Joint Waveform & Wavefront Engineering for Terahertz Communications in 6G

Terahertz (THz) band communications are positioned to provide the broad available bandwidths needed to fulfill society’s increasing demand for faster data rates. However, due to the relatively small wavelength of THz signals and the need for high gain (i.e., electrically large compared to the wavelength) antennas, many devices that are anticipated to implement THz communications in the future will likely be operating in the near field. Of the exciting near-field solutions for THz communications, many rely on the ability to control the propagating wavefront with a thin programmable device such as an antenna array or a metasurface. However, when using a phased array or metasurface to manipulate a broadband beam, systems suffer from the well-known beam squint effect, which results in different frequencies within the signal spectrum being focused in slightly different directions. This effect can impart a frequency-selective response on a broadband signal observed by a receiver. This paper characterizes the frequency-selective response of a reconfigurable intelligent surface (RIS) and demonstrates that considering the waveform design (i.e., modulation scheme) can help improve the performance of a RIS operating on a broadband signal.