128 antennas, 6X-10X improvement in throughput to ?1.5 gigabits. The fearless Masayoshi Son wants to change the world again, this time by launching 5G. They have deployed 100 cell sites in 43 cities, with many in Tokyo. He may be a year ahead of anyone else. Masa-san wasn't patient enough to wait. (I'm mostly working in Google translation from the Japanese, included below. I'm sure I'll be updating a few details. The Softbank folks aren't answering my emails.)
Softbank subsidiary Wireless City Networks is taking charge. They may be using the ZTE 128 antenna unit demoed last year. Cell siting is particularly difficult in Japan and MIMO is the best way to get much more out of your existing network and spectrum. On the other hand, Softbank controlled Sprint has 120 MHz of unused spectrum. Sprint's logical path forward begins with carrier aggregation putting that spectrum to use.