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[EN] Recent standardization of portions of the millimeter-wave (mm-wave) band for fifth-generation (5G) operation has called for further research on how short-term fading behaves in that unexplored part of the spectrum. With such a target, this paper reports on a thorough measurement campaign conducted in an indoor environment characterized by rich-multipath scattering, a part of a modern building, with floor and ceiling constructed of reinforced concrete over steel plates with wood and plasterboard-paneled walls. Particularly, measurements have been performed in a variety of scenarios, under line-of-sight (LoS) and non-line-ofsight (nLoS) conditions, for a wide range of frequencies, namely from 25 to 40 GHz - a span of 15 GHz - therefore, including 26, 28 and 39 GHz. First and second order statistics of representative fading models, namely Rayleigh, Rice, Nakagami, folded normal, alpha-mu, eta-mu, kappa-mu and alpha-eta-kappa-mu have been investigated. The metrics used in the analysis were the normalized mean square error (NMSE), the Kolmogorov-Smirnov (KS), and the Akaike information criterion (AIC). Additionally, the study of the kappa-mu. model is advanced, in which new, exact, simple closed-form expressions for probability density function, cumulative distribution function, and level crossing rate are derived for some particular cases, namely for mu = n + 1/2 in which n is an element of N., This work was supported in part by the Rede Nacional de Ensino e Pesquisa (RNP) with resources from Ministerio da Ciencia, Tecnologia e Inovacoes (MCTIC) through the 6G Mobile Communications Systems Project of the Centro de Referencia em Radio-comunicacoes (CRR), Instituto Nacional de Telecomunicacoes (Inatel), Brazil, under Grant 01245.010604/2020-14, in part by the Fundacao de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG), in part by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) under Grant 304248/2014-2, and in part by the Ministerio de Economia, Industria y Competitividad of the Spanish Government under the National Project through the Agencia Estatal de Investigacion (AEI) and the Fondo Europeo de Desarrollo Regional (FEDER) under Grant TEC2017-86779-C2-2-R.

In this paper, new, exact expressions for the probability density functions and the cumulative distribution functions of the ratio of random envelopes involving the α-μ, η-μ, and κ-μ fading distributions are derived. The expressions are obtained in terms of easily computable infinite series and also in terms of the multivariable Fox H-function. Some special cases of these ratios, namely, Hoyt/Hoyt, η-μ/Nakagami-m, κ-μ/Nakagami-m, κ-μ/η-μ with an integer μ for the η-μ variate, η-μ/η-μ with an integer μ for only one of the η-μ, and their reciprocals are found in novel exact closed-form expressions. In addition, simple closed-form expressions for the asymptotes of the probability density functions and cumulative distribution functions of all ratios, both for the lower and upper tails of the distributions are derived. In the same way, asymptotes for the bit error rate on a binary signaling channel are obtained in closed-form expressions. To demonstrate the practical utility of these new formulations, an application example is provided. In particular, the secrecy capacity of a Gaussian wire-tap channel used for device-to-device and vehicle-to-vehicle communications is characterized using data obtained from field measurements conducted at 5.8 GHz.