6G ISAC-THz Opens New Possibilities for Wireless Communication Systems

In his speech at China's first 6G Symposium on September 16, 2021, Huawei Wireless CTO Dr. Wen Tong said, "6G is no longer just a platform that connects everything. It is an intelligent platform that offers both integrated sensing and communications (ISAC) and integrated computing and communications. This platform will provide intelligent services and applications for industries to create greater social value. Bit transmission is not the only function of the 6G network.

We will reconstruct and represent the physical world using the propagation properties of radio waves such as reflection, scattering, refraction, and multipath. The 6G network will serve as a sensing network and 6G terminals will serve as sensing terminals. With network sensing and terminal sensing working in tandem, we can model the physical world covered by the entire network based on 6G. This will create two new features – sensing-assisted communications and network-wide crowdfunding for AI big data."

The sensing data extracted from the 6G network will not just be used for modeling the physical world, it will also serve as a big data source and entry for AI learning. Network sensing enables a new usage scenario beyond communications, covering a series of use cases, including device-based or device-free target positioning, imaging, environment reconstruction and monitoring, and gesture and action recognition. Such use cases will be widely applied to industries, including human-machine coordination, environment reconstruction for smart cities, climate sensing, healthcare, and security detection. More application examples are given in the article 6G: The Next Horizon^[1].

ISAC-THz Prototype Verification

THz lies between the mmWave and infrared frequencies, and is considered an important alternative solution for achieving Tbit/s communication rates thanks to the ultra-large communication bandwidth. Due to its high frequency, THz has a millimeter-level and even sub-millimeter-level wavelength, so it can be applied to relatively small handheld or wearable devices to achieve functions like high-precision positioning, high-res 3D imaging, and mass spectrometry analysis for materials. Unlike optical cameras, THz can penetrate certain obstacles, which achieves high-precision imaging and all-weather sensing in invisible conditions with enhanced privacy protection. As such, THz bands can be used in many daily life and production scenarios, including non-invasive health monitoring, checking food safety, finding defects in high-precision manufacturing, monitoring pollution, and supporting machine vision^[2]. As a result, THz is one of the most important techniques for ISAC.

The Huawei 6G Research Team has built a unified platform for integrated sensing and communications in the terahertz (ISAC-THz) band, which is applicable to the 100-300 GHz bands. The team has also verified the technical feasibility and prototype in two challenging scenarios: high-precision sensing and imaging on terminals, and outdoor medium-distance ultra-high-speed transmission.

Millimeter-level, high-precision sensing and imaging

Figure 1 Huawei's ISAC-THz prototype achieves millimeter-level imaging resolution for objects in a closed box

As shown in Figure 1, a robotic arm simulates a person holding a THz terminal to scan and produce an image of an object in a closed box. The prototype uses 140 GHz carrier frequency, 8 GHz bandwidth, and a 4TX16R MIMO array. The THz wave is sent from the antenna array, penetrates the box, and is reflected back to the antenna by the object inside the box. After sampling and processing in real time using an algorithm, the imaging result is generated and displayed.

To achieve millimeter-level imaging resolution, the research team proposes a virtual aperture MIMO array architecture. As shown in Figure 2, a limited number of physical antenna elements on the terminal forms a small array. By moving the handheld terminal and scanning the target object, users can form a virtual aperture with greater degrees of freedom, which is equivalent to a real physical aperture formed by thousands of antenna elements without increasing the terminal size. Because the scanning traces are generally sparse and irregular, the prototype uses compressed sensing, tomography, and sparse aperture algorithms to process the sparsely sampled signal waveforms to obtain millimeter-level high-resolution images. Click the link for more details^[3].

Figure 2 Sparse sampling enables high-precision imaging with a virtual aperture

Outdoor medium-distance ultra-high-speed transmission

As shown in Figure 3, the THz communications prototype is tested in an urban scenario. The transmitter simulates a typical base station installed on the roof of a building and the terminal receiver on the ground level of a city street. The roof-to-ground distance is about 500 meters, and a line-of-sight (LOS) link between the base station and terminal is available. This prototype operates at 220 GHz with a bandwidth of 13.5 GHz. It combines a 2x2 polarized MIMO architecture and ultra-wideband, low-bit quantized digital baseband processing technology to perform channel estimation, equalization, non-linear compensation, demodulation, and decoding.

This is the first prototype to achieve 240 Gbps outdoor transmission over medium-to long-distance LOS air interfaces, proving that it is technically feasible to use THz for outdoor ultra-high-speed communications.

Figure 3 Huawei's ISAC-THz outdoor communications prototype achieves the industry's highest transmission data rate of 240 Gbit/s at a 500-m distance

Outlook

The Huawei 6G Research Team will continue researching and verifying ISAC-THz technology together with channel measurement and modeling in different frequency bands and scenarios^[4–5], and exploring areas such as miniaturization implementation, 3D stereoscopic imaging, THz mass spectrometry, THz networking, and mobility.

The ISAC concept will not be limited to THz bands. Instead, it will be integrated into the full spectrum to meet different sensing range and precision requirements. We look forward to cooperating with more partners in this field to make "Intelligence of Everything" a reality.

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