AT&T has outlines its plan for 5G networks using millimeter wave technology at the 2016 Texas Wireless Summit. The company also demonstrated its 5G network technology with Ericsson. Arunabha Ghosh, the director of wireless communications at AT&T laboratories, said that the millimeter wave spectrum is plentiful but can be tricky to use. Due to noise limitations, power amplifiers are less efficient in millimeter wave networks than they are in lower bands. The answer to this problem? Placing cell sites very close to each other.
“If you want to use 500 megahertz of spectrum and deliver 100 megabits per second, you have to have the sites like 200 meters apart maximum, 100 meters for autonomous vehicles,” he said. “A city block in Austin is 200-250 meters. You are talking about several small cells deployed every block to support this 100 megabits per second that we need.”
A concentrated network of small cells will require more fiber, Ghosh said adding that in some desirable small cell locations, fiber may just be a couple feet away, but could still be very hard to access. Ghosh also mentioned that highways will need to provide support to support connected cars, and fiber is often unavailable near highways.
He said that 5G networks will need new network architectures, noting that millimeter wave offers large amounts of bandwidth and a number of the envisioned use cases will have considerable variability in the number of users. Therefore AT&T is beginning to look at architectures that use the same frequencies for access and over the air backhaul. The company also foresees multihub networks. Ghosh said that there are at least two possible ways to design these networks. The first of which would be true mesh networks that could let each node decide the route of the packet. However, Ghosh said that in this scenario, the worst case quality of service may be insufficient for some critical applications. The second approach would be to use the uplink and downlink to create a more hierarchical system.
Transitions will need to be much quicker in millimeter wave networks than in current networks. For millimeter networks, blocking is actually beneficial because the distances between access points are so short. When blocking does happen, traditional handovers won’t be able to be fast enough to keep the users connected.
“When you have blocking in mm wave the transition periods are much faster,” Ghosh said. “Once a base station starts to fade, you can do a handover in the networks we are used to. But not in mm wave … very fast switching is needed.”