T-Mobile Netherlands and True Thailand choose 8 antennas for FDD spectrum, joining Kuwait in choosing Huawei gear. 8 antennas will rarely deliver twice the throughput of four antennas but will often deliver a 50% increase in capacity. 32/64 antenna Massive MIMO has been chosen by BT, DT, & FT but that is in higher frequencies running TDD.
32 antennas for 700 MHz spectrum would be too large to be practical most places. 256 antennas at mmWave frequencies are the size of a large chip and can go almost anywhere. 64 antennas at mid-frequencies like 2500 MHz need 1.5 to 3 meters, practical on most towers. Antenna size goes up with lower frequencies, making 32 antennas at 700 MHz a challenge and often impractical at 1800 MHz.
In addition, most frequencies below 2000 MHz all dedicated to FDD transmission, with upstream and downstream in separate bands. This produces problems with upstream signaling for Massive that lets the base know the state of the mobile and other receivers.
The base needs to know the ever-changing.status of each receiver to perform the MM magic.A truck or even a person passing between the base and receiver can result in a call drop with MM. As Apple discovered in an iPhone model, even a change in how you hold the receiver than cause a problem.
Professor Erik Larsson believes the FDD problems are unsolvable but all three MM vendors offer an FDD system. One solution is to use a codebook to send upstream just a pointer, reducing congestion. I have several papers with other ingenious approaches. Until we get results from units in the field with numerous connections, we won't have clear answers.
"Don't go beyond 8 antennas for FDD for now," a very senior engineer believes.