Tom Marzetta and other researchers have been very skeptical about the performance of Massive MIMO using FDD (Frequency Division Duplex) rather than TDD (Time Division.) Massive MIMO requires regular and robust reporting from the cell phone, which they feared would be impractical with FDD. Huawei, ZTE, and now Ericsson have done field trials they believe prove otherwise.
In trial with T-Mobile. Nishant Batra is confident many antenna Massive MIMO will add to performance in FDD frequencies as well as the TDD frequencies beginning to widely deploy. While the performance gain may not be exponential, directing the beam significantly increases capacity even in FDD systems.