BOSTON -- Wireless and cable operators may see millimeter wave spectrum bands as a key element in the 5G roadmap, but questions about network propagation and interoperability with existing LTE services remain.

While higher band millimeter wave bands such as 28, 39 and 60 GHz bands offer higher bandwidth and throughput, the differences in topology can affect how they perform, particularly in dense urban environments.

In particular, wireless operators will see issues with delivering services if they are in a dense city area where there a number of potential obstructions from buildings and other sources exist.

"Millimeter wave actually does travel far and if you do have an open area and no obstacles, 28 GHz will propagate well," said Belal Hamzeh, director of network technologies with CableLabs, during a FierceWireless event collocated with the INTX trade show. "If you are in a dense area like Boston or downtown New York, you have a lot of obstacles and that's where millimeter does fail."

Hamzeh added that "you will need millimeter wave for 5G capacity, but to be able to provide necessary coverage, you're going to have to have many small cells."

With plans to offer a point to multipoint fixed 5G service sometime next year, C-Spire Wireless will use a mix of frequency bands to address low and high bandwidth needs.

"We all operate multiple frequency bands from a wireless perspective and one spectrum band does not cover everything and we have to deliver low bandwidth and high bandwidth," said Steven Bye, CTO of C-Spire. "Customers expect ubiquity of coverage whether they are in the Delta or New York City so the networks are heterogeneous by nature."

C-Spire continues to work on improving its growing 4G LTE network. The service provider currently delivers service to 1.2 million customers in Mississippi, the Memphis metropolitan area, the Florida Panhandle, and parts of Alabama including Mobile.

Bye said that any operator migrating to 5G will gain a complementary network that will coexist with the existing 3G and 4G networks already in place.

"We're still maturing our LTE network and there are still years of functionality to make it better so 5G is on top of that LTE network to take advantage of new technologies like MIMO to give us improved performance and lower latency," Bye said. "For us even in the fixed context, it's a complement because we still operate a 4G network and a 3G network and what's critical will be the ability for customers to move between those domains."

By added that while 28 GHz is limited "in terms of propagation from that last 2-500 feet from the fiber node to the side of the house so when subscribers go out of the house you could use Wi-Fi and unlicensed because that's an important delivery mechanism for data services."

Meanwhile, T-Mobile (NYSE:TMUS) which has never been afraid to try new technologies, said LTE will continue to be important in moving to 5G.

"Operating 5G and 4G at the same time enables us to make best out of both and that is another illustration of how 5G is going to complement and extend LTE," said Karri Kuoppamaki, VP, network technology development and strategy for T-Mobile, US.

Ericsson (NASDAQ: ERIC) acknowledged that while the propagation is an inevitable challenge with millimeter wave wireless, the benefit for wireless operators is that it requires smaller antennas.

This will allow operators to better mitigate interference issues in challenging urban environments and provide consistent performance to users who don't care what access technology their operator is using.

"You could do a lot more things in terms of beamforming and do very focused beams and that helps with reducing your interference in the network and increases the performance," said Manish Jindal, head of technology and strategy development for Ericsson in North America. "What we have seen in our simulation results is when you make a 4G and 5G network interwork with one another we have seen that the combined networks will perform way better than performing independently."

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