  Millimeter-wave (mmWave) communication systems require narrow beams to
increase communication range. If the dominant communication direction is
blocked by an obstacle, an alternative and reliable spatial communication path
should be quickly identified to maintain connectivity. In this paper, we
introduce a new metric to quantify the effective multipath richness (EMR) of a
directional communication channel by considering the strength and spatial
diversity of the resolved paths, while also taking into account beamwidth and
blockage characteristics. The metric is defined as a weighted sum of the number
of multipath component (MPC) clusters, where clustering is performed based on
the cosine-distance between the MPCs that have power above a certain threshold.
This process returns a single scalar value for a transmitter (TX)/receiver (RX)
location pair in a given environment. It is also possible to represent the EMR
of the whole environment with a probability distribution function of the metric
by considering a set of TX/RX locations. Using this proposed metric, one can
assess the scattering richness of different communication environments to
achieve a particular quality of service (QoS). This metric is especially
informative and useful at higher frequencies, such as mmWave and terahertz
(THz), where the propagation path loss and penetration loss are high, and
directional non-light-of-sight (NLOS) communication is critical for the success
of the network. We evaluate the proposed metric using our channel measurements
at 28 GHz in a large indoor environment at a library setting for LOS and NLOS
scenarios.
