Page 61 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 2

EMR: A NEW METRIC TO ASSESS THE RESILIENCE OF DIRECTIONAL MMWAVE CHANNELS TO
BLOCKAGES

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Fatih Erden , Ozgur Ozdemir , Ismail Guvenc , David W. Matolak 2
1 Department of Electrical and Computer Engineering, NC State University, Raleigh, NC 27606, Department of Electrical
2
Engineering, University of South Carolina, Columbia, SC 29208
NOTE: Corresponding author: Fatih Erden, ferden@ncsu.edu

Abstract – Millimeter‑wave (mmWave) communication systems require narrow beams to compensate for high path loss
and to increase the communication range. If an obstacle blocks the dominant communication direction, alternative paths
(directions) should be quickly identi ied to maintain reliable connectivity. In this paper, we introduce a new metric to quantify
the Effective Multipath Richness (EMR) of a directional communication channel in the angular domain. In particular, the
proposed metric takes into account the strength and spatial diversity of the resolved Multipath Components (MPCs), while also
considering the beamwidth of the communication link and the blockage characteristics. The metric is de ined as a weighted
sum of the number of distinct MPC clusters in the angular domain, where the clustering of the MPCs is performed based on
the cosine‑distance between the dominant MPCs.
For a given transmitter (TX) and receiver (RX) pair, the EMR is a single scalar value that characterizes the robustness of the
communication link against blockages, as it captures the number of unique communication directions that can be utilized.
It is also possible to characterize the blockage robustness for the whole environment by evaluating the spatial distribution
of the EMR metric considering various different TX/RX locations. Using our proposed metric, one can assess the scattering
richness of different environments to achieve a particular service quality. We evaluate the proposed metric using our 28 GHz
channel measurements in a library environment for Line‑of‑Sight (LOS) and NLOS scenarios, and compare it with some other
commonly used propagation metrics. We argue that EMR is especially informative at higher frequencies, e.g., mmWave and
terahertz (THz), where the propagation attenuation is high, and directional Non‑Light‑of‑Sight (NLOS) communication is
critical for the success of the network.
Keywords – 28 GHz, 5G, 6G, angular spread, blockage, delay spread, millimeter‑wave (mmWave), multipath components
(MPCs), multipath richness

1. INTRODUCTION reasonably achieved within Quality‑of‑Service (QoS) con‑
straints) at mmWave frequencies to only a few [4], i.e.,
High‑frequency bands, such as millimeter‑ Line‑of‑Sight (LOS) path, if available, and a few additional
wave (mmWave) and terahertz (THz), have attracted paths through strong re lections from obstacles. The
increasing attention as a solution to the continuously number of dominant paths may further decrease, or there
growing data rate demand. Due to very large amounts may be none at all, if one or more paths are blocked due to
of available spectrum at these higher frequencies, they mobile obstacles, as illustrated in Fig. 1. However, it is still
have received major attention for 5G, and recently 6G, possible to establish multi‑gigabit links between the user
standardization efforts. For ef icient planning of wireless and the serving Base Station (BS)/Access Point (AP) pro‑
networks, a thorough understanding of the propagation vided that the user location has the minimum necessary
channel characteristics in the respective deployment Received Signal Strength (RSS) over at least one path. The
band is critical. Even though comprehensive knowledge most popular solution for compensating the additional
of the sub‑6 GHz bands have been acquired already loss at high frequencies is to use phased array antennas.
through extensive channel measurements and modeling, Phased array antennas can electrically create and steer
there is still much to investigate about mmWave bands beams in different directions so that alternate paths (with
for successful deployment and operation of the wireless more gain due to beamforming) can be utilized in case the
networks using these bands. most preferable path(s) is (are) blocked [3].

Due to their high frequency, mmWave signals are more Given the foregoing background, the number of backup
sensitive to blockages and attenuate much faster than paths (i.e., useful paths that are dominant over any weak
the sub‑6 GHz signals [1, 2]. For the same reason, free‑ scattered components and which can be utilized when
space path loss of a typical mmWave link is more than stronger paths are blocked) will increase the chance of es‑
an order‑of‑magnitude larger than that of a traditional tablishing and maintaining communication between the
sub‑6 GHz link [3]. These factors restrict the number users and the BSs/APs. Accurate knowledge of these
of dominant paths (over which communication can be paths under varying channel conditions is important and

© International Telecommunication Union, 2021 47