[T-Mobile Note 7  Guides, News, & Discussion] milan03:
I did ELI5 in another post, but I'll drop it in here as well.

Original post is actually here: https://www.reddit.com/r/tmobile/comments/4wtsdy/galaxy_note_7_the_first_4x4_mimo_capable/

- Up until now, all LTE smartphones had 2 Receive antennas in order to match the amount of Transmit antennas at the cell site, which sends up to two independent "spatial streams" in each transmission. In other words, when your signal is good, your phone is able to receive and demodulate both of those streams which provides faster download speeds.

- Over the past several years, T-Mobile begun equipping their LTE cell sites with 4 Transmit (and receive) antennas, which added additional diversity. Think of it as an additional set of "ears" at the cell site, able to "hear" your phone better since our phones transmit power is many times lower than Tx power at the cell site.

- Even though T-Mobile still sends out two spatial streams from the cell site, additional diversity allows operators to transmit data more efficiently, and essentially prepared operators like T-Mobile for the next step, 4x4 MIMO. This also helps with things like VoLTE, as the added antenna diversity improves overall signal resilience.

- In the meantime, there's been an ongoing struggle for OEMs to figure out how to implement 4 LTE Receive antennas in such a small smartphone form factor, and achieve acceptable antenna separation and isolation, and mitigate the interference. At the same time, in order to achieve the highest efficiency gain, signals that are coming into such a small device have to be uncorrelated, leveraging fading and multipath. I won't dive into that today.

- So adding 4 Receive antennas in the smartphone, just like T-Mobile has done at the cell sites, Samsung is effectively adding a set of "ears" and creating that same diversity on the user's end, which benefits not only users, but also operators as they're now delivering data more efficiently, especially to devices that happened to be in poor signal conditions.

- The best part happens when both the operator has 4 Transmit and the smartphone has the matching 4 Receive antennas. When that happens an operator can roll out 4x4 MIMO (Multiple Input Multiple Output) LTE feature, which effectively doubles download speeds, assuming good signal conditions, on top of improved resiliency when in sub-optimal signal conditions.

- So in T-Mobile's or Verizon's case, they typically have 20MHz Band 4 spectrum deployed for their LTE. In the existing 2x2 MIMO use case that yields up to 150Mbps peak downlink data rates, in good signal conditions.

But once they activate 4x4 MIMO, speeds double up to 300Mbps out of the same 20MHz Band spectrum, without operators deploying additional spectrum resources.

- Even in less the optimal signal conditions, in places where you'd see 5Mbps speeds, we should be able to see improved speeds because our phones now have more antennas and are able to maintain that connectivity with the cell site.

It's easy to see that we end up winning on multiple levels now that 4Rx antenna implementation in a smartphone became a reality. I plan on reviewing LTE Performance in Note 7 and other smartphones on my site http://cellularinsights.com so keep an eye on it if interested in this kind of stuff.