1 Ray-Optics Simulations of Outdoor-to-Indoor Multipath Channels at 4 and 14 GHz

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Ray-Optics Simulations of Outdoor-to-Indoor

Multipath Channels at 4 and 14 GHz

Pasi Koivum¨

aki, Aki Karttunen, and Katsuyuki Haneda

Abstract—Radio wave propagation simulations based on the

ray-optical approximation have been widely adopted in coverage

analysis for a range of situations, including the outdoor-to-indoor

scenario. This work presents O2I ray-tracing simulations utilizing

a complete ofﬁce building ﬂoor plan in the form of a laser-scanned

point cloud. The simulated radio channels are compared to their

measured counterparts at 4and 14 GHz in terms of path loss

and delay and angular spreads. Validation of channel simulations

for the O2I case is rare, and so far non-existent for above-6GHz

bands. This work reveals the importance of a ﬂoor plan model in

accurately simulating the channel; it is conﬁrmed that path loss

can be replicated with a simple interior path loss model in place

of a detailed building interior model, but neglecting to model

the interior results in high delay and angular spread errors.

By modeling the interior, the ray-tracing simulations achieve

relative mean error of under 10% for delay and angular spreads.

Finally, effects of multi-layer insulating window on propagation

simulations are reported. Noticeable variation of the penetration

loss on a small change of the incident angle of a propagation

path causes large changes in estimated coverage.

Index Terms—Point cloud, ray-tracing (RT), outdoor-to-indoor

(O2I), propagation, penetration loss.

I. INTRODUCTION

PROVIDING wireless service of sufﬁcient quality to in-

door users is an essential goal for network operators.

Operators seek to utilize previously unused frequencies, in-

cluding for example, the above-6GHz new radio frequency

range 2 (NR FR2) [1] in addition to the below-6GHz legacy

NR FR1 [2] radio frequency (RF). In the legacy NR FR1,

most indoor users are served by outdoor cellular infrastructure.

The same service coverage becomes much more challenging

in the FR2, given the higher penetration losses through e.g.,

building walls, experienced by radio signals. Additionally,

increasing demand for energy efﬁciency [3] has resulted in

better insulation of buildings achieved by e.g. multi-layered

windows and insulating ﬁlms. This has generated interest in

studying indoor coverage for energy efﬁcient smart cities of

the future [4].

To this effect, there is a continued interest in outdoor-

to-indoor (O2I) channel measurements [5]–[11]. The most

commonly reported quantity is effect on signal strength inside

while being serviced from outside [9]–[11], but many studies

also report large scale parameters (LSPs) of multipath channels

such as delay and angular statistics [5]–[8].

P. Koivum¨

aki and K. Haneda are with Aalto University, Depart-

ment of Electronics and Nanoengineering, 02150 Espoo, Finland. e-mail:

pasi.koivumaki@aalto.ﬁ

Aki Karttunen is with Tampere University, Faculty of Information Technol-

ogy and Communication Sciences.

Given the difﬁculty involved in conducting large-scale

measurement campaigns, measurement-calibrated site-speciﬁc

simulations are an interesting alternative for coverage estima-

tion. Most published results of wave propagation simulations

showcase either wholly outdoor or indoor simulations instead

of the O2I case, given that obtaining a complete three-

dimensional (3D) model of a building can be more difﬁcult

than using exteriors obtainable from e.g. public databases. A

method that has attracted recent interest is a laser-scanned

point cloud of the environment used in ray-tracing [12]–[17].

Laser-scanning can be utilized to obtain a complete model

of the building and its ﬂoor plan. A number of simulation

approaches have been published for O2I scenarios, e.g. [18]–

[23]. In [18], [19] ray-based propagation was combined with

ﬁnite difference methods using ﬂoor plan of the building.

In [20], [21] a path loss model was applied to indoor propaga-

tion without a model of the building interior. In [22] a “virtual

ﬂoor plan” was generated to approximate building interior

effects on propagation, while [23] utilized a commercial ray-

tracing tool with complete ﬂoor plan of the building. To the

authors’ best knowledge, O2I propagation simulations have so

far only been compared to measurements in terms of path loss,

and only for the below-6GHz band by e.g. [18], [19], [21]–

[23]. Similarly, while many approaches to O2I simulations

have been published, the effects of the building interior model

on LSP accuracy have not been studied. Effects of insulating

structures of e.g. windows on propagation simulations and

estimated coverage due to penetration loss angular selectivity

is a similarly unaddressed question.

To these open questions, the novel contributions of this work

are as follows:

1) Results of point cloud ray-tracing utilizing a 3D model of

the building interior are presented at two frequency bands,

4and 14 GHz. The frequency bands were chosen as part

of LuxTurrim5G [4] to study O2I coverage at below and

above-6GHz bands. By comparing to measurements, path

loss error is found to be in line with earlier publications

reporting O2I channel simulations. Relative error of less

than 10% is achieved for delay and angular spreads at

both bands, a result so far unaccomplished for the O2I

channel.

2) Effects of modeling the building interior on simulated

channel LSPs are studied. It is shown that while path

loss can be replicated with reasonable accuracy without

having knowledge of the building interior, a ﬂoor plan

of the building is required for accurate delay and angular

spreads.

arXiv:2210.03159v2 [eess.SP] 13 Jan 2023

Direct path

Paths from nearby

buildings, interior

Excluded

distant paths

Fig. 1: PADP obtained at 14.25 GHz for link Tx2Rx1. Distant

paths and a limit of 350 ns to exclude them is shown with a

dashed red line.

3) Effects of a special multi-layered insulating window on

the simulated channel are elaborated. It is shown that

small changes of the incident angles of propagation paths

cause signiﬁcant changes in channel LSPs and estimated

coverage due to penetration loss angular selectivity of the

multi-layered windows.

The rest of the paper is organized as follows. Section II

describes the O2I site and its laser-scanned point cloud where

spatio-temporal channel measurements were performed for

validating the ray-tracing results. Section III introduces the

point cloud based ray-tracing methods. Section IV presents

comparisons between measured and simulated O2I radio chan-

nels. The paper is concluded in Section V.

II. OUTDOOR-TO-INDOOR PROPAGATION ENVIRONMENT

This Section describes the laser-scanned point cloud model

utilized in ray-tracing and the measured channel data at the

same site, which were used as ground truth to optimize and

validate ray-tracing results.

A. Channel Sounding Campaign

Measured channels are used as a ground truth for ray-

tracing. The O2I measurement campaign has been the subject

of the authors’ previous publications [6], [24], where a more

detailed description of the measurement set-up, methodology

and site is provided. A total of two transmit (Tx) antenna

locations and 69 receive (Rx) antenna locations were mea-

sured at center frequencies of 4.65 and 14.25 GHz. The Tx

locations were outside the ofﬁce building and Rx locations

were inside the second ﬂoor of an ofﬁce building, distributed

across three different rooms. The Tx antenna was elevated

using a personnel lift to be on the same level with the Rx

antenna. Both measurements used the same bandwidth of

500 MHz. Directionally-revolved channel impulse responses

were obtained by mechanically rotating a horn antenna on the

Rx side [24].

Figure 1 shows an exemplary Power Angular Delay Proﬁle

(PADP) obtained from one of the links. Note that weakest

gain of the PADP is limited to -150 dB, a noise threshold

determined from the PADP. This is done to highlight the

excluded distant paths. Signals exist below this threshold,

but they are not considered meaningful to represent. For all

following analysis, the studied delay range is limited to up

to τ= 350 ns, illustrated with the dashed red line. This

is to compensate for the effect of distant buildings which

sometimes contribute strong propagation paths [24]. These

buildings are not represented in the point cloud model used

in ray-tracing, and hence measured paths from them were

omitted for comparison. A propagation path is deﬁned as a

distinct local maxima in the measured PADP. A search over

the PADP [15], [25] derived a set of discrete propagation paths

to obtain comparable results to the ray-tracing simulations.

B. Point Cloud Acquisition and Processing

The point clouds are captured with a Z+F IMAGER®5006h

3D laser-scanner [26]. The device uses movable mirrors to

steer a laser beam in different directions to detect distances to

reﬂective surfaces. A number of locations outside the building

and inside on the 2nd ﬂoor are scanned and combined into

a complete model of the environment. Resolution of the

point cloud used in this work is approximately 10 cm. To

obtain a point cloud appropriate for ray-tracing simulations,

the following steps were performed.

1) Point clouds obtained outside and inside the ofﬁce build-

ing were aligned and merged into one complete point

cloud using common reference points.

2) Vertical interior walls of the second ﬂoor and the exterior

walls on the level of the Tx-Rx links are extracted

from the laser-scanned point cloud by detecting large

ﬂat sections [12]. They are shown in Fig. 2 in red and

black, respectively, the black wall opposite to the ofﬁce

building being a parking structure. Ceilings and ﬂoors of

the 2nd ﬂoor are removed along with the ground outside

the building to reduce the size of the point cloud.

3) Individual trees and their canopies are extracted from the

laser-scanned point cloud manually. They are shown in

Fig. 2 in various colors.

The complete point cloud model used in ray-tracing is

shown in Fig. 2. All 69 measured Rx locations across three

different rooms are shown with red triangles. Room 1is

a square corner room with triple-glass windows facing the

outside housing Rx locations 1-21. Room 2is a rectangular

room with triple-glass housing Rx locations 22-41. The third

area consists of a kitchen with a double-glass window facing

the outside and a corridor that runs behind rooms 1 and 2,

housing Rx locations 42-69. The exterior walls with triple-

and double-glass windows are highlighted in Fig. 2.

III. POINT CLOUD RAY-TRACING

This Section describes the ray-tracing methods for deter-

mining propagation paths between the Tx and Rx. Gains of

the traced paths are estimated separately as introduced in

Section IV. The direct propagation path between Tx and Rx

Room 1, Rx1−21

Room 2,

Rx22 −41

Corridor,

Rx42 −69

Triple-glass

windows

Double-glass

windows &

Kitchen

Concrete parking structure

Fig. 2: Point cloud ray-tracing model extracted from a laser-scanned point cloud. Exterior walls are shown in black, interior

walls are shown in red. Trees outside the ofﬁce building are shown in various colors. Reference directions of the measurement

campaign are shown in degrees.

along with specular reﬂections are considered. Each traced

path was subject to determine if it undergoes shadowing due

to building walls and vegetation.

A. Direct Path

The direct path between Tx and Rx is determined with the

Tx and Rx locations illustrated in Fig. 2. The Tx constitutes

a starting point of the propagation path and the Rx its ending

point.

B. Specular Reﬂections

Specular reﬂection is an interaction of a plane wave with

an electrically large surface where the angles of incidence and

departure are equal. Our method for detecting specular reﬂec-

tions in a point cloud environment is based on an established

technique [12], [15], [27], which utilizes the image method

and the 1st Fresnel zone. Detection of single-bounce specular

reﬂections from a section of a point cloud is illustrated in

Fig. 3. An image of the Tx is calculated for each point in the

Fig. 3: Detecting a single-bounce specular reﬂection from a

point cloud. Points which satisfy Eq. (1) are colored with red.

point cloud using its normal vector. To ﬁnd all valid single-

bounce reﬂected paths, it is determined if the point lies within

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1Ray-OpticsSimulationsofOutdoor-to-IndoorMultipathChannelsat4and14GHzPasiKoivum¨aki,AkiKarttunen,andKatsuyukiHanedaAbstractRadiowavepropagationsimulationsbasedontheray-opticalapproximationhavebeenwidelyadoptedincoverageanalysisforarangeofsituations,includingtheoutdoor-to-indoorscenario.Thisworkpres...

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