Wireless friendliness evaluation and optimisation for sandwich building materials as reflectors

Sandwich-structured building materials are extensively used for energy saving, sound insulation and fire safety purposes. The electromagnetic (EM) waves incident on a sandwich building material will undergo complicated multiple reflections across all its layers and affect the indoor wireless performance in ways that are not yet fully understood. In this article, we establish a framework to evaluate and optimise the wireless friendliness of a sandwich building material. First, we derive and analyse the equivalent reflection coefficient of a sandwich building material. Then, based on a multipath channel model incorporating the line-of-sight (LOS) path and the reflections from the sandwich building material, we propose a metric for evaluating the wireless friendliness of a sandwich building material, i.e., the spatially averaged capacity over a room. Finally, we propose an iterative algorithm to maximise the spatially averaged capacity by jointly optimising the relative permittivity and thickness of each layer of the sandwich building material, while ensuring its mechanical and thermal insulation requirements. Numerical results show that the spatially averaged capacity can be increased by 25-42% via the joint optimisation of the relative permittivities and thickness of all layers of a sandwich building material.