## 1 Introduction  
A society without wireless communication is unthinkable nowadays. In the latest decades, people and enterprises have used wireless communication every day and the evolution in wireless technologies has redefined how people work and communicate. Beside consumers, enterprises have also embraced this revolution to work faster and more efficiently. In 1982, the first generation of mobile communications (1G) was released: Analog-based and very limited, as voice service was its only capability. Ten years later, the digital version appeared (2G), offering the possibility of sending data through the network in the form of what was called short message system (SMS). The following generation, 3G, along with the then recently introduced smartphones, offered the possibility of sharing multimedia data, faster web browsing and even video calling. The fourth generation, 4G, increased the performance of the previous generation in terms of data rate. The next revolution in wireless communication is right around the corner: The fifth generation (5G). This technology promises higher speeds and lower latencies. To accommodate this new generation, frequency bands both below and over current 4G ones will be used [1]. Nowadays, with the emergence of streaming platforms allowing via a mobile connection to watch movies and series or even to play games, the consumer is demanding even more data rate in order to be able to keep up with the increasing quality of the content. 5G will try to provide a solution to all these demands and even more, like the low latency required in vehicle to vehicle or vehicle to infrastructure communications for autonomous cars. When designing 5G, some doubts arose on the usage of millimeter wave frequency bands. [PERSON] et al. supported that 5G would get consistent coverage at 38 GHz by installing base stations with a 200-meter cell-radius in dense urban environments [2]. Once any worry related to the usability of such frequencies was removed, a normalization process was started by consortia (like 3 GPP) and institutions (like ITU). Today some public administrations (like FCC) have announced auctions of spectrum parts at millimeter wave frequencies for 5G [3]. Alternatively, there is an unlicensed band around 60 GHz that is also well positioned to succeed as a candidate for providing high performance services based on radio communication systems [4]. Both frequency bands allow high rate traffic and the final decision depends on a deep analysis of their advantages and disadvantages. Facing the 5G expectation, a precise channel characterization is necessary, as network planners are concerned on achieving the required quality of service. However, indoor scenarios still need deeper analysis to assure that propagation within such environments fits within the expected performance of those frequency bands. Thus, this paper proposes an experimental analysis from the results of a measurement campaign at two frequency channels in the millimeter wave range: 40 GHz and 60 GHz. The measurements are performed in a corridor in both a line of sight (LOS) and obstructed line of sight (OLOS) scenarios, and in an ordinary auditorium used for teaching activities. As a result, this paper provides wideband propagation parameters at both frequency bands and in different environments, giving an insight into their characteristics and also helping network planners in selecting the most adequate frequency channels. We detected that the effect of multipath would be less important in the 60 GHz band, as attenuations by propagation mechanisms will reduce the influence of contributions with larger delays. In general, the root mean square (RMS) delay spread is larger at 40 GHz than at 60 GHz, which could limit the usability for wideband systems. The worst case also occurs in OLOS conditions, which cannot be neglected in a real world indoor radio link. After this introduction, the setup of the measurements is discussed in Section 2, devoted to materials and methods.