The document discusses reconfigurable intelligent surfaces (RIS), also known as intelligent reflecting surfaces. It provides details on: 1) How RIS uses small antennas/reflecting elements that can individually tune their phase shifts to control signal propagation and overcome issues like shadowing. 2) The design of RIS reflecting elements, which consist of three layers - a top patch layer connected to varactor diodes, a middle ground plane, and a bottom biasing layer. 3) Experiments testing the RIS's voltage-phase response, channel reciprocity, and beamforming performance to validate its ability to control signal reflection.

1. Reconfigurable
Intelligent Surface
Or
Intelligent Reflecting
Surfaces
Our mentor : Ankur bansal sir
By:
Ajeet Singh Rawat
Ashish Kumar Meena

2. Communication Technology And 5G
● lower-frequency radio waves can travel long distances and penetrate walls and
obstacles.
● That means that carriers can deploy much larger networks without having to build a
vast number of new cell towers.
● MIMO (Multiple Input Multiple Output ) technology is used as
core of 5th generation cellular network
● Use of arrays of 64 or more antenna-integrated radios to
enable precise beamforming towards any location in the cell
● And to enable spatial multiplexing of many user terminals.

3. Little Bit on MIMO
● wireless technology that uses multiple transmitters and receivers to transfer more data at the same
time.
● sends the same data as several signals simultaneously through multiple antennas, while still utilizing
a single radio channel.
● This is a form of antenna diversity, which uses multiple antennas to improve signal quality and
strength of an RF link.

4. What is shadowing ??
● Shadowing effects are defined as the effects of received signal power
fluctuations due to obstruction between the transmitter and receiver.
● Therefore, the signal changes as a result of the shadowing mainly come from
reflection and scattering during transmittal.

5. How we overcome it in Past??
● The traditional approach to overcome such propagation limitations is
to utilize relays in between the base station and intended receivers
to fill coverage holes
● We used anything from passive repeaters (e.g., a carefully rotated
copper plate that reflects signals in a predetermined direction) to
relays with baseband processing (e.g., using a decode-and-forward
protocol where an amplified signal is retransmitted after noise removal). Image: Passive repeaters

6. Intelligent Reflecting Surface
• 2D surface.
• Composed of a large number of sub-wavelength reflecting elements (small antennas such as micro-strip
patches).
• Each reflecting element is connected to a tunable chip to change its load impedance such as PIN diode or
varactor.
• The on/off state of the PIN diodes results in different load impedances and generate a phase-shift
difference of π.

7. Intelligent Reflecting surface
● Also if we control bias voltage of the varactors, this results in continuously tunable load impedance
and induces in continuous phase shift.
● In addition, Variable resistor can be attached to change the amplitude of reflection coefficient.
● So, we control the reflection coefficient (amplitude and phase) of each reflecting element
individually.

8. IRS practical advantages for Implementation
IRS possesses various practical advantages for implementation.
● First, its reflecting elements (e.g., low-cost printed dipoles) only passively reflect the impinging signals
without requiring any transmit radio-frequency (RF) chains, thus can be implemented/operated with
orders-of-magnitude lower hardware/energy cost as compared to traditional active antenna arrays or
the recently proposed active surfaces.
● IRS operates in full-duplex (FD) mode and is free of any antenna noise amplification as well as self-
interference, which thus offers competitive advantages over traditional active relays.
● IRS is generally of low profile, light weight, and conformal geometry, it can be easily mounted
on/removed from environment objects for deployment/replacement.

9. Elements of IRS
● So we know that RIS can be built using artificial electromagnetic metamaterial,
which consists of periodic arrangements of specifically designed subwavelength-
sized structural elements .
● Such metamaterials have unique electromagnetic properties that do not exist
in nature , such as, negative refraction , perfect absorption, and anomalous
reflection/scattering.
● The reflective elements are densely packed without spacing. Each element consists
of three layers.
● The top layer contains two pairs of rectangular metallic patches, each of which is
connected by a varactor diode with the junction capacitance controlled by the external bias voltage.
● The middle layer is a ground plane for reflecting impinging waves. The ground plane is connected to the patches on
both sides of the top layer through four via-holes serving as ground, for design convenience.
● The bottom layer contains direct current (DC) biasing lines that regulate the varactor diodes on the top layer.

10. Shadowing and Intelligent Reflecting
surfaces(IRS)
● To overcome the shadowing effect we can use the intelligent
reflecting surfaces to get the desired signal.
● We will use these intelligent reflecting surfaces(IRS) on building walls,
and many other places so the signal does not scatter and become
weak.

11. Intelligent Reflecting surfaces for
wireless communications
Use IRS technology to:
● Control propagation environment
● Example: Improve signal-to-noise ratio.
A signal processing problem
● Learn propagation channels
● Determine how to configure the RIS

12. IRS Reflection
Whenever the transmitter sends the signal, sometimes the
signals scatter and the user doesn’t get the maximum signal.
We can change the properties of each elements as we require due to this we can control
the reflection of the signals and transmit to them in the required direction.
We control each elements with the help of the IRS controller as shown in the figure.

13. Reflective Element Design

14. Reflective Element Design
They designed a metasurface tuned by varactor diodes for operation in the C-band.
The reflective elements are densely packed without spacing. Each element consists of three
layers. The top layer contains two pairs of rectangular metallic patches, each of which is
connected by a varactor diode with the junction capacitance controlled by the external bias
voltage.
The middle layer is a ground plane for reflecting impinging waves. The ground plane is
connected to the patches on both sides of the top layer through four via-holes serving as
ground, for design convenience.
The bottom layer contains direct current (DC) biasing lines that regulate the varactor diodes
on the top layer. Two via-holes connect the central patches to biasing lines.

15. Phase Shift
Let vn = βn(θn)e^jθn with θn ∈ [−π, π) and βn(θn) ∈ [0, 1] respectively denote the phase shift and the
corresponding amplitude.
Specifically, βn(θn) can be expressed as:
where βmin ≥ 0, φ ≥ 0, and k ≥ 0 are the constants related to the specific circuit implementation.
βmin is the minimum amplitude,
φ is the horizontal distance between −π/2 and βmin, and
k controls the steepness of the function curve. Note that for k = 0 is equivalent to the ideal phase shift model,
i.e., βn(θn) = 1, ∀n.

16. Concept of Phase Shift
we illustrate one typical architecture of IRS, which consists of three layers and a smart controller.
● In the third/inside layer that is a control circuit board responsible for exciting the reflecting elements
as well as tuning their reflection amplitudes and/or phase-shifts in real time.
● It is observed that the minimum amplitude occurs near zero phase shift and approaches unity (the
maximum) at the phase shift of π or −π.
● It is observed that a reflecting element is capable of achieving almost 2π full phase tuning.
● It is desirable to have independent control of the amplitude and phase shift of each IRS element for
optimizing the reflection design.
● The IRS reflection amplitude and phase shift per element can be independently and continuously
tuned

17. Increasing Efficiency through Codes
● Compelling advantage of the RIS technology exists when the transmitter and receiver have a single
dominant path to the RIS, and the RIS is configured based on these paths
● Insight is utilized in our prototype to greatly reduce the training/feedback overhead when configuring
the surface.
● Use this property to develop an algorithm that gradually changes the phase shifts on a column - by-
column or row-by-row basis to gradually increase the SNR of the end-to-end link.
● The basic idea of our proposed greedy algorithm is to invert the phases of a certain row or column of
the RIS. If this configuration state increases the power of the received signal, compared to the
previous state, the new state is utilized. The algorithm is enabled by our UE-RIS feedback module.

18. Discussion on test system

19. Results of Test
Bias Voltage Response Test:
● Due to the tolerances of the PCB manufacturing process, the difference in the dielectric constant of the material, and
the non-ideal characteristics of the varactor diode, the electromagnetic characteristics of a fabricated RIS are often
different from simulated one
● Used horn antennas to transmit and receive signals precise control of the reflected signal, we measure the relationship
between the reflection coefficients and the bias voltage
● Tested the voltage-phase response at different incident angles: 15◦, 30◦, and 45◦ in the azimuth plane.
● experimental results show response of the RIS elements is angle-dependent, thus one may not find a pair of bias
voltages that will result in exactly 180 degrees phase difference for any incident angles.
● A larger incident angle results in smaller phase variations.
Channel Reciprocity Test
● The time division duplex (TDD) mode is widely adoptedin 5G mobile communication systems and Wi-Fi.

20. ● When it comes to RIS-aided communications, UL/DL channel reciprocity would imply that the RIS
can be configured to beamforming signals in UL and then work equally well in the DL without
having to be reconfigured.
● It means a channel estimate computed in one TX-RX direction can be reused when transmitting in
the opposite direction, provided that the configuration remains unchanged.
● It also implies that if we configure the RIS for one direction, then we may keep the configuration
when transmitting in the opposite direction.
Radiation Pattern Test
● beamforming performance of the fabricated RIS, carried out in a microwave anechoic chamber
● The RIS and transmit antenna are fixed on rotating platform, with transmit antenna facing the RIS.
● Reflection coefficients according to 2D-DFT codebook to form a beam in the azimuth direction of
30 degrees. The codewords were 1-bit quantized , The platform was rotated to measure the
radiation pattern.
● The measurement shows that a high gain beam is generated in a direction of 30◦ . The half-power
beamwidth is 5.2◦. The largest side lobes is −8.79 dB and is located to the left, while the largest
sidelobe at the right side is −11.8 dB.
Indoor Over-the-Air Test

21. ● Transmitter is placed in the corridor and the receiver in a room next. The transmitter and receiver are separated by a 30 cm thick
concrete wall, which includes a 53 cm thick pillar in the middle. In such a case without an LoS path, the transmitted signal suffers from
penetration loss before reaching the RIS. Nevertheless, the proposed Algorithm 1 can be utilized to configure the RIS to gradually
increase the received signal power.
● position and angle of the copper plate are the same as the RIS.
● The results shows configured using the proposed algorithm brings a power gain of around 26 dB. experiment shows that an RIS can be
very effective also in non-LoS scenarios, at least over short distances.
Outdoor Over-the-Air Test
● 50 Meter Test: field trial was carried out on the roof. The transmit power was 13 dBm.
● The results show a 27 dB power gain when using the RIS, as compared to using the copper plate. This number is well aligned with the
power gain observed in the indoor test.
● It is generally hard to compare measurement results like this with theory since there are many sources of uncertainty. However, to
put the numbers into perspective, suppose the copper plate would behave similarly to an RIS that has a random configuration.
● a fully optimized RIS beamforming could provide an average power gain up to 1100 = 30.4 dB. Furthermore, it is known that 1-bit RIS
configurations suffer a −3.9 dB loss on average [18]. Hence the predicted power gain in this setup is 26.5 dB, which is very much
aligned with the measurement results., it gives a first-order indication that the RIS prototype with the proposed algorithm performs
according to theory.
● 500 Meter Test:The experiment was carried out between two buildings. The transmit power was 23 dBm. Due to the long distance
and limited transmit power, high-order modulation was not supported. The maximum transmission rate in the measurement was
32.1 Mbps, which was achieved using 20 MHz of bandwidth and 16 QAM modulation.
● In this experiment, the power gain is 14 dB, which was sufficient to enable real-time transmission of a video with 1920 × 1080
resolution. The video was only playing smoothly when using the RIS.
● The measured power gain is smaller than in the short distance experiments. There are several possible explanations for this result. In
general, the total received power originates both from the path via the RIS (or copper plate) and the combination of the multi-paths
that are not involving the RIS.

22. ● possibility is that the more complicated propagation environment makes the channels via the RIS frequency-selective, which
effectively reduces the beamforming gain since no RIS configuration fitsfor the entire band.
● gain of the RIS depends on multiple parameters , which includes the field pattern of the RIS elements,the distances between
the RIS and the antennas, the angles of incidence/reflection, the number of the reflection elements, etc.
Power Consumption
● It is interesting to note that the varactor diodes consume little power, although they are large in number. This is because
reverse bias voltages are applied to them. Most of the power is spent on the chips of the level regulators,to enable
continuous bias voltage adjustment. This is useful for experiments.
● Apart from the RIS board, the high-end FPGA controller
consumes 1.5 W of power in our prototype. Nevertheless, the
Proposed beamforming algorithm could also be implemented
on an ordinary microcontroller.
● This would bring down the power consumption of the
● controller to around 10 mW. In summary,
● we predict that an equal-sized RIS designed for minimum
● power consumption could consume far below 1 W.