A Glimpse into the Future of Massive MIMO

In recent ABI Research insights, we wrote about The Transformative Potential of Massive MIMO (IN-5420) and The Massive MIMO Innovation Pipeline (IN-5549) and outlined the benefits and challenges of massive Multiple Input, Multiple Output (MIMO) and future massive MIMO research and development areas.

One promising research area concerns Extremely Large Aperture Arrays (ELAAs), often called distributed massive MIMO or cell-free massive MIMO. In this insight we take an in-depth look at the topic as the future of massive MIMO.

The motivation behind ELAAs relies on the spectral efficiency of the array increasing as the number of antenna elements increases, and since, according to the Shannon-Hartley theorem, spectral efficiency dictates sector throughput, having hundreds or even thousands of antenna elements should drive orders of magnitude increases in sector throughput compared to conventional “compact” massive MIMO.

Distributed massive MIMO enhances network efficiency by leveraging this high spectral efficiency and scalability since intercell interference is drastically minimized thanks to channel hardening (since many antennas are now more likely to be close to the end user); link reliability and spectral efficiency becomes limited only by signal propagation losses.

Cell-free massive MIMO is a form of distributed massive MIMO that implements coherent user-centric transmission to overcome the intercell interference limitations in a conventional Radio Access Network (RAN) and provides macrodiversity and system scalability. With the antenna elements distributed in the RAN in this way, the user is surrounded by antennas—instead of the more conventional scenario where the antenna is surrounded by users—so that highly desirable line-of-sight propagation conditions apply to the radio channel and cell boundaries are eliminated.

One of the main challenges to building the distributed massive MIMO RAN has been the high cost of deploying large numbers of antennas, and since all the antennas need to be phase synchronized and have access to the same data, a lot of high capacity fronthaul cables are needed—particularly if a star topology is used for the RAN topology—and is impractical. Alternatively, by connecting the fronthaul in series, the complexity is reduced; however, signal processing must also be distributed with each of the antenna elements. This type of arrangement is called cell-free serial massive MIMO

Ericsson is researching how to do this in practice by disaggregating the antenna elements from inside the conventional antenna enclosure and placing them on a building’s outside windows or placing them on walls or ceilings indoors, for instance—much like today’s Distributed Antenna Systems (DAS). The result—called radio stripes—is a tape, which can be many hundreds of meters long, containing the antenna elements. In the following section we examine how Ericsson plans to do this in practice.

At the 2019 Mobile World Congress (MWC), Ericsson unveiled the radio stripe concept.

As can be seen in the figure below, Ericsson has placed the antennas and Antenna Processing Units (APUs) in the radio stripe and has connected them in series inside the cable. The cable provides synchronization, data transfer, and power supply to the APUs, and each radio stripe is then connected to one or more baseband Central Processing Units (CPUs). More specifically, the stripe contains antenna elements and circuit-mounted semiconductors, including power amplifiers, phase shifters, filters, modulators, and analog-to-digital and digital-to-analog converters inside the protective casing of the stripe.

With this concept a series of radio stripes can be deployed in much the same way as decorative lights are deployed. In this way, many radio stripes share one common fronthaul connection to the baseband CPU. Because there are many antenna elements in the radio stripe, the hardware can be constructed using low-cost smartphone-grade hardware.

Radio stripe operation in Time Division Duplex (TDD) mode and its inherent distributed signal processing allows the system to scale up to very large numbers of antenna elements. Since channel state information and beamforming calculations occur locally in the APUs, there is no need to transfer large amounts of data over the fronthaul to the baseband CPUs, and fronthaul overhead demands are reduced.

While this remains a concept for the moment with Ericsson proceeding to plan for product implementation, we believe that Ericsson’s radio stripes show enormous promise for the future of antennas, particularly for indoor and venue coverage in locations including stadiums, factories, and enterprises. With the advent of 5G in the sub–6 GHz and millimeter wave bands, the radio stripe promises to be an elegant and low-cost method of bringing the benefits of massive MIMO indoors, thus avoiding the power, cost, and aesthetic challenges inherent in conventional compact massive MIMO approaches.

ABI Research looks forward to hearing more about this approach in the future.