A lot of the wireless companies' 5G experimentation is taking place in super-high frequencies -- as high as 73,000 MHz. Today's cell phone networks broadcast signal in a range of 700 MHz to 3,500 MHz.
The advantage of high-frequency signals is that they're capable of providing significantly faster data speeds. The disadvantage is that they travel much shorter distances and they can't easily penetrate walls. That means thousands -- perhaps even millions -- of mini cell towers, or "small cells" would need to be placed on top of every lamp post, every building, inside every home and potentially every room.