Better than Rician: modelling millimetre wave channels as two-wave with diffuse power

Zöchmann et al. EURASIP Journal on Wireless Communications and (2019) 2019:21 Networking https://doi.org/10.1186/s13638-018-1336-6 RESEARCH Open Access Better than Rician: modelling millimetre wave channels as two-wave with diffuse power Erich Zöchmann1,2,3* , Sebastian Caban1,2 , Christoph F. Mecklenbräuker2 , Stefan Pratschner1,2 , Martin Lerch2 , Stefan Schwarz1,2 and Markus Rupp2 Abstract This contribution provides experimental evidence for the two-wave with diffuse power (TWDP) fading model. We have conducted two indoor millimetre wave measurement campaigns with directive horn antennas at both link ends. One horn antenna is mounted in a corner of our laboratory, while the other is steerable and scans azimuth and elevation. Our first measurement campaign is based on scalar network analysis with 7 GHz of bandwidth. Our second measurement campaign obtains magnitude and phase information; it is additionally sampled directionally at several positions in space. We apply Akaike’s information criterion to decide whether Rician fading sufficiently explains the data or the generalised TWDP fading model is necessary. Our results indicate that the TWDP fading hypothesis is favoured over Rician fading in situations where the steerable antenna is pointing towards reflecting objects or is slightly misaligned at line-of-sight. We demonstrate TWDP fading in several different domains, namely, frequency, space, and time. Keywords: Millimetre wave, 60 GHz, Measurements, Fading, Hypothesis testing, Rician fading, TWDP fading 1 Introduction Accurate modelling of wireless propagation effects is a fundamental prerequisite for a proper communication system design. After the introduction of the doubledirectional radio channel model [1], wireless propagation research (< 6 GHz) started to model the wireless channel agnostic to the antennas used. More than a decade later, propagation research focusses now on millimetre wave bands to unlock the large bandwidths available in this regime [2–5]. At millimetre waves (mmWaves), omnidirectional antennas have small effective antenna areas, resulting in a high path loss [6–10]. To overcome this high path loss, researchers have proposed to apply highly directive antennas on both link ends [11–14]. Most researchers aim to achieve high directivity with antenna arrays [15–20] and a few with dielectric lenses [21–23]. When the link quality depends so much on the achieved beamforming gain, antennas must be considered as part of the *Correspondence: Christian Doppler Laboratory for Dependable Wireless Connectivity for the Society in Motion, TU Wien, Gußhaustraße 25, 1040 Vienna, Austria 2 Institute of Telecommunications, TU Wien, Gußhaustraße 25, 1040 Vienna, Austria Full list of author information is available at the end of the article 1 wireless channel again. Small-scale fading is then influenced by the antenna. According to Durgin [24, p. 137], “The use of directive antennas or arrays at a receiver, for example, amplifies several of the strongest multipath waves that arrive in one particular direction while attenuating the remaining waves. This effectively increases the ratio of specular to nonspecular received power, turning a Rayleigh or Rician fading channel into a TWDP fading channel.” The mentioned two-wave with diffuse power (TWDP) fading channel describes this spatial filtering effect by two nonfluctuating receive signals together with many smaller diffuse components. 1.1 Related work The authors of [25] investigated a simple wall scattering scenario and analysed how fading scales with various antenna directivities and different bandwidths. Increasing directivity [25], as well as increasing bandwidth [25, 26], results in an increased Rician K-factor. The authors of [27] analysed fading at 28 GHz with high gain horn antennas on both link ends. They observe high Rician K-factors even at non-line-of-sight (NLOS). This effect is explained by spatial filtering of directive antennas, © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zöchmann et al. EURASIP Journal on Wireless Communications and Networking as they suppress many multipath components [25]. Outdoor measurements in [28, 29], show a graphical agreement with the Rice fit, but especially Fig. 10 in [29] might be better explained as TWDP fading. TWDP fading has already successfully been applied to describe 60 GHz near body shadowing [30]. Furthermore, as quoted above, TWDP must be considered for arrays, as they act as spatial filters [24, 31]. While theoretical work on TWDP fading is already advanced [32–37], experimental evidence, especially at millimetre waves, is still limited. For enclosed structures, such as aircraft cabins and buses, the applicability of the TWDP model is demonstrated by Frolik [38–42]. A deterministic two ray behaviour in ray tracing data of mmWave train-to-infrastructure communications is shown in [43]. A further extension of the TWDP-fading model, the so-called fluctuating tworay fading model, was also successfully applied to fit mmWave measurement data [44–46]. This model brings in another degree of freedom and allows for common shadowing of both specular waves. The wireless channels in this present study are unblocked; thus, this model is not considered here. Our group has conducted two measurement campaigns [47, 48] to directionally analyse receive power and smallscale fading parameters for mmWaves. This contribution is based on the measurement data gathered in [47, 48]. 1.2 Outline and contributions With this contribution, we aim to bring scientific rigour to the small-scale fading analysis of millimetre wave indoor channels. We show in Section 2 —by means of an information-theoretic approach [49] and null hypothesis testing [50]—that the TWDP model has evidence in mmWave communications. We have conducted two measurement campaigns within the same laboratory with different channel sounding concepts. Our measurements are carried out in the V-band; the applied centre frequency is 60 GHz. For both measurement campaigns, 20 dBi horn antennas are used at the transmitter and at the receiver. The first measurement campaign (MC1) samples the channel in azimuth (ϕ) and elevation (θ), keeping the antenna’s (apparent) phase centre ([51, pp. 799] ) at a fixed (x, y) coordinate. The transmitter is mounted in a corner of our laboratory. The sounded environment as well as the mechanical set-ups are explained in Section 3. For MC1, we sounded the channel in the frequency domain by aid of scalar network analysis, described in Section 4. These channel measurements span over 7 GHz bandwidth, supporting (...truncated)