Hierarchical Bandwidth Modulations for Ultra-Broadband Communications in the Terahertz Band

Terahertz (THz)-band (0.1-10 THz) communication will be key in enabling high speed wireless links due to the wide available bandwidths. At THz frequencies, the path-loss is governed by high spreading loss due to small antenna apertures and by molecular absorption loss due to water vapor. The latter also determines the available transmission bandwidth, which shrinks with distance. Modulations that consider the high propagation loss and the distance-dependent bandwidth are needed to fully exploit the THz channel's bandwidth. Using a hierarchical constellation to simultaneously service users at symbol rates, Hierarchical Bandwidth Modulation (HBM) leverages molecular absorption to increase aggregate data rates in a broadcast system while offering flexibility to receivers experiencing high path loss. This paper introduces HBM and evaluates its performance. The symbol error rate performance for a 4/M-QAM HBM system is derived and verified using simulations. These results are used to define the design constraints for an HBM system: the HBM functional region and transition region. The functional region is verified using an experimental testbed for ultrabroadband communications. The results show that with proper design HBM successfully achieves its goal to exploit the distance-dependent characteristics of the THz channel, to spatially multiplex users, and to increase the system capacity.