If you’re guessing, its massive MIMO, spectrum, and 5G.

I was reading a lot of articles about massive MIMO this morning. Great articles, it has to be balanced with what’s practical for deployment.

When looking at who is using it below 6GHz, we need to look at Sprint and T-Mobile.

Sprint has between 100MHz and 120MHz of TDD contiguous 2.5GHz spectrum, band 41, in the USA.
T-Mobile has over 30MHz of spectrum nationwide in 600MHz, FDD paired spectrum, band 71.

The way I see it, these 2 spectrums offer a lot, but they cover quite differently. Many people are comparing these with mmwave. As I said before, mmwave is a different use case, so for mobile coverage, it will suck.

However, 600MHz is ideal for rural coverage. It covers a wide area and has good penetration. It’s going to help T-Mobile roll out usable LTE across the USA.

Then you have 2.5GHz which does not carry as far in rural settings. It does OK, but not like 600MHz. It’s ideal in urban environments where it covers very well and has good penetration.

Now, let’s compare urban and rural quickly before we move on.

Rural is wide open space, more or less, and you want spectrum that will go far, meaning the air will not degrade it, and penetrate vegetation well. The rule of thumb is that the lower the spectrum the farther the signal will carry through the air and over the terrain.
Urban is considered dense city environments where there is a high concentration of buildings, black top, and people. Generally, any spectrum will cover well here, but the 2.5GHz has good building penetration if the building’s windows will allow it. Many modern buildings have something in them that will keep the sun’s rays to a minimum and this also keeps RF to a minimum.

Why did I point that out, because of the type of signal matter in a cell and wireless data system? The rules about coverage can get confusing if you only look at one aspect.

What about technology?

TDD in Band 41

TDD is where the system will transmit and receive on the same antenna. This means that you only need one antenna to do everything. The system will transmit a burst of data then that system will listen for a burst of data.

So in 2.5GHz, Sprint has 120MHz of contiguous spectrum, allocated to be used as TDD.

FDD in Band 71

The FDD system will have separate transmit and receive systems. An antenna for each service.

That means that the transmit and receive are split. Like Band 71 covers the range of 617 – 698 MHz but it’s paired. 617 – 652 MHz transmits from the tower, but the UE listens for this spectrum while the US will transmit 663 – 698 MHz and the tower listens for this.

Notes

When using CDMA, FDD was the spectrum of choice. It had a specific uplink and downlink spectrum. It was geared towards mostly voice, even though it was data, you had allocated spectrum for uplink and downlink.

Then came LTE. LTE at first worked so well on FDD because it could designate uplink and downlink spectrum. It was solid. Then advances came and aggregation happened to help us pass more data. Aggregation is when we can take multiple carriers and aggregate them into looking like one big carrier. Aggregate means that they all work together. If you’ve ever worked with muxing and demuxing, it’s like that. Multiple streams that look like one big stream at each end.

Now, LTE can work well with either technology. It’s all about how the device interacts with the base station.

When coverage was a major issue, you could put in a receive antenna that had very high gain to hear the cell phone.

That means that the BTS had a lot of control in how the system worked.

Then what happened?

Then a strange thing happened, cell site coverage started to shrink. Loading became a major issue when voice and data traffic picked up. Spectrum became a commodity. Everything pointed to smaller coverage areas which means more down tit, smaller cells, more cell sites, more data, higher densification, higher self-interference, more handoffs, and so on.

The problems would mount. Technology had to advance, thus, LTE solved many of those problems. Aggregation was a key factor in more throughput. Voice became VoLTE. Download speeds were much more important than upload speeds.

By the way, one more factor was all the tower issues like:

The tower companies, i.e: American Tower and Crown Castle Inc were getting more and more money based on size and weight.
The OEMs moved the radios from the shelter up to the top of the tower, adding weight and complexity to the site design.
Fiber was a key material between the BBU and the radio head.
Power had to be run from the shelter to the radio head.
PIM on the tower became a problem.
2T2R, 4T4R, and 8T8R MIMO became a key differentiator in coverage and throughput to the end user.
Many FDD systems needed a filter on the tower top.

All of the above became problems for the carriers. Now, the more connections you had become a problem.

Now, with the growth of all data systems and smaller cell sites, TDD looks more and more attractive.

Who knew, Wi-Fi had it right all along and it took the cell industry 20 years to catch up.

Different Environments?

Rural areas:

Well, in rural areas you can have good coverage out, but a UE device also has to transmit back. So while 600MHz travels well, the base station needs to hear the end device. So you can just push more power through it. You have to make sure the receiving system has enough gain to “hear” the incoming signal.

That is why FDD has worked so well in rural systems. The receive antennas can have more gain than the transmit antennas. An antenna setup to receive with high gain antennas has a slight advantage because you don’t need balancing in and out.

Although TDD works well, it just has a smaller coverage area.

Now, when we look at 2.5GHz we see that it will not travel as far nor penetrate as well. It does offer more speed when you are in its coverage area. It will offer good coverage if it has line of sight.

Urban areas:

Urban areas are a challenge because you’re trying to cover as many users as possible in a smaller area. So you want the cites to cover a smaller area so that the loading can be managed properly.

The thing about 600MHz is how far it travels. That can be a problem in an urban area. The RF engineering needs to be looked at carefully, especially with antenna gain and down tilt. Those areas need to be carefully engineered.

The good news about 600MHz is that building penetration is good. So the outside in scenarios play out very well, assuming the building doesn’t have energy-saving windows. That could inhibit what it can do in many buildings. As you know, many new buildings are energy-efficient.

That’s where 2.5GHz will do very well. While it won’t penetrate buildings as well, it does go through windows. That may be good enough in the city where many places rely on Wi-Fi once inside.

With 2.5GHz the cells are smaller and the lack of penetration actually aids in high-speed data system planning. You have less self-interference and efficient handoffs if it’s designed properly.

Sprint has 100MHz to 120MHz of the spectrum. They can split it between LTE and 5G and still have an amazing system.

UE Backhaul

One thing that I have seen Sprint do very well is to put together a great system with UE backhaul. Now, with massive MIMO and 5G, imagine that UE backhaul on steroids! Can you picture a system where the backhaul is using 2.5GHz and can be placed anywhere with line of sight to another macro site?

So if the merger happens, can you see a system that has 2.5GHz as backhaul and 600MHz as the last mile? It would work very well and I could see small cells being deployed almost anywhere across any city. If you have enough height and a decent signal back to a macro site, then you can densify quicker than anyone else.

If the merger happens, it seems like it may not if it continues to delay.

I talked about UE backhaul back in 2015, https://wade4wireless.com/2015/10/19/what-is-lte-ue-backhaul/, then again in 2017, https://wade4wireless.com/2017/05/08/sprint-and-ue-backhaul-equals-a-magic-box/, if you’re interested. Mobility deployed this model with Sprint in many areas.

For the Magic Box, Sprint and Airspan got this to work and it is very successful. It’s good enough for small business and indoor coverage.

Mobilitie deployed this solution for Sprint across many cities. It had success to fill in the coverage in many areas.

With the 2.5GHz 5G backhaul, maybe it could be used more and more. Especially if the 2.5GHz is using massive MIMO. Massive MIMO opens up so many new doors because it’s not just MIMO as we know it, it will open up MU-MIMO, Multiple User MIMO so that the antenna can carry on distinct communication with multiple UEs at the same time with degradation in performance.

This takes the UE backhaul to the next level when the site can effectively dedicate signal to a UE will serving the local UEs without degrading any performance. Pretty awesome, right?

If this happens, Sprint and T-Mobile could deploy a 5G system very quickly. It would happen as fast as they could get through Site Acquisition, permitting, and leasing. OK, that really slows things down, but it’s quicker than waiting for fiber at every location.

You could go with UE backhaul, bring the site live the minute you commission and integrate, then wait 10 weeks for the fiber to get even more capacity.

Maybe, you could bring the site live, and only order fiber if you need it.

I see the same model with mmwave spectrum. If someone other than the carriers could build out a mmwave system, they could dedicate it as a fiber extender. That’s what many people have done in E band. It is very “line of sight” because it’s over 60GHz. Rain may be a factor, but you can get 1Gbps second links.

Wireless Fiber Extensions:

Wireless fiber extension is the dream. Wireless backhaul just can’t keep up. Now, fiber can’t keep up because everyone wants only one thing, more! They all want more throughput which puts a strain on the fiber being deployed. It’s never enough. If you have 6 pairs, then you want 24 pairs. It’s growing exponentially.

Hey, it’s not the carriers fault, or the cable companies, it’s your fault. The more you want, or need, the bigger the demand on them.

You want more, yet you’re too cheap to run your own fiber from a real ISP. Instead you, like me, rely on the cable company and the wireless carriers to provide all of this for a flat fee every month.

Hence, this is the business model of mmwave system, to bring you an alternative to cable and fiber.

I see Sprint offering you something in the 2.5GHz band that will be enough for most of you. In fact, I see it as better than mmwave because Sprint has it already deployed everywhere. They just need to replace the current LTE with massive MIMO 5G. Something they are doing as I write this.

This is going to be a game changer, in my opinion. Sprint has the tools to do something amazing! They are on the cusp if they can execute and stay out of bankruptcy.

References:

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