ETSI GR RIS 005 V1.1.1 (2025-02)

Reconfigurable Intelligent Surfaces (RIS); Diversity and Multiplexing of RIS-aided Communications

DGR/RIS-005

General Information

• Status: Not Published
• Technical Committee: RIS - Reconfigurable Intelligent Surfaces
• Current Stage: 12 - Citation in the OJ (auto-insert)
• Due Date: 19-Feb-2025
• Completion Date: 26-Feb-2025

Standards Content (Sample)

GROUP REPORT
Reconfigurable Intelligent Surfaces (RIS);
Diversity and Multiplexing of RIS-aided Communications
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 005 V1.1.1 (2025-02)

Reference
DGR/RIS-005
Keywords
RIS
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ETSI
3 ETSI GR RIS 005 V1.1.1 (2025-02)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
Executive summary . 5
Introduction . 6
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definition of terms, symbols and abbreviations . 8
3.1 Terms . 8
3.2 Symbols . 8
3.3 Abbreviations . 8
4 General aspects of RIS-based diversity and multiplexing schemes . 9
4.0 Motivation of RIS-based diversity and multiplexing . 9
4.1 Overview of RIS-aided multi-antenna systems . 9
4.2 Use cases and deployment scenarios . 9
4.3 General characteristics of RIS-aided channels . 11
5 Diversity schemes for RIS-aided systems . 11
5.1 Time Diversity . 11
5.1.0 Time Diversity schemes . 11
5.1.1 Requirements for RIS hardware. 11
5.1.2 Requirements for RIS operating mode. 11
5.1.3 Characteristics of RIS-aided channels . 11
5.1.4 Diversity scheme design . 14
5.1.5 Impact on transmitter and receiver complexity . 15
5.2 Frequency Diversity . 15
5.2.0 Frequency Diversity schemes . 15
5.2.1 Requirements for RIS hardware. 15
5.2.2 Requirements for RIS operating mode. 16
5.2.3 Characteristics of RIS-aided channels . 16
5.2.4 Diversity scheme design . 17
5.2.5 Impact on transmitter and receiver complexity . 19
5.3 Tx and Rx Antenna Diversity . 19
5.3.0 Tx and Rx Antenna Diversity schemes . 19
5.3.1 Requirements for RIS hardware. 20
5.3.2 Requirements for RIS operating mode. 20
5.3.3 Characteristics of RIS-aided channels . 20
5.3.4 Diversity scheme design . 22
5.3.5 Impact on transmitter and receiver complexity . 22
5.4 Polarization Diversity . 22
5.4.0 Polarization Diversity schemes . 22
5.4.1 Requirements for RIS hardware. 22
5.4.2 Requirements for RIS operating mode. 22
5.4.3 Characteristics of RIS-aided channels . 22
5.4.4 Diversity Scheme Design . 23
5.5 Spatial Diversity . 23
5.5.1 Requirements for RIS hardware. 23
5.5.2 Requirements for RIS operating mode. 24
5.5.3 Diversity scheme design . 24
5.5.3.1 RIS-Aided Space-Frequency Block Coding (SFBC) . 24
5.5.3.2 RIS-Aided Space-Time Block Coding (STBC) . 25
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4 ETSI GR RIS 005 V1.1.1 (2025-02)
5.5.4 Impact on transmitter and receiver complexity . 26
5.6 Diversity Scheme Comparisons . 26
6 Multiplexing schemes for RIS-aided systems . 27
6.1 Space-Division Multiplexing. 27
6.1.1 Space-Division Multiplexing based on Superposition Method and Multi-Tiles RIS . 27
6.1.2 Space-Division Multiplexing for Discrete Phase Control RIS . 30
6.2 Other Multiplexing Techniques . 34
7 Other Multi-Antenna Technologies . 35
7.1 Beamforming . 35
7.2 RIS Antenna Array Selection . 36
7.2.0 RIS Antenna Array Selection schemes . 36
7.2.1 RIS Selection for Robust Capacity Enhancement . 38
7.2.2 RIS Selection for Outage Probability Reduction . 39
8 Channel Estimation and CSI Acquisition Schemes . 40
8.0 CSI Considerations . 40
8.1 Estimating Composite Channel . 40
8.2 Estimating Separate Channel . 41
8.2.1 Time Separated Channel Estimation . 41
8.2.1.1 On-Off Channel Estimation . 41
8.2.1.2 Spatially Separated Channel Estimation on Different Time Resources . 41
8.2.1.3 Separate Channel Estimation Based on Windowing Method . 42
8.2.2 Frequency Separated Channel Estimation . 43
8.2.2.1 Frequency Separated Channel Estimation on Same Time-Resources but Different Beams . 43
8.2.2.2 Separate Channel Estimation with Orthogonal Reference Signal (RS) in Frequency Domain . 44
8.2.3 Spatially Separated Channel Estimation . 44
8.2.3.1 Spatially Separated Channel Estimation on Same Time-Frequency Resources but Different
Beams . 44
8.2.4 Code Division Multiplexed Based Channel Estimation . 45
8.2.4.1 Separate Channel Estimation with Orthogonal Reference Signal (RS) in Code Domain . 45
9 Conclusions and Recommendation . 45
Annex A: Change history . 46
History . 47

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5 ETSI GR RIS 005 V1.1.1 (2025-02)
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
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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.
Executive summary
The present document focuses on the array technologies for Reconfigurable Intelligent Surface (RIS)-aided
communication systems. First, the present document summarizes use cases and deployment scenarios for RIS. Next, the
present document introduces different diversity schemes for RIS-aided communication, such as time diversity,
frequency diversity, spatial diversity, etc. For each diversity scheme, the present document introduces the corresponding
requirement for RIS hardware and operating mode, the channel model, the impact on Tx and Rx devices, and the
diversity scheme design. Then the present document introduces different multiplexing schemes in RIS-aided
communication. Finally, the present document introduces other techniques related to array technologies in RIS-aided
communication, including beamforming, RIS selection, and channel estimation.
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6 ETSI GR RIS 005 V1.1.1 (2025-02)
Introduction
RIS has been viewed as one of the enabling technologies for the next-generation communication systems due to its
capability to adapt the channel conditions. Utilizing an array of radiating elements, RIS can re-direct incident signals to
improve the coverage and reliability of communication against channel impairments such as blockage and
Outdoor-to-Indoor (O2I) loss.
As one of the most important technologies in Long Term Evolution (LTE) / 5G, array systems and
Multiple-Input-and-Multiple-Output (MIMO) provides significant performance gains to the wireless communication
system. RIS, which is also an array system consisting of scattering unit-cells by itself, can potentially provide additional
degrees of freedom and further increased gains through diversity / multiplexing schemes. Therefore, it is important to
build a comprehensive understanding of the array technologies applicable to RIS-aided communication, as well as the
benefits and limitations of these technologies for future system design and standardization.
The present document mainly focuses on an introduction of array technologies applicable to RIS-aided communication
systems, including diversity schemes, multiplexing schemes, RIS-aided beamforming, RIS selection, channel
estimation, etc. The aim of the present document is to provide a comprehensive survey to analyse the feasibility,
requirement, performance, and impact of these technologies, contributing to the continuing evolution and practical
deployment of RIS-aided communication systems.

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7 ETSI GR RIS 005 V1.1.1 (2025-02)
1 Scope
The present document focuses on different array technologies applicable to RIS-aided communication, mainly
emphasizing diversity and multiplexing schemes. Other technologies such as beamforming, RIS selection, and channel
estimation have also been covered in the present document.
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] ETSI GR RIS 001 (V1.1.1): "Reconfigurable Intelligent Surfaces (RIS); Use Cases, Deployment
Scenarios and Requirements".
[i.2] ETSI GR RIS 003 (V1.1.1): "Reconfigurable Intelligent Surfaces (RIS); Communication Models,
Channel Models, Channel Estimation and Evaluation Methodology".
[i.3] ETSI TR 138 901 (V18.0.0): "5G; Study on channel model for frequencies from 0.5 to 100 GHz
(3GPP TR 38.901 version 18.0.0 Release 18)".
[i.4] Ramaccia Davide, Dimitrios L. Sounas, Andrea Alu, Alessandro Toscano and Filiberto Bilotti:
"Phase-induced frequency conversion and Doppler effect with time-modulated metasurfaces",
™
IEEE Transactions on Antennas and Propagation 68, no. 3 (2019): pp. 1607-1617.
[i.5] Chian De-Ming, Chao-Kai Wen, Chi-Hung Wu, Fu-Kang Wang and Kai-Kit Wong: "A novel
channel model for reconfigurable intelligent surfaces with consideration of polarization and switch
™
impairments", IEEE Transactions on Antennas and Propagation (2024).
[i.6] Yuan Jide, Elisabeth De Carvalho, Robin Jess Williams, Emil Björnson and Petar Popovski:
™
"Frequency-mixing intelligent reflecting surfaces for nonlinear wireless propagation", IEEE
Wireless Communications Letters 10, no. 8 (2021): pp. 1672-1676.
[i.7] Heng Liang and Louay MA Jalloul: "Performance of the 3GPP LTE space–frequency block codes
™
in frequency-selective channels with imperfect channel estimation", IEEE Transactions on
Vehicular Technology 64, no. 5 (2014): pp. 1848-1855.
[i.8] Alex Sam P. and Louay MA Jalloul: "Performance evaluation of MIMO in IEEE802.
™
16e/WiMAX", IEEE Journal of Selected Topics in Signal Processing 2, no. 2 (2008):
pp. 181-190.
[i.9] Wang Bolei, Mengnan Jian, Feifei Gao, Geoffrey Ye Li and Hai Lin: "Beam squint and channel
™
estimation for wideband mmWave massive MIMO-OFDM systems", IEEE transactions on signal
processing 67, no. 23 (2019): pp. 5893-5908.
ETSI
8 ETSI GR RIS 005 V1.1.1 (2025-02)
[i.10] Chian De-Ming, Chao-Kai Wen, Chi-Hung Wu, Fu-Kang Wang and Kai-Kit Wong: "Joint
™
phase-time arrays: A paradigm for frequency-dependent analog beamforming in 6G", IEEE
Access, vol. 10, pp. 73364-73377, 2022.
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:
AOA Angle of Arrival
AOD Angle of Departure
ASK Amplitude-Shift Keying
BPSK Binary Phase-Shift Keying
BS Base Station
CDL Clustered Delay Line
CP Cyclic Prefix
CSI-RS Channel State Information Reference Signal
DFT Discrete Fourier Transform
ELAA Extremely Large Antenna Arrays
HPBW Half Power BeamWidth
IoT Internet of Things
KPI Key Performance Indicator
LC Inductor-Capacitor
LOS Line of Sight
LTE Long Term Evolution
MIMO Multiple-Input and Multiple-Output
mmWave millimeter Wave
NLOS Non-Line Of Sight
O2I Outdoor-to-Indoor
OFDM Orthogonal Frequency Division Multiplexing
PAM Pulse-Amplitude Modulation
RC Resistor–Capacitor
RIS Reconfigurable Intelligent Surface
RS Reference Signal
Rx Receiver
SFBC Space-Frequency Block Coding
SINR Signal-to-Interference-plus-Noise Ratio
SNR Signal-to-Noise Ratio
STBC Space-Time Block Coding
THz TeraHertz
TRP Transmission Reception Point
Tx Transmitter
UE User Equipment
UL UpLink
UMi Urban Microcell
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9 ETSI GR RIS 005 V1.1.1 (2025-02)
4 General aspects of RIS-based diversity and
multiplexing schemes
4.0 Motivation of RIS-based diversity and multiplexing
As the most important technologies in LTE / 5G, multi-antenna systems and MIMO have brought significant
performance gains to the communication system. Such gains are mainly realized in the form of diversity, multiplexing,
beamforming, etc. Introducing RIS into a network generally provides additional paths for links and therefore
emphasizes the greater necessity of diversity and multiplexing schemes. Furthermore, as a multi-antenna system by
itself, RIS can provide extra degrees of freedom and potentially enable more diversity / multiplexing options to further
improve the performance of the communication system. In the following clauses, the present document will discuss
diversity / multiplexing analysis for RIS-aided communication and also cover other multi-antenna technologies such as
beamforming.
4.1 Overview of RIS-aided multi-antenna systems
A RIS-aided multi-antenna system generally consists of multi-antenna Tx / Rx and RIS arrays, in which RIS configures
the electromagnetic environment to enhance the communication performance. A RIS usually consists of a large number
of re-radiation elements, each with at least configurable phase controlled by switches, or diodes, etc. Compared to a
conventional multi-antenna system that integrates all antenna elements on the same array, RIS-aided communication
has better channel conditions for the usage of diversity and multiplexing schemes. Distributed antenna systems may
have comparable channel conditions but require more deployment cost. Besides, when a RIS has additional capabilities,
such as fast phase adaptation, frequency shifting, polarization manipulation, etc., it can enable additional
diversity / multiplexing schemes by itself and reduce the complexity and requirement of Tx / Rx devices.
4.2 Use cases and deployment scenarios
As is discussed in ETSI GR RIS 001 [i.1] and ETSI GR RIS 003 [i.2], RIS can be deployed in many different scenarios
and aid communication in different manners. For a Tx-Rx link, one of the most common scenarios is that the direct
Tx-Rx path is not available due to, e.g. high pathloss, blockage, O2I loss, antenna limitations, etc. as is shown in ETSI
GR RIS 003 [i.2]. Figure 4.2-1 shows one of such scenarios, in which RIS extends the coverage of BS to the indoor
region by avoiding the O2I loss. In such scenarios, RIS can enhance the coverage mainly by providing an additional
path and large beamforming gain, while the interaction between Tx-Rx path and Tx-RIS-Rx path is negligible.
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10 ETSI GR RIS 005 V1.1.1 (2025-02)

Figure 4.2-1: Illustration of scenarios with only Tx-RIS-Rx paths due to O2I loss
Another scenario is when both Tx-Rx path and Tx-RIS-Rx path are available, while the channel conditions of the two
paths are also comparable. A typical use case is shown in figure 4.2-2, in which an indoor RIS is deployed to aid the
existing direct link from BS to UE. The interaction between the Tx-Rx and Tx-RIS-Rx paths can influence the
composite channel conditions and require adaptation of transmission schemes. Similar use cases can also be deployed in
outdoor spaces as is shown in ETSI GR RIS 003 [i.2]. In such scenarios, RIS is usually deployed to improve the link
performance (e.g. throughput, outage, etc.) instead of the signal strength. When the channels suffer from uncertainty
(e.g. dynamic blockage, high mobility, etc.) and deep fading, diversity schemes can usually be enabled at Tx, Rx, or
RIS to enhance the reliability of communication. However, if the channel conditions of both paths are generally good
and stable, multiplexing schemes can be used to take the advantage of the additional path and transmit multiple streams
simultaneously.
Figure 4.2-2: Illustration of scenarios with coexistence of Tx-Rx and Tx-RIS-Rx paths
In multi-user scenarios, RIS can also be deployed for diversity and multiplexing purposes. A single RIS can be
configured to generate multiple beams simultaneously to support transmission from BS to multiple UEs, either in
broadcast manner or using different sub-bands for different UEs. Such schemes can also be extended to scenarios with
multiple BSs and multiple RIS. In some scenarios, RIS can support the space-division multiplexing of multiple BS-UE
pairs if they can utilize the same RIS re-radiation pattern. The deployment of RIS can also diversify the channel
conditions and allow the BS to better utilize user diversity and achieve high throughput. These scenarios will be
discussed in more details in clauses 5 and 6.
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11 ETSI GR RIS 005 V1.1.1 (2025-02)
4.3 General characteristics of RIS-aided channels
In most scenarios, RIS aids a link by re-radiating incident signals towards the Rx and redirects the propagation.
Therefore, the RIS path can generally be viewed as an additional multipath with path gain enhanced by a configurable
phased array. For common reflective RIS, the AOA and AOD of such RIS paths are mostly different from that of LOS
paths. Besides, the RIS path can also have its own Tx-RIS multipaths and RIS-Rx multipaths. Therefore, the channel of
a RIS-aided link is expected to have rich multipaths and more significant fading effects, especially when the LOS path
is available. A detailed modelling of the RIS-aided channel will be discussed in following clauses.
5 Diversity schemes for RIS-aided systems
5.1 Time Diversity
5.1.0 Time Diversity schemes
Time diversity schemes are usually designed for robust communication with time-varying channel conditions. Burst
errors can occur when symbols are transmitted in slots with deep fading channels and can cause outage. The channel of
RIS-aided communication can be time-varying when mobility and blockage happens. Therefore, time diversity
schemes, e.g. repetition coding / interleaving, can also be applied to RIS-aided communication systems to enhance the
robustness. Such time diversity schemes are mostly applied on Tx side. In addition, by cooperating with traditional Tx
diversity schemes, RIS can also dynamically adapt the channel to further improve the time diversity.
5.1.1 Requirements for RIS hardware
RIS needs to be fabricated and configured to re-radiate signals from Tx to Rx. Besides, to dynamically adapt the
channel and further improve time diversity, RIS elements need to be able to change their phases with a high frequency
and short delay.
5.1.2 Requirements for RIS operating mode
RIS should be able to work in reflection / refraction mode to re-radiate signals from Tx to Rx.
5.1.3 Characteristics of RIS-aided channels
RIS-aided channels can be time-varying when Tx / Rx is mobile, as is demonstrated in the following example in
figure 5.1.3-1. It is assumed that the lengths of BS-UE, BS-RIS, and RIS-UE paths are � , � , and � , respectively.
�� �� ��
��
The corresponding path gains are � , � , and � , respectively. The RIS re-radiation gain is represented by � � ,
�� �� �� �
where � is assumed to be a common base phase applied to all RIS elements and does not affect the re-radiation beam
pattern. The sub-band carrier frequency considered in this example is �, and the light speed is �. Then the overall
channel from BS to UE is:
������ �����(� �� ) ������ ����(� �� ��
�� �� �� �� �� �� ��
�� ���
��
� � � �
ℎ= � � + � � � � � = � �� + � � � � � (1)
�� �� � �� �� �� � ��
������ �� �� �
�� �� ��
Depending on the term + �, the channel gain ranges from |� − � � � | to � + � � � .
�� �� � �� �� �� � ��
�
When UE is mobile, the channel gain on this sub-band can drop from maximum to minimum within half wavelength of
movement, which is a small distance for mmWave band. The channel gain difference in RIS-aided communication can
be even larger than that caused by fading, as the RIS provides a much larger re-radiation gain compared to ordinary
clusters. Doppler effect can also result in similar time-domain channel variations. Such channel gain variation can result
in burst errors and even outage, requiring time diversity schemes to be applied on Tx or RIS side to reduce the outage
probability.
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12 ETSI GR RIS 005 V1.1.1 (2025-02)

Figure 5.1.3-1: Dependence of RIS-aided channel on path lengths, which leads to
time-varying channel when UE is mobile
Figures 5.1.3-2 to 5.1.3-4 illustrate the simulated time-domain channel with / without RIS. At a carrier frequency of
4 GHz, the CDL-D channel model defined in ETSI TR 138 901 [i.3] is used to generate the BS-UE and RIS-UE fading
channel with LOS paths. The Doppler effect and the path length change both contribute to the channel variations.
Different UE speeds (3 km/h, 30 km/h, and 100 km/h) are considered in this evaluation to better illustrate the time
variation of channel. In these simulations with and without RIS, the multi-path component contributed by the surface
(e.g. a wall) on which RIS would be mounted is not considered. However, it is expected that the gain provided by such
a surface is usually much smaller than the RIS gain due to a lack of beamforming.
-88.5
-89
-89.5
-90
CDL-D without RIS
CDL-D with RIS
-90.5
-91
0 5 10 15 20 25 30
Time (ms)
NOTE: The UE speed is assumed to be 3 km/h.

Figure 5.1.3-2: Illustration of time-varying channel with / without RIS caused by
Doppler effect and path length change
ETSI
Channel gain (dB)
13 ETSI GR RIS 005 V1.1.1 (2025-02)
-86
-86.5
-87
-87.5
-88
-88.5
-89
-89.5
CDL-D without RIS
CDL-D with RIS
-90
-90.5
0 5 10 15 20 25 30
Time (ms)
NOTE: The UE speed is assumed to be 30 km/h.

Figure 5.1.3-3: Illustration of time-varying channel with / without RIS caused by
Doppler effect and path length change
-85
-86
-87
-88
-89
-90
-91
CDL-D without RIS
-92 CDL-D with RIS
-93
0 5 10 15 20 25 30
Time (ms)
NOTE: The UE speed is assumed to be 100 km/h.

Figure 5.1.3-4: Illustration of time-varying channel with / without RIS caused by
Doppler effect and path length change
ETSI
Channel gain (dB)
Channel gain (dB)
14 ETSI GR RIS 005 V1.1.1 (2025-02)
It is clear from figures 5.1.3-2 to 5.1.3-4 that the time selectivity of channel with RIS is more significant than the
channel without RIS, since the gain of RIS path can be much closer to the gain of the main BS-UE path compared to
those via scattering clusters. The channel also changes much faster when the UE speed increases in the three figures.

Figure 5.1.3-5: Time-varying channel caused by dynamic blockage
RIS-aided channel can also be time-varying when blockage status dynamically changes, as is shown in figure 5.1.3-5.
Although RIS can provide an alternative path for the link and reduce the probability of complete outage, the SNR of the
link can still change significantly when a blocker appears / disappears. Time diversity schemes can also be enabled in
this scenario to further improve the robustness of communication.
5.1.4 Diversity scheme design
Conventional time diversity schemes, based on the combination of adding redundancy via coding and interleaving, can
be applied to RIS-aided systems. Tx can repeat or encode the information bits, and interleave the coded bits in the time
domain before transmission, as is shown in figure 5.1.4-1. When the number of diversity branch is sufficiently large, all
information bits experience similar channel gains regardless of mobility / dynamic blockage, which alleviates burst
errors and improves link robustness. Such time diversity schemes can be applied to counter the time-varying channel
caused by both mobility and blockage.

NOTE: Coding schemes other than repetition can also exploit time diversity in RIS-aided communication systems.

Figure 5.1.4-1: Example time diversity schemes that can be used by Tx
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15 ETSI GR RIS 005 V1.1.1 (2025-02)
When mobility results in the time-varying channel, a new diversity scheme can be enabled by RIS to replace the
interleaving function at Tx and cooperate with Tx coding schemes to further enhance the robustness of communication.
As is illustrated in figure 5.1.3-1, the fast fading mainly results from the phase difference between BS-UE path and
BS-RIS-UE path. Interleaving takes advantage of phase difference changes over time and allows adjacent coded bits to
be transmitted with different channel conditions. RIS, however, can adapt its phase � actively (e.g. randomly) to change
channel conditions faster so that adjacent coded bits do not need to be transmitted after the channel evolves. This
function of RIS can be enabled together with coding schemes at Tx to better enhance the time diversity of the
transmission.
In addition, if the transmission happens on a very narrow band, RIS can also adapt it phase so that the waveforms via
both BS-UE channel and BS-RIS-UE channel can always be constructively combined at Rx. This method requires
BS / RIS to have the knowledge of locations and UE mobility in order to compensate for the phase difference caused by
path length difference and Doppler effect.
5.1.5 Impact on transmitter and receiver complexity
Applying time diversity schemes for RIS-aided communication systems can effectively reduce error bursts and simplify
the error correction on Rx side. When RIS is used to implement time diversity methods, Tx complexity can be reduced,
since coding and interleaving functions can be simplified. If RIS is able to adapt its phase by tracking the movement of
UE, both BS-UE and BS-RIS-UE transmissions will always combine constructively, and time diversity schemes at Tx
can be minimally enabled.
5.2 Frequency Diversity
5.2.0 Frequency Diversity schemes
Multipath effect is very common in wireless communications, causing signals on different carrier frequencies to
experience selective fading conditions. Frequency diversity schemes are usually designed for robust communication
with such frequency selective channel conditions. RIS-aided communication has inherent frequency selectivity, i.e. the
frequency domain channel is not flat even if both paths with / without RIS have frequency-flat channels, and the
frequency selectivity is more significant than multipath channels due to the high re-radiation gain. Therefore, frequency
diversity schemes, e.g. coding / frequency hopping, can also be applied to RIS-aided communication systems to
enhance the performance. Such frequency diversity schemes are mostly applied on Tx side. However, by cooperating
with traditional Tx diversity schemes, RIS can also dynamically adapt the channel to further improve the frequency
diversity.
5.2.1 Requirements for RIS hardware
Basic RIS needs to be fabricated and configured to re-radiate signals from Tx to Rx. Besides, to dynamically adapt the
channel and further improve frequency diversity, RIS elements can be designed to modulate the incident signals and
shift the frequency of incident waveforms (e.g. as is demonstrated in [i.4]), which requires RIS to be able to change
their phases with a high frequency and short delay.
A basic architecture of RIS supporting frequency-shifting is demonstrated in figure 5.2.1-1. At each RIS element, the
�� ��
incident signal is re-radiated after traveling through a time-varying phase shifter � �(�). The term � corresponds to
the beamforming capability of RIS, while �(�) shifts the frequency of incident signals. The simplest realization of �(�)
can be a square wave, implying that the phase shift at this element oscillates between � and −�. In this case, the RIS
does not need any active signal processing components to shift the carriers and can operate in a relatively passive
manner. Advanced options for �(�) include, e.g. sinusoidal waves.
ETSI
16 ETSI GR RIS 005 V1.1.1 (2025-02)

Figure 5.2.1-1: General architecture of RIS element supporting frequency-shifting
5.2.2 Requirements for RIS operating mode
RIS should be able to work in reflection / refraction mode to re-radiate signals from Tx to Rx.
5.2.3 Characteristics of RIS-aided channels
Frequency selectivity is an inherent property of RIS-aided communication. As is demonstrated in clause 5.1.3, when
BS-UE, BS-RIS and RIS-UE channels are all LOS, the gain of the composite channel from Tx to Rx aided by RIS still
depends on constructive / destructive superposition of BS-UE and BS-RIS-UE channels. The phase difference between
������ �� �� �
�� �� ��
the BS-UE path and BS-RIS-UE path is ��, which changes with respect to the subcarrier
�
frequency � and determines the frequency selectivity of RIS-aided channels. When multipath effect is considered, the
frequency selectivity of a RIS-aided channel is generally more significant than that of a channel without RIS, as the RIS
provides a much larger re-radiation gain compared to ordinary clusters. Figure 5.2.3-1 illustrates the simulated
frequency-domain channel with / without RIS given simulation setups in table 5.2.3-1. At a carrier frequency of 4 GHz,
the CDL-D channel model defined in ETSI TR 138 901 [i.3] is used to generate the BS-UE and BS-RIS-UE fading
channel with LOS paths (note that the composite BS-RIS-UE path is modelled as a single CDL-D channel without
losing generality). In these simulations without RIS, the multi-path component contributed by the surface (e.g. a wall)
on which RIS would be mounted is not considered. However, it is expected that the gain provided by such a surface is
usually much smaller than the RIS gain due to a lack of beamforming. It is clear in figure 5.2.3-1 that the frequency
selectivity of RIS-aided channel is indeed more significant than that without RIS.
Table 5.2.3-1: Simulation Assumptions
Simulation parameters Values / setups
BS-UE distance (m) About 40
BS-RIS distance (m) About 20
Carrier frequency (GHz) 4
BS-UE Path loss model UMi NLOS model
BS-RIS / RIS-UE path loss model UMi LOS model
BS antenna 1 omni-directional antenna
UE antenna 1 omni-directional antenna
RIS 576 omni-directional elements
Fading channel model CDL-D
Delay spread (ns) 100
Tx power (dBm) 23
Noise density (dBm / Hz) -167 (including noise figure)
Bandwidth (MHz) 100
ETSI
17 ETSI GR RIS 005 V1.1.1 (2025-02)
CDL-D with RIS
CDL-D without RIS
-50 -40 -30 -20 -10 0 10 20 30 40 50
Frequency (MHz)
Fi
...