Page 26 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 6 – Wireless communication systems in beyond 5G era

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 6

Wavelength Mux Although RIS have great potentials to implement ad‐
Spatial Mux
vanced electromagnetic wave manipulations, several fun‐
damental and implementation problems are still un‐
Digital to Light 2026
solved. At the physical layer, only simple functionalities,
• µLed 30µm 4×16×2 10 pix
2024 • RGB multiplexing
• ASIC demo such as electronic beam‐steering and multibeam scatter‐
• µLed 30µm 16×2 10 pixels • 15 bps/Hz+
• 16× Spatial Division 100 Gbps
• ASIC fab
• 10 bps/Hz ing, have been demonstrated in the literature. In addi‐
2022
20 Gbps
• µLed 60µm 2 10 pixels tion, problems such as channel state information esti‐
• Digital OFDM
• ASIC design
• 5 bps/Hz mation and acquisition, passive information transfer and
1 Gbps
Massive
2020 parallelization transceiver design are still open. At the network layer,
• Sensor array 2 10 pix 0101010
• Multiusers • One to one projection the propagation settings of installed RIS might be adapted
• µLed 30µm 2 10 pix 2027
100 Mbps
1 Tbps depending on scenarios, application needs and on real‐
time/predicted network dynamics. As today, open chal‐
Fig. 8 – Visible light communication road map from Mbit s −1 to Tbit s −1 . lenges remain on how to de ine the network architecture
Figure courtesy of [21].
incorporating multiple RIS and how to orchestrate the
recon iguration of multiple RIS devices in time‐space, to
aspects of the communication‐computation‐control chain meet speci ic suitable (goal‐oriented) deployment strate‐
[79]. Following the Shannon’s mantra, the common as‐ gies for effectively exploiting RIS technology. Such RIS
sumption in wireless communications has been that the network adaptation capabilities should enable dynamic
channel is given and it cannot be altered according to the programming of the wireless propagation environment
communication needs. However, with the advent of RIS, while meeting speci ic legislation and regulation require‐
there is the possibility to adjust the communication chan‐ ments on spectrum use and electromagnetic ield emis‐
nel to control wireless connectivity and mitigate interfer‐ sion, that might vary for speci ic locations and evolve over
ence. In this way, it is possible, for example, to increase time. Finally, it is still an open question to check under
the channel capacity without necessarily increasing nei‐ what conditions RIS‐empowered networks can provide a
ther the transmit power nor the bandwidth, or to reduce signi icant reduction of the overall network energy usage.
the associated electromagnetic ield footprint. Radical
technological advances based on the emerging paradigm
5. WHAT SHOULD 6G BE?
of RIS [80] are offering today the opportunity to forge
a new generation of dynamically‐programmable wireless Previously, Section 2 and Section 3 discussed wireless
propagation environments with minimal redesign and re‐ cellular networks and 5G, while Section 4 generally
con iguration costs for the connect‐compute network. described the technological, architectural, and metric‐
related trends envisioned for 6G. These important aspects
RIS can act especially as a transmitter, receiver or as an were aimed at providing the fundamental background for
anomalous re lector, where the direction of the re lected now trying to answer the questions, which were originally
wave is no longer specular according to natural re lec‐ stated in the Introduction: What can 6G be? Do we re‑
tion laws, but instead adaptively controllable. This can of‐ ally need 6G? In the last few years some articles [86], [87]
fer unprecedented opportunities to locally support a dy‐ have already tried to pose and to initially address these
namic adaptation to stringent and highly‐varying 6G ser‐ questions, showing some concerns regarding the need for
vice requirements such as momentary link capacity, local‐ a new ’network generation’ after 5G. So, we now address
isation accuracy, energy ef iciency, electromagnetic ield various critical points in order to justify how 6G can really
emission and secrecy guarantee. By de inition, RIS are ar‐ differentiate from 5G. Then, the following general analysis
ti icial intelligent controlled surfaces, constituted by hun‐ will help justifying the need for the so‐called new genera‐
dreds or thousands of recon igurable unit elements. They tion, underlining how 6G is not merely the answer to the
can be embedded in parts of the environment, such as disregarded promises of 5G, but it is a new disruptive gen‐
walls, mirrors, ceilings, etc. and can operate as a nearly‐ eration of communication networks. The following will
passive tunable anomalous lector or as a transmit‐ assume the perspective of KPI and metrics and the one
ter/receiver, when equipped with active radio‐frequency of new verticals (and subsequent needed technologies),
elements. Nowadays, RIS operate at low frequency but re‐ which were the main drivers for the advent of 5G and have
search is actively designing a solution to support wide‐ also been the main forces for the rise of 6G.
band operations up to the sub‐THz spectrum. In par‐
ticular, RIS can be implemented using a variety of tech‐ 5.1 Performance indicators and metrics
nologies and, through its property of modifying the ra‐
Let us now discuss the current set of performance indi‐
dio wave propagation, can provide extraordinary bene its
cators and metrics for 6G (see the list in Section 4.1) to
for diverse wireless goal‐oriented communications. Dif‐
see if they are really motivated or if they just represent
ferent antennas’ technologies can be adopted to design
a needless increase of the ones of 5G. The irst criticism
RIS, along with re lect‐arrays [81], transmit‐arrays [82],
refers to the KPI linked to higher bandwidth and data rate.
[83] and, smart, programmable or software de ined meta‐ We agree with [86], which correctly states that increasing
surfaces [84] [85].

14 © International Telecommunication Union, 2021