Know your 5G antennas

Stacked? Interleaved? Antenna options for 5G sites.

CommScope and Nokia have announced jointly developed antenna options that integrate 5G active midband antennas with passive antenna elements for lower bands. The announcement follows a similar one from Ericsson in June, although CommScope and Nokia say their product is in fact the first to market, as it is commercially available now while Ericsson’s is planned for 2021.

Both product ranges come with “interleaved” versions, a method that combines antenna elements in a more integrated manner, rather than merely stacking one antenna on top of the other within the same enclosure. That means you can design a full length passive antenna and an active antenna in the same length product as the previous passive-only panel. With a stacked option, you had to shorten the passive panels to make space for the active unit within the same size enclosure.

“Operators…  are beginning to question the cost to performance benefits of 64T64R type massive MIMO”

But there’s also more to it than that, as Colin Bryce, Director of Mobile Network Engineering at CommScope, explains.

There are two aspects we’re beginning to find,” he says.  One, I think the operators, and it varies operator to operator, are beginning to question the cost to performance benefits of 64T64R type massive MIMO. Sometimes the performance does not always seem to live up to expectation when you hit the harsh realities of the world of the radio propagation environment.

“We’re not entirely sure as an industry why that is. Have we been a little bit over-optimistic on the modeling? I think the world in general is beginning to understand that complex mathematical models are simply a representation of the world, they are not reality. We’re seeing that with climate change and Covid modelling, and the radio propagation environment is a hugely difficult system to model. So is it that? Or is it that we still haven’t deployed the full algorithms for the full capabilities for some of these solutions?

“So a lot of operators are looking at other types of solutions, 32T/32R, 8T/8R type solutions – and our view is the operators need a toolbox of different antenna and radio solutions to deploy 5G cost effectively and efficiently.”

That’s the unique aspect. It’s the first time we have delivered m-mimo capability with this interleaved technology allowing a full column height in the low band arrays

The second aspect is the practical reality of deploying additional products at cell sites. So by and large operators like to re-use their current cellsites, meaning they are trying to deploy 5G onto cell sites that were designed around a 1800 or 2100 MHz propagation model.The problem is m-mimo antennas are quite heavy, they are quite power consuming and they add surface area to the top of tower which increases wind load. So the operators are asking if there are practical ways of deploying this technology that takes some of these these problems away. And one of the things is trying to integrate 5G capability with standard passive antennas.” 

“Passive antennas will cover lowband frequencies (600-900 MHz) and then there are some midband arrays in current products for 14-2600 MHz. And over the past ten years we have seen integration of more antenna arrays, more port count, more bands to be able to deliver all these technologies into a single passive antenna. The question was then, can you also include beam forming arrays with those passive antennas?

“Previously, integrating the massive mimo antennas into a single box was effectively a conjuring trick. We took a passive antenna and added the m-mimo on top of it and made it look like a single antenna. So for zoning or planning permission, or from the site owner’s point of view, the operator is not adding an extra box to the site. But the fact is that the antenna is two separate systems, one stacked on top of the other.

“Now the problem of that is in the low band. The gain of an antenna is [about] the laws of physics. You stack antenna arrays into columns, and the length of that column impacts the vertical width of the beam coming from the antenna – and as you narrow that vertical width you increase the gain of the antenna, you get improved propagation which improves coverage and also capacity, because it also improves the signal to noise ratio of the antenna.

“There tends to be a limit on the width and height of antennas, so they usually come in 1, 1.5, 2 and 2.5m metre lengths. But if the top of the antenna for these massive mimos tends to be at 0.5m and you have a given  permission to use a 2m antenna on a tower, then effectively to use this use topbox solution you would have to reduce the passive antenna by 0.5m. And that reduces the gain and performance of all your low band radios. And operators don’t want that. They want to get the same performance in lowband that they had previously.”

Faced with this issue, antenna designers came up with interleaving the active elements with the passive elements.

Bryce: “Interleaved technology allows us put the high band arrays behind the low band arrays, and yet the lowband is almost transparent to the high band. We use ‘choking and cloaking’ filtering technology that goes into the low band arrays and makes a band pass, effectively, for the higher band. And this allows you to deploy  a full height lowband antenna within this complete solution.

“So you can deploy a single box on the tower that gives you the midband performance that you had before, gives you the ability to deploy a m-mimo antenna up to 64T/64R or 32T32R or an 8T/8R beam former antenna – and deploy those behind the lowband arrays which still run the full length of the antenna and give you that higher gain performance that you need for low band coverage. 

“That’s the unique aspect. It’s the first time we have delivered m-mimo capability with this interleaved technology allowing a full column height in the low band arrays.”

As Bryce said, the CommScope/Nokia passive base module comes in different port and antenna array configurations to cover a variety of deployment scenarios. The module, with interleaved low-band elements, is available in 8T8R and 64T64R configurations (a 32T32R version will become available in 2021). 

Beam steering

“We have utilised this interleaved technology in what we call simple beam formers, to create a capability of beam steering on an 8T/8R 5G radio,” Bryce said.

“So that would be a passive antenna with low and midband elements with up to four 3.5 or 2.6 GHz columns behind it. Each of those columns have dipoles, meaning they are cross polarised antenna arrays, giving you 8 radio feeds. But because those four columns are correlated half a wavelength apart you can do beam forming, deploying this interleaved technology in a  standard antenna and supporting up to 8T/8R.

“As soon as you go above 8T/8R, you need tighter integration, really integrating the antenna array and the radio with some sort of blind mate type connectors at the back.  You’re not running 64 cables between a radio and an antenna. That’s why we tend to draw the definition of massive mimo at 16T/16R and above:  at 16T/16R you end up having to run so many jumper cables between a radio and an antenna that it starts to make sense to have those tightly integrated together.”

Bryce said that the work with Nokia has involved a lot of testing and development to ensure performance wasn’t impinged.

“A large part of this was the packaging of these two products, bringing them together, making sure we met environmental specifications and also running large sets of tests against performance, trying to validate both the legacy performance and the m-mimo and 5G beam forming We’ve run a real joint development process with Nokia, both in terms of developing the technology but also testing it.”

Passive-Active antennas and the market

Nokia actually started down the active-passive combo road with Kathrein in 2017/8, announcing development of a combined passive-active antenna enclosure for Orange. Orange was keen enough on the product to have made the announcement at its annual R&D update held in the Paris in early 2018. Huawei had its own antenna technology in place, and was already down the path of producing combined active-passive antennas. In fact Huawei first proposed its “antenna 1+1 strategy” in 2017: the idea at that point was to use one passive antenna to incorporate 2G, 3G, and 4G frequency bands and use one antenna to deploy 5G, such as C-band Massive MIMO.

By 2019, the market had changed somewhat, as Ericsson had bought Kathrein with, we were told, Kathrein’s passive antenna expertise one of the principal reasons for the purchase.  This left Nokia needing a new partner, which in this instance is CommScope.