Creating a Sub-Terahertz Testbed with the NI mmWave Transceiver System

Overview

The wireless community is buzzing about the use of millimeter wave (mmWave) frequencies for 5G communications systems. Although a significant amount of research in 5G is being directed at different use cases, researchers are beginning to look at even higher frequencies in what is considered by many as one of the first research topics in 6G. 

Why Sub-THz and THz?

mmWave frequencies offer large swaths of available spectrum for wireless communications, which is one of the key reasons it’s important to 5G. There are numerous challenges involved in taking advantage of this spectrum, such as propagation loss, power consumption of devices, and the cost to manufacture mmWave chipsets. So if these challenges remain unsolved, why bother analyzing the THz spectrum for communications?

Figure 1. Radio frequency and applications[1]

Like mmWave, there’s a significant amount of spectrum available for THz. But unlike mmWave, THz can address additional applications. These applications may be lumped into five broad categories:

  • Wireless cognition, including robotic control
  • Sensing from air quality detection to gesture detection for touchless smartphones
  • Imaging, including body scanners
  • Communications, such as short-range, high-data throughput like data centers or intra-device radio communication
  • Positioning for centimeter-level accuracy

Sub-THz Testbed

Like 5G, creating testbeds and prototypes helps researchers take their theories and ideas beyond simulations and into the real world. To evaluate the applications listed above, it’s important to have a testbed that can communicate over the air. These applications have intensive signal processing requirements as well as high-frequency analog demands. Subsequently, it’s necessary to process and stream data at the ultra-wide bandwidths used at these frequencies in addition to being able to transmit and receive sub-THz and THz signals.

NI has significant experience creating mmWave testbeds for this type of research using its mmWave Transceiver System (MTS). The MTS employees a modular design, so the same hardware and software can be used for sub-THz research by the simple act of replacing the radio head. Virginia Diodes (VDI) offers a line of upconverters and downconverters that can be used with the MTS to create a sub-THz testbed.

Figure 2. mmWave Transceiver System with VDI radio heads

By using the baseband processing from the MTS, the same software can be reused with minimal modification even though the radio heads are different. There are currently two software reference architectures available for the MTS: One reference architecture is a physical layer for a single carrier communications link, and the other is used for channel sounding.

For channel sounding, a SISO system can be built using a unidirectional SISO MTS baseband and IF hardware setup, one upconverter and one downconverter from VDI, and a rubidium clock source. The figure below shows the basic signal processing that occurs on the MTS when it is used as a channel sounder.

Figure 3. Channel sounding block diagram

The system processes the data received in real time in order to display information like the channel impulse response (CIR) on the software GUI. This interface provides the user with a quick way to visually understand the channel. Measurement data is recorded and can be further processed offline. If there’s a need to store raw, unprocessed signals, more storage can be added to the system. The front panel GUI of the channel sounding reference design is shown below. When using this software, the user must provide system calibration. This can be performed with a power meter; however, additional calibration may be required depending on the complexity of the antenna being used.

Figure 4. Channel sounder front panel GUI

When using the MTS and VDI radios as a wireless communications link, a single carrier based physical layer reference architecture is available. This software is designed for a 2 GHz instantaneous bandwidth and up to 4x4 MIMO. It uses the same base hardware as the channel sounder but requires additional FPGAs to enable real-time decoding of the signal. The FPGAs process the signal, as shown below in Figure 5.

Figure 5. Single carrier PHY processing block diagram

The single carrier reference architecture is a full physical layer with light MAC layer that is capable of roughly 7.3 Gbps of data throughput per channel. The GUI is shown in Figure 6.

Figure 6. Single carrier physical layer GUI

When combined, the MTS and VDI radio heads can be used for a variety of wireless communications research purposes. Whether it’s intended for 5G mmWave or 6G sub-THz research, the modular, software-defined system is designed for maximum reuse, helping researchers innovate faster.