"TDD (Time Division) Massive MIMO represents the only effective implementation of Massive MIMO at the frequency bands under consideration."!? So says Professor Erik Larssen, a leading researcher at Linkoping University in Sweden. Larsson blogged the controversial paper on April 4, the day after it was published at https://arxiv.org/pdf/1704.00623.pdf. The blog and the abstract of the paper is below.
On the other hand, China Telecom, China Unicom, Huawei, and ZTE have announced successful trials of FDD (Frequency Division) Massive MIMO. A joint press release by China Telecom and ZTE asserts the opposite: "The FDD Massive MIMO solution is predictable to be deployed in China Telecom in 2017." Obviously, I'm not qualified to judge engineers of that quality when they disagree. I'll do my best to explain the issues and direct you back to the principals.
The primary issue, as I understand it, is whether FDD overhead is inevitably too high for FDD to be practical. With line of sight (LOS), the two techniques appear to have similar results. Without decent Line of Sight. the new paper reports a significant difference.
Bristol and Lund showed what's possible in spectrum below 6 GHz, rather than millimeter wave (below.) That was in May, 2016. In 2017 as I write, production Massive MIMO systems are getting 3x-10 more capacity, a remarkable return for a relatively modest investment. Lund Professor Emil Björnson believes, "20x-40x improvements in spectral efficiency over IMT Advanced are what to expect from Massive MIMO."
Peak speeds in MM shipping systems for 20 MHz typically are > 400 megabits. The universities, working with National Instruments, achieved 145.6 bits/s/Hz on a single 20 MHz radio channel. Today's single channel LTE systems get 2-4 bits/s/Hz, a small fraction of what Massive can do.
Most systems building today use 60-80 MHz (4 bands.) That would allow speeds of well over a gigabit with multi-band Massive. As far as I know, no one has reported field results for more than 20 MHz on Massive. That should come shortly.
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.
TDD Massive MIMO is already going wide at SoftBank Japan, yielding a 4x-10x capacity improvement on a single channel. Eric Zhao believes Massive MIMO is often the right choice where you need high capacity. Zhao finds early 32 antenna system averaging 450% more capacity in a single band. Many leading engineers agree MM is the way to go.
Worldwide, chief technologists are enthusiastic about Massive MIMO. At Huawei's MBB Forum, Hidebumi Kitahara of SoftBank confirmed the reported results, especially in crowded cells. Later in the day, Huang Yuhong of China Mobile also reported success. Her performance improvement estimates were "only" 3x, but it's early days for the technology. At SoftBank owned Sprint, Günther Ottendorfer reports a 10x improvement. Sanyogita Shamsunder of Verizon a year ago told me, "We must have massive MIMO." Nasser Al Nasser of Saudi Telecom in the release below reports a 6x improvement.
4x4 MIMO is sweeping the world and 64 antennas Massive MIMO is starting to deploy by the thousand. Huawei also offers a middle ground: Eight antennas. They work with ordinary phones that only have two antennas. That's a big advantage since phones with four antennas are just hitting the market and won't be common for several years.
The eight antenna improvement over 2x2 is "almost double" according to Zarrar Khan, CTO of Kuwait VIVA, a rapidly growing telco controlled by Saudi Telecom. It offers significantly more capacity than 4x4 rigs. In comparison, Huawei reports 4x4 testing by MTN, the South African giant, improves downlink improvement of 74% and uplink 39%.
Important caveat: MIMO results are drastically different depending on the terrain.
Massive MIMO is the next great step in mobile networks. Huang Yuhong of China Mobile and Hidebumi Kitahara of SoftBank are deploying hundreds of nodes and reporting performance improvements of 2X to 10X. Sprint in the U.S. and 3 in Austria are firmly committed. Gig LTE - 4x4 MIMO, 3/4 band aggregation, and 256 QAM is spreading rapidly in 2017; the next step will be the 64+ antennas and beamforming of Massive MIMO, some at millimeter wave. I had a chance to ask some experts a few questions so put together these. Others welcome.
in 2018-2019, what's a smart Massive MIMO deployment strategy for a network with growing traffic?
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