Performance analysis of relay aided terahertz systems

Abstract

Next-generation communication networks are expected to support Terabits per second (Tbps) data rates, which require an extensive bandwidth. Terahertz (THz) communication is promising for next-generation networks due to the broad and unoccupied bandwidth in the THz band, which spans frequencies from 0.1 THz to 10 THz. Despite THz communication's high potential for future networks, THz channel conditions entail significant challenges. Channel impairments observed in THz wireless channels, such as high path loss, multipath, and misalignment fading, degrade communication performance and restrict the transmission range of THz communication systems. Adverse weather conditions can further degrade the performance of THz communication systems deployed outdoors. This thesis primarily focuses on employing various relaying techniques in THz communication systems to overcome the performance degradation caused by the deteriorating channel conditions associated with the THz wireless environment. The THz channel model adopted throughout this thesis incorporates components including path loss, misalignment fading, and multipath fading. Molecular absorption and free-space losses at THz frequencies cause high path loss, resulting in a shorter range, thus necessitating highly directive antennas in THz communications. Using highly directive antennas leads to antenna misalignment, making it essential to consider misalignment fading in the channel model. The multipath fading model, commonly utilized in THz communication literature, is adopted throughout this thesis. Chapter 1 includes a comprehensive literature review on channel modeling for THz communications. In Chapter 2, the adopted THz channel model is detailed along with a commonly utilized stochastic model for foggy channels, and the random numbers to model channel coefficients' envelopes used in simulations for misalignment fading, multipath fading, and foggy channels are generated using the inverse transform sampling method. Compared to their corresponding probability density functions (PDF), the histograms of the generated random numbers validate the accuracy of the random numbers being drawn from the required distributions. Relaying techniques are widely employed in the THz communication literature to overcome the impacts of high attenuation associated with the THz wireless environment by improving the signal quality at the receiver and extending the coverage range. Chapter 1 includes a comprehensive literature review on the utilization of relays in THz communication systems, and Chapter 3 opens with a general overview of dual-hop systems. Subsequently, a THz-THz dual-hop fixed-gain AF relaying system, comprising a single S, R, and D node, each deployed at fixed locations, is presented in Chapter 3. Monte Carlo simulations are used to investigate the system's performance under different levels of foggy weather conditions, considering α-µ multipath fading and the presence of antenna misalignment in the THz channel model. The outage performance of a dual-hop DF and a single-link THz communication system are also investigated through simulations for comparison. The numerical results demonstrate that the dual-hop fixed-gain AF THz system consistently outperforms both the single-link and dual-hop DF THz systems in all simulated foggy weather conditions. The study indicates that dual-hop systems can improve the outage performance of THz systems and extend their communication range, especially under severe channel conditions. Focusing on a specific S-R-D link and assuming fixed deployment of the nodes provide valuable insights into the performance of relaying systems and enable more tractable analysis due to constant distances among the nodes. Nevertheless, considering the spatial randomness of nodes allows for more realistic performance analyses of communication networks. The Poisson Point Process (PPP) is widely used in the literature to model the spatially random locations of the nodes in wireless networks. After providing an overview of PPP, Chapter 4 focuses on a network model with PPP-distributed DF relays, where S communicates with D through single or multiple relay nodes. Initially, the case that a single relay is utilized, selected according to the best relay selection method, is considered. The serving relay is chosen as the one providing the highest average rate to D within the communication range of S when the best relay selection method is employed. The analysis in the literature for the coverage probability of the network operating in the radio frequency (RF) band, employing the best relay, is presented. Subsequently, a relay clustering approach that utilizes multiple relays within a configurable distance from D, located within the communication range of S, including the best relay, is proposed and utilized in the RF network. The coverage probability of this extended network is theoretically analyzed using tools from stochastic geometry. Monte Carlo simulations are performed to evaluate the performance of relay clustering in the RF network. The theoretical analysis is highly consistent with the Monte Carlo simulation results. Numerical results demonstrate the impact of varying system parameters, such as relay density, dynamic clustering parameter, rate threshold, transmit power, and S-D distance, on system performance. These results indicate that the relay clustering approach improves performance compared to using only a single relay and suggest that employing relay clustering in THz networks could be beneficial, especially when there may be poor coverage due to channel impairments like high path loss and misalignment fading. A dense THz network with DF relays distributed according to a PPP is considered in Chapter 5. The coverage probability of the THz network employing a single relay determined by the best relay selection method is theoretically analyzed considering multipath fading channel and misalignment fading in the THz channel model. Monte Carlo simulations are performed to verify the accuracy of the analysis for the best relay selection and to evaluate the effectiveness of relay clustering in the THz network. Simulations investigate the impact of various system parameters and different severities of misalignment fading on system performance. Numerical results show that the coverage performance of THz networks is degraded due to the higher prominence of disruptive effects in the wireless channel and that relay clustering is a promising solution for enhancing network performance even under deteriorating channel conditions. Overall, this thesis has shown that integrating relays into THz communication systems can significantly enhance outage and coverage performance and extend communication range despite challenging channel conditions, thereby facilitating the employment of the THz band for next-generation networks. Furthermore, it has demonstrated that employing multiple relays through relay clustering can offer additional performance improvements in THz networks.