Page 65 - ITU Journal Future and evolving technologies – Volume 2 (2021), Issue 2

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

-30
20 3.1 Thresholding with the minimum desired
-40 power level
15
10 -50 As beamforming will be used at higher frequencies, it
would be misleading to include the low‑power paths in
5 -60 the calculation of the EMR because these paths will not be
AoD-El (° ) 0 -70 Power (dBm) of any (or only be a little) help to the communication per‑

-5 -80 formance. Therefore, after the extraction of the MPCs, the
irst step is to identify the MPCs that are above a minimum
-10
-90
received power level. This power level should be deter‑
-15 mined based on the application‑speci ic communication
-100
-20 needs, operating frequency, and the available environ‑
-110
-150 -100 -50 0 50 100 150 mental parameters, such as the blockage size/probability
AoD-Az (°) statistics. For example, in [12], where a learning‑assisted
(a) beam search scheme is proposed for indoor mmWave net‑
works at 60 GHz, the minimum required RSS is de ined
-30
20 as −60 dBm; otherwise the User Equipment (UE) is trig‑
gered to ind a stronger link. On the other hand, in [13],
15 -35
the RSS threshold is considered to be −84 dBm for a reli‑
10 able communication at 60 GHz between the vehicles and
-40
5 the Road Side Units (RSUs) in an urban environment from
AoD-El (° ) 0 -45 Power (dBm) Manhattan, NY. Similar to [12, 13], we consider a thresh‑
old in this work to discard paths in the angular domain
-5 that are weaker than that threshold. In Section 4, we will
-50
-10 investigate how the EMR changes with different thresh‑
olds.
-15 -55
-20 Let X = { , , … , } be the set of all MPCs with
2
1

-60
-150 -100 -50 0 50 100 150 being the total number of MPCs. Each MPC in X can be
AoD-Az (°) characterized by six parameters as follows:
(b)
= { AoD,Az , AoD,El , AoA,Az , AoA,El , , }, (9)

Fig. 3 – MPCs in the AoD‑power space: (a) All MPCs and (b) MPCs after
thresholding with min = −60 dBm. Points with multiple colors indi‑
cate multiple MPCs at the same point. where = 1, … , . Then, the subset of MPCs whose
power are above a predetermined level min can be de‑
ined as
3. EFFECTIVE MULTIPATH RICHNESS
(EMR) X = { | > min , = 1, … , } . (10)

To calculate the EMR, two user‑de ined inputs are re‑ Fig. 3(a) shows all the MPCs (extracted from the mea‑
quired: a minimum power level (at the RX) for the MPCs, surements shown in Fig. 5(a) at TX1‑RX1 pair) in the
and a beam separation threshold. The irst input is used AoD‑power space. We note that the number of extracted
to select the MPCs through which the communication MPCs depends on several factors, such as multipath delay
can be maintained, whereas the second one is used in resolution of the channel sounder or the MPC extraction
clustering the MPCs to estimate the number of backup algorithm. Based on the choice of these factors, one
paths that are well‑spread over the angular space. Fol‑ can end up with a much smaller number of MPCs than
lowing the power thresholding and clustering processes, shown in Fig. 3(a). Considering the requirements, such
the EMR is expressed as the sum of the weighted number as minimum average date rate per user, let us assume the
of clusters, and the effective richness of the channel in minimum power level min for an MPC to be eligible as a
terms of useful paths is revealed. The procedure to ind backup path (after a blockage, in a different beam direc‑
the EMR is explained in detail next. The outcomes of tion) to be −60 dBm. Then, the number of MPCs reduces
the intermediate steps are demonstrated using a sample to only 17, and the resulting MPCs are shown in Fig. 3(b).
measurement (performed in a library environment for The parameters of these MPCs are provided in Table 3.
TX1‑RX1 pair shown in Fig. 5(a)). We acknowledge that, As it is clear from Fig. 3(b), three‑dimensional AoDs for
in this study, we are only interested in the dominant some of the MPCs are very close to each other, which may
MPCs and their overall spatial diversity. That is, we are result in multiple MPCs being blocked in the presence of
not concerned with whether the MPCs reach the receiver an obstacle (of certain size) that is at a certain distance
through diffraction, diffuse or specular scattering, or to the TX. As a result of this observation, the likelihood of
after penetrating through obstacles.

© International Telecommunication Union, 2021 51