RIS-assisted Terahertz communications systems design
Σχεδίαση συστημάτων υποβοηθούμενων από RIS σε Terahertz συχνότητες
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Keywords
Reconfigurable intelligent surfaces ; THz ; Analytical assessment ; Optimal placement ; Localization ; Beam-trackingAbstract
Millimeter wave and terahertz frequencies are attractive for future networks due to the huge bandwidth they offer, which will greatly benefit applications that can take advantage of it. However, these frequency bands suffer from high pathloss and are susceptible to blockage. While directional antennas can mitigate pathloss due to the high gain they offer, they require appropriate alignment between the transmitter and the receiver beams, or the links will suffer from misalignment, which reduces the power at the receiver and can potentially cause outage. The need for antenna alignment becomes especially important in the case of mobile users, where the antenna alignment needs to be updated frequently. Blockage, however, becomes more relevant with highly directional antennas, as the narrow beams they generate can be entirely blocked depending on the size of the obstacles and their distance from the transmitter.
This thesis explores the possibility of using reconfigurable intelligent surfaces (RISs) to reinforce high frequency communications. As they are expected to, at least partially, replace relays, they can offset the high pathloss in these frequency bands, offset the effect of misalignment in a link, and restore blocked links. Regarding the joint mitigation of blockage and misalignment, the use relays investigated in a scenario where the user equipments are potential relays, and the users are potential blockers. Moreover, two relay selection methods are evaluated and compared with each other. It is shown that relays can be restore blocked links and links that suffer from misalignment, but their performance depends on parameters, such as, the density of users in a specific area of interest, which affects the number of potential relays and blockers, and the average intensity of the misalignment of the relays.
The challenges that beam-tracking algorithms face in order to align the antennas between a transmitter and a mobile receiver, are investigated. Specifically, the parameters that affect their performance are presented, such as the point of view of the tracker that depends on the antenna orientation, the antenna beam-width, the number of directions to scan with relatively large antenna arrays, and the tracking frequency. Moreover, the concept of cooperative beam-tracking with multiple access points tracking a single user equipment is proposed and evaluated.
The performance of RISs, along with the optimal location and orientation with regards to their placement, is evaluated. In this regard, an analytical model RIS-aided links is demonstrated, that works for small transmitter beam footprints on the RIS. This model gives insight into how crucial parameters, such as the antenna gain of the transmitter, the transmitter-RIS, and RIS-receiver distances, affect the received power. Moreover, the optimal antenna gain is found with regard to the location of the receiver, and the optimal placement for the RIS is explored for both static and mobile receivers.
A localization algorithm is presented for detecting the location of a user equipment. The algorithm utilizes beam-forming and beam-focusing techniques to estimate the direction and distance of the user equipment. The performance of the algorithm is evaluated in two scenarios with different maximum distances from the transmitter with static and mobile users. Moreover, a beam-forming codebook is designed that improves the performance of the algorithm in the near-field under the presence of additive white Gaussian noise. The performance of the codebook is compared to the performance of a conventional beam-forming codebook in the localization algorithm.
The potentially large size of RISs allows for advanced wavefront engineering. This enables the use of non-conventional beam profiles, such as beam-focusing, self-healing, and self-accelerating beams to be used. It is presented how beam-focusing can be used to concentrate the power of the incident beam on the receiver, in order to significantly increase the received power without increasing the transmitted power. Furthermore, self-healing and self-accelerating beams allow wireless links to operate even with the line-of-sight blocked.