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A smartphone in Japan.
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It’s a good time to be alive for pixel peepers. TV makers are pushing 4K-resolution sets to replace our present 1080p screens; Apple’s iMacs sport a 5K resolution; and NHK, Japan’s national broadcaster, is testing 8K broadcasting equipment, targeting 2020 and the Tokyo Olympics for its introduction.

To help wireless devices cope with the higher speeds demanded by such applications, Fujitsu has developed a 300-GHz prototype receiver compact enough to fit into a cellphone. Though limited to about 1 meter in range, the company says the device can download 4K and 8K video almost instantly.

Today’s cellphones operate in frequency ranges between 0.8 to 2.5-GHz, and are capable of download speeds of around 230 megabits per second, while the top speed for 802.11n Wi-Fi operating in the same frequency range can reach speeds as high as 600 Mb/s. Fujitsu touts its new receiver as operating in the terahertz band—frequencies of over 300 GHz—where terminals can communicate at speeds hundreds of times faster than today’s mobile handsets.

Devices to enable such high speeds have been developed, but because terahertz-band waves quickly attenuate, receiver-amplifier chips need to be sensitive enough to deal with a weak signal. Present designs rely on a separate antenna, which in turn requires a waveguide component to transport the incoming signal from the antenna to the chip. This makes the overall combination far too bulky for cellphone use, says Fujitsu.

The goal, then, is to create a receiver-amplifier module with a built-in antenna  to increase miniaturization. This has been achieved for devices employed in millimeter wave-band equipment operating at 60-GHz to 80-GHz frequencies, for instance, and used in applications such as collision-avoidance radar. These modules connect the antenna to the receiver-amplifier  chip through an internal printed-circuit substrate making a waveguide unnecessary.

imgIllustration: Fujitsu

“Typical printed-circuit-substrate materials used in these higher frequency ranges are ceramics, quartz ,and Teflon,” says Yasuhiro Nakasha, a research manager at Fujitsu’s Devices & Materials Lab. “But when these are used in terahertz-band communications, there is significant signal attenuation and loss of receiving sensitivity.”

To get round this, Fujitsu has micro-fabricated a printed-circuit substrate using a polyimide (a heat-resistant synthetic polymer) material.  Signals from the antenna are transmitted to the receiver-amplifier chip through a connecting circuit on the substrate.

In order to ensure stable signal transmission with low loss, the top and bottom faces of the printed circuit substrate are grounded and connected using through-hole metalized vias. This and the connecting circuit together form a grounded coplanar-waveguide structure: a transmission pathway designed to enhance high frequency signal propagation. To reduce signal interference from the printed circuit substrate, the vias need to be spaced apart less than one-tenth of the signal’s wavelength—in this case less than a few tens of micrometers.

Though the polyimide material experiences a signal loss ten-percent greater than quartz, Fujitsu says the material’s processing accuracy is more than four times higher than the latter. This makes it possible to space the vias closer together, thereby halving the overall signal loss compared to using a quartz substrate.

To facilitate a strong connection between the antenna connecting-circuit on the printed-circuit substrate and the receiver-amplifier chip, Fujitsu adapted a millimeter-mounting technology to handle terahertz transmission. This method let the receiver-amplifier circuitry directly face the printed circuit substrate.

The outcome is a module with an overall volume of just 0.75 cubic centimeters—not including output terminals—small enough to be incorporated into a mobile phone. Download speeds obtained so far in the lab reached 20 Gb/s.

Fujitsu will begin field-testing by the end of March 2016, and aims to launch the technology in 2020. The application the engineers envision include instant downloading of large volumes of data from servers and terminals, electronic versions of printed guides and brochures used at events, and downloading video and music from kiosks.

Nakasha isn’t looking beyond 2020 at the moment, but he believes the technology has the potential to one day achieve speeds of 100 Gb/s.

Part of the research used was obtained from an R&D project on expanding radio spectrum resources commissioned by Japan’s Ministry of Internal Affairs and Communications.

The Conversation (0)

Metamaterials Could Solve One of 6G’s Big Problems

There’s plenty of bandwidth available if we use reconfigurable intelligent surfaces

12 min read
An illustration depicting cellphone users at street level in a city, with wireless signals reaching them via reflecting surfaces.

Ground level in a typical urban canyon, shielded by tall buildings, will be inaccessible to some 6G frequencies. Deft placement of reconfigurable intelligent surfaces [yellow] will enable the signals to pervade these areas.

Chris Philpot

For all the tumultuous revolution in wireless technology over the past several decades, there have been a couple of constants. One is the overcrowding of radio bands, and the other is the move to escape that congestion by exploiting higher and higher frequencies. And today, as engineers roll out 5G and plan for 6G wireless, they find themselves at a crossroads: After years of designing superefficient transmitters and receivers, and of compensating for the signal losses at the end points of a radio channel, they’re beginning to realize that they are approaching the practical limits of transmitter and receiver efficiency. From now on, to get high performance as we go to higher frequencies, we will need to engineer the wireless channel itself. But how can we possibly engineer and control a wireless environment, which is determined by a host of factors, many of them random and therefore unpredictable?

Perhaps the most promising solution, right now, is to use reconfigurable intelligent surfaces. These are planar structures typically ranging in size from about 100 square centimeters to about 5 square meters or more, depending on the frequency and other factors. These surfaces use advanced substances called metamaterials to reflect and refract electromagnetic waves. Thin two-dimensional metamaterials, known as metasurfaces, can be designed to sense the local electromagnetic environment and tune the wave’s key properties, such as its amplitude, phase, and polarization, as the wave is reflected or refracted by the surface. So as the waves fall on such a surface, it can alter the incident waves’ direction so as to strengthen the channel. In fact, these metasurfaces can be programmed to make these changes dynamically, reconfiguring the signal in real time in response to changes in the wireless channel. Think of reconfigurable intelligent surfaces as the next evolution of the repeater concept.

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