Massive FD-MIMO technology is proven in the field – will Distributed FD-MIMO be next?

In 5G news nowadays, the Massive MIMO technology (also known as Full Dimension MIMO or FD-MIMO in 3GPP jargon) appears frequently because of the excitement around its network capacity and enhanced user experience benefits. Massive MIMO is a prime example of how the industry stakeholders get together, take an initial theoretical concept and transform it to a mainstream 5G technology in less than a decade. Such innovation speed is no less than warp speed if one considers the typical pace of evolution in wireless space!  

From the simulations and early tests, it was clear the network capacity of LTE caps out at 1.5-2 bits per second per Hertz (bps/Hz) in a typical urban cell, after averaging across near and far users. As mobile data demand continues its exponential rise, many proposals were made to improve this capacity further, including relay, carrier aggregation, small cell, and multi-user MIMO (MU-MIMO). MU-MIMO reuses the same radio resources for different users in a given cell, significantly increasing the overall cell capacity or throughput. However, this normalized bps/Hz capacity number did not seem to budge much.

Around that time, the concept of Massive MIMO was proposed in academic papers. These papers proposed the idea of making the signal dimension at the base station much bigger by using a massive number of antennas such that all inter and intra-cell interference asymptotically go to zero. MU-MIMO performance would be improved significantly with a much lower interference level, therefore leading to capacity gain. It looked promising, but no one knew how to bring it to reality, since arranging 10s or 100s of antenna elements in the conventional way (i.e., in the horizontal plane) would lead to a base station that is longer than a bus, so obviously it was not going to work in a real deployment.  

An important breakthrough came when engineers at Samsung noticed that a concept called Active Antenna Systems (AAS), could be exploited to organize 64 or 128 antennas into a 2D active antenna array that is similar in size with a conventional 4-TX system as shown in the middle portion of Figure 1. Such a system is called a Full Dimension MIMO (FD-MIMO) system. Initial evaluation of the FD-MIMO system coupled with high-order MU-MIMO showed a capacity gain by a factor of 3-4 times for a 64 or 128-TX FD-MIMO compared to a 2-TX LTE system, as was summarized in a 2012 Globecom paper , “Fulfilling the promise of massive MIMO with 2D active antenna array”, and later in a 2013 IEEE magazine paper , “Full-dimension MIMO (FD-MIMO) for next generation cellular technology”. 

Samsung has been actively leading the FD-MIMO standardization process in 3GPP from the beginning, including the 3D channel model study in 2012 that paved the way for subsequent system design, the 4G LTE version of elevation beamforming and the FD-MIMO work from 2014, and more recently the 5G NR-MIMO version of FD-MIMO. Samsung has also been a leader in prototyping and testing the feasibility of the technology and was the first to demonstrate an FD-MIMO system supporting 12 simultaneous MU-MIMO users achieving a record aggregate capacity of > 20 bps/Hz in early 2015. These feasibility study result was later published in a 2017 IEEE JSAC paper , “Full Dimension MIMO (FD-MIMO): demonstrating commercial feasibility”.

Fast forward to the present day of year 2020, 5G is front and center in the news, and the whole world is excited by the transformative potential of this new generation of wireless technology. In countries that are actively deploying 5G networks in sub-6 GHz spectrum (formally called Frequency Range 1 or FR1) such as South Korea, Japan, and United States, Massive MIMO is increasingly becoming the default choice of radio technology. Things are looking good on the technology side, but for those of us striving for even better performance, the natural question is: what’s next? Can we continue the innovation stream and come up with another success story similar to FD-MIMO to take the industry forward to the next level?

One performance enhancement idea emerged when we examined the pain point of the current FD-MIMO technology for the premium sub-1 GHz band. In this band, the wavelength of the RF signal becomes too large for a large-scale antenna array system like FD-MIMO to be feasible. A FD-MIMO RF Unit (RFU) at 700MHz would roughly be the size of a Mini Cooper car. It is impractical to deploy such bulky antenna system at the tower top, due to excessive weight and size.  

To bring the benefit of FD-MIMO to low frequency bands, one promising idea that came to our attention is the Distributed FD-MIMO (D-FD-MIMO) technology. In an implementation example shown in the figure below, we break down a large 32-antenna FD-MIMO system into four modules (similar to the Lego toy pieces, and hence the nickname Lego MIMO!). The Lego MIMO modules have much smaller size and they can be deployed far apart. 

Initial system level simulations show that the D-FD-MIMO system achieves up to 2 times cell average throughput gain compared to the FD-MIMO system, lifting both cell capacity as well as average user throughput. Such a cellular system can be flexibly deployed to “blanket” a given geographical area and provide better service for both outdoor and indoor users. 

We have developed a hardware prototype and performed field test to verify the feasibility and the performance gain of the D-FD-MIMO system. In the field test, 3 distributed LEGO MIMO RFUs and 7 UE emulators were used. When the number of active RFUs increased from one to three, the overall throughput improved by about 4 times.

A significant amount of work needs to be done before we can accurately quantify the benefits of the D-FD-MIMO technology, but these initial results are certainly promising and show a great potential for this new breakthrough of the MIMO technology.

There has been a huge potential in Massive MIMO technology and Samsung is proud to be at the forefront of its development. We will remain committed in advancing the technology and look forward to seeing what is ahead of us as we continue to strive for technology innovations.