Since the deployment of 5G cellular systems, the wireless communications community has already begun to shift its attention toward the next frontier of 6G and beyond technologies. With the recent push to unlock extensive spectrum in “mid bands” (7–24 GHz) for data exchange, it is almost inevitable that next-generation wireless communications will eventually utilize higher frequencies laying in the sub-terahertz (sub-THz) band between 100 and 300 GHz. As a result, high-frequency ultra-broadband sub-THz wireless systems have been identified as a candidate enabler for extremely high-rate, low-latency, and high-reliability communications.

One principal difference between state-of-the-art 5G mmWave systems and beyond-5G sub-THz wireless communications is the fact that the latter will often have to operate in the near field, which leads to additional challenges and research questions that must be addressed to ensure that these systems work optimally. 

Near-Field vs. Far-Field: What is the Difference?

In most wireless communication scenarios, signals are transmitted far enough such that the transmitting and receiving antennas appear small when compared to the signal wavelength. Transmissions at distances like this are considered to be in the “far field”. In the far field, we can make some simplifying assumptions about how beams propagate. However, when your wavelength gets smaller and your antenna gets bigger (which is precisely the case for sub-THz systems), “far enough” becomes far. For example, the far field of an antenna that is around twenty centimeters long and operating at 2.4 GHz, which is the operating frequency for WiFi, the far field starts at sixty-four centimeters. Since most computers and smart phones will be more than sixty-four centimeters away from their router or access point, the assumptions made in the far field work for communication systems at 2.4 GHz. Meanwhile, for an antenna or an antenna array of the same size but operating at 120 GHz, the far field starts at a whopping thirty-two meters. Furthermore, a similar-sized antenna array operating at 300 GHz has a far field that begins at eighty meters. The trend continues as the frequency increases, which means as we begin using higher frequencies for communications, more and more transmissions will occur in the near field. 

Some everyday technologies already use near-field communications such as contactless payment options. However, developing high-speed sub-THz systems that work in the near field will be a bit more challenging. Because traditional models for predicting the propagation assume far-field propagation, one key challenge will be designing new models to characterize how sub-THz signals propagate in the near field. 

Advantages of the Near Field

 The fact that sub-THz communications must often operate in the near field is not necessarily bad news. There are actually different kinds of beams that can only exist in the near field that could help sub-THz systems to focus their power more densely in a certain region and increase power efficiency or even to propagate past or around blockages that would normally cause a severe dip in performance. One of these kinds of beams is called a Bessel beam. A Bessel beam is very directive, focusing the power of the beam along a narrow angle. Additionally, if the main beam is blocked, the main beam will reappear after some distance. These near-field beams have much potential for sub-THz communication systems, but there are few propagation experiments and models to demonstrate how they behave for sub-THz systems.

Experimental Analysis

In our recent paper, “Sub-terahertz near field channel measurements and analysis with beamforming and Bessel beams” we explore the difference in the propagation loss and antenna radiation pattern between the far and near fields. We do this analysis for traditional Gaussian (i.e. “far-field”) beams as well as Bessel beams. Our results confirm that using the traditional far-field propagation models for near-field sub-THz communications would lead to substantial errors. Although we could use Maxwell’s equations when the far-field path loss equations are invalid, ideally, there should be an alternative that does not require as much computational complexity but is reasonably accurate. Fortunately, the results in this paper suggest that it may be possible to find such an expression. 

The results also show that the radiation pattern of the sub-THz antenna is substantially different in the near field compared to the far field (which is the one that is generally shown in any spec sheet for an antenna). Additionally, our results indicate the promise of non-diffracting beams (specifically, Bessel beams) for future sub-THz communications, given higher gains with these beams in the near field and their notably more focused radiation patterns.  

In general, our study highlights the key dependencies crucial for future system design choices and may serve as a reference for next-generation channel models capturing the near-field propagation of (beyond-)6G sub-THz communications.