Only Massive MIMO in TDD spectrum has gone beyond testing but China Telecom, Sprint, Huawei, and ZTE are confident commercial FDD products are close. Their press releases about FDD tests are enthusiastic. However, systems based on TDD (Time-division duplexing,) are deploying by the hundreds and soon by the thousands, but FDD (Frequency-Division duplexing) systems are just in early testing. The choice between FDD and TDD is driven by spectrum availability. The majority of spectrum in use is FDD.
Massive MIMO requires constantly updated Customer State Information (CSI.) The cell needs constantly updated receiver location and capability to steer the many antennas. The overhead is considerable, possibly too high to use FDD in many places. FDD systems appear to have lower capacity but there is almost no data from the field.
In a 2015 paper by the inventor of Massive MIMO and two colleagues, abstract below.Key experts Emil Bjornson, Erik Larsson, and Tom Marzetta point to what they suggest is a typical example. CSI requires about 12.5% of the capacity in TDD. FDD would have overhead four times as high, enough to raise questions about whether TDD systems could ever become cost-effective. Their chart is above and a larger version below.
Based on data then available, they observed, "It is still unclear to what extent Massive MIMO can be applied in FDD mode. ...It appears that FDD can only support Massive MIMO in special low-mobility and low-frequency scenarios." That is a discouraging answer to the crucial question, "Can Massive MIMO work in FDD operation?"
They clearly hope to be proven wrong and call for "intensive research activities and channel measurements." They point to "proposed methods to reduce the overhead. Generally speaking, these methods assume that there is some kind of channel sparsity that can be utilized; for example, a strong spatial correlation where only a few strong eigendirections need to be estimated or that the impulse responses are sparse in time. While methods of these kinds achieve their goals, we stress that the underlying sparsity assumptions are so far only hypotheses. ...We encourage researchers to investigate this thoroughly in the coming years, to determine if any of the sparsity hypotheses are indeed true or if there are some other ways to reduce the overhead signaling."
Another promising technology is an analog 96 antenna box from Blue Danube. AT&T has invested and has good results from the analog system.
Bjornson, Larsson, and Marzetta deserve a strong answer. Marzetta invented Massive MIMO. Larsson and Bjornson are Swedish professors who have led a large EU research project. Larsen was Marzetta's co-author on the textbook.
They call TDD vs FDD the "One critical question."
Massive MIMO: Ten Myths and One Critical Question
Emil Bjornson, Erik G. Larsson, and Thomas L. Marzetta
Abstract—Wireless communications is one of the most successful technologies in modern years, given that an exponential growth rate in wireless traffic has been sustained for over a century (known as Cooper’s law). This trend will certainly continue driven by new innovative applications; for example, augmented reality and internet-of-things. Massive MIMO (multiple-input multiple-output) has been identified as a key technology to handle orders of magnitude more data traffic. Despite the attention it is receiving from the communication community, we have personally witnessed that Massive MIMO is subject to several widespread misunderstandings, as epitomized by following (fictional) abstract: “The Massive MIMO technology uses a nearly infinite number of high-quality antennas at the base stations. By having at least an order of magnitude more antennas than active terminals, one can exploit asymptotic behaviors that some special kinds of wireless channels have. This technology looks great at first sight, but unfortunately the signal processing complexity is off the charts and the antenna arrays would be so huge that it can only be implemented in millimeter wave bands.” The statements above are, in fact, completely false. In this overview article, we identify ten myths and explain why they are not true. We also ask a question that is critical for the practical adoption of the technology and which will require intense future research activities to answer properly. We provide references to key technical papers that support our claims, while a further list of related overview and technical papers can be found at the Massive MIMO Info Point: http://massivemimo.eu