In the past year, our first major activity is to study the communication potential and challenge over the non-line-of-sight (NLoS) path compared to the line-of-sight path (LoS) for leaky-wave antenna at terahertz. Since the fundamental beam steering mechanism for leaky-wave antenna is via frequency control, communication to different angular locations requires transmitting over different frequencies. This unique characteristic suggests that LoS and NLoS communications using leaky-wave antennas have different transmit frequency requirements. As a result, the communication capacity of the LoS and NLoS paths not only depends on their pathloss, but also their corresponding frequency spectrum. Notice that this frequency dependency in different communication paths is unique to the leaky-wave antenna and therefore has not been studied in the prior works.
Understanding the path characteristics at terahertz is essential for terahertz communication system design. Path characteristics including signal strength, bandwidth, or even sensitivity to alignment are crucial for establishing and maintaining a terahertz communication link. In the past year, we focus on the specular NLoS paths, in which the signal is reflected from a surface as if it were a mirror. Note that the specular NLoS paths are unique to millimeter-wave and terahertz links due to the scale of the wavelength and the roughness of the reflected surface, and therefore offering both new challenges and new opportunities for system design. While the link budget of NLoS paths has been studied extensively at lower frequencies below ~6 GHz, little quantitative evaluation has been done at terahertz to understand how we can exploit NLoS paths in THz networks.
The key activity over the past year was to perform measurements over LoS and specular NLoS paths at terahertz and analyze their characteristics in both power and frequency spectrum. In addition to conducting the experiment for the leaky-wave antenna, as a comparison, we also conduct the experiment using a substrate-lens coupled dipole which has a known narrow cone antenna pattern.
The role of reflected paths has been extensively studied in microwave systems. In such systems, NLoS paths are conventionally known to affect channel fading, inter-user interference, or offer spatial diversity for multiplexing via increasing the channel’s degrees of freedom. However, at higher frequencies millimeter wave and terahertz frequencies, the situation is quite different, more reminiscent of the realm of optics. Since the transmission at higher frequencies becomes more directional to overcome the higher pathloss, possibly at both the transmitter and the receiver, the communication link is likely to rely on a single path rather than a collection of paths as in the lower frequencies. The high directionality and the smaller wavelength combined suggest the possibility of specular NLoS paths, in which the signal is reflected from a surface as if it were a mirror, in the frequencies above 100 GHz. Despite the rapidly growing interest in exploiting millimeter and terahertz waves for wireless data transfer, there has been little attention devoted to the understanding and use of NLoS paths in this spectral range, as most of the work has focused on LoS paths.
Our goal is to investigate the idea of harnessing these specular NLoS paths for communication in directional networks at frequencies above 100 GHz. We study the implication of using high-gain directional antennas that are likely to be less sensitive to signals arriving from NLoS directions. To this end, we explore two illustrative transmitter architectures: (i) The transmitter and receiver are each equipped with a substrate-lens coupled dipole with a known antenna pattern that is a fairly narrow cone, as a representative of conventional high-gain antennas. (ii) A leaky-wave antenna is employed at the transmitter so that the antenna radiation pattern is strongly frequency-dependent. For both antenna architectures, we employ a transmitter emitting wideband THz pulses covering frequencies from 100 GHz to over 500 GHz.
To understand how the highly directional transmitter and receiver resolve angularly close paths, for both transmitter architectures, we setup an experiment in which two reflective flat objects are place alongside the LoS path (off the TX-RX axis on both sides) to create two specular NLoS paths that are plus and minus 10 degrees from the LoS angle. We manually rotate the directional receiver to emulate beam steering and investigate the signal strength and power spectrum at different receiving angles.
Based on the experimental setup as described above to explore the LoS and specular NLoS paths at terahertz, our key findings are:
(i) For the conventional antenna (radiation pattern is not frequency dependent) setup, we observe that the received power decreases when the receiver rotates off the LoS axis, indicating that the received power is highly sensitive to the RX’s orientation. As the rotation angle increases, the received power increases at rotation angles of plus and minus 10 degrees, when a reflected path is captured. Yet, despite capturing the NLoS path, the power when the rotation angle is plus or minus 10 degrees from the LoS angle is about 15dB less than the LoS path.
(ii) When examining the spectral content of the received signal for different rotation directions, we find that the change in the spectral content is independent of the rotation direction. For example, the received spectrum when the receiver rotates 1 degree clockwise or counterclockwise appears identical. We also observe that the lower frequency components are always more resilient against the RX orientation, since they produce a larger spot size at the receiver.
(iii) When a leaky-wave antenna (radiation pattern is frequency dependent) is employed at the transmitter, we observe a similar trend in the received power as before: the received power decreases when the receiver rotates off the LoS axis and increases when the rotation angle reaches plus or minus 10 degrees from the LoS angle (when the NLoS path is captured). However, in contrast to the 15dB loss for NLoS path when employing a conventional directional antenna, we observe that the received power for the NLoS path can be comparable to the LoS path when a leaky-wave antenna is employed. Interestingly, the power loss is not symmetric for the two NLoS paths at plus or minus 10 degrees: we observe one NLoS path has a higher power loss (~5dB) than the other NLoS path.
(iv) When examining the spectral content of the received signal for different receiver orientations, we observe that the spectral characteristics depend on the path angle due to the leaky-wave antenna spatial-spectral coupling. We observe that even misaligning the RX from the optimum LoS direction by 1 degree on either side leads to different spectral content: one rotation direction retains more higher frequency components compared to the other.
(v) Further analysis on the bandwidth of the LoS and NLoS paths suggests potentially a higher data rate over a NLoS path when employing a leaky-wave antenna at the transmitter. Notice that the available bandwidth for different paths is different when a leaky-wave antenna is employed at the transmitter. For leaky-wave antenna, a smaller emission angle corresponds to a wider bandwidth. As a result, when an existing NLoS path corresponds to a smaller leaky-wave antenna emission angle, the wider bandwidth can potentially overcome the power loss and provide higher or comparable data rate compared to the LoS path.
Publications
Ghasempour, Y. Amarasinghe, C.-Y Yeh, E. Knightly, and D. Mittleman, “Line-of-Sight and Non-Line-of-Sight Links for Dispersive Terahertz Wireless Networks,” APL Photonics 6, 041304, April 2021.