6G Challenges

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 ?

When we talk about moving towards 6G technology, a big question is whether we understand the THz (terahertz) channel well enough and if we have a good model for it. This is similar to past concerns about antenna technology for very high frequencies. Just like when we moved from 4G to 5G and started using many more antennas to handle higher frequencies, with 6G and THz frequencies, we face even bigger challenges.

The THz frequency is very high and can carry a lot of data quickly, but it's also new territory. We don't yet have good models that show us how signals behave at these frequencies. Signals at THz frequencies can be absorbed by the atmosphere more easily and are affected by things like water vapor, which makes it hard to predict how they will travel.

Understanding and overcoming these challenges is key to making 6G technology work and getting the most out of its ability to transmit data at very high speeds.

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

With 6G and THz frequencies, this becomes much harder than low frequencies in previous technology. The reason is that signals at these high frequencies are more sensitive to obstacles, they can get absorbed by materials easily, and even the air or weather can affect them a lot. This makes it difficult to predict how the signal will behave, which is what we need to do for good channel estimation and CSI reporting.

To handle these issues, we need to develop new tools and techniques. These might include advanced algorithms that can quickly and accurately figure out the state of the channel, even when conditions change rapidly. We also need to design systems that can automatically adjust how they send data based on the latest channel information to ensure that communication remains clear and fast.

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.

How to develop technologies to meet the end-to-end latency ?

From 4G, there has always been key user cases that requires very tight latency. But in most cases the specification that wireless industry came out was about the effort to reduce the latency within radio protocol stack. In some aspect, this is understanble because wireless industry does not have full control over the specification of the whole network end-to-end, but it would be better be more cautious about promoting new technologies and take into acount those factors controlled by other industries (e.g, IP networks).

6G KPI / Requirement is good enough for everything that we promise ?

Looking back to what industry promised (at least vaguely claim to deliver) at the early conceptual discussion, I got the impression that the industry commited too much (sounds like commiting what the technology is not capable of). This kind of over-commitment lead to over-disappointment among user when the technology is delivered and such a disappointment would raise a negative impact on the success of the technology. I think it would be good for us to become a little bit more cautious when we try to promote new comming technology. We all know what would be the eventual outcome of 'overcommit - underdelivery'.

For now, one thing poping up in my mind whenever I see 6G KPI and 6G use case. It is Hologram Application. At least with the current technology (as of Feb 2022), it would require several Tera bps of throughput for hologram application for single user. Then my question is 'With 1 Tbps max throughput proposed in 6G KPI, how can we implement this kind of application ?'. Are we just expecting some super genious engineers to come out with another disruptive technology to greatly reduce the required throughput for the transmission of holograms ?

And many other challenges

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



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