Over the past several decades, the number of devices connected and communicating wirelessly has increased drastically. From cell phones to wireless headphones to smart thermostats to laptop computers to smart televisions, more and more devices are using wireless signals to communicate. In addition to more devices wirelessly communicating, the expectations for the data rates achieved by wireless communication links have steadily increased as well. To keep up with the demand, wireless communications engineers are turning to higher frequencies in the electromagnetic spectrum, namely millimeter-Wave (mm-Wave), sub-terahertz, and terahertz (THz) frequencies. These bands have wider available bandwidths that have the potential to enable wireless data rates from 40 Gbps to 1 Tbps. The recently released 5th Generation (5G) of wireless systems included mm-Wave frequencies between 24-71 GHz, and future systems are expected to use even higher frequencies.
Moving to these higher frequencies can enable higher data rates and the available spectrum to support the growing number of connected devices, however these frequency bands – especially the sub-THz and THz band – come with their own challenges. Two of these challenges are the need to focus the beam toward the receiver and to overcome blockage. Traditional wireless communication systems generally propagate signals in almost every direction. The signals travel well enough in the air and through many common objects to ensure that the receiving device will likely still be able to capture enough of the transmitted signal to recover the message that was sent. Sub-THz and THz frequency signals, however, attenuate quickly and do not propagate as well. To compensate for this, most (sub-)THz systems focus all the transmitted power in the direction of the receiver. In this way, the power is concentrated in the desired direction to make sure enough of the signal is captured for the device to recover the message. This approach works very well until something or someone physically blocks the path the signal is taking between the two devices. In other words, unlike your traditional Wi-Fi, Bluetooth, or cellular device, (sub-)THz communication systems struggle to recover if there is not a clear, direct line-of-sight path between the transmitting and the receiving device.
On top of this, the act of focusing the transmitted power in a certain direction is more challenging for (sub-)THz frequencies than it is for lower frequencies. Focusing a radiated beam in a certain direction is not a new concept. It is perhaps most well-known in radar systems that steer beams to follow or track an object. Wireless communication systems can implement the same technology to allow the transmitter to focus the radiated power in the direction of the receiver. This process is called beamforming. Traditional beamforming techniques, however, assume that the ratio of the antenna size to the wavelength is much smaller than the transmission distance. This ratio often holds for lower-frequency devices, but at higher frequencies, the wavelength is smaller, and it no longer holds for many practical distances. As a result, new techniques to focus a beam in the desired direction are needed.
In our recent paper, “Ultrabroadband Terahertz-band Communications with Self-healing Bessel Beams,” we present a potential solution to these challenges by using a Bessel Beam. The most common beam is a Gaussian Beam. For example, if you shine a flashlight at a wall, you will notice a bright central spot that gradually dims farther away from the center of the beam. That is the intensity profile of a Gaussian Beam.
A Bessel Beam also has a bright spot in the center, but it also has rings surrounding it according to a certain function called the Bessel Function.
Bessel Beams have some interesting properties that make them especially attractive for high-frequency communications. First, they are designed to propagate in the region close to the antenna. Thus, unlike the traditional beamforming techniques, Bessel Beams will successfully focus the power of these high-frequency signals for practical transmission distances. Additionally, Bessel Beams focus along a line, and along this line, they do not attenuate, which is especially helpful for (sub-)THz frequencies that generally struggle with propagation when transmitted as a Gaussian Beam. Finally, these Bessel Beams can fully reconstruct themselves if the central line is blocked. Because these properties make Bessel Beams seem so promising for future wireless communication systems, we wanted to test them out on a real sub-THz communication system to see how they worked. In our paper, we demonstrate Bessel Beams showing improved performance compared to Gaussian Beams for sub-THz communications and their ability to reconstruct after encountering partial blockage. We also explored some practical solutions for generating Bessel Beams in future wireless communication systems. Our paper provides initial proof that Bessel Beams are a viable option for high-frequency communication systems, and we hope it will inspire more exploration of this potential solution for the future of wireless communications.