A Novel Feature of 6G: Asymmetric Uplink/Downlink Channels in the Terahertz Near Field - Modeling and Characterization

G and beyond wireless communication technologies are expected to utilize communications bands in the millimeter wave and terahertz (0.1-10 THz) frequency bands. THz-band communications can help unleash a broad range of wireless applications, owing to the large available bandwidth. However, the large electrical size of the antennas needed to overcome the path loss results in a large near-field distance. Operating in the near-field, the link budget cannot be determined by the Friis path loss equation, and a novel link budget mechanism must be formulated. In this paper, a simple methodology that modifies the Friis link budget for the near-field is presented and validated with experimental measurements. Most importantly, it is shown that the communication channel faces asymmetrical path losses when the transmitter and receiver are not designed reciprocally. This novel feature can affect MAC and networking strategies, as well as adaptive link design for uplink/downlink channels.