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R&D / Technology – Massive-element antenna systems technology for 5G base stations

Using our own ideas and technology to make the fifth-generation mobile communications a practical reality

The widespread adoption of smartphones has seen a dramatic increase in mobile communications traffic in the last few years. There is little doubt that the pace of this growth will accelerate in years to come, and research into the fifth-generation (5G) mobile communications has been proceeding apace as the world looks for ways to cater to this demand. The expectation is that 5G communications will be ready to use from 2020 onwards, and advances in a range of technologies are essential for that to happen.

At Mitsubishi Electric, we are engaged in research into "massive-element antenna systems technology for 5G base stations". In 5G technology, a composite beam made up of transmissions from arrayed multiple antenna elements is sent to each terminal to create a fine communications environment. However, the problem with using existing technologies is that the transceiver becomes so large that it is virtually impossible to set up base stations. With revolutionary technology that uses an "analog-digital hybrid configuration" capable of meeting the need for extreme miniaturization, coupled with highly integrated RF circuitry, Mitsubishi Electric has been able to build massive-element antennas feasibly. And using multibeam multiplexing technology that provides data transmission speeds 50 times faster*1 than current rates and nonlinear multi-diagonalization precoding technology capable of providing stress-free communication service to individual users even in crowded cities, we have achieved our goal of building a system that makes 5G communication a reality.

Society is moving towards an IoT era in which devices of all kinds are connected to networks. In mobile communications, of which importance is increasing as social infrastructure, the visionary ideas and advanced technologies offered by Mitsubishi Electric are providing solutions that will move society forward.

The outcomes of these advances include a part of the results of "The research and development project for realization of the fifth-generation mobile communications system" commissioned by Japan's Ministry of Internal Affairs and Communications (MIC).

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Massive-element antenna as the key to 5G communications using high-frequency bands

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5G communications are expected to use high-frequency bands such as the high-SHF band*2. High-frequency signals by their nature are particularly prone to propagation attenuation, which tends to limit their effective range. To combat this, we are investigating ways to direct narrow beams by concentrating the signal's energy, rather than the broadcast signal transmission methods used in existing base stations. In a massive-element antenna array, the beam can be formed by combining the radio waves emitted by individual elements, and its direction can be controlled by regulating each of the radio waves.

5G also requires a wide frequency band to provide high-speed communications. But available frequency resources are limited and for realizing high-speed communication in limited bandwidth, the problem must be approached from a different angle. That is where spatial multiplexing comes in. Using narrow beams that target the respective terminals, different data for each terminal can be transmitted in parallel, thereby achieving spatial multiplexing in radio communications. 5G attempts to spatially multiplex more signals than the fourth-generation (4G) mobile communications, making higher capacities possible. Massive-element antenna arrays in which large numbers of antennas are clustered together allow highly accurate beams to be generated, which in turn enable extremely dense spatial multiplexing. So by transmitting signals in high-frequency bands over long distances and providing high capacities through spatial multiplexing, a massive-element antenna provides the solution to the two problems that beset 5G communications.

At Mitsubishi Electric, we are taking the Active Phased Array Antenna (APAA) technology we have been developing for artificial satellites and using it to build massive-element antennas that enable high-precision beamforming.

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Aiming to find out just how small a massive-element antenna can be

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For a massive-element antenna to be usable in an urban 5G base station, compactness is essential. To achieve this, the high-SHF band massive-element antenna being developed by Mitsubishi Electric employed a new approach known as the analog-digital hybrid configuration. In the many existing fully digital configurations proposed for low-SHF band multi-element antenna systems, a set of digital signal converters, from analog to digital (ADC) and from digital to analog (DAC), was needed for each antenna. If we used a fully digital configuration for the high-SHF band massive-element antenna system, we would need an ADC/DAC set for every antenna element, making the resulting device too large, too power-hungry and too expensive to be practicable.

But by grouping multiple elements in our high-SHF band massive-element antenna system to form an APAA and then converting the signals using one ADC/DAC combination for each APAA, we dramatically reduced the number of signal converters compared with existing fully digital configurations. So in the analog-digital hybrid configuration, the beamforming is analog and the signal processing across beams is digital. Using this configuration, we successfully developed a compact massive-element antenna system.

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Integrating high-frequency circuitry for even greater compactness

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As well as the analog-digital hybrid configuration, Mitsubishi Electric has been working to make RF devices still smaller and slimmer by integrating the RF circuitry.

In existing massive-element antennas, there are modules behind the antenna panel in a three-dimensional configuration that makes the antenna apparatus thicker. These modules contain RF circuit components such as the phase shifter that enables arbitrary beamforming and a power amplifier to boost the signal. The development process included attempts to substantially reduce this thickness. The approach we used was to focus on integrating the RF circuit components.

We developed our own compact, high-performance RFIC*3 with multiple integrated circuits and mounted the RFIC flat onto the back surface of the antenna panel. This made the antenna and RF circuits into a single component, making the unit much slimmer. In terms of an evolutionary leap making something far slimmer, this is akin to the switch from thick CRTs to thin LCD panels in TVs.

With these two technological breakthroughs ― the analog-digital hybrid configuration and the integration of RF circuit components ― we successfully developed the "panel-type massive-element antenna". Once this technology is ready for use in the real world, it will be possible to easily mount base stations on the sides of buildings and similar locations, facilitating the rollout of 5G.

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Supplying stress-free communications to single individuals in a crowded world

Current base stations broadcast signals widely throughout the surrounding area, whereas the massive-element antennas used for 5G communications beam the signals direct to individual terminals. While analog beamforming can send signals long distance, it causes inherently spatial signal leakage in unwanted directions, resulting interference with other terminals. Using the analog-digital hybrid configuration, this interference is prevented by using multibeam multiplexing technology to suppress unwanted signals using digital precoding. This makes it possible to send data to multiple terminals at the same time.

The other technology we have developed to achieve more reliable and precise communication is nonlinear multi-diagonalization precoding technology. Even where multibeam multiplexing technology directs beams to individual terminals, there may still be overlap between beams in crowded locations where terminals are in proximity. Inter-beam interference may result in slower data rates. The method used to cut the overlapping portions of the beams and prevent the interference is nonlinear multi-diagonalization precoding technology.

This technology combines both multi-diagonalization and nonlinear operations as the signal processing techniques. In multi-diagonalization operations, a strong beam is transmitted on the assumption that it will overlap with beams for nearby terminals. However, the strength of the beam gives rise to interference. So nonlinear operations are used to remove the excess sections of beam. In conventional diagonalization precoding technology, the only way to avoid interference is by weakening the beam. This new technology ensures clear and precise communication by sending out strong signal beams without causing interference. Until now, achieving the goal of nonlinear operation-based precoding has been a core focus for academic investigation, so this constitutes a truly revolutionary breakthrough. Using this combination of multibeam multiplexing technology and nonlinear multi-diagonalization precoding technology, we can now provide high-capacity communication even in crowded urban areas at speeds of 20 Gbps, roughly 50 times the speed of existing technology.

*1
Compared with the latest 4G mobile base stations as of October 2016
*2
SHF (Super High Frequency, 3−30 GHz radio frequencies)
*3
RFIC: Radio Frequency Integrated Circuit
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