controlled by AI in 6G.

Source : 6G: The Next Horizon

Source : 6G: The Next Horizon

YouTube

Revolutionizing The World Through 6G Technology (May 2021)
What is 6G Technology | Samsung White Paper | What is 6G Speed | Tutorial Explained (Jul 2020)

Readings

6G: The Next Horizon

6G KPIs (Key Performance Indicator)

Following is the 6G target KPIs that are mentioned in many 6G related documents/forum. Obviously it looks very challenging. As of wriing this (May 2021), we have long way to go to hit the 5G target and I cannot imagine how we can meet these target listed in 6G column. Let's trust again the great minds in engineering.

< Comparision between 5G KPI and 6G KPI >

KPI (Key Performance Indicator)	5G	6G
Peak Data Rate	20 Gb/s	1 Tb/s
Experienced Data Rate	0.1 Gb/s	1 Gb/s
Peak Spectral Efficiency	30 b/s/Hz	60 b/s/Hz
Experienced Spectral Efficiency	0.3 b/s/Hz	3 b/s/Hz
Maximum Bandwidth	1 Ghz	100 Ghz
Area Traffic Capacity	10 Mb/s/m^2	1 Gb/s/m^2
Connection Density	10^6 devices/km^2	10^7 devices/km^2
Energy Efficiency	N/A	1 Tb/J
Latency	1 ms	100 us
Jitter	N/A	1 us
Reliability	1x10^-5	1x10^-9
Mobility	500 Km/h	1000 Km/h

Peak Data Rate : The final target of 6G Max throughput is 1 Tb/s. If you compare it to 5G max throughput as of now (Sep 2022, assuming it around 10 Gb/s), it is around 100 times of 5G max throughput. If you compare it to ideal 5G max throughput (20 Gb/s), it is still around 50 times of 5G throughput.

Expected Data Rate : Expected the data rate is the date rate we can achieve even in harsh enviroment (e.g, cell edge). In 6G, it is expected to achieve 1 Gb/s data rate even in such a harsh condition. This is also around 100 times higher than 5G.

Maximum Bandwidth : As of today(Sep 2022), the most common 5G bandwidth would be 100 Mhz per single carrier (both in FR1 and FR2) and it will soon get extended to 400 Mhz per single carrier in case of FR2. According to Release 17 specification, 5G max bandwidth can go as wide as 2 Ghz. Considering this, 6G max bandwidth would be 50 or 100 times wider than 5G max bandwidth.

Latency : As of today(Sep 2022), it would be a little bit tricky to achieve 1ms Latency in 5G, but at least it is doable. In 6G, it is targeted to achieve the latency target of 0.1 ms (100 us) which is 10 times shorter than 5G.

There are various ways of describing the 6G KPI with a little bit different perspectives. I am trying to consolidating various types of those KPIs here and linked the source at the bottom of each figures.

Source : SamSung 6G Whitepaper

Source : Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

Source : 6G WHITE PAPER-ON VALIDATION AND TRIALS FOR VERTICALS TOWARDS 2030’S

Source : A Vision of 6G Wireless Systems: Applications, Trends,Technologies, and Open Research Problems

6G Use Cases / 6G Application

The use case for 6G (6G Application) doesn't seem much different from the 5G use case. Are we running out of imagination ? -:). But the KPI requirement (e.g, throughput, latency etc) is expected to be much tigher than 5G. Some example of use cases in comparision with 5G is shown below.

Source : Towards 6G Networks: Use Cases and Technologies

NOTE : Some people often add Hologram to the list of 6G use case as well. But we would need additional technologies (i.e, data compression algorithm) to make Hologram as any wireless communication technology. Even Tbps data rate may not be enough for uncompressed holographic streaming.

Source : Towards 6G Networks: Use Cases and Technologies (Mar 2019)

Here goes a little bit more ambitious use cases for 6G from Hera-X : Deliverable D1.1 6G Vision, use cases and key societal values.

Source : Hera-X

YouTube

Revolutionizing The World Through 6G Technology (May 2021)
What is 6G Technology | Samsung White Paper | What is 6G Speed | Tutorial Explained (Jul 2020)

Challenges / Questions

As of now (Feb 2021), nothing is fixed about 6G. It may not make much sense of having questions about something that is not clearly defined. But at least there is one thing that seems to be relatively clear. It would be that 6G would be based on Thz (from a few hundreds Ghz to a few Ghz) and my question is based on this assumption.

Isn't it too early to get into this ?

I asked the same question when I started looking into things about 5G around mid 2013. 5G activities in some pioneering organizations started even earlier. Now I heard some leading company in 5G has started their research back to 2007 or so.

I would say, "Yes it is too early if you are interested only in solid/determined 3GPP specification". I don't think you would see much formal activities in 3GPP until release 18 or later (probably sometime around 2026 or later ?).

However if you are interested in observing the whole process of how a new technology is being formed and evolved and finally turns into operational products, nothing is too learly. To me, it is always enjoyable to follow up from the very early conception through the full developmental process.

Do we have enough time ?

Assuming that 6G is targeted to be deployed in 2030, we have roughly 10 years as of now (Feb 2021). It implies that most of critical component of technology (especially the technologies related to physical layer implementation) should be ready a few years before the deployment target. It means that we have only around 5 years or so until those PHY related technology is ready. Would this be enough time ? I think it would be very challenging.

Looking backwards to the process of conceptualization to realization for 5G which also took almost 10 years, there wasn't much difference in terms of physics between 4G and 5G. You may say that regular radio frequency (mostly under 2Ghz) to mmWave transition is a big difference in terms of physics, but I think the technology gap is small enough that most of 4G technology (semiconductor, OFDM, Antenna Technology etc) can be reutilized. How much of 5G technology can be reutilized in Thz technology ? I don't think there would be much of overlap between 5G and 6G physical layer implementation and a lot of technology need to be reinvented.

A way to shorten the time line for the technical readiness would be to redefine the definition of 6G PHY. For example, instead of targetting 300 Ghz and above as the first target, setting D(110Ghz-170Ghz) or G(110-300Ghz) band as the first target and try to extend it to higher frequency as 6G evolution. In this case, there would be disputes over whether we can call the D band as Thz technology or not... but this kind of approach has been employed very often. Even in 5G case, under 40 Ghz was set as the first mmWave target and 50~70 Ghz as 5G evolution target.

==> (May 2021) I think it is worth noting the statement from this whitepaper saying "it is evident that we are still far from achieving Tbps speed even in test beds with a relatively low technology readiness level. For mass volume consumer products, we still lack proven technologies for all areas, from digital, through packaging increasing the integration level, to antennas,which will be a challenge for both academia and industry in many ways in the coming years"

NOTE : As far as I recall, there was proof of concept implementation / demonstration for 5G around early 2013. With the similar time frame, can we have something like this for 6G around 2023 ? To be honest, I think the chance is high. However, if we set D band as the first target as the first target I don't think it is impossible to come up with something to show in a few years.

How to generate Thz Signal ?

I think the first step for PHY implementation is to develop signal (like CW or pulse) for the targeted frequency. In this case, we need to develop a device that can generate the signal at Thz range. There has been a few conventional approach for this. One is to downconvert the optical signal to Thz range and the other one is to upconvert high frequency mmWave to Thz range signal. Recently some researches has been done to develop devices to generate Thz signal directly in Thz range.

Once this type of device is developed, next step would be to reduce the size enough to fit for the communication device (especially size reduction for mobile device would be the most challenging) and develop the process for mass production.

How to modulate Thz Signal ?

Can we use the OFDM which is good for wideband implementation ? or do we need to turn to single carrier modulation ? If we need to turn to single carrier technology, how can we implement ultra wideband ?

==> (May 2021) I think it is worth noting the statement from this whitepaper saying "In practice, this bandwidth requirement is then even wider for single carrier modulation due to the larger guard band compared with OFDM. Yet the digital signal processing (DSP) of OFDM signals consumes so much energy that it is unlikely to be a viable solution for any DSP in the foreseeable future"

Do we have proper antenna technology for Thz ?

One of the hot topics when we were talking about 5G technology in comparison to 4G. Now experts are talking about ultra massive mimo. Applying the logic explained here, it would be understandable that we would need to put a lot of more antenna elements to work in such a high frequency like Thz, but there would be a lot of challenges to integrate very high number of elements. In addition, it would be critical to develop analog and digital solutions required for beam foraming (e.g, phase shift and amplification control) that can work in this high frequency region.

Do we know about the characteristics of Thz Channel and do we have any good model for it ?

How to handle the issue of estimating channel and reporting channel state information ?

How to handle such a high sampling rate requirement of ADC for ultra-wide bandwidth ?

I think this has always been a serious challenges for every new technology. Evolving from 4G to 5G, we needed to revolutionize the ADC handling 20 Mhz BW to 400 Mhz BW. As of writing (Feb 2021), it seems that ADC handling 200 Mhz BW seems to be ready with reasonal cost even for the mobile device, but not sure of the one that can handle 400 Mhz. In 6G, we are talking about 100 Ghz Bandwidth and we can easily guess how hard it would be.

Do we have such a high performance of DSP or revolutionaly channel coding algorithm ?

Achieving such a high data rate like 1 Tbs implies that we would need to perform such a wide baseband processing. Among these process, channel coding would be the most performance demanding. So we may need to completely redesign the channel coding algorith to be more efficient or to be possible for parallel processing.

We may need AI-driven Smart Hardware

In order to enable dynamic communication and networking solutions needed to support the applications of THz networks agile reconfigurable hardware orchestrated. Considering the complexity of these configurations and unavailability of deterministic (closed form) of solution, we may need AI based solution for this (Ref [1])

How to overcome the difficulties of Heterogeneous integration and fabrication ?

In current technology (as of Sep 2022), most of the electronic components are based on the similar materials (i.e, silicon CMOS), but in 6G for super high frequency it would be likely that we need to depend on various different materials and technologies for the hardware components (e.g, combination of CMOS, III-V, graphene, nano-materials and others). In this case, how to integrate / fabricate this kind of different materials/technologies on a same chip.

And many other challenges

Followings are the list of challenges or Open Questions from other papers/white papers.

Source : 6G WHITE PAPER-ON VALIDATION AND TRIALS FOR VERTICALS TOWARDS 2030’S

Reference

[1] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

6G Enabler Techologies

Like every new technology, 6G also will require many component technologies that would make it possible. There is nothing we can say for sure about the component technology as of yet (Feb 2021), but following illustration can be a good summary of possible candidates. This is based on Scoring the Terabit/s Goal: Broadband Connectivity in 6G and I think it is one of the best summary with focus on low layer implementation. Try to get familiar with the keywords shown here and it will be much easier for me to read other technical documents once you get familiar with these keywords. I strongly recommend you to read through the paper.

Image Source : (Modified from) N. Rajatheva et al.: Scoring the Terabit/s Goal: Broadband Connectivity in 6G

Following table shows some of the 6G component technologies and what kind of use caes that those technology can provide.

Source : Towards 6G Networks: Use Cases and Technologies (Mar 2019)

Spectrum

At least for now (early 2021), it seems that teraHeartz frequency (0.3 ~ 1 THz) is taken as the strongest candiates. However the debate over the best frequency for 6G will continue and we may end up with multiple segements of spectrum sitting far apart from each other being used for 6G. It may proceed as it has gone through in 5G. In 5G/NR, mmWave was taken as the major spectrum for 5G and this was a strongest trigger for 5G at early discussion. And then at the phase of formalizing 3GPP, another segment (FR1) sitting far away from mmWave(FR2) was introduced. Finally in real deployment, at least as of now (Oct 2021) FR1 seems to be the dominent spectrum for 5G which is far away from the initial 5G dream.

6G Spectrum Candidate

For now (as of writing this, May 2021), the strongest candidates for 6G spectrum seems to be between 0.3 THz(300 GHz) and 1 THz. This frequency would be an interesting region which is right above the maximum radio frequency and right below the lowest spectrum of light. But electronics, test equipment etc are not fully prepared for the research in this are for communication purpose and majority of the research on communication in this epctrum is being done in a few spectrums lower than this are as highlighed in the figure below.

Reference : ETSI, NYU Wireless

Source : Terahertz Communication: The Opportunities of Wireless Technology Beyond 5G

Following is the frequency range and the frequency candidate for 6G defined by Hera-X deliverable document.

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

Source : Keynote 1: The road to 6G and what 5G still has in store (PIMRC 2021)

How much bandwidth we need for 6G ?

The answer to this question would vary depending on many factors like modulation scheme, spectrum efficiency (how many bits can be carried by 1 Hz frequency spectrum).

The rough estimation done by Hera-X is as follows. According to this, even with 64QAM we would need around 20GHz BW. It would be challenging to achieve this bandwidth in single carrier... so likely to require multiple carrier (carrier aggregation) to achieve this.

Source : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

Are we sure of going with Terahertz for 6G ?

When the 6G discussion started a couple of years ago and suggested Terahertz to be a major spectrum, everybody seems to accept the idea (at least no strong resistance against the idea as far as I remember), but as time goes on the debate over the teraheartz idea started heating up. As of now (Oct 2021), I see roughly three categories of opinions as summarized below.

Optimistic : I think we will see Terahertz as main spectrum for 6G. I know there are a lot of obstacles to overcome for now but they will be removed by the time we deploy 6G. We had similar pessimism about mmWave when we first talked about utilizing mmWave for main spectrum of 5G, but now we see the mmWave being used in real deployment.

Pessimistic : mmWave being used for 5G ? I don't think mmWave is really working as we expected and I don't see any clear vision that the mmWave issues will be cleared in near future. Considering this situation, I think it is premature to talk about Terahertz range spectrum in cellular communication system.

Neutral : I agree that it will be challenging (probably less likely) to see Terahertz being the major spectrum for 6G, but I don't think the terahertz will go completely out of the scope of 6G. Even though it would be less likely for Terahertz to be used for UE (e.g, mobile phone) with mobility, there would still be possibility to be utilized for some part of cellular system like wireless backhaul.

Why Terahertz ?

This may be supporting 'Optimistic' opinions mentioned above. Why we are considering Terahertz as a major spectrum for 6G ?

Some of the advantage of Terzhertz spectrum can be listed as follows (Ref[5])

Wide bandwidth Availability : from tens and up to hundreds of GHz of contiguous bandwidth,
Super short symbol duration : picosecond-level symbol duration,
Huge number of Antenna in small dimension : integration of thousands of sub-millimeter-long antennas,
Easy Coexistece : ease of coexistence with other regulated and standardized spectrum

NOTE : In another perspective, some of these advantage can be challenges in real implementation of the technology.

Wide bandwidth Availability : from tens and up to hundreds of GHz of contiguous bandwidth

==> this requires various electronic components (especially DAC/ADC) that can handle super wide bandwidth.

Super short symbol duration : picosecond-level symbol duration

==> this requires extremely high performance of baseband processing power.

Huge number of Antenna in small dimension : integration of thousands of sub-millimeter-long antennas

==> this implies that it is hard to establish the communication with small number of antenna elements. With the huge number of antenna elements, more more advanced technlogy of beam management (beam forming, beam scan, beam selection, beam measurement etc)

Reference :

[1] ETSI GR mWT 022 V1.1.1 (2021-04) : Analysis of Spectrum, License Schemes and Network Scenarios in the RF bands above 174,8 GHz

[2] ETSI GR mWT 018 V1.1.1 (2019-08) : Analysis of Spectrum, License Schemes and Network Scenarios in the W-band

[3] ETSI GR mWT 008 V1.1.1 (2018-08) : millimetre Wave Transmission (mWT); Analysis of Spectrum, License Schemes and Network Scenarios in the D-band

[4] Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

[5] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

Electronics

One of the most critical technology required for the realization of 6G would be to implement the hardware working at Subteraherz and Teraherts range in Electronic way. The hardware working in this frequency range had traditionally been implemented in Photonics but it is less likely that the photonics solution can easily been utilized in mobile commuincation system especially on UE implementation.

Realization of hardware in this range with electronic technology is relatively new area but I think it is progressing rapidly and this note is to follow up the researchs on this field (i.e, electrical implementation of Subterahertz / Teraherts Hardware).

From Conventional to Native Terahertz Frontend

Following is brief summary showing the pathways to realize subteraherts and terahertz frontend based on this paper (Ref [1]). I think it would be the best if we find solutions to achieve subteraherts and terahertz frontend using the section [A] technology, but at least as of now (Sep 2022) it doesn't seem to be the case and putting more focus on section [B] technology as an alternative.

Semi Conductor Readiness

Up until 5G implementation, we didn't have to worry too much of this level of readiness : Semi Conductor/Transistor level readiness since the technology in semi conductor was mostly going ahead of Wireless communication technology requirement and the semicoductor technology to meet 5G requirement was available before 5G requirement was finalized. But the situation seems to be different in 6G. The semiconductor technology available now seems to be far from being ready to meet 6G requirement. Therefore, we would need to keep close eye on the technical evolution in this aready as we form the detailed 6G requirement and check the feasibility of achieving those requirement.

Following is the summary of status on the semiconductor technologies that would be critical to implement the electrical components for 6G.

Source : Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis(Ref [2])

Source : ETSI GR mWT 022 V1.1.1 (2021-04) (Ref [3])

Highligts to note : Followings are some of the highlights that you would notice from the data shown above.

Most of the technologies can cover the 10~100 Ghz range.
Below 10 Ghz : CMOS would be dominant for low/mid power and GaAs would be dominant for high power.
Over 100 Ghz : SeGe would be better option.
Over 200 Ghz : InP would be dominent option.
In general, as frequency goes higher. The saturation Tx power tend to decrease.

Some important parameters to be noted for the performance / characteristics of this technology are as follows :

f_T : the frequency at which Transistor Current Gain drop to 1 (0 dB)
f_max : the frequency at which Transistor Power Gain drop to 1 (0 dB)
Power Generation Capability : Important for PA
Noise Figure : Important for LNA
Linearity
Signal Combining
Power Consumption
Spectral Efficiency
Form Factor

NOTE : As a rule of thumb, the transistor f_T/f_max should be more than twice the operating frequency (i.e. the RF carrier frequency) to obtain decent gain and efficiency in amplifier design (quoted from (Ref[4])

It is stated as follows (Ref[4])

Although there are already technologies that can achieve operation in the range of 100–1,000 GHz, the implementation of any larger system like an RF transceiver will be much more difficult than for 5G frequencies in the lower mmW region. One must remember that even at lower mmW frequencies, it is impossible to achieve similar performance to frequencies below 6 GHz with the same power consumption. The limitations arise from physics and the boundaries of different semiconductor technologies.

It is stated as follows in (Ref [5]) :

Fundamentally, semiconductor device performance will need to be a minimum of 3x to 5x better than the wireless carrier frequency to implement radios with acceptable range, power dissipation, and link margin characteristics. For example, utilization of the sub-THz 100-300GHz spectrum will therefore require semiconductor technologies with 0.5THz to > 1THz performance. Silicon and III-V semiconductors are candidate technologies, with advances in SiGe and InP promising >1THz performance.

Considerations on the overal Radio Front End

As the technologies for individual transistor evolves to meet the 6G requirement, we have to re-evaluate those technologies in terms of the structure of the whole front end. The overall sturcture of transmiter and reciver front end can be illustrated as below and we need to check if the technology for each segment of the frontend is ready and also need to consider how to integrate each of the segment to build the whole frontend module.

A,H - ADC/DAC : The critical question at this stage would be 'Is there ADC/DAC which can provide such a huge sampling rate and enough bit resolultion to meet the 6G Bandwidth requirement ?'. Usually as sampling rate and bit resolution goes higher in ADC/DAC, power consumption increase. Then another question is 'how to improve power efficiency ?'

B,G - PLL : Would there be any PLL technology that can operate in 6G spectrum in fundamental frequency mode ? If it cannot operate in fundamental mode in 6G spectrum, we need to multiply the frequency or use harmonic frequency, but the performance would degrade in such cases.

C - Power Amplifier : The critical questions at this stage would be 'Is there Power Amplifier working at such a high frequency with enough power ?'. Usually the operating frequency goes very high like 6G spectrum, the maximum power that a power transistor can output tend to be limited.

C - Low Noise Amplifier : Would there be any LNA operating in 6G spectrum with low enough Noise Figure ?

AD Converter

Regarding AD Converters, generally several questions as follows are the important ones to be answered.

What is the maximum sampling rate ?
What is the bit resolution ? (i.e, How many bits are required for each sample ?)
What is the power consumption ?

In general, maximum sampling rate would be limited by the available semiconductor technology, the bit resolution would vary depending on processing performance and the power consumption would be affected by sampling rate and bit resolution.

A common way to estimate a ADC taking all three factors is to use FOM (Figure of Merits) as described below.

According to Ref [1], state-of-the-art DACs able to sample at frequencies in excess of 100 Giga-samples-per-second (GSaps) have been experimentally demonstrated. For example, two 128 GSaps DACs are multiplexed to effectively sample at 256 GSaps. This translates to a signal bandwidth of at most 128 GHz with 2bit resolution

Noise Factors on Rx Chain

As described above, the major issue on Tx chain is how to develop Power Amplifiers that can pump up enough power. What is the major issue (critical factor) on Rx chain. It is how to decrease the noise factor of the whole reciever chain. The estimation of noise factor on reciever chain can be summarized as below based on Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis (Ref [2])

Reference

[1] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

[2] Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

[3] ETSI GR mWT 022 V1.1.1 (2021-04)

[4] WHITE PAPER ON RF ENABLING 6G – OPPORTUNITIES AND CHALLENGES FROM TECHNOLOGY TO SPECTRUM

[5] Roadmap to 6G

6G Channel Characterization

I think the way we investigate on channel characterization would be similar regardless of the frequency range. So if you have any knowledge on channel characterization for any of previous technology (e.g, 5G channel characterization), it would be much easier to follow up on Terahertz Channel Characterization. If you are interested in my note about 5G channel characterized done in before and early 5G standization, check this page.

As of initial writing of this note (May 2021), I haven't got much of the information about 300Ghz ~ 1 Thz yet. Most of the information (e.g, tech articles or papers) on this topic is largely around D-Band (110~170 Ghz) or W-Band(75~110Ghz).

: In early 2021, some papers and white papers are published on channel sounder in 300 Ghz region, but with distance between Transmitter and Reciever is just around a few meters. (Check here )

Atmospheric Attenuation

This is about Atmospheric Attenuation. As commonly understood, we see the general tendency of increasing attenuation as frequency goes higher. As a result, we will see higher attenuation in Tera Hertz candate (C) frequency comparing to 5G or D band (B). In the 6G Candidate ranges, we see much less attenuation in dry environment ((1) & (2)) comparing to standard environment ((3) & (4)) implying that interacting with water molecule would play an important role. The attenuation at higher altitude ((1) & (3)) is lower than the case at sea level ((2) & (4)).

Source : NYT Wireless - Publication

Attenuation by Rain

In terms of Attenuation by Rain, it is obvious that the precipitation affects a lot on attenuation, but regarding the frequency effects there is not much differences between 5G FR2(C) and 6G Candiate Frequency(D, E).

Source : NYT Wireless - Publication

Path Loss Model

Source : NYT Wireless - Publication

Attenuation by Foliage

Source : NYT Wireless - Publication

Source : NYT Wireless - Publication

Channel Sounding

Example : THz Channel Sounding: Design and Validation of a High Performance Channel Sounder at 300 GHz

Example : Channel Sounding Techniques for Applications in THz Communications

Reference

YouTube

THz Channel Sounding and Characterization for Smart Rail Mobility in the Era of 6G (Apr 2021)
Spectrum Frontiers: Terahertz (Mar 2021)
Channel Characteristics for Terahertz Wireless Communications

(Nov 2018)

Future Wireless Technologies: mmWave, THz, & Beyond - mmWave Coalition - Ted Rappaport (Oct 2018)

6G Radio Technologies

6G Radio will be targeted for covering many area that is not covered by the current technology or doing the similar thing as current technology in more advanced way. For those new features and implementation, 6G radio would incorporated many component technologies as illustrated below.

Research Areas on 6G Radio Technlogy

Following illustration is based on the contents from Roadmap to 6G (Next G Alliance).

Building Blocks of 6G Radio Hardware

The building blocks of 6G Radio (actually any kind of digital communication radio) can be summarized as follows (this is based on [1]). Since these building blocks are mostly implemented by hardware electronics, you would find more detailed information from the note : 6G Electronics.

NOTE : In such a high frequency like sub Thz or Thz, the Antenna System may be integrated with Lenses or other reflective structures.

Reference

[1] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

[2] Roadmap to 6G

6G Antenna

As you may understand, the size of the most of RF components is inversally proportional to the frequency of the operating frequency. In other words, the size of the RF components gets smaller as the operating frequency increases. Antenna elements (single antenna) would be the components that best complies to this rule.

Considering very high operating frequency of 6G, we would easily guess that the size of an antenna elements would be very small.

The size of single dipole antenna element on PCB working at 1 Thz can be only 150 um and the size of graphene antenna (or any other antenna made of plasmonic materials) can be even smaller - e.g, 1umlong /10nm wide (Ref [1]).

This small size can be beneficial in that it can be applicable for on-chip communication or wearable communication but the small aperture would be limiting factor in terms of transmission power.

To overcome the power limi due to small aperture, multiple antenna elements in an array format is likely to be the one being used in real communication system (same logic as we use massive MIMO in 5G).

metallic antenna (like PCB antenna) arrays utilized in conjunction with electronic front-ends have been demonstrated at frequencies under 300 GHz with up to 16 controllable elements. (Ref [1]).
in conjunction with plasmonic front-ends, very large graphene-based plasmonic antenna arrays with up to 1,024 elements have been proposed (this may be called as ultra-massive MIMO) (Ref [1]).

MIMO/BeamForming Implementation

Thanks to the very small size of the antenna elements in the 6G frequency range enables huge number of antenna elements (array, e.g, 1024, 4096 or higher) can be integrated into Tx and Rx chipset. With this number of antenna elements, it would be easier to more sophisticated / accurate MU MIMO.

As in 5G FR2, it is highly likely that Hybrid Beamforming will be used in 6G as well. For the analog precoder part in the hybrid beamforming, usually two different types are used : Fully-Connected or Array of Subarrays (I think the array of subarray type is more commonly used). In 6G with utilizing much larger number of antenna elements, we can think of another types for the analog precoder part called dynamic array-of-subarrays which can change the configuration of the sub-arrays dinamically as illustrated below.

Source : Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

Reference

[1] Terahertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

[2] Hera-X : Deliverable D2.1 Towards Tbps Communications in 6G: Use Cases and Gap Analysis

[3] ETSI GR mWT 022 V1.1.1 (2021-04)

[4] WHITE PAPER ON RF ENABLING 6G – OPPORTUNITIES AND CHALLENGES FROM TECHNOLOGY TO SPECTRUM

[5] Roadmap to 6G

References

Institutions/Organizations/Forums

ETSI/ITU/FCC

Event / Workshop / Symposium

R&D / Test Solution

Papers/White Papers

Evolution of Physical-Layer Communications Research in the Post-5G Era (2019)
6G: the Wireless Communications Network for Collaborative and AI Applications (Apr 2019)
Towards 6G Networks: Use Cases and Technologies (Mar 2019)
Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond

(Feb 2019)

Wireless Communication and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond (Mar 2019)
Artificial Intelligence-Enabled Cellular Networks : A Critical Path to Beyond-5G and 6G

(Jul 2019)

6G: The Next Frontier (May 2019)
5G Evolution and 6G (NTT DOCOMO White Paper)
Towards 6G Networks: Use Cases and Technologies (Feb 2020)
6G The Next Hyper Connected Experience for All (SamSung Whitepaper) (Jul 2020)
Various Whitepapers on 6G Channel

(2020)

A research outlook towards 6G.
Scoring the Terabit/s Goal: Broadband Connectivity in 6G (2021)
Towards 6G wireless communication networks: vision, enabling technologies, and new paradigm shifts (2021)
White Paper On RF Enabling 6G - Opportunities and Challenges From Technology To Spectrum (Apr 2021)
Sub-Terahertz Channel Sounding and FPGA Customization of a 6G Testbed Receiver (May 2021)
Ever-present intelligent communication (Ericsson Whitepaper) (2021)
Hera-X : Deliverable D2.1 - Towards Tbps Communications in 6G: Use Cases and Gap Analysis (Jun 2021)
Hexa-X: 6G technology and its evolution so far (Jul 2021)
6G WAVES (Autum 2021)
Roadmap to 6G - Next G Alliance (Feb 2022)
Mapping the Path Toward Sustainable 6G (Whitepaper)Next G Alliance(Mar 2022)
Mapping the Path Toward Sustainable 6G (Slides)Next G Alliance(Mar 2022)
Mapping the Path Toward Sustainable 6G (Webinar)Next G Alliance(Mar 2022)
6G Applications and Use CasesNext G Alliance(Jun 2022)
6G Distributed Cloud and Communications System - Next G Alliance(Aug 2022)
Trust, Security, and Resilience for 6G Systems - Next G Alliance(Aug 2022)
Mobile Communications Towards 2030 - 5G America (Nov 2022)

Videos - Qualcomm

MWC 2022: Qualcomm demos Advanced MIMO antenna designs

(Watch Demo 3, Feb 2022)

Videos - R&S

Videos - 1st 6G Summit

Videos - 2nd 6G Summit

Videos - 6G Summit 2021

Videos - 6G Symposium 2021

Videos - PIMRC 2021 / 6G Flagship

International Workshop on Future Communications (2021)

Demo Videos from 6G WAVES (Magazine)

Video - 6G Research Visions Webinar Series (2020)

Video - Webinar

The Potential of Non-Terrestrial Networks for 6G (Dec 2021)
WEBINAR: Towards 6G - Why and how? (Mar 2021)
Trends and Future of Wireless Communications (Jan 2021)
Opportunities and Challenges in 6G Wireless Technologies (Jan 2021)
Toward 6G Networks: Use Cases and Technologies - IEEE VTS YP Seminar Series (Jan 2021)
6G Architecture to connect the Worlds (Jan 2021)
6G Vision for 2030+ (Nov 2020)
Future is Made in Finland, Episode 4: First in 6G (full recording) (Nov 2020)

Videos - Terahertz Technology

(2019)

Introduction to Terahertz (THz) Technology and Applications (Part II)

(2019)

(2018)

Wireless Above 100GHz

(2018)

Circuits: Terahertz (THz) and Beyond - NYU Wireless Live Stream

(2018)

M. Bonn - Graphene in the (Terahertz) Microwave

(2018)

(2017)

Millimeter-wave and Terahertz Integrated Circuits in Silicon Technologies: Challenges and Solutions

(2017)

(2014)

Active Terahertz and High-Frequency Electronics

(2013)

Terahertz Basics with Dr. David Daughton

(2013)

Introduction to Terahertz Lecture Part 1

(2013)

Introduction to Terahertz Lecture Part 2

(2013)

(2012)

Video - D/G band

R&S #ThinkSix – OTA measurements for sub-THz communication in D-band (Dec 2020)
SubTHz Testbed for 6G Research (Apr 2020)
6G - Extension to a D-Band Antenna Measurement system (Feb 2021)
Accurate propagation measurements at D-band, 160 GHz mmWave frequency (Jun 2019)

Videos - General

Ep 18. Ever-Present Intelligent 6G Communications (With Magnus Frodigh) (Jul 2021)
Evolving Mobile Broadband Connectivity Towards 6G (Jun 2021)
Ep 16. 6G and the Physical Layer (with Angel Lozano) (May 2021)
#ThinkSix – Main 6G research areas (Mar 2021)
Why Apple Already Started Developing 6G Technology? (Feb 2021)
Trends and Future of Wireless Communications (Jan 2021)
6G the next frontier of research (Jan 2021)
R&S #ThinkSix – Spectrum for 5G, Beyond 5G and 6G research (Dec 2020)
The Rise Of China's 6G Technology Revealed! More Than 100X Faster Than 5G (Dec 2020)
China outpaces US with 6G tech — how it happened (Nov 2020)
China’s 6G: Technology Dominance (Nov 2020)
How do we put Europe at the forefront of 6G Research and Innovation? (Nov 2020)
6G Use Cases, Scenarios, Applications, and Terahertz Communication by Young JoKo (Oct 2020)
Beyong 5G: The 6G revolution is ushering a new era (Sep 2020)
Going Towards 6G (Apr 2020)
6G NETWORK TECHNOLOGY FOR 2030 (Feb 2020)
Reinventing the Wireless Network Architecture Towards 6G: Cell-free Massive MIMO and Radio Stripes (2019)
"Why we will need 6G to make the Tactile Internet successful" (Gerhard Fettweis) (2019)
Finland's 6G vision for 2030

(2019)

Why all the 6G wireless talk before we even have 5G?

(2019)

Making 5G Wireless Technology a Reality and Initiatives Towards 6G (2019)
What Comes After 5G? | Unveiled
China is now working on 6G networks!
Tech Rant: Will 6G Destroy Human Society?
Configuring MIMO Communication Links with Machine Learning
Opportunistic Beam Training with Hybrid Analog/Digital Codebooks for mmWave Systems

Following along Evolutionary Path - Physical Layer/MAC Layer

I will write down some technical progess and my learning process as I am writiing tech diary. This is not necessarily very technical .. just a little memo on my learning process.

[ Jul 2019 ]

For now (Jul 2019), I am writing comments on this based on the assumption that 6G physical layer will be based on THz(Tera Hertz) technology,
Roughly two major technology of Thz implementation is being discussed. One is based on Optics and the other is based on electronics, but it is highly likely that electonics based technology would be adopted (at least for communication between mobile station and base station).
There are many options to implement this, but preferred one would be transistor based on implementation. Then need to check if a source(osciallator), mixer, power amplifiers can be implemented for the selected technology. I am not the expert on this area, so not in the position to share the latest information on this. If you are interested, you may refer to Ref [11] as a start and follow through the references listed there. It is published in 2011, so I think the technology has been much more matured by now(Jul 2019).
Once the basic components (e.g, Source, Mixer, Amplifer etc) is developed and commercialized, some pioneering test equipment vendor would come up with signal generator and analyzer. Then the industry or academia would start researching on physical channel properties for the target frequency. This is what had done by 5G several years back (see this as an example).
The result on this physical channel properties and propagation model would be a critical step to determine whether the selected technology can be adopted for the new communication technology or not. Once it is determined to be feasible to use the technology, you may start hearing on possible concepts of physical layer protocol issues like waveform, modulation scheme, frame structure etc.
Channel Modeling

: As they did for 5G, NYU Wireless is doing pionioring work in this area as well. It seems that they starting researching on this and sharing the research result for a while. Check on NYU Wireless / NYUSIM site.

MAC for Terahertz Communicaition System

: Even if there has been some changes in detail in terms of MAC layer between LTE and 5G, the fundmantel mechanism is almost same in LTE and 5G/NR. But it is likely to be drastic change in Terahertz MAC comparing to conventional cellular technology (e.g, LTE, 5G/NR) since the physical layer properties in Terahertz communication would be drastically different. As of now, I don't see any concrete proposals or specification on this, but I would suggest you to start with this survey paper(Ref [15]) as a starter. If you are interested in protocol side (rather than physical layer hardware implementation), this can be a good starting reference.

[ Aug 2019 ]

This month my study has motly focused on reading papers on Machine learning application to this area. At first, I thought the machine learning would mostly for core network optimization, but I learned there has been a lot of research on the application to physical layer process of wireless communication (e.g, detection of modulation scheme, auto encoder etc). See Ref[19]~[26]. I personally recommend you to start with Ref [26] and pick one of specific direction that you want to follow through.

[ Nov 2019 ]

I realized that there are TeraHz physical layer deliverables are defined in Teranova. This seems to be similar activities that we saw from METIS specification at early 5G research. As we saw in the difference between METIS specification and final 3GPP specification, it is not sure how much of this early specification will be adopted by the final industry specification (3GPP), but at least it will be good to shape the overal pictures of Terahertz PHY/MAC.
I see more and more papers coming out in 2019 about applying Machine Learning to Wirless PHY layer especially signal detection, modulation detection ([31],[32],[33])

[ May 2020 ]

A few month ago(Mar 17, 2020), 2nd 6G Summit was held and videos for presentations are posted here. Comparing to those from the 1st summit, most presentations shows more on technical aspect of 6G. I think it is worth watching.

[ Jul 2020 ]

SamSung released a good white paper on 6G : 6G The Next Hyper Connected Experience for All

. At least to me, this is one of the best white papers on 6G giving huge insights on 6G on technical aspects. For me, the section 4 (Candidate Technologies) is the most interesting and informative. Esepcially those topics on Terahertz Technologies, Novel Antenna Technologies,Comprehensive AI were the most intersting ones to me.

[ Oct 2020 ]

I read a white paper from 6G channel on the subject of Machine Learning application for 6G : White Paper on Machine Learning in 6G Wireless Communication Networks. I gives a good insight on what is the limitation on the method that are used now and how ML would contribute at each layers of the wireless communication system. The Webinar based on this whitepaper is open in public here : Deep Dive into the White Paper on Machine Learning in 6G Wireless Communication Networks

[ Nov 2020 ]

A good Webinar video is shared in public about PHY and low layer aspect/idea on 6G. Status of Radio Module development for Teraherz and a lot of talks on massive MIMO for 6G etc. See this video on YouTube.

[ Feb 2021 ]

I found a very good paper summarizing the current status of Thz technology and challenges. I think this is the in-depth version of first presentation of this webinar which is also very good.

[ Apr 2021 ]

Here goes an excellent webinar presented in Jan 2021 about trend and future of telecommunication from 1G through 6G with focus on 5G/6H. I also found another very nice webinar (6G Use cases and Technlogies) presented in Jan 2021. This is more with technical point of view and you will get the big picture of several important technical concepts that are very frequently treated in 6G related tech papers.
A well written Whitepaper is out from 6G Channel : White Paper On RF Enabling 6G - Opportunities and Challenges From Technology To Spectrum

[ Jul 2021 ]

I would recommend a few podcast that Wireless Future posted recently : Ep 16. 6G and the Physical Layer (with Angel Lozano)Evolving Mobile Broadband Connectivity Towards 6G Ep 18. Ever-Present Intelligent 6G Communications (With Magnus Frodigh)
Hera-X released a detailed technical deliverable document : Deliverable D2.1 - Towards Tbps Communications in 6G: Use Cases and Gap Analysis

[ Oct 2021 ]

I just noticed that all the session in 6G Symposium 2021 is now open to public in YouTube. Look into the list of the sessions here. I lot of great technical insights there.

[ Feb 2022 ]

I noticed that Next G alliance issues a report titled 'Roadmap to 6G' you can download here

. You would get the good summary of technical issues and roadmap, especially North Americal focus on 6G.

In MWC 2022, Qualcomm demonstrated a Sub-Thz MIMO working at 140 Ghz carrier frequency and 7.5 Ghz Bandwidth and achieving over 100 Gbps data rate. Watch the demo 3 of this video : MWC 2022: Qualcomm demos Advanced MIMO antenna designs.

Articles/Media

Followings are the list of articles mostly for general public. If you think it is too early to talk about 6G when 5G just start being deployed (as of Jan 2020). See my list of articles on 5G and check how early they started talk about 5G. Not very technical, but I found it intresting to observe how public opinion is formed and new technology is planned and evolve.