Line of Sight (LoS) Multiple Input Multiple Output (MIMO) terahertz communication channels suffer from a decrease in capacity in the far field due to the channel matrix only having single rank. Prior work in millimeter wave (mmWave) systems has considered the design of Intelligent Reflective Surfaces (IRSs) to mitigate this problem by introducing phase variation in reflected signals. Given the high cost of IRSs, we propose inexpensive Random Reflective Surfaces (RRSs) to create a rich multi-path environment for terahertz.In this work, we build three RRSs using high-density polyethylene (HDPE) sheets embedded with tiny copper flakes at different densities. The reflection behavior of these systems is studied in a terahertz testbed where we focus on the frequency band centered at 410GHz. Using 2x2 MIMO measurements in this testbed, we show that our RRS designs indeed do improve capacity as compared to reflections from polished aluminium (which behaves like a LoS channel). In addition, we examine the range of reflected phase produced and show that, indeed, the high variation in phase leads to improved channel capacity for the case when the channel state information is only available at the receiver (CSIR) for 2x2 MIMO.

Communication at the terahertz band is a challenging yet exciting technology. It promises to deliver terabit/sec data rates for short distances but, due to its unique propagation characteristics, it suffers from significant impairments. To enable these high data rates, we need to use MIMO (Multiple Input Multiple Output) systems. However, given that natural reflected paths display enormous attenuation, we need to rely either on LoS (Line of Sight) MIMO or carefully placed reflectors to overcome environmental obstacles. We have observed that when using polished reflectors (e.g., metal plates), the channel behaves similarly to a LoS MIMO channel in that there is a decrease in capacity with distance and a periodicity. In this paper, we describe the construction of a novel reflector that maintains a steady capacity with distance. The reflector creates multiple independent paths between the transmit and receive antennas such that the eigenvalues of the channel matrix are non-trivial. This provides us with the capacity improvement. Our work is experimental and we focus on the frequency band around 410 GHz. We constructed a variety of reflectors and identified a design that improves capacity. We show that our reflector maintains a high capacity with increasing distance while a LoS path and a path constructed with a polished reflector show a decrease in capacity. We also used the same reflector at other frequencies and observed poorer behavior illustrating a dependence between wavelength and reflector design. Our results show that we can maintain high-capacity indoor channels by carefully designed reflectors.

Communication at the terahertz band is increasingly seen as vital for future short-range very high datarate channels. However, these channels suffer from significant environmental impairments and, as a result, providing coverage in indoor settings requires the use of directional line of sight (LoS) paths to visible users and reflected paths, using smooth metal reflectors, for users in the shadow of an obstruction. Previous work has shown that these types of reflected paths display similar characteristics to LoS paths and we call them R-LoS (reflected LoS). MIMO for LoS and R-LoS channels is feasible at terahertz frequencies and delivers very high capacity at some distances. Unfortunately, channel capacity varies greatly with small changes in distance (the channel matrix fluctuates between full rank and rank 1) which is undesirable for communication systems. In this paper, we utilize diffusive reflectors to create multiple reflections such that the MIMO channel capacity for R-LoS is better behaved. We conduct experiments at 410 GHz for reflections from different artificially created diffuse surfaces. We use measurements to estimate channel capacity for 2×2 MIMO when the only path is the diffuse reflected one. We show that by creating multiple controlled reflections, it is possible to achieve relatively stable capacity up to 13 - 16 bits/sec/Hz at varying distances. We also analyze the phase of the received signals and the beam profile in detail. Overall, our results indicate that by utilizing artificially created reflections, we can maintain a stable MIMO channel at high capacity.

Terahertz frequencies are an untapped resource for providing high-speed short-range communications. As a result, it is of interest to study the propagation characteristics of terahertz waves and to develop channel models. In previous work we used a measurement-based approach to develop an accurate channel model for line of sight (LoS) links. In this paper we extend that work by developing channel models for non-line of sight (NLoS) links where the signal suffers one reflection. We study reflections that occur off a metal plate as well as a piece of wood.Our model for received magnitude includes the effects of standing waves that develop between the transmitter and receiver. Measurements show an excellent agreement between empirical data and the model. In addition, we have analyzed the received phase of the reflected signal at frequencies in the range 320-480 GHz. We observed a linear error between the predicted and actual phase and developed a model to accommodate that discrepancy. The final model we have developed for predicting received phase is very accurate for the entire range 320 - 480 GHz and for both materials.

This paper examines the performance of a 2×2 Line of Sight (LoS) Multiple Input Multiple Output (MIMO) channel at three terahertz frequencies-340 GHz, 410 Ghz, and 460 GHz. While theoretical models predict very high channel capacities, we observe lower capacity which is explained by asymmetric transmit-to-receive signal strengths as well as due to signal attenuation over longer distances. Overall, however, we note that at 460 Ghz, channel capacity of higher than 12 bps/hz is possible even at sub-optimal inter-antenna spacings (for different distances). An important observation is also that we need to maintain appropriate receive signal levels at receive antennas in order to improve capacity.

This paper examines a 2x2 LoS (Line of Sight) MIMO (Multiple Input Multiple Output) channel for terahertz communications. Utilizing measurements, we extract channel models which are then used in a simulation to produce bit error rate curves for uncoded QPSK (Quadrature Phase Shift Keying). The specific focus of this work is on studying the impact of changing transmitter receiver distances and relative orientation. Since the terahertz band has a small wavelength, such small movements can have a large impact on the channel. The results support this intuition and show that horizontal displacements of one antenna relative to another by distances as small as 1-2 cm can cause the bit error rate to increase by an order of magnitude. Similarly, utilizing sub- optimal antenna spacing at the transmitter and receiver can also result in similar types of performance degradation. This study illustrates that while LoS MIMO is an effective approach for utilizing the large terahertz bandwidth, we need to consider the effects of antenna deployments.