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To determine the propagation characteristics in building for fifth-generation (5G) wireless communication, it is necessary to obtain the complex permittivities of the construction materials at millimeter wave (mmWave) band. Cloth materials are widely used in house, which have a great impact on the propagation of mmWave; however, their dielectric properties in mmWave band are rarely reported. In this communication, a new method for the estimation of the complex permittivities based on quantum-behaved particle swarm optimization (QPSO) algorithm and free-space measurement is proposed. Compared with classical particle swarm optimization (PSO) and chaos PSO (CPSO), QPSO is robust and efficient in searching the global optima. The complex permittivities of four cloth materials (linen, leatherette, polyester fiber, and latex mattress) are extracted from the measured transmission and reflection coefficients for perpendicular polarization over 40–50 GHz, and the accuracy is verified by comparing the calculated and measured reflection coefficients at other incident angles. The results presented in this communication are helpful for modeling the propagation of mmWave in indoor environment, and the proposed estimation method can serve as a good tool for the determination of dielectric properties of construction materials in wireless network planning. [ABSTRACT FROM AUTHOR]