Nippon Telegraph and Telephone Corporation and Tokyo Institute of Technology have jointly developed an ultra high-speed IC for wireless front-ends that operates in the terahertz frequency band. This new IC technology has achieved 100 gigabit per second (Gbps) wireless transmission data rates in the 300 GHz band. The details of the technology were presented at the 2018 IEEE MTT-S International Microwave Symposium (IMS 2018) in Philadelphia last week.
Terahertz waves are being used for a wide range of research projects as this is one frequency where wide frequency bands can be secured. The researchers of this project, implemented a mixer circuit that has a unique proprietary high isolation design with an Indium phosphide high electron mobility transistor (InP-HEMT). This enlarged the transmission bandwidth, which is a problem in the conventional 300 GHz band wireless front end. It also improved the signal-to-noise ratio (SNR). Using this circuit, they created a 300 GHz wireless front end module and were able achieve wireless transmissions of up to 100 Gbps.
The researchers achived 100 Gbps wireless transmission using a single carrier. In the future, they can use multiple carriers and use technologies like MIMO and OAM to achieve even higher data rates. According to the researchers, this ultra high-speed IC technology can enable high-capacity wireless transmission of up to 400 Gbps. This is about 400 times the datarates offered by current LTE and Wi-Fi technologies, and 40 times than those offered by 5G technologies. It is also expected to be a technology that opens up utilization of the unused terahertz wave frequency band in the communications field and non-communication fields.
Research Background
With the spread of broadband networks, high-capacity wireless transmission technology of 100 Gbps has attracted worldwide attention. There are three ways of further increasing the capacity of wireless transmissions - expanding the transmission bandwidth, increasing the modulation multi-level number, and increasing the spatial multiplexing number. To realize future large capacity wireless transmission technology from a level of 400 Gbps to that of one terabit per second (Tbps), it is necessary in one wave (one carrier) to expand both the transmission bandwidth and the modulation multi-level number simultaneously and to increase the number of spatial multiplexing transmissions by superimposing them multiple times.
In the carrier frequencies from 28 GHz to 110 GHz that are currently being researched and developed, the transmission bandwidth is limited. So, researchers are studying the use of frequencies which make it easier to expand the transmission band area, from the 300 GHz band to the terahertz wave frequency band. The 300 GHz band has a frequency that is 10 or more times higher than the 28 GHz band which is being studied for 5G, which will be the next generation mobile communication technology. With the 300 GHz band, it will be easier to secure a wide transmission bandwidth, as this frequency band has not been assigned to any specific applications yet.
On the other hand, with a high frequency, leakage of unnecessary signals between the ports inside the IC and mounting tends to occur, and so far, it has been impossible to obtain a sufficiently high signal-to-noise ratio (SNR). For this reason, even if a 300 GHz band is used, it is impossible to obtain both a wide transmission bandwidth and a high modulation multi-level value at the same time, and so wireless transmission up to now has remained at the rate of several-tens Gbps.
Technical Features
The team devised a unique high isolation design technology and used this technology to realize a mixer circuit. A mixer circuit has three ports: a Local Oscillation Frequency Port (LO), a Radio Frequency Port (RF), and an Intermediate Frequency Port (IF). When operating with the very high frequency signals of terahertz waves, unnecessary signals leak easily between ports as the result of a small parasitic capacitance on the mixer circuit and external mounting.
By adding a quarter-wave line and series capacitance, they were able to create a unique design that dramatically improved isolation between ports. The high isolation characteristics realized in this way can suppress unnecessary signals, contributing not only to improvement of SNR, but also to prevention of deterioration of frequency characteristics when the mixer IC is mounted on a module. As a result, they achieved both broadband characteristics and high SNR characteristics for a wireless front-end module.
Research Results
For this project, the research team devised its own high isolation design technology and applied this technology to a mixer circuit, which is a key component responsible for frequency conversion in the 300 GHz band wireless front end. They also developed an IC with an indium phosphide high electron mobility transistor (InP-HEMT). By applying high isolation design technology, they succeeded in suppressing leakage of unnecessary signals inside each IC and between ports in the IC. They also succeeded in improving signal noise reduction and expanding band width, which had been issues confronting the use of conventional 300 GHz band wireless front end technology up to now. With these technologies, a 300 GHz band wireless front-end module was realized and the team confirmed reception of a good 16QAM signal in back-to-back transmissions. They also confirmed transmission at a speed of 100 Gbps in the 300 GHz band.
Future Developments
Since the team has already realized 100 Gbps transmission speeds with a single carrier, in the future they will expand their research to use multiple carriers or spatial multiplexing technology such as MIMO and OAM together resulting in even higher data rates. With this combination, they expect to attain an ultra high-speed IC technology that enables high-capacity wireless transmission speeds of over 400 Gbps. In addition, they expect that the approach can be applied in various fields such as imaging and sensing in which terahertz waves are expected to be used. Through collaboration with partners, NTT aims to create new services and new industries using ultra high-speed ICs, and it aims to further develop ultra high-speed IC technology.
