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Irregular MultiFocal Reflector for Efficient mmWave Propagation in Indoor Enviroments

Irregular MultiFocal Reflector for Efficient
mmWave Propagation in Indoor Enviroments
J. Samuel Romero-Pe˜
na , Narcis Cardona
iTEAM Research Institute, Universitat Polit`
ecnica de Val`
encia , Val`
encia , Spain
{jhrope, ncardona}
Abstract—In future implementations of 5G systems , it is
essential the use of the spectrum in the range of mm-Waves
frequencies , in order to offer to the users the bandwidth
proposed in the standard. However , using this frequency range
lead to many technical difficulties in which the most important
challenge is the critical attenuation of the signal in non-line-of-
sight (NLOS) environments in indoor environments. Therefore is
essential to plan strategies that allow us to mitigate the problem
of signal attenuation in this kind of complex environments and
ensure the viability of using this technology in short term. Then
the objective of this research is the design of a passive reflector
that allow us to redirect the energy of the transmitting antenna
efficiently in order to avoid the obstacles of the environment ,
and therefore avoid excessive losses .
Index Terms—5G Systems, mmWave, Reflector, Reflection ,
During the last years there has been a substantial increase
in the demands of greater bandwidths , because today the
use of the mobile phone is more widespread , not only for
mobile services that consume little bandwidth such as email
, calls , Twitter , What-App , etc ; as also more asymmetric
use of the spectrum like the streaming uses such YouTube ,
Netflix , Instagram , Facebook , etc, that have more and more
multimedia content [1] .
For this reason , the user does and will do a more demanding
use of the spectrum and therefore the industry sees a need
to have a greater bandwidth to offer a better multimedia
experience. But due to technical limitations nowadays is
impossible to offer this speed that is being demanded, because
the spectrum is almost full. Therefore the telecommunication
industry has proposed to take a technological leap and explore
the spectrum in the range of mm-Waves.
One of the greatest technical challenges to offer services at
the mm-Wave frequency are the very high propagation losses
in free space that are an intrinsic behaviour as the frequency
increases .Additionally the NLOS losses that are also high in
indoor environments because all the obstacles are electrically
larges [2] .
Several solutions have been proposed thought in literature
to avoid the issue of high signal attenuation in both indoor
and outdoor environments. Some of these proposed solutions
include beam-forming [2] and beam-steering techniques [3]
using multiple antennas , high transmit power and high sensi-
tivity receivers [4] , and use of multiple active repeaters [5].
However , all these strategies are very complex and expen-
sive techniques , for the reduced coverage of each base station
in mm-Wave would be the equivalent of the current peak cells
in the best cases (100 mt2) [6]. For this reason , the initial
investment will be considerably high in the early stages .
In addition to the technical and coverage problems ,the max-
imum power allowed by the telecommunications regulatory
entities , for possible health problems due to energy adsorption
in the human tissues at mm-Wave [7].
Therefore , and according to all these previously issues , the
best technical and economic solution will be the design of a
passive reflector in mm-Wave . This reflector allow to redirect
the energy from the base station transmitter antenna to the
user efficiently, with the aim to avoid the excessive losses and
consequently increasing the coverage.
To begin with the design of this passive reflector , the
starting point idea [8] was worked with a rough surface,
because is the best way to increase mmWave coverage with a
diffuse reflection.
To the best of the authors’ knowledge there is no works in
literature devoted to this issue. Although there are some studies
related to reflectors for bidirectional communication links [9]–
[13] and also studies about radar reflectors [14]–[16].
The reference paper [8] was the only one that has been
considered irregular reflector surfaces for mmWaves. The
paper [8] refers to a flat reflector with wedges that emulates a
parabolic reflector with a fixed focal position, that can be used
for satellite communications , because it reduces its volume .
So our goal is to adapt this idea to design a diffuse reflector
according to our research.
The objective of this research was to design a reflector that
allow to distribute energy over a wide coverage area. Due
to which the reflector cannot be a flat reflector , because in
the future 5G systems at mm-Wave will be offered with very
directive antennas to counteract the highest losses in the free
space . Therefore with a flat reflector is the output radiation ,
will depend of the transmitter antenna directivity and its angle
of incidence on the reflector . Therefore a flat reflector will
have a specular behaviour , that is not useful for the indoor
scenarios (See Figure 1 ).
So the main objective of this new passive reflector will be
homogeneously spread the energy received by the transmitter
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antenna over a specific coverage region , that will be indepen-
dent of the reflector location and incidence on the reflector
(See Figure 1).
Fig. 1. Specular reflector vs Diffuse reflector for mmWave coverage man-
agement in 5G Systems
A. Design guidelines
Figure 2 depicts the scenario considered for this study. As
can be shown, The active antenna is that highly directive array
antenna that feeds the reflector. The angle of entry (θin) is
the angle at which the radiation of the active antenna hits
the reflector and the coverage area (θcoverage ) offered by
the reflector will depend on it. The output radiation (θout)
refers to the angular area radiated by the reflector, which
depends on the input radiation of the active antenna. The
input margin (θinmargin) is the angular range in which the
transmitting antenna can hit the reflector without affecting
the coverage of the reflector (θcoverage ), according to the
design guidelines.Reflector losses (Lref lector ) is the difference
between the input gain(Gin) and the output gain (Gout ), in
other words it is the maximum gain lost when dispersed by
the reflector.
Fig. 2. Nomenclature used for reflector design guidelines
It should be taken into account that the energy should be
homogeneously distributed in indoor environments. To achieve
this goal, there are some guidelines that should be considered:
The reflector must maintain a fixed coverage area of
θcoverage = 40 , where the angle of entry can have a
margin of (θinmargin = 20). Therefore the reflector
does not have a focal position.
The output radiation of the reflector must be homoge-
neous enough to assure that in a given area there is no
fading of the signal of more than 6dB. Ensuring reflector
losses (Lref lector ) not greater than 12dB.
The reflector bandwidth has to be 10 GHz, in order to be
versatile enough to be used in future 5G systems in the
millimeter band. The operating range will be 30 GHz to
40 GHz.
B. Design Methodology
Starting from the premise proposed in the paper [8], we
design the reflector. One of the main requirements to start the
design of the reflector was that it be a periodic, symmetrical
and parametrizable surface. The surface has to be parametriz-
able depending on the wavelengths and the angular incidence
of the radiation.
As seen in figure 3, the angle of incidence on the reflector is
a critical parameter, because according to where the transmitter
radiation hits the reflector, the energy will be dispersed in
unwanted areas (Path B,C), losing the energy that wants to
be redirected and conserve like the (Path A). It can also be
observed that depending on the angle of incidence of radiation
there will be areas of shadow between wedge and wedge,
which reduce the effective surface and do not redirect as much
energy to the area of interest (lost energy).
Fig. 3. Design considerations for the passive reflector
Therefore, the reflector cannot be designed lightly and it
is necessary to make several iterations with different surface
parameters (θn,An,hn,λnand Shapen) according to the
design guidelines. The design parameters were adjusted using
a ray tracing logic, trying to predict where each ray would go
based on its specular reflection. Each surface was designed as
a coefficient matrix of its parameters (See Figures 3,5 ), so
that the reflector has a gradual behavior , in order to avoid
the shadows between wedges and reflections in unwanted
Therefore, different types of reflectors with different geo-
metric shapes began to be designed, in order to identify which
are with more potential .(See Figure 4).
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Fig. 4. Proposed geometric shapes for reflector design
A. Simulation Set-Up
The simulations were carried out using the electronic analy-
sis platform ”CST (Computer System Technology) [20]”. The
methodology consisted of locating the reflector in the far field
area of the transmitter, with the objective that , the incident
radiation in the reflector will be a plane wave (See Figure 5).
The transmitting antenna used was a horn antenna in the Ka
band from 26GHz to 40 GHz [19]. The reflector is an irregular
metal surface that we have previously designed(See Figure 5).
Once the antenna and the reflector are located, we simulate the
final far field together of the two elements (Tx + Reflector).
Fig. 5. Simulation scenario used to characterize reflector radiation
Therefore, the resulting far field will have the contributions
of the transmitting antenna and the reflector, where the radi-
ation pattern of the reflector will be where according to the
orientation of the reflector θbase = 45with respect to the
transmitting antenna θin = 0this would have a maximum
radiation at θout = 90(See Figure 5). Therefore reflector
radiation pattern results will be centered at 90(See Figure
During the testing and design process of the reflector they
were tested with different types of surfaces, with different
electrical periodicity of the roughnesses. This was a very
arduous process because it was necessary to design several
types of surfaces with a 3D professional modeling software
[21]. Additionally each surface was simulated several times
because we had to hit on the same surface with different
angle of entry (θin) to observe the diffuse behavior (θcoverage )
resulting from the reflection (See Figure 2).
Because part of our research work was based on paper [8],
where irregular flat surfaces were designed with wedges in
order to emulate parabolic reflectors. We started designing
this type of reflectors in order to meet the proposed design
guidelines. As previously mentioned, these surfaces were
designed with a matrix of coefficients in which each coefficient
determines each wedge parameter (θn,An,hn,λn) as can
be shown in figure 6-b.
Figure 6 depicts an example of one of the many flat reflec-
tors that have been designed. where the matrix of coefficients
of the slopes of each wedge is observed, and it is observed how
each section of the reflector has different slope, some negative
(green cells) and others positive (red cells) (See Figure Figure
6-b). Each wedge has different slopes because the objective
of the reflector is to direct the energy in different positions in
order to obtain a diffuse reflection.
Fig. 6. Antenna design with geometric flat slopes with coefficient matrix ,
(a)Proposed irregular reflector, (b) Coefficient matrix
In figure 7 shows the field radiated by the reflector that
has been designed in figure 6. This result is compared with
the field radiated by a completely flat reflector (Red Line),
where its maximum radiation follows a fully specular behavior
θout = 90, and also in these results the radiation pattern of
the horn antenna is observed (Blue Line) θin = 0.
Fig. 7. Results of the radiation pattern of the irregular reflector proposed in
Figure 6
After the results obtained not only from this example design,
but from several designs previously considered with different
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coefficient matrices. It is concluded that for the proposed
design guidelines, where a fundamental premise is that there
should not be a fixed focal position. All surfaces created
with flat slopes (See figure 4-a ) have a strong specular
behavior, although each wedge has a different parameter to
that of its adjacent. These surfaces with flat slopes had bad
results , because there are usually phase problems between the
contributions, which cause the resulting pattern of the reflector
to attenuate the signal considerably and have strong fading as
seen in the figure 7 (Yellow Line”Irregular-Reflector”).
Finally after several unsatisfactory designs it was decided to
design surfaces where there was a smooth transition between
wedges.Surfaces with gradual and continuous slopes, in order
to avoid abrupt discontinuities on the surface that could
be affecting the sum consistent (in phase) of the multiple
contributions of the transmitting antenna (radiation received
by the reflector) . For this reason the surfaces (See figure 4-
b,c,d ) were proposed.
With the objective that the reflector maintains a fixed cover-
age area, the strategy used was to ensure that the reflector had a
radiation pattern as wide as possible, in order to keep the fixed
coverage area independent of the angle of incidence, while stay
on the design margins (θinmargin ).With this strategy, power
is exchanged for coverage, between more coverage, less power
and vice versa. Because the reflector is a passive element,
its only function is to distribute energy efficiently, so it is
important that this reflector will be supported by very high
gain directive mmWave antennas.
The proposed design is in Figure 8, where it is composed of
two types of surfaces. The first surface is a concave surface,
where it is designed with a cosine function with a period of 0
to 90with amplitude (Z axis) of 15 mm and with dimensions
of its base (X-Y axis) of 100mm x 100mm. The orientation
of the second surface is orthogonal to the first surface, and
is a convex surface, formed by a cosine function, where only
negative cycles (convex) are considered, with a period 10 times
lower than the first surface and with an amplitude (Z axis) of
0.5 mm.
This surface was not designed with a matrix of coefficients,
because this surface is composed entirely of sinusoidal func-
tions, so it does not apply to the design system proposed above
for flat slope surfaces of Figure 6.
Fig. 8. Proposed design of the best reflector to improve indoor coverage
The reflector that has been designed is a periodic reflector,
which means that a larger reflector may be available if the
same reflector is placed consecutively. The objective of making
the reflector larger will depend on the directivity of the
transmitting antenna that will feed the reflector, because the
less directive the antenna is, the greater the surface of the
reflector should be.
This reflector design avoid that the contributions were in
counter-phase and therefore that there was a considerable
fading in the coverage area.The final design of the reflector
is show in the Figure 9-b.
Figure 9-b shows the field radiated by the reflector that
has been designed (See Figure 8). This result shows the field
radiated by the reflector at 30GHz, with different positions of
the transmitting antenna(See Figure 9-a ). In each position of
the Txthis hits in the reflector different with different input
angles θin = 0,10,10, according to the design guidelines
there should not be a fixed focal position, but a input range
of θinmargin = 20.
Fig. 9. (a)Simulated reflector scenario with different Tx positions.(b) 2-D
radiation pattern by the diffuse reflector with different input angles θin =
0,10,10of Tx at 30GHz
According to the results of the radiation pattern (See Figure
9-b), it is concluded that, from a specular reflection at 90,
with a beam width of 15to 12dB and a maximum direc-
tivity of 23dBi. The proposed diffuse reflector increased the
coverage area (θcoverage ) from 15to 100to 12 dB, ho-
mogeneously distributing the energy throughout the coverage
range, in the case of any fixed position of the transmitter. But
in the case where the angle of entry is changed in a range
of θinmargin = 20, the coverage area in common becomes
approximately 72. In the case of this reflector, maximum gain
was exchanged for coverage and flexibility.
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The following results (See Figure 10) show the behav-
ior of the radiation pattern of the reflector, as it changes
between three operating frequencies (30GHz-35GHz-40GHz)
and different input angles ( θin = 0,10,10) in order to
determine the bandwidth proposed in the design guidelines in
the mmWave spectrum.
Fig. 10. Field coverage radiated by the reflector at different frequencies
(30GHz,35GHz,40GHz) and input angles ( θin = 0,10,10)
According to the previous results, it is concluded that the
reflector operates in the proposed bandwidth from 30GHz to
40GHz, for any position of the transmitting antenna in the
range of θinmargin = 20.
This paper has presented a solution to distribute energy
in indoor environments efficiently in the mmwave range.
Therefore, a passive reflector has been designed that distributes
the received power homogeneously, increasing the coverage
area by 450% in exchange for a maximum gain reduction of
approximately 10 dB in an operating bandwidth of 10GHz
(30GHz-40GHz ) .This reflector aims to solve coverage prob-
lems in 5G systems in indoor environments, in order to avoid
losses due to obstacles.
This work was supported by the H2020 Marie Curie pro-
gram, with project grant no: 766231 WAVECOMBE-ITN-
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... Recently, with the extensive exploration of beyond the fifth-generation (B5G) and the sixth-generation (6G) technologies, wireless communication using metal reflectors has gained renewed interest, since it provides a cost-effective solution for coverage expansion, signal enhancement, rank improvement and blindzone compensation, as illustrated in Fig. 1. For instance, fully passive metal reflectors can be used for indoor communication improvement [6]- [8], millimeter-wave (mmWave) signal coverage enhancement [9]- [14], and energy redirection [15]. Compared to the extensively studied semi-passive techniques like intelligent reflecting surfaces (IRSs) -assisted communications [16]- [18], though metal reflectors cannot achieve dynamic reflection, they possess many attractive advantages, including zero energy consumption, ultra low cost, signaling- and maintenance-free, easy deployment and full compatibility with existing and future wireless systems. ...
... It is observed from (15) and (17), that the maximum reflection direction of metal plates also depends on the polarization angle of the incident wave, rather than always occurs exactly at the specular reflection direction with θ r = θ t . ...
... In this section, we validate our developed model with experimental measurements in an open space. For convenience of measurement, the models (15) and (17) are verified. We use Table I. ...
Wireless communication using fully passive metal reflectors is a promising technique for coverage expansion, signal enhancement, rank improvement and blind-zone compensation, thanks to its appealing features including zero energy consumption, ultra low cost, signaling- and maintenance-free, easy deployment and full compatibility with existing and future wireless systems. However, a prevalent understanding for reflection by metal plates is based on Snell's Law, i.e., signal can only be received when the observation angle equals to the incident angle, which is valid only when the electrical dimension of the metal plate is extremely large. In this paper, we rigorously derive a general reflection model that is applicable to metal reflectors of any size, any orientation, and any linear polarization. The derived model is given compactly in terms of the radar cross section (RCS) of the metal plate, as a function of its physical dimensions and orientation vectors, as well as the wave polarization and the wave deflection vector, i.e., the change of direction from the incident wave direction to the observation direction. Furthermore, experimental results based on actual field measurements are provided to validate the accuracy of our developed model and demonstrate the great potential of communications using metal reflectors.
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