ETSI GR RIS 001 V1.1.1 (2023-04)

Reconfigurable Intelligent Surfaces (RIS); Use Cases, Deployment Scenarios and Requirements

DGR/RIS-001

General Information

• Status: Not Published
• Technical Committee: RIS - Reconfigurable Intelligent Surfaces
• Current Stage: 12 - Completion
• Due Date: 11-Apr-2023
• Completion Date: 19-Apr-2023

Standards Content (Sample)

GROUP REPORT
Reconfigurable Intelligent Surfaces (RIS);
Use Cases, Deployment Scenarios and Requirements
Disclaimer
The present document has been produced and approved by the Reconfigurable Intelligent Surfaces (RIS) ETSI Industry
Specification Group (ISG) and represents the views of those members who participated in this ISG.
It does not necessarily represent the views of the entire ETSI membership.

2 ETSI GR RIS 001 V1.1.1 (2023-04)

Reference
DGR/RIS-001
Keywords
3GPP, architecture, MIMO, radio

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3 ETSI GR RIS 001 V1.1.1 (2023-04)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 6
3 Definition of terms, symbols and abbreviations . 6
3.1 Terms . 6
3.2 Symbols . 6
3.3 Abbreviations . 6
4 Definition . 7
4.0 RIS definition . 7
4.1 Structure . 8
4.1.0 General overview . 8
4.1.1 Metamaterials . 8
4.1.2 Reflectarray. 8
4.2 Hardware design . 9
4.2.0 Types of hardware design . 9
4.2.1 Active RIS . 9
4.2.2 Passive RIS . 9
4.2.3 Hybrid RIS . 9
4.3 Operating mode . 10
4.3.1 Reflection mode . 10
4.3.2 Refraction mode . 10
4.3.3 Absorption mode . 10
4.3.4 Backscattering mode . 10
4.3.5 Transmitting mode . 11
4.3.6 Receiving mode . 11
4.4 Operating frequency . 11
4.4.0 Description . 11
4.4.1 RIS bandwidth of influence . 12
4.4.2 Sub-6GHz band (FR1) . 12
4.4.3 mmWave band (FR2). 12
4.4.4 Terahertz band . 12
4.4.5 Unlicensed bands . 13
4.5 Communication duplex mode. 13
4.5.1 TDD . 13
4.5.2 FDD . 14
4.5.3 Full duplex . 14
5 Description of use cases . 15
5.0 General description. 15
5.1 Coverage enhancement . 15
5.2 Spectral efficiency . 17
5.3 Beam management . 18
5.4 Physical layer security . 18
5.5 Localization accuracy . 19
5.6 Sensing capabilities . 19
5.7 Energy efficiency . 19
5.7.1 Wireless power transfer . 19
5.7.2 Energy harvesting . 20
5.7.3 Power saving . 20
5.7.4 EMF exposure minimization. 20
5.8 Link Management. 21
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5.8.1 Programmable Wireless Data Centers . 21
6 Deployment scenarios . 21
6.0 General description. 21
6.1 Operating environment . 22
6.1.1 Indoor scenarios . 22
6.1.2 Outdoor scenarios . 22
6.1.3 Hybrid scenarios . 22
6.2 RIS deployment . 22
6.2.1 Static RIS . 22
6.2.2 Nomadic RIS . 22
6.2.2.0 Description . 22
6.2.2.1 Personal RIS scenario . 23
6.2.2.2 UE-integrated RIS scenario . 23
6.2.2.3 Vehicle-integrated RIS scenario . 23
6.3 RIS control plane . 23
6.3.0 Description . 23
6.3.1 Centralized management . 24
6.3.2 Distributed management . 24
6.3.3 Autonomous RIS . 24
6.3.4 UE-controlled RIS . 25
7 Requirements . 25
7.1 Hardware Cost . 25
7.2 Ease of Deployment and Maintenance . 26
7.3 Signal Power Boosting . 26
7.4 Reconfigurability . 26
7.5 Interoperability . 27
7.6 Regulatory requirements . 28
History . 29

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5 ETSI GR RIS 001 V1.1.1 (2023-04)
Intellectual Property Rights
Essential patents
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pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
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referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
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oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association. ®
BLUETOOTH is a trademark registered and owned by Bluetooth SIG, Inc.
Foreword
This Group Report (GR) has been produced by ETSI Industry Specification Group (ISG) Reconfigurable Intelligent
Surfaces (RIS).
Modal verbs terminology
In the present document "should", "should not", "may", "need not", "will", "will not", "can" and "cannot" are to be
interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

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1 Scope
The present document identifies Reconfigurable Intelligent Surfaces (RIS) relevant use cases with corresponding
general Key Performance Indicators (KPIs), deployment scenarios operational requirements for each identified use case.
KPIs and operational requirements will include system/link performance, spectrum, co-existence, and security.
2 References
2.1 Normative references
Normative references are not applicable in the present document.
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are not necessary for the application of the present document, but they assist the
user with regard to a particular subject area.
[i.1] 3GPP TR 22.858 (V18.2.0): "Study of enhancements for residential 5G (Release 18)".
[i.2] 3GPP TR 22.859 (V18.2.0): "Study on Personal Internet of Things (PIoT) networks (Release 18)".
[i.3] 3GPP TS 38.104 (V18.0.0): "NR; Base Station (BS) radio transmission and reception
(Release 18)".
3 Definition of terms, symbols and abbreviations
3.1 Terms
Void.
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
BS Base Station
CPN Customer Premises Network
DC Direct Current
DL Downlink
DMA Dynamic Metasurface Antenna
EIRP Equivalent Isotropically Radiated Power
EM ElectroMagnetic
EMF ElectroMagnetic Field
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eRG evolved Residential Gateway
FDD Frequency Division Duplex
FR Frequency Range
IoT Internet of Things
ISAC Integrated Sensing And Communication
KPI Key Performance Indicator
LBT Listen Before Talk
LoS Line of Sight
LTE Long-Term Evolution
M2M Machine to Machine
MIMO Multi-Input Multi-Output
NLoS Non-Line of Sight
NR New Radio
O2I Outdoor to Indoor
OFDM Orthogonal Frequency Division Multiplexing
PIN Personal Internet of Things Network
QoS Quality of Service
RAT Radio Access Technology
RF Radio Frequency
RIS Reconfigurable Intelligent Surfaces
SNR Signal to Noise Ratio
TDD Time Division Duplex
TRP Transmission and Reception Point
UAV Unmanned Aerial Vehicle
UE User Equipment
UL Uplink
4 Definition
4.0 RIS definition
RIS is considered a key candidate wireless technology trend for future networks. RIS corresponds to a new network
node composed of an arrangement of scattering elements called unit-cells, whose properties can be dynamically
controlled to change its electromagnetic behaviour. The response of RIS can be controlled dynamically and/or
semi-statically through control signalling such as to tune the incident wireless signals through reflection, refraction,
focusing, collimation, modulation, absorption or any combination of these. An illustrative diagram of RIS is provided in
Figure 4.0-1, as a new network node dynamically and/or semi-statically configured by the RIS controller, turning the
wireless environment from a passive to an intelligent actor such that the channel becomes programmable. This trend
will expand basic wireless system design paradigms, creating innovation opportunities which will progressively impact
the evolution of wireless system architecture, access technologies, and networking protocols.
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Figure 4.0-1: Illustrative diagram of RIS, a new type of network node where
its response can be adapted to the status of the propagation environment through control signalling
4.1 Structure
4.1.0 General overview
RIS can be implemented using mostly passive components without requiring high-cost active components such as
power amplifiers, resulting in low implementation cost and energy consumption. This allows easy and flexible
deployment of RIS, with the possibility of RIS taking any shape and to be integrated onto objects (e.g. walls, buildings,
lamp posts, etc.). RIS are supposed to run as nearly-passive devices and hence are unlikely to increase exposure to
EMF, and in fact, they can potentially be used to reduce EM pollution in legacy deployments. These associated
characteristics suggest RIS may be considered as a sustainable environmentally friendly technology solution. RIS may
have different structures with considerations of cost, form factor, design and integration.
4.1.1 Metamaterials
Metamaterials and meta-surfaces is an approach to implement RIS.
Metamaterials are artificial materials whose properties can be engineered. They are typically synthesized using multiple
elements made from composite materials such as metals and plastics.
A thin metamaterial layer, also called a meta-surface, could realize a desired transformation of transmitted, received, or
reflected ElectroMagnetic waves. A meta-surface typically consists of periodically arranged unit cells.
The ElectroMagnetic properties of a meta-surface may be electronically tuneable using various components integrated
in the surface such as PIN diodes, varactor diodes, liquid crystals, etc.
4.1.2 Reflectarray
Reflectarrays use elementary antennas as reflecting elements.
The reflection properties, such as the phase, of the elements can be changed by, e.g. varying a controllable load
connected to an antenna element. The reflection of the impinging electromagnetic wave can be controlled by creating a
phase gradient on the array by selecting the appropriate phase responses of the contiguous elements of the array. Hence,
reflectarrays can be used to implement RIS units.
When the element spacing and antenna elements on a reflectarray are reduced, reflectarrays tend to behave as
meta-surfaces.
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4.2 Hardware design
4.2.0 Types of hardware design
In this clause, different types of circuit designs of RIS are provided. RIS can be seen as a generic hardware ranging
from meta-surfaces able to manipulate wave propagation in very-rich scattering environments to those able to realize
desired anomalous reflection beyond the well-known Snell's law. RIS can be designed to operate in different modes
while exhibiting comparable energy efficiency with their reflective counterparts.
NOTE: The definition in this clause is described from manufacturing perspective, not from operating perspective.
This means that a RIS defined in this clause can work under an operating mode which does not consume
power, though it would still be classified as an active RIS from circuit design perspective.
4.2.1 Active RIS
The term active RIS is adopted when energy-intensive RF circuits and consecutive signal processing units are
embedded in RIS. On another note, active RIS systems comprise a natural evolution of conventional massive MIMO
systems, by packing more and more software-controlled antenna elements onto a two-dimensional surface of finite size.
The active RIS structure can be used to transmit and receive signals across the entire surface or using a portion of
elements, making it capable of conducting more tasks than passive RIS. A RIS structure in which only a portion of the
elements are capable of transmission and/or reception is sometimes called semi-active.
The discrete photonic antenna array is another practical implementation of active RIS. It integrates active optical-
electrical detectors, converters, and modulators for performing transmission, reception, and conversion of optical or
RF signals.
4.2.2 Passive RIS
Passive RIS acts like a passive metal mirror or wave collector which can be programmed to change an impinging EM
field in a customizable way. Compared with its active counterpart, a passive RIS is usually composed of low-cost and
almost passive elements that do not require dedicated power sources. Their circuitry and embedded sensors can be
powered with energy harvesting modules, an approach that has the potential of making them truly energy neutral.
Regardless of their specific implementations, what makes the passive RIS technology attractive from an energy
consumption standpoint, is their capability to shape radio waves impinging upon them, forwarding the incoming signal
without employing any power amplifier nor RF chain, and even without applying sophisticated signal processing.
Moreover, in addition to half-duplex mode, passive RIS can also work in full duplex mode without significant self
interference or increased noise level, and require only low-rate control link or backhaul connections. Finally, passive
RIS structures can be easily integrated into the wireless communication environment, since their extremely low power
consumption and hardware costs allow them to be deployed into building facades, room and factory ceilings, laptop
cases, or even human clothing.
4.2.3 Hybrid RIS
A hybrid RIS is capable of reflecting their impinging signal, while simultaneously sensing a portion of it. Hybrid RIS
bear the potential of significantly facilitating coherent communications without notably affecting the energy efficiency
and coverage extension advantages offered by passive RIS.
An example of an implementation of a Hybrid RIS is a surface that is loaded by a varactor, whose capacitance can be
changed by an external DC signal. The varying capacitance can change the phase of the reflected wave. In this way, the
phase variation along the Hybrid RIS can steer the reflected beam towards desired directions.
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4.3 Operating mode
4.3.1 Reflection mode
The concept of the RIS-empowered smart wireless environments initially considered only passive RIS with almost zero
power consumption unit elements. Their envisioned prominent role lies on the capability of the surface to reconfigure
the reflection characteristics of its elements, enabling programmable manipulation of incoming EM waves in a wide
variety of functionalities. It is essential to achieve a fine-grained control over the reflected EM field for quasi-free space
beam manipulation so as to realize accurate beamforming. Meta-atoms of sub-wavelength size are a favourable choice,
although inevitable strong mutual coupling, and well-defined grey-scale-tuneable EM properties exist.
Conversely, in rich scattering environments, the wave energy is statistically equally spread throughout the wireless
medium. The ensuing ray chaos implies that rays impact the RIS from all possible, rather than one well-defined,
directions. The goal becomes the manipulation of as many ray paths as possible, which is different from the common
goal of creating a directive beam. This manipulation has two kind of aims, including tailoring those rays to create
constructive superposition at a target location and steering the field efficiently. These manipulations can be efficiently
realized with RIS equipped with half-wavelength-sized meta-atoms, enabling the control of more rays with a fixed
amount of electronic components (PIN diodes, etc.). The meta-atoms are usually half-wavelength-sized in lower
frequency bands, whereas in higher frequency bands like FR2, their sizes depend on manufacturing constraints.
RIS working in reflection mode can act as a reflector in the environment, and it can be used to improve coverage,
mitigate interference and increase capacity.
4.3.2 Refraction mode
The refraction mode allows incident EM waves passing through the RIS and refract them to different target directions
by adjusting their phase. The main difference between refraction and the reflection mode characterized in clause 4.3.1 is
the missing of the shielding layer inside the RIS panel, which enables the EM waves to pass through the panel.
One typical use case of refraction mode is outdoor to indoor scenario. In order to improve the coverage for some certain
areas inside the building, the RIS will be used as the window glasses and it will focus the incident EM waves to
different target areas.
4.3.3 Absorption mode
Under the absorption mode, the impinging radio wave of a certain center frequency and a certain bandwidth can,
ideally, be totally absorbed and no reflection wave can be observed. The absorption mode, that allows RIS to have
almost zero output waves, can be beneficial to interference mitigation, privacy and information security industry. One
typical use case is to implement RIS on the building facade to shield electromagnetic wave, so that the electromagnetic
wave of indoor and outdoor or different indoor rooms would be isolated from each other. RIS plane will absorb the
incident wave to prevent them from penetrating building walls. The switch of RIS between absorption and refraction or
reflection mode can be controlled by bias voltage.
One example of absorption RIS is graphene based RIS, which can reach nearly 100 % absorption in some given bands
according to the design. The perfect absorption is achieved by electrically reconfiguring the meta atom response via the
chemical potential of the graphene.
4.3.4 Backscattering mode
For a RIS in backscattering mode, the reflected wave is to cover a large area instead of an exact location. Therefore, the
balance between gain and effective area is necessary for realizing wide-angle blindspot coverage. Backscattering mode
can be used for passive RIS, which are manufactured to reflect an impinging EM signal into a certain direction.
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4.3.5 Transmitting mode
A RIS in transmitting mode is incorporated in a radio transmitter with the RIS assisting in shaping the transmitted radio
wave.
As an example, Dynamic Metasurface Antennas (DMAs) have been recently proposed as an efficient realization of
extreme massive antenna arrays. DMAs have beam tailoring capabilities and facilitate processing of the transmitted and
received signals in the analog domain. DMAs work in a dynamically configurable manner with simplified transceiver
hardware. Additionally, compared with conventional antenna arrays, DMA-based architectures require much less power
and cost. In this way, eliminating the need for complicated corporate feed and/or active phase shifters becomes possible.
Another promising advantage of DMAs is that they can comprise massive numbers of tuneable metamaterial-based
antenna elements fitting into small physical areas and providing wide range of operating frequencies.
DMA architecture that consists of multiple separate waveguide-fed element arrays with each connected to a single
input/output port is a typical reflecting RIS. A large number of radiating elements can be accommodated in waveguides,
and the sub-wavelength spaced character allows each input/output port to feed a multitude of possibly coupled radiators.
For 2D waveguides, a scattered wave from each element propagates in all directions. Since the proposed waveguide is
typically designed to be single mode and the wave can only propagate along one line, its analysis is much easier than
2D waveguides. Furthermore, ensuring isolation between different ports is easier in 1D waveguides than in multiple
ports of a 2D waveguide.
4.3.6 Receiving mode
A RIS in receiving mode is capable of receiving and processing radio signals. This can be accomplished by embedding
waveguides at each RIS element, or group of elements, to direct the impinging radio signals to reception hardware. This
hardware may include, for example, a low noise amplifier, a mixer down converting the signal from RF to baseband,
and an analog-to-digital converter.
In the example illustrated in Figure 4.3.6-1, an impinging EM training signal at the RIS elements is received in the RF
domain via M RIS phase configurations, which are randomly selected through a random spatial sampling unit. This
collection of spatially random analog combined versions of the impinging radio signals facilitates, for example, the
application of compressed-sensing-based channel estimation techniques, enabling signal reception at the RIS with much
less reception RF chains (even with one) than the number of RIS elements.

Figure 4.3.6-1: Block diagram of a RIS hardware architecture including
a single active reception RF chain, enabling the sensing of the impinging signal in baseband
4.4 Operating frequency
4.4.0 Description
This clause describes possible operating frequencies for RIS to be integrated into wireless networks. Two Frequency
Ranges (FR) are described, namely FR1 and FR2. Corresponding frequency range for FR1 is 410 MHz - 7 125 MHz
and corresponding frequency range for FR2 is 24 250 MHz - 71 000 MHz, as defined in Table 5.2-1 in 3GPP ®
TS 38.104 [i.3]. Many Radio Access Technologies (RATs) work on FR1, such as WiFi , LTE and part of NR. Some
RATs work on FR2, such as part of NR.
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Deployment scenarios, use cases and relevant recommendations are in many cases described separately for different
frequency ranges, since the channel conditions for FR1 and FR2 are different.
The operating frequency and channel arrangements presented in this clause are based on the frequency ranges and
operating bands defined in 3GPP TS 38.104 [i.3].
4.4.1 RIS bandwidth of influence
Since the bandwidth of signals reflected by RIS might be different compared to that of the incident signal, when
considering the working frequency of RIS it is necessary to also consider the unwanted emissions created by RIS
reflections. The unwanted emissions may cause interference to different bands than the incident signal.
The working frequency bands in clause 4.4.0 are described by the incident signal. The characteristics of potential
unwanted emissions generated by RIS are described in 3GPP TS 38.104 [i.3].
4.4.2 Sub-6GHz band (FR1)
The corresponding frequency range for FR1 is 410 MHz - 7 125 MHz. FR1 is sometimes referred to as sub-6 GHz in
some references and literature, while in fact it has now been extended to 7,125 GHz due to additional spectrum
allocations.
The operating frequency bands belonging to FR1 for NR are defined in Table 5.2-1 in 3GPP TS 38.104 [i.3], where
both TDD and FDD bands are considered supported for RIS operations.
In cellular networks such as the LTE and NR, frequency bands in FR1 are envisaged to carry much of the traditional
cellular mobile communications traffic and provide good coverage.
4.4.3 mmWave band (FR2)
The corresponding frequency range for FR2 is 24 250 MHz - 71 000 MHz. FR2 is sometimes referred to as millimeter
wave (mmWave) in some references and literature.
The operating frequency bands belonging to FR2 for NR are defined in Table 5.2-2 in 3GPP TS 38.104 [i.3], where
both TDD and FDD bands are considered supported for RIS operations.
In cellular networks such as the NR, frequency bands in FR2 are aimed to provide ultra high data rate since larger
bandwidth can be provided in higher frequencies.
4.4.4 Terahertz band
TeraHertz (THz) bands are defined as the frequency region between 0,1 THz and 10 THz. Sub-THz bands are
sometimes referred to as the lowest part of the THz spectrum between 0,1 THz and 0,3 THz. In these frequencies, only
TDD deployments are being considered.
Despite the fact that most immediate RIS applications are envisaged for FR1 and FR2 frequencies as currently defined
in 3GPP specifications, THz and sub-THz communications are gaining traction for future beyond-5G networks due to
their intrinsic high bandwidth and data rate support. Since communication at these frequencies is inherently
short-ranged and characterized by unreliable intermittent links impaired by blockage and absorption, a continuous Line
of Sight (LoS) link should be guaranteed. Thus, RIS deployments can play a key role in customizing the propagation
environment to ensure a continuous LoS link. Moreover, the deployment of multiple RIS can provide richer multipath
characteristics to an otherwise single-rank channel by creating additional signal path(s) artificially, thus improving its
spatial multiplexing capabilities. Furthermore, deployment of RIS could be used to extend coverage of communications
for very high frequency range such as in sub-THz and THz bands.
As in some cellular networks such as the 5G NR, THz communications can exploit synergies with the lower bands in
FR1 and FR2 to maximize network coverage.
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4.4.5 Unlicensed bands
In both FR1 and FR2, unlicensed bands have been allocated in different regions. For RIS to operate in these unlicensed
bands, channel access mechanisms may need to be considered. Similar to cellular networks such as LTE and NR,
depending on the regional requirements and deployment scenarios, multiple channel access mechanisms can be
considered for RIS operation in unlicensed bands. One such example is that an active RIS may perform Listen Before
Talk (LBT) based channel access to operate in unlicensed band. The controlling node for RIS, for example a BS or a
UE, could configure RIS to perform such channel access mechanism. Another important aspect for RIS in unlicensed
bands is to operate within the power limits such as maximum EIRP limits that are typically based on the regulatory and
standards requirements.
4.5 Communication duplex mode
4.5.1 TDD
For TDD mode, it is well known that there exists channel reciprocity between DL and UL channels which utilize the
same frequency band. Whether RIS preserves channel reciprocity depends on the fabrication methods. In the cases
where reciprocity holds for RIS TDD systems, the same configuration of RIS, such as phase shifts and amplitudes, or in
other words beam directions, can be used in both the DL and the UL. Specifically, in the TDD mode, RIS can receive
downlink transmission from the network and reflect to UE(s) in one time unit (for example, one or multiple OFDM
symbols) and it can receive uplink transmission from UE(s) and reflect to the network in another time unit. The
reception/reflection in UL and DL may occur on same frequency but in different time units. In addition, a guard period
may be configured for RIS when switching between DL/UL to ensure that the UL and DL reflection/reception at the
RIS do not interfere. For the TDD, two modes can be included: reciprocity-constrained mode and reciprocity-non-
constrained mode. Typically, channel reciprocity can be maintained for most of the RIS hardware implementations.
However, as one possibility, depending on the fabrication methods of RIS, reciprocity may not always be maintained.
Therefore, to operate in reciprocity-constrained mode, channel reciprocity can be maintained by at least configuring the
phase shifts at the RIS elements to reflect UL/DL towards BS/UE such that the BS/UE can transmit DL/UL and receive
UL/DL using the same respective beams. For the reciprocity-non-constrained mode, channel reciprocity may not need
to be maintained and the phase shifts at RIS can be configured independent of the UL/DL beams at the UE/BS. In
Figures 4.5.1-1(a) and 4.5.1-1(b), illustrations are provided for the two cases of reciprocity constrained and reciprocity-
non-constrained mode.
UE
UE
BS
BS
Slot N+1
Slot N
(UL reflection from RIS towards BS
(DL reflection from RIS towards UE)
using same set of beams)
Figure 4.5.1-1(a): Illustration of TDD communication with reciprocity-constrained mode at
UE using same set of phase shifts for UL and DL at RIS
ETSI
14 ETSI GR RIS 001 V1.1.1 (2023-04)
UE
UE
BS
BS
Slot N+1
Slot N
(UL reflection from RIS towards BS
(DL reflection from RIS towards UE)
using different set of beams)
Figure 4.5.1-1(b): Illustration of TDD communication with reciprocity-non-constrained mode at
UE using different set of phase shifts for UL and DL at RIS
4.5.2 FDD
For FDD mode, DL and UL channels utilize different frequency bands. Hence channel reciprocity does not hold for RIS
FDD systems. In such cases, using the same configuration of RIS, such as phase shifts and amplitudes, or in other
words beam directions, in the DL and the UL may lead to performance degradation. RIS design and operation in F
...