Behind every good 5G signal today is a 4G connection that's often working just as hard to deliver data to a user. If Batman is 5G, then Robin would definitely be 4G.

At least, that's how Sprint and a number of other operators have configured their networks. And the result, according to one of Sprint's top networking executives, is a user experience that's better than either 5G or 4G could provide on its own.

"We have seen the benefits of dual connectivity," Herkole Sava, director of technology, innovation and architecture at Sprint, told Light Reading. "This was fundamental to the 5G strategy of Sprint."

To be clear, Sprint isn't the only operator making use of "EN-DC," which is the technology that supports simultaneous transmissions across both 5G and 4G. But the operator's unique 5G deployment across 16 million users helps to highlight not only the reciprocal relationship between 4G LTE and 5G NR but also the core reasons behind the global wireless industry's move into a new generation of network technology.

It's not either/or, it's both

EN-DC is a part of the 3GPP's initial 5G standards, which means it's not exclusive to Sprint and can be used by a variety of wireless network operators, including T-Mobile. Indeed, today's 5G networks require a 4G network to function. That's because all of today's initial 5G deployments use the "non-standalone" (NSA) version of 5G, so named because the 5G part of the network can't stand up without a 4G crutch.

The "standalone" (SA) version of 5G is scheduled to be released early next year, and will allow operators to launch 5G without a 4G network handling things like authentication and cell site handoff like they do in NSA today.

But while today's NSA version of 5G requires 4G for some base functions, it also opens the door to using 4G for a speed boost as well. That's exactly what Sprint is doing, with noteworthy results. For example, consulting and testing firm Signals Research Group recently assessed Sprint's network in Chicago and found that, in one case, their LG V50 5G smartphone was simultaneously clocking around 200Mbit/s on Sprint's 5G connection and around 40Mbit/s on its LTE connection. The result was a total top speed of 240 Mbit/s.

And that's just the start, said Sprint's Sava said. "We have further optimized the markets" since those tests, he noted.

A unique 5G

Sprint's 5G is not like the 5G from Verizon, AT&T or T-Mobile. Whereas all three of those carriers are launching 5G in millimeter-wave (mmWave) spectrum first with plans to expand it to lower spectrum bands in the future, Sprint is leveraging its vast 2.5GHz holdings for a 5G network that's more like the ones in China and South Korea than the ones in the US. That's because 2.5GHz is midband spectrum that toes the line between covering large geographic areas and transmitting lots of data. Conversely, mmWave is a highband spectrum that can't cover large geographic areas, while a lowband spectrum like T-Mobile's 600MHz can't transmit as much data.

Moreover, Sprint has a lot of 2.5GHz spectrum. Like, roughly 120MHz across most major US metro areas. To put that into perspective, most operators like AT&T and Verizon typically deal with spectrum blocks in 10MHz and 20MHz chunks (though they have cobbled together a lot of those chunks over the years). The more spectrum a carrier has, the faster their network can go.

For its network in Chicago, Sprint is allocating three 20MHz channels for LTE (for a total of 60MHz) and a full 60MHz for 5G. Importantly, Sprint is using a technology called "carrier aggregation" to essentially glue its LTE channels together for faster download speeds -- but it doesn't need to do that with 5G because the 5G standard was designed to handle channels 60MHz and wider.

A/B testing for 5G

Because Sprint is devoting equal amounts of spectrum to 5G and LTE, the operator is creating an almost perfect A/B test for the efficiency of 5G. A/B testing is a way to compare two versions of a single variable and is used across a wide range of industries to find efficiencies. In this case, Sprint's network can show exactly how much more efficient 5G is when compared with 4G.

So how good is 5G? Sprint isn't saying.

"Our results are positive," Sava said, explaining the carrier isn't ready yet to provide specifics. "What we've seen so far is extremely encouraging."

But the network testing done by Signals Research Group in Chicago recently offers some clues: "The big silver lining from this study is that 5G delivered a significant performance advantage over LTE, especially when network loading existed on the LTE network," the firm wrote. "A smartphone with 5G capabilities can easily surpass the capabilities of an LTE-only smartphone."

Putting it all together

So, given Sprint's setup, it's reasonable to assume the operator is providing blazing fast speeds thanks to the 4G-5G dynamic duo, right? Well, the speeds are fast, yes -- SRG recorded peak speeds of 561Mbit/s on Sprint's Chicago network -- but the details were messy.

For example, the firm found that one test that lasted just three minutes showed a connection that bounced across several different spectrum bands and in and out of 5G.

Both SRG and Sprint's Sava noted that Sprint's LTE network in Chicago is pretty congested and that Sprint was still tuning its 5G network during the time of the tests. "In theory, the peak speed could have been at least 2x higher with 60 MHz of LTE," SRG noted.

One more thing: dynamic spectrum sharing

EN-DC is available to all 5G operators, but unused spectrum is not. As Verizon, AT&T and T-Mobile work to expand their 5G networks into their lowband spectrum holdings, they will need to contend with the fact that many of those bands are already working overtime with 4G LTE connections.

That's why they're all planning to use "Dynamic Spectrum Sharing" (DSS) technology to introduce 5G into their 4G spectrum bands. DSS promises to allow operators to transmit 5G and 4G signals in the same spectrum band by basically allowing 4G and 5G users to take turns using the exact same chunk of spectrum in 1-millisecond increments.

This type of dynamic sharing is a big change from how the wireless industry transitioned from 3G to 4G. In that transition, operators had to carve out a chunk of spectrum that was dedicated to 4G, regardless of how many 4G users they actually had.

But according to Sprint's Sava, DSS is not a technology Sprint will implement in the near term because it already has 60MHz of clear 2.5GHz spectrum for 5G. And, he said, it a technology that does have drawbacks.

"Dynamic spectrum sharing comes with a penalty and a cost in terms of spectrum efficiency," Sava said, explaining that the application of DSS results in a 20% loss of spectrum efficiency, mainly on the LTE side. DSS "doesn't come for free... Not many people talk about this."

Finally, in referencing a Light Reading article from earlier this year detailing the highly questionable usage of the 5G signal icon among most operators, Sava said that Sprint is "taking the high road" and would only display its 5G signal icon when the service is available and accessible.

 Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano