Millimeter-wave frequency bands will play an important role in next-generation wireless (including military) communications (5G) communications systems due to the enormous amount of available bandwidth in this frequency range. While mm-wave outdoor systems have been proposed and thoroughly investigated for many years, it is only the availability of CMOS for the mm-wave range that has enabled recent commercial success, which is not only important in the civilian area, but will also drive military systems based on COTS components.
For the design, performance assessment, and deployment planning of wireless systems, understanding of the propagation mechanisms and creation of suitable channel models is a conditio sine qua non. For mm-wave systems, measurement and modeling of the corresponding propagation channels is thus of the utmost interest. This is particularly relevant since many of the dominant propagation effects are significantly different from those at the traditional cm-wave frequencies (i.e., microwave range).
Due to the complexity of mm-wave systems, also the channel models have to correctly account for a variety of channel parameters. Pathloss and shadowing are of obvious importance, since they determine range and interference level. Since pathloss at mm-wave frequencies is very high, beam forming gain is needed to compensate for it; to correctly account for the beam forming potential, the angular dispersion needs to be characterized. Not only the angular spread, but also the temporal changes of the angular dispersion have to be described, so that it can be judged how fast beamformers need to adapt. Due to the wideband nature of mm-wave systems, delay dispersion needs to be characterized, both with respect to delay spread, and the number of multipath components (which might be sparse in mm-wave systems). Frequency correlation within the mm-wave band, and between this band and the microwave band is vital for assessing the potential and performance of multi band systems.
This tutorial aims to give a comprehensive overview of mm-wave channel measurements and models, including the following aspects: • Channel measurement techniques: the first step for any channel investigation has to be an accurate measurement. We will review different types of channel sounders, including narrowband, wideband, and directional (MIMO) sounding techniques, with special attention to the unique challenges that arise at mm-wave frequencies. • We will also discuss ray tracing and its challenges arising from the fact that most surface structures are larger than a wavelength. • Channel measurement results in the literature: the tutorial will next give an overview of the measurement results in the literature. We have recently performed an extensive literature review of hundreds of papers and will present a concise summary of the main observed trends. • Channel models: in order to serve for system design and testing, the measurement results have to be turned into suitable models. We will review the main classes of channel models, namely tapped delay line, geometry-based stochastic, and quasi-deterministic, and discuss the pros and cons of these approaches for mm-wave systems. An assessment of existing standardized models such as the 3GPP model will also be included.