Investigating the EM performance of building materials for mid-band 5G in urban built environment: an exploratory study

• Main Author: Ng, Sean Jake Peng
• Other Authors: Soong Boon Hee
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Electromagnetic propagation; Building materials; Scattering parameters measurement
• Online Access: https://hdl.handle.net/10356/180978
• Institution: Nanyang Technological University

The diversity of composite building materials in an urban built environment can play a pivotal role in shaping the frequency response arising from their chemical composition and inherent material properties. With the advent of fifth-generation (5G) wireless technologies, there is a growing motivation to study the electromagnetic (EM) performance of cementitious composites, particularly in urban environments and civil engineering applications. In the scope of this thesis, a quantitative investigation is conducted to assess the EM wave propagation behaviour through cementitious composites within the harmonized 3.5 GHz 5G frequency band, under a limitation related to Ordinary Portland Cement (OPC), fine sand, and fly-ash cenosphere (FAC) with varying weight percentages of micro-sized iron(III) oxide (α-Fe2O3) inclusions. For the initial measurement campaign, a randomized controlled statistical study is conducted on 15-cm thick Fe2O3-enhanced mortar slabs in a two-factor 6 × 10 factorial experiment. From the analysis of the free-space transmission coefficients (|S21|) data, samples from the 2 wt% Fe2O3 treatment group demonstrated statistically significant mean improvements in microwave transparency (up to 2.28 dB) and 100% fractional bandwidth performance across the 3.40 to 3.60 GHz frequency range. Subsequently, the dielectric parameters of the fabricated mortar slabs were evaluated prior to the analysis of the dielectric and transmission losses. The proposed analysis contributes to EM enhancements for 5G wireless communication systems where potential benefits of incorporating Fe2O3 in building materials are explored. For the second measurement campaign, transmission losses of the Fe2O3-enhanced mortar slabs (4-cm and 6-cm thickness) and an emulated wall structure are assessed under different urban scenarios (Anechoic chamber and indoor environment). Transmission characteristics were rigorously analyzed across the 3.40 to 3.60 GHz frequency range using tabulated mean |S21|, Δ|S21| and maximum Δ|S21| derived from categorical |S21| parameter plots. Improvements of up to 2.21 dB in performance were observed by the 2 wt% Fe2O3 samples, showing excellent agreement with findings from the first measurement campaign. The third measurement campaign focuses on the microwave characterization and EM optimization of cementitious composites. This comprehensive study encompasses various factors, including the choice of aggregate types (FAC and fine sand), different levels of Fe2O3 inclusions (2 wt% and 4 wt%), and channel frequencies within the 3300 to 3800 MHz frequency band (divided into five 100 MHz channels). Prior to the statistical analysis, |S21| parameters were measured using an S-band waveguide experiment setup. In the channel with center frequency of 3650 MHz, a notably enhanced performance was observed for samples which comprised FAC and 2 wt% Fe2O3 compared to other treatment groups. Finally, a 3D least square polynomial surface fit is performed on the sampled |S21| data to model the relationship between weight percentage of Fe2O3 and frequency, based on aggregate types. The resulting polynomial model delineates the change in frequency response to a corresponding change in continuous factor levels within the parameter space of the experimental design. The thesis is concluded with the investigation of material loading on an evolved antecedent hexagonal-stubbed complementary split-ring resonator (CSRR)-loaded electrically small antenna (ESA) designed to cover the 3.5 GHz 5G frequency band. Optimization of the narrowband antenna system was performed in a simulation environment during the conceptualization phase to achieve EM resonance at 3.50 GHz. The design features an impedance bandwidth of 1.57% with theoretical gain values and maximum return loss of 1.80 dBi and 20.0 dB, respectively. Thereafter, the antenna design was integrated into a printed circuit board and a physical prototype was fabricated as a proof-of-concept. Subsequently, the antenna prototypes were embedded into OPC pastes comprising different weight percentages of Fe2O3 inclusions (1 wt% ¬– 4 wt%). Experimental and simulation-derived results were cross-validated using their |S11| parameter plots, demonstrating a systemic downward shift in the resonant frequency and associated variations in impedance matching due to changes in loading reactance of the CSRR-loaded ESA. Finally, a novel inversion modelling procedure was developed based on perturbation theory to numerically extrapolate the relative permittivity of the various dielectric-loaded materials. The analysis contributes to the optimization of concrete-embedded 5G antenna sensor designs and provides a foundational framework to estimate unknown EM parameters of cementitious composites.