T-Mobile's "5G" network will be built in 600 MHz spectrum and will only be 25-50% faster than its 4G LTE network, perhaps 400-700 megabits. Ted Rappaport of NYU Wireless, a world-class expert on 5G, emails me:
The low band (600 MHz) systems will always be limited in capacity and are not future proof no matter what special massive Mimo is used- the antennas will be too large and the bandwidths are too small, and simply can't carry the bandwidths of mmwave-- the RF channel allocations are simply too small. The wider bandwidth channels at mmwave is the only way to carry the multi Gbps data rates that will be seen in 5G. And to simultaneously also provide backhaul between cells in the same network.
The secret sauce in millimeter wave is 400 MHz and 800 MHz channels. The 600 MHz and other low band use 20 and 40 MHz. Tmo' 600 MHz will probably be 400-700 megabits. mmWave is delivering 10gig and 20 gig in the laboratory. Verizon is getting 1 gig to each customer in their trials and can go higher.
In 4G LTE/LAA, Tmo's 4G is delivering 500 megabits to live customers in Manhattan, pretty darn good. To do much better than that in 20 MHz 5G NR would require breaking the Laws of Physics.
The difference between 4G LTE and 5G NR is actually very small in a fair comparison: same spectrum, same antennas, same generation of equipment. The 2018 LTE goes beyond the gigabit delivering 9 ms latency. Short TTI is in the LTE Release 14 and 5G Release 15. It's most of the latency improvement and it works in 2018 models of both.
Ted also points out the antenna problem. mmWave antennas are very small; 256 of them fit on a unit the size of a paperback novel. (We have it on film we still need to edit.) Antennas for 600 MHz are much bigger, often only two fit on a tower. When you get to 2500 & 3500, you can use 64 antenna Massive MIMO. That's why 3500 suddenly became hot. Massive MIMO is working very well and solves most of the very short reach problem.
The NY Times is reporting "implausible promises" because they are too cautious to say "lies."