# LoS Theoretical and Experimental MIMO Study from 1-40 GHz in Indoor Environments
## 1 Introduction  
Millimeter wave (mm-wave) frequencies for 5G and beyond has gained considerable attention within the wireless industry. The main reason is the availability of large bandwidths from 30 GHz to 300 GHz that will be used to increase the wireless binary rates above 10 Gbps [1]. The large blocks of spectrum in the mm-wave range allows the designers to tradeoff spectral efficiency for bandwidth in the early stages of deployment of this technology. Furthermore, the traditional sub-6 GHz bands cannot accommodate the enormous amount of data to be generated in 5G respect to previous generations. The mobile data will reasonably grow up from 12 exabytes per month at the end of 2017 up to 77 exabytes per month in 2022 [2]. The International Telecommunication Union (ITU) forecasted in its report of 2015 a global mobile data traffic of 5 zettabytes by the end of the 2020s [3]. Thus, there is no doubt that a more comprehensive understanding of radio wave propagation is crucial to any further development of wireless networks. It is important to highlight the importance of advanced techniques as massive line-of-sight MIMO to achieve higher bit rates as seen in [4] in vehicular communications, [5; 6; 7] in wireless sensor networks and [8] in ultra-dense networks; without a thorough analysis of the radio channel, massive MIMO will be of difficult application [9]. The World Radio Conference established 5G frequency bands in WRC'15 [10], and more recently in WRC'19 [11]. Moreover, the first phase of EU's Horizon 2020 5G Public Private Partnership initiative isinvestigating the 6-100 GHz frequencies, including mm-wave frequencies, for 5G's ultra-high data rate mobile broadband, among other applications. While significant studies of the channel characteristics are available in the 28 GHz, 38 GHz, and 60 GHz bands for indoor and outdoor environments [12; 13; 14; 15; 16; 17], only a few trials have been conducted in other mm-wave bands and for outdoor and outdoor-indoor environments, as well as scenarios with mobility. However, due to the higher frequencies and wider bandwidths, compared to existing standards below 6 GHz as 4G Long Term Evolution (LTE) and Wireless Local Area Network (WLAN), mm-wave applications need specific considerations. Usually, the experimental analysis of the channel is completed using simulation tools. Ray-tracing and ray-launching techniques are among the most useful ones, due to their good tradeoff between accuracy and computational effort. Another advantage lies in the completeness of the analysis; path loss (PL), angular and delay parameters, frequency parameters as well as statistical parameters can be extracted from ray based simulations. Regarding the simulation of the mm-wave channel using the mentioned techniques, different analysis were done at 2 GHz and 18 GHz [18], 28 GHz [19], 28 GHz and 38 GHz [20] and 60 GHz [21]. To the best of the authors' knowledge, little research dealing with MIMO propagation and channel modeling in the whole 1-40 GHz has been reported [22]. As a preliminary work from the authors in [23], the whole band was measured and several channel parameters were analyzed; particularly, the PL, the 1-slope model of the PL, the RMS DS, the 1-slope model of the RMS DS, and the K-factor were extracted. To compare the behavior of the channel throughout the whole band, the measured band was divided in sub-bands centered at 2 GHz, 10 GHz, 20 GHz, 30 GHz, 40 GHz with a 2 GHz bandwidth in all cases. Some works have analyzed the channel behavior in specific bands between 1 GHz and 40 GHz; for example, the path loss was studied at 26 GHz and 39 GHz in [24] and at 28 GHz in [25; 26; 27; 28], but a comparison between the path loss model values of different sub-bands is still missing in the literature; we can find the same absence in the RMS DS studies; analysis of delay dispersion at specific frequencies or sub-bands can be found as, for instance, the work carried out in [25] and [28] at 28 GHz and in [29] at 26 GHz. Therefore, the objectives of the paper are the following.