1Intelligent Reflecting Surface Design Using
Reflectarray with Reduced Number of Elements
Qi Luo, Senior Member, IEEE, Steven Gao, Fellow, IEEE, Wei Hu, Member, IEEE, Wei Liu, Senior
Member, IEEE, Mohammed Sobhy, Life Member, IEEE, Yichuang Sun, Senior Member, IEEE
Abstract—The development of reconfigurable intelligent sur-
face (RIS) enables the possibility to control the wireless channels,
which is an attractive technique for the future 6G and beyond
wireless networks. The last few years have seen many studies on
this topic, and these studies show that the use of reconfigurable
intelligent surfaces can effectively control the waveform of the
wireless signals and improve the overall system performance
through creating smart radio environments. The intelligent sur-
face consists of a large number of elements, and ideally, each of
the elements should be independently controllable. This imposes
a big challenge in physically realizing a reconfigurable intelligent
surface with reasonable system complexity and overall cost. In
this article, a proof-of-concept study on reducing the number of
array elements to design an intelligent reflecting surface using
reflectarray is presented. A hybrid design technique is developed,
which mixes two types of antenna elements to realize a reflecting
surface. Compared to the conventional designs that only use
microstrip antennas to realize a reflecting surface, given a fixed
area the presented design uses 25% fewer array elements while
shows comparable performance in terms of manipulating the
waveforms of the incident waves. As a result of using less number
of elements, the overall system complexity and cost of the RIS
are reduced. To verify the proposed design concept, two passive
prototypes with center frequency at 12.5 GHz were designed
and fabricated. Good agreement between the simulation and
measurement results is obtained.
I. INTRODUCTION
Intelligent reflecting surface (IRS) and reconfigurable intel-
ligent surface (RIS) have been identified as one of the key
enabling technologies for the next generation wireless com-
munications, as it can help to reduce the energy consumption
and to improve the spectral efficiency of wireless networks by
effectively controlling the signal propagation environment [1],
[2].
Generally speaking, there are two methods to realize an
intelligent reflecting surface. The first method is using reflec-
tarray (RA), and the other one is using metasurface [3]. Using
either method, the designed reflecting surface would consist
of a larger number of reflecting elements and the reflection
response (e.g. phase and amplitude) of each element should
be independently controlled. For the reflecting surface to have
reconfigurable responses, RF switches, varactors, or phase
shifters should be incorporated into each reflecting element.
For example, a RIS consists of 256 two-bit elements was
reported in [4]. In this design, five positive intrinsic-negative
(PIN) diodes are integrated into each of the RIS elements, and
by controlling the ON/OFF states of the PINs, four states of
phase shifting are obtained. In order to effectively control the
wireless environment, the size of RIS should be electrically
large. In other word, a large number of reflecting elements
is required. For instance, MIT’s RFocus prototype [5] has
more than 3000 antenna elements. Although the cost of the
reflecting surface is relatively low thanks to the modern printed
circuit board (PCB) manufacturing technology, the largest part
of the cost of a RIS is on the controlling components such
as the phase shifters and monolithic microwave integrated
components (MMICs). Moreover, the cost of RF components
increases with the frequency. Thus, from the aspect of physical
layer design, it is important to develop feasible solutions to
reduce the number of reflecting elements with a given aperture
size, while not compromising the performance of the reflecting
surface. The outcome of such a technique would lead to a
reduction in the overall cost and system complexity of the RIS
system, which makes the RIS one step closer to its practical
implementation.
In the field of antenna engineering, there are many studies
on reducing the number of antenna elements for an array
antenna. The first approach is to increase the distance between
the neighbouring antenna elements, as far as the grating lobe
would not appear when the beam of the array is scanned in
the required angle range [6]. For example, a 16×16 array was
reported in [7], where high directivity horn antenna was used
as the array element and the distance between the elements was
increased to 0.87λ0, λ0 being the wavelength in free space at
the center frequency of the antenna. Another approach is using
the sparse array technique. The sparse array uses aperiodically
antenna elements and by synthesising the phase distribution,
beam-steering with relatively low sidelobes can be obtained
using a reduced number of elements [8], [9]. The challenge
for this approach is that not all the elements are radiating
simultaneously, which leads to low aperture efficiency of the
array antenna. Instead of reducing the array element number,
another method that can be used to reduce the complexity
of the beamforming network of the array antenna is through
the use of subarray technique. Using the concept of subarray,
several antennas or metasurface elements are grouped as a
subarray. In this way, the total number of modules required
to control the array is less than the total number of array
elements. Yet, one problem with this technique is that the
resulted array has a limited control over the waveform of
the radiated signal or requires a closer spacing between the
array elements, which may not be feasible for a general array
configuration [10], [11].
The objective of this study is to investigate a feasible
approach to design an intelligent reflecting surface using RA of
reduced antenna elements without sacrificing the performance
2Fig. 1. The concept of the developed intelligent reflecting surface using hybrid reflectarray with reduced number of array elements.
of the reflecting surface in terms of manipulating the waveform
of incident waves. Fig. 1 shows the concept of the developed
intelligent reflecting surface. As shown, the reflecting surface
has two different types of unit cells, microstrip patch and
dielectric lens. The patches are printed on a dielectric substrate
and are manufactured using PCB technology. The dielectric
lenses are fabricated through 3D printing and are placed at
the edges of the reflecting surface. To control each of the
reflecting elements, a phase shifter can be interconnected to
each array element. It should be noted that the use of phase
shifters is not the only solution to realize phase control,
and there are other low-cost solutions as well [12]. In this
study, the configuration of the developed reflecting surface
is compatible to be incorporated with these different phase
control techniques. Since the lens has a size four times as
large as the patch unit cell, one lens is equivalent to four
microstrip patch antennas. In this case, instead of using four
phase shifters, only one phase shifter is required. As a result,
the total number of associated RF components is decreased
and the complexity of the control circuit is reduced.
In this study, for the demonstration purpose, we chose to
design reflecting surfaces operating at the center frequency of
12.5 GHz. The presented design can be scaled for operating
at other frequencies. As a proof-of-concept, the prototypes are
passive demonstrators and the phase shifters are substituted
by microstrip delay lines to mimic the ideal lossless phase
shifters. It will be demonstrated that by using the proposed
hybrid design method, compared to a regular reflecting surface
of the same aperture size, the presented design uses 25% fewer
elements but with a comparable gain and beam-steering perfor-
mance. In the following, the design and working principles of
the RA elements is first presented, followed by EM simulation
and lab measurement results. At the end of the article, we
extend our discussions onto the scalability issue and future
work of this study.
II. ELEMENT DESIGN
As shown in Fig. 1, the developed reflecting surface consists
of two different types of elements, one is microstrip patch and
the other one is dielectric lens. The lens is used to replace the
microstrip patches at the edges of the conventional reflecting
surface, in order to reduce the total number of the radiating
elements and its associated RF components.
The microstrip patch unit cell is a probe-fed patch and the
patch is designed to have a center frequency at 12.5 GHz. Fig.
2 shows the configuration of the patch antenna. The patch is
printed on a 0.5 mm thick RO4003C substrate (εr = 3.55,
tanδ = 0.0027). The period (P) of the unit cell is 15 mm,
which is about 0.625λ12.5GHz . The width (WPatch) of the
square patch is 5.9 mm. The phase shifter is placed on another
0.3 mm thick RO4003C. A 0.1mm thick Pre-peg (εr = 3.54)
is used to bond the two substrates. A clearance hole is etched
on the ground plane through which a via is used to connect the
patch and the microstrip line that is interfaced with the phase
shifter. The working principle of this patch element is that the
patch receives the incident wave and then convert the RF signal
into the electrical current. The electrical current then transmits
to the phase shifter through the interconnections. After adding
some phase shift from the phase shifter, the electrical current
re-transmits to the patch and then the patch re-radiates. In this
way, by controlling the phase shifter, the patch re-radiates the
incident wave with a controllable phase.
Fig. 3 shows the configuration of the Lens antenna. A
pair of connected patches are used as the feed sources of
the lens. The two square patches are probe-fed and have the
same configurations as the microstrip patch element presented
above except that the two patches are connected to each other
through a 50Ω microstrip printed at the bottom layer where
a phase shifter is integrated. The width of the square patch
is 5.4 mm. Patch 2 is positioned at the center of the lens
while Patch 1 is placed at an offset position. The lens is an
extended hemispherical lens. The material of the lens is PTFE
3Fig. 2. The configuration and working principle of the microstrip patch
element for the reflecting surface.
(εr = 2.2) and the dimensions of the extended hemispherical
lens are calculated using the formulas given in [13]. Because
lens antenna has a much narrower beamwidth compared to the
microstrip patch, it would receive very low RF power if the
incident wave is from large angles. As a result, the illumination
efficiency of the RA would be low. To mitigate this issue,
the concept of using connected patches as the feed sources
of the Lens is developed. The working principle of the lens
antenna element is as follows. Here we considered the incident
wave is pointing at the center of the reflecting surface and at
the edge of the reflecting surface, the incident waves come
with certain angles (e.g. 40o). The offset feed patch (Patch 1)
generates a tilted beam from the lens so the lens will efficiently
receive the incident waves. After that, the received RF energy
is then transmitted to the center patch (Patch 2) with added
phase delays from the phase shifter. Finally, Patch 2 feeds the
lens and generates a high directivity beam in the boresight. To
better demonstrate how Patch 1 and Patch 2 operate, Fig. 3 also
shows the simulated current distribution on Patch 1 and Patch
2 when the lens is receiving and re-radiating, respectively.
As shown, when the lens is receiving the RF signal from the
incident wave, there is a strong current on Patch 1. When it is
re-radiating, there is a strong current on Patch 2. To improve
the illumination efficiency of the lens, the position of Patch 1
needs to be optimized. Patch 1 is located on a circle with a
radius of R and angle of Φp relative to Patch 2. These two
parameters respectively determine the angles of the title beam
(θt, ϕt) of the lens.
In this design, the diameter of the lens is chosen to be
28 mm, which is approximately twice the period of the patch
unit cell. In this way, one lens can be used to replace four
patches as in a regular RA. The parameters for the lens are
a = 14mm, b = 17.3mm, and L = 10.2mm. The distance
(R) between Patch 1 and Patch 2 is 8.2 mm (0.34λ12.5GHz).
Having a low mutual coupling between Patch 1 and Patch 2 is
crucial for the presented design because high mutual coupling
would affect the re-radiated waveform of the RA. For example,
when Patch 2 is radiating, if Patch 1 also gets excited due to
high mutual coupling from Patch 2, then unwanted radiation
is generated. The results of the EM simulation show that with
these given parameters, when Patch 2 is excited, the lens
radiates in the boresight with a gain of 12.8 dB. When Patch
1 is active, the beam of the lens titled to 37o with a gain of
Fig. 3. The configuration of the lens antenna and its working principle.
12.5 dB. The simulated isolation between Patch 1 and Patch 2
is higher than 15 dB.
III. REFLECTARRAY DESIGN, SIMULATION, AND
MEASUREMENT
A. Reflectarray Design
To prove the design concept, passive reflectarrays were
designed and simulated in EM simulators. At this stage of our
study, ideal phase shifters are considered. It is worth noting
that the present design is compatible to be integrated with
the phase shifters or RF switches such as PIN diodes and RF
micro-electromechanical system (MEMS) switches [14]. For
example, by placing RF switches on the microstrip line of
the designed array element, similar to the design reported in
[15], 1-bit or 2-bit phase quantization can be obtained. Then,
the antenna elements of the reflecting surface can be coded
to provide different reflection phase distributions and realize
different reflected waveforms.
As mentioned in the last section, to avoid unwanted ra-
diations from the lens, it is crucial to maintain low mutual
coupling between Patch 1 and Patch 2. In this design, the
distance between Patch 1 and Patch 2 is 0.34λ12.5GHz , which
is quite small. Thus, to reduce the mutual coupling, Patch
1 and Patch 2 are recommended to be placed in a diagonal
position relative to each other. This means that the lens would
be placed at the four corners of the aperture. The designed RA
is equivalent to a conventional patch-only reflecting surface
4with 12 × 12 elements. The size of the reflecting surface is
180mm× 180mm (7.5λ12.5GHz × 7.5λ12.5GHz). Compared
to the conventional RA, the twelve patches in each corner
of the aperture are replaced by three lens antennas. Thus,
with the same aperture size, instead of using 144 elements,
the presented design uses 108 elements, corresponding to a
reduction of the RA elements to 75%.
To check the performance of the presented reflecting surface
in terms of reflecting the incident waves to different angles,
four passive RAs are designed and simulated, which have the
reflected beam pointing at elevation angles of 0o, 10o, 20o,
and 30o in the E-plane (ϕ = 0o plane), respectively. These
RAs have the same configuration and the only difference is
the phase delays added to the reflected signal.
B. Simulation Results
Fig. 4 shows the simulated far-field radiation patterns of
the designed RAs in the E-Plane when the reflected beam
is pointing at different angles, including (θ = 0o, ϕ = 0o),
(θ = 10o, ϕ = 0o), (θ = 20o, ϕ = 0o), and (θ = 30o, ϕ = 0o).
In the EM simulations, a microstrip line of varying lengths
is used to adjust the reflected phase of each array element,
representing the case that an ideal lossless phase shifter is
employed. The EM simulations also assume that the incident
wave is pointing at the center of the reflecting surface and
a circular waveguide antenna, which has a gain of 14 dBi at
12.5 GHz, is placed 120 mm above the center of the reflecting
surface to generate the incident waves.
-100 -80 -60 -40 -20 0 20 40 60 80 100
Theta (Degree)
-30
-20
-10
0
10
20
30
G
ai
n 
(dB
i)
Reflected beam angle=30deg
Reflected beam angle=20deg
Reflected beam angle=10deg
Reflected beam angle=0deg
Fig. 4. The simulated reflected waveforms of the developed hybrid RAs.
In order to compare the performance of the developed
hybrid reflecting surface with the conventional design that
only uses patch as the array element, four passive patch-only
reflecting surfaces are also designed and simulated. These
four RAs have the same aperture size and configuration as
the developed hybrid RAs. The radiation performances of
these two types of designs at the centre frequency (12.5 GHz)
are summarized in Table I. As shown in this table, when
configuring the reflected waves to point at different angles,
these two types of RAs show similar waveforms. The gain of
the hybrid RA is 1.4 dB lower than the patch-only RA when
the reflected waveform is directing to 30o. This is due to the
narrow beamwidth of the lens and is the trade-off when using
lens in the design of reflecting surfaces. Except for this, with
25% less reflecting elements, the developed hybrid RAs show
comparable radiation performances to the conventional patch-
only RAs.
C. Measurement Results
To validate the radiation performance of the developed re-
flecting surface, two prototypes were fabricated and measured.
One prototype has the reflected beam pointing at boresight,
and the other prototype has the beam pointing at 20o in the E-
plane. The lenses were fabricated through 3D printing which
has low-cost and each of the lens is attached to the PCB
by four screws. Fig. 5 compares the normalized radiation
patterns of the measured and simulated results. There is
good agreement between the simulations and measurements.
In order to fix the reflecting surface and the waveguide
antenna to the testing platform, some plastic fixing structures
were fabricated. The unwanted reflections from these fixing
structures have some affects on the sidelobes of the measured
radiation patterns.
-90 -60 -30 0 30 60 90
Theta (Degree)
-40
-35
-30
-25
-20
-15
-10
-5
0
N
or
m
ia
liz
ed
 G
ai
n 
(dB
i)
Simulated
Measured
Simulated
Measured
Fig. 5. Comparison of the normalized radiation patterns of the measured and
simulated radiation patterns.
IV. DISCUSSION
This study is a proof-of-concept for the proposed hybrid
design method to reduce the number of elements in a reflecting
surface with a given aperture size. At the current stage of
this study, before extending the presented design concept to
an electrically reconfigurable reflecting surface, we designed,
fabricated and measured passive prototypes as demonstrators.
During the design of the demonstrators, some assumptions are
made which to some extent simplifies the reflecting surface
design. In this section, we would like to discuss how to extend
the present design concept in our future work of this study.
• Frequency scalability. In terms of frequency scalabil-
ity of the presented design, considering the fabrication
accuracy of the PCB technology and the resolution of
3D printing, the developed reflecting surface can be
scaled to operating in the millimetre frequency range.
For Terahertz (THz) design, due to the loss associated
with the dielectric materials and the relatively narrow
bandwidth of the microstrip antennas, waveguide antenna
would be preferred. Thus, the presented method may not
5TABLE I
COMPARISON OF THE PRESENTED DESIGN WITH THE CONVENTIONAL PATCH-ONLY RA WITH BEAM POINTING AT DIFFERENT ANGLES.
Presented Hybrid RA Conventional Patch RA
Beam an-
gle
θ = 0o θ = 10o θ = 20o θ = 30o θ = 0o θ = 10o θ = 20o θ = 30o
Aperture
size
7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ 7.5λ× 7.5λ
No. of el-
ements
108 108 108 108 144 144 144 144
Gain 22.9 dBi 22.2 dBi 21.4 dBi 19 dBi 23 dBi 22.5 dBi 21.8 dBi 20.4 dBi
Half
power
beamwidth
8o 8.1o 11.2o 13.6o 8o 8.1o 11.3o 12.8o
Sidelobe
level
15 dB 14.4 dB 13.1 dB 11 dB 11 dB 15.3 dB 16.9 dB 11.4 dB
be suitable to be directly extended to the Terahertz (THz)
reflecting surface design.
• Size scalability. In terms of size scalability, the reflecting
surface can be enlarged by increasing the number of array
elements. The first step would always be designing the
reflecting surface using the patch-only elements and then
using lens to replace the microstrip patches at the edge of
the aperture. In the presented design, three Lens antennas
are placed at each corner of the reflecting surface. The
reason for choosing this approach is that the size of the
patch-only RA is 12 × 12. When the size of the RA is
larger, the proposed design technique can be extended
to increase the number of lens antennas. We would
recommend to place the lens at the corners of the aperture
and place its feed patches in a diagonal position, in order
to maintain low mutual coupling between the feed patches
of the Lens.
• Reflected waveform control. In this work, it is assumed
that the incident wave is pointing at the center of the
reflecting surface and we only investigated reflecting the
incident waves up to 30o off the boresight. In practical
applications, it is desirable to direct the reflected waves
to larger angles and the angle of incident waves may
be different. To mitigate these issues, multiple patches
can be employed as the feed sources of the lens and RF
switches can be introduced to activate the receiving and
transmitting patch feeds. As a trade-off, the improved re-
configurability comes with the price of increased control
complexity but the number of required phase shifters are
still decreased. This needs to be further investigated in
future studies.
• Efficiency. From the simulation and measurement results,
the developed reflecting surface using hybrid RA shows
comparable radiation performance when the reflected
waveform is directed within the angle range from 0o to
30o. In principle, similar waveforms will be obtained if
the reflected waveform is directed within the angle range
from −30o to 0o. Thus, it can be concluded that the
developed hybrid design approach does not compromise
the efficiency of the reflecting surface if the reflected
waves are within the angle range of ±30o. With cur-
rent configuration, the efficiency of the presented design
would decrease when the angle of the reflected wave is
larger than 30o but this issue can be solved by employing
multiple patch feeds in the lens, as discussed above.
V. CONCLUSION
In this article, a hybrid design method to reduce the number
of elements in a reflecting surface has been introduced. This
method uses both lens antennas and microstrip patches to
realize a reflectarray, leading to a reduction of 25% of the
reflecting elements compared to a regular 12×12 patch array.
With a reduced number of elements, the developed reflecting
surface shows similar gain and radiation patterns to the patch
only RA of the same aperture size when the reflected wave
is within the angle range of ±30o. This study is a proof-of-
concept and passive demonstrators are designed and fabricated
to verify the proposed design concept. Future work for this
study includes improving the beam reconfigurability of the
reflected waves and realizing an electrically reconfigurable
demonstrator.
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Qi Luo (S’08–M’12) is a Senior Lecturer at the School of Physics, Engineer-
ing and Computer Science, University of Hertfordshire, the UK. His research
interests are reconfigurable intelligent surfaces, smart antennas, circularly po-
larized antennas, phased arrays, metasurfaces, multiband microstrip antennas,
and electrically small antenna design.
Steven Gao (M’01–SM’16–F’19) is currently a Professor and the Chair of RF
and Microwave Engineering, and the Director of Postgraduate Research with
the School of Engineering and Digital Arts, University of Kent, Canterbury,
U.K. His research interests include smart antenna, phased array, multi-in
multi-out (MIMO), broadband and multi-band antennas, small antennas, RF
front ends, FSS, and their applications into 5G mobile communications,
satellite communication, small satellites, radars, energy harvesting and medical
systems.
Wei Hu (Member, IEEE) is currently an Associate Professor. From 2018
to 2019, he visited the University of Kent, U.K., as an Academic Visitor.
His current research interests include multiband and wideband antennas,
circularly polarized antennas, MIMO antenna arrays, and wideband wide-
scanning phased arrays.
Wei Liu (Senior Member, IEEE) has been with the Department of
Electronic and Electrical Engineering, The University of Sheffield, Sheffield,
U.K. since 2005, first as a Lecturer and then a Senior Lecturer. His research
interests cover a wide range of topics in signal processing, with a focus on
sensor array signal processing and its various applications, such as robotics
and autonomous systems, human computer interface, radar, sonar, satellite
navigation, and wireless communications.
Mohammed Sobhy (LM’90) is currently an Emeritus Professor of Elec-
tronics with the University of Kent, Canterbury, U.K. His current research
interests include RF circuits design, analysis and applications of nonlinear
electronic systems.
Yichuang Sun (Senior Member, IEEE) a Professor of Communications and
Electronics (since January 2001), Head of Communications and Intelligent
Systems Research Group, and Head of Electronic, Communications and
Electrical Engineering Division in the School of Engineering and Technology
at the University of Hertfordshire, UK. Professor Sun has been conducting and
leading research in wireless and mobile communications, RF/microelectronic
circuits and systems, instruments and measurement, and neural networks and
machine learning in the School.