Sub-terahertz research in 6G wireless: Where should you start?

Article By : Jessy Cavazos. Keysight Technologies

Though practical 6G implementations are expected to emerge in 2030, 6G research is well underway. So, it's not too late to be a part of it.

6G is not just the evolution of 5G. Every generation of cellular technology is transformational. 4G gave life to the mobile Internet while 5G expands cellular communications beyond smartphones. 6G will continue taking mobile communications to new heights, beyond traditional cellular communication devices and applications.

The wide bandwidths available at the sub-terahertz (THz) frequencies under consideration for 6G will enable the transfer of massive amounts of information compared to those currently used for 4G and 5G. These frequencies could enable virtual reality (VR) and augmented reality (AR) applications benefiting from immersive holograms.

Enabling devices to operate at sub-THz frequencies starts with researching and understanding material properties, semiconductors, antennas, and even new digital signal processing (DSP) technologies. Researchers are looking into how to use materials like indium phosphide (InP) and silicon germanium (SiGe) to develop high-power and highly-integrated devices. Fortunately, universities, commercial entities, and the defense industry have been researching and using these compound semiconductor technologies for years with a continued push to increase the upper frequency limits and improve performance in other areas like noise and linearity. Understanding system performance is one of the key challenges ahead of the industry for the commercialization of 6G.

New technologies for 6G

Take the case of level of error vector magnitude (EVM) in a communication system, a key performance indicator used to evaluate complex radio modulation schemes. It’s a function of the noise and linearity performance of the radio transmit and receive chains. This aspect drives many aspects of the design of components and circuits for 6G systems. Today, several projects in 6G focus on the devices and understanding of the noise level of integrated devices like amplifiers, antennas, and filters at sub-THz frequencies.

The relentless demand for higher data rates pushes the industry to higher frequencies that have larger swathes of available bandwidth. This is a continuous trend across generations of cellular technology, the most recent one being the expansion of 5G into the bands between 24 and 71 GHz, which helps to illustrate the path this part of 6G research is likely to take. With commercial systems using frequency range 2 (FR2) bands launched, the industry continues to improve the technology and the 3rd Generation Partnership Project (3GPP) keeps on advancing the standard to help mobility, link management, and power management. This is all informed by use cases that demand such high data rates. While the “killer app” may not have arisen yet, the increasing demands for data are in full force.

Regulatory bodies have yet to finalize policy regarding these bands, other than some experimental requirements and specifics related to radio astronomy. However, given the requirement for system integration for such short wavelengths, interconnection between measurement and devices under test is possible through only three means: wafer-probe, waveguide, or over-the-air (OTA) via antennas. Wafer-probing only works for the validation of exposed ICs. Waveguide rectangular (WR) 6.5 covers 110 to 170 GHz frequencies; WR3.4 refers to 220 to 330 GHz frequencies; and many off-the-shelf interconnection accessories are also available. For OTA, at least in the United States, the Federal Communications Commission (FCC) has opened the 95 to 330 GHz bands for experimentation and provides experimental licenses for these bands.

For these frequencies, there is only one standard that may apply for general communications transceiver specifications and there are no commercial systems using this standard (802.15.3d). So, a good place to start is to explore modulation and coding formats that could be used at these bands.

You’ll then need to test the performance of your system at sub-THz frequencies. There are many challenges to address, but they can be categorized as follows:

  • Generating enough power to overcome higher propagation loss and limits in semiconductors.
  • Antenna design and integration with both the transmitter and receivers.
  • Designing receivers with the lowest possible noise figure.
  • High-fidelity modulation across the entire available band.
  • High-speed digital signal processing to accommodate the high data rates from wide bandwidth chunks.

Overcoming the physical barriers from material properties and reducing noise in the system are essential aspects to focus your efforts. That calls for the development of new technologies to reach high frequencies as well as for digitization and test and measurement. Wide bandwidth test instruments are necessary to research sub-THz systems.

Figure 1 provides an example of a sub-THz testbed able to perform measurements at 220-330 GHz frequencies known as the H band. The testbed transmitter features a multichannel arbitrary waveform generator (AWG) with an analog bandwidth of 32 GHz, generating a modulated direct intermediate frequency (IF), an upconverter to convert the IF signals of the AWG to sub-THz frequencies, and a vector signal generator to provide a low-phase-noise local oscillator (LO) for the up (and down) converters.

Figure 1 The sub-THz testbed operates in the H band spanning 220-330GHz. Source: Keysight

The testbed also includes a power meter to perform power measurements. On the receive path, a downconverter brings the sub-THz frequency to a measurable IF, and a high-performance multichannel oscilloscope digitizes the IF signal. The same testbed can perform measurements at lower frequencies by using different up and down converters for the requisite bands.

6G channel characterization

Once you have a working system, you may want to characterize the channel through which these signals propagate. Channel sounding research is especially relevant in the context of novel frequency bands for communications—specifically the sub-THz region for 6G. Channel sounding characterization is required so that a mathematical model of the radio channel encompassing inter-city reflectors like cars, buildings, and people can be used to design the rest of the transceiver technology. That includes the modulation schemes, encoding to overcome channel variations, and forward error correction (FEC) encoding.

Figure 2 The channel sounding setup is created for D band that operates at 110-170GHz. Source: Keysight

Figure 2 shows a channel sounding configuration for the D band (110-170 GHz). This setup can also perform EVM measurements at these frequencies and higher (G band at 140-220 GHz). The same hardware setup, as shown in Figure 1, can perform channel sounding measurements by adding channel sounding signal generation and analysis software (Figure 3).

Figure 3 This channel sounding measurement is set up at 144GHz. Source: Keysight

An important aspect to note is that receiver systems often use baseband processing to mitigate channel impairments. Customizing the field programmable gate arrays (FPGAs) of your test system receiver can enable you to evaluate real-time baseband algorithms during the transmission of a wide bandwidth signal across an OTA channel.

Ready to Tackle 6G?

Practical 6G implementations are expected to emerge in 2030. It might sound far away, but given the technology improvements required, it will take that long and arrive sooner than you think. 6G research is well underway. This technology will expand upon and go far beyond the capabilities of 5G, marking the start of a new era of wireless that accelerates digitalization and drives business innovation. So, it’s not too late to be a part of it.

You can start by downloading the white paper “6G: Going Beyond 100 Gbps to 1 Tbps” that covers the three fundamental approaches to increasing data throughput for communication systems and provides measurement examples at sub-THz frequencies for extreme data throughput applications. You can also find more information on 6G channel sounding in the paper “Sub-Terahertz Channel Sounding and FPGA Customization of a 6G Testbed Receiver”.

This article was originally published on EDN.

Jessy Cavazos is part of Keysight’s Industry Solutions Marketing team.

 

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