Empirical Modeling of Radiowave Angular Power Distributions in Different Propagation Environments at 60 GHz for 5G  
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1 Signal Theory and Communications Department, University of Vigo, 36310 Vigo, Spain;  
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[PERSON] and [PERSON]  
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bands, the one where a larger bandwidth is available is 60 GHz. This is the frequency band considered in our study. The high propagation losses at these frequencies, compared to frequencies below 6 GHz, will reduce the received power, while the large bandwidths used will yield high noise levels. In order to improve the signal-to-noise ratio (SNR) at the receiver (Rx) and to enable the required high data rate transmission, the use of directional steerable antennas has been proposed [4; 5; 6]. Directional patterns will also mitigate the effect of inter-symbol interference, improve spatial selectivity, and, hence, increase the spatial frequency reuse. Massive multiple-input-multiple-output (MIMO) techniques [7; 8] will have to be implemented in 5G systems to deal with the data traffic growth. Base stations are expected to be equipped with a very high number (even hundreds) of antennas [9] that will serve simultaneously several terminals, which will be also equipped with a number of antennas. The evaluation of these systems will not be possible using classical techniques because of the high number of antennas to be connected or because the integrated antenna ports would not be accessible [10]. New over-the-air (OTA) testing techniques have been proposed to overcome this problem. These OTA techniques require detailed channel models that should describe the angular power distribution (APD) of the radiowave energy around the base and terminal antennas [11; 12; 13]. In this way, the improvement of the radio link performance due to combining the energy received through different propagation paths can be assessed. This has raised the interest on measuring, analyzing, and modelling the APDs under different propagation conditions. Propagation studies at 60 GHz show that, in most situations, several propagation paths, with different propagation delays, attenuation, angles of departure (AODs), and angles of arrival (AOAs), may exist [14; 15; 16; 17; 18; 14; 18]. Early 60 GHz propagation studies [14; 15; 16; 17; 18] already used directional antenna steering measurements to resolve propagation paths and to gain some insight on the main scattering mechanisms. It was soon clear [14] that directional antenna steering could partially compensate the high propagation losses, providing a considerable link improvement. This raised the interest on the knowledge of the radio channel APDs. At first, most of the studies concentrated on indoor radio channels and azimuth (horizontal) APDs [14; 16; 17; 18]. It was with the proposal of the millimeter wave band for 5G communications [4] that the characterization of APDs in outdoor urban radio channels at 60 GHz attracted the attention of researchers [19; 20; 21; 22]. As 5G communications will work using a wide set of frequency ranges, on almost every possible scenario, and using a variety of new techniques such as massive-MIMO, a considerable number of channel models have been developed. Recent surveys on 5G channel models can be found in [23; 24]. Channel models are classified as deterministic, map-based (like METIS [25; 26]), and stochastic. These can be further classified as geometry-based stochastic models, like COST2100 [27], WINNER II [28], 3 GPP [29], or non-geometry-based stochastic models, such as Saleh-Valenzuela model [30]. However, this is not a clear-cut division, as some models incorporate both deterministic and stochastic characteristics. If we take a closer look at models that consider millimeter wave frequencies, we find that most of them describe an angular spread and clustering that add to the spread and clustering in the time domain. The modified Saleh-Valenzuela model [31] considers angular clustering where the AOAs of rays within an angular cluster are described using a Laplacian distribution.