Investigation of indoor Terahertz channel characteristics with deterministic and stochastic approaches

In this study, indoor Terahertz channel properties were examined with deterministic and stochastic approaches. In order to observe the effect of multipath fading on the channel, channel modeling was performed using ray tracing technique in a room to obtain channel impulse response and channel frequency response graphs as in Figure A. As can be seen, when the reflection of all surfaces is reduced, frequency selectivity decreases significantly. Purpose: This study examines the characteristics of indoor multipath THz band channels, and make a contribution to the development of an appropriate statistical model. Theory and Methods: First, free space path loss and molecular absorption is examined with MATLAB simulations. Then, Ray Tracing technique is used for analyzing the multipath structure of indoor THz communication channels. Finally, Saleh-Valenzuela model is adapted for indoor THz channels. Results: In the 0.1- 10 THz band, free space path loss increases as the frequency and/or distance between transceiver and receiver increases. Further molecular absorption is very critical in THz band. There is a direct relationship between temperature and molecular absorption coefficient, the coefficient increases as the temperature increases. The coefficient also increases as humidity increases or as pressure changes. For multipath structure investigation, Ray tracing technique is used where channels are formed considering different shaped and sized rooms, for various transmitter and receiver locations. The results have shown that, when the transmitter is located at the corner (as the usual case in THz communication systems), the channel is critically frequency selective and dependent on the receiver location. Placing the receiver on the ceiling decreases the frequency selectivity and the dependence on receiver location get loose. But for safer communication reflections should be limited. For a 4x5x3 m rectangular room, sample channel impulse and frequency response graphics are given in Figure A. In the first case, there is no reflection from the floor (i.e. there is a carpet) and in the second case reflections from all surfaces are decreased by a constant 0.3, which may obtained by a special paint on the walls. As can be seen from the figure, coating the surfaces to minimize reflections makes the channel flat fading. Conclusion: THz band communication is basically limited by distance and molecular absorption. In order to get a healthy communication, the sub-bands where molecular absorption is low should be selected The observed indoor THz channel is also highly frequency selective. For a flat fading channel, the solution is reducing the reflectivity of the surfaces. Further, adapting Saleh-Valenzuela channel model resulted with good performance, with reduced complexity.