Page 96 - 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

“smart radio environments” as a means of Based on the above motivation, we propose a
propagation path control by IRS reflectors. It also massive distributed IRS based on a 1-bit control IRS
models the basic characteristics of IRS reflectors reflector with few elements in which all elements
and clarifies the differences with other relay-system can be controlled for reflection (ON) or penetration
devices. Furthermore, as specific use cases, there (OFF) at the same time. Although 1-bit control is a
are intelligent walls [4-6] and hyper-surfaces [7-9]. rough control, the squared power gain is asymptotic
An intelligent wall is an IRS reflector that can to that of continuous control even for 1-bit
enable/disable reflection and control phase [0, π] controlled IRS in [14]. Instead of controlling the
with respect to the incident radio wave. It can be amplitude and phase parameters for each element
achieved using a PIN diode or variable resonator. of the IRS reflector, this paper proposes a
Hyper-surfaces, meanwhile, centrally control IRS formulation to select the ON/OFF combination of
reflectors through a control unit equipped with an IRS reflectors. In particular, we formulate the joint
IoT gateway. In addition, a propagation path control ON/OFF IRS selection method and interference
is being theoretically formulated as a suppression as a non-linear binary integer
programmable wireless environment in which the programming problem. We then solve the
maximizing of received signal intensity and the formulated problem of IRS reflectors ON/OFF using
minimizing of maximum delay spread are objective the optimization solver while maintaining the
functions. interference level below a certain threshold.
Further, a clustering method of IRS reflector is also
Past studies have targeted a single, relatively large
area IRS reflector, but to improve the performance proposed to reduce the computational complexity
of an entire wireless service area beyond indoor during the massive deployment of IRS. The
environments and certain small areas, there are proposed method performs clustering IRS
also reports of obtaining even better effects by reflectors having similar characteristics and selects
deploying and controlling distributed IRS reflectors only the representative value of a cluster as a
at multiple locations [10][11]. There are also combination candidate to keep computational costs
studies on maximizing the channel capacity of all constant even in the case of many IRS reflectors. We
single-antenna terminals by jointly optimizing the evaluate the effectiveness of the proposed method
phase control of distributed IRS reflectors and base- using simulation and experimental data.
station beam-forming control [12][13]. In order to The rest of this paper is organized as follows.
flexibly meet various requirements such as higher Section 2 describes the system model used in this
transmission capacity and reliability in the beyond study and formulates the problem of suppressing
5G era, it is necessary to expand to "massive" interference by selecting IRS reflectors. Section 3
distributed IRS aided wireless communication as describes the method of clustering IRS reflectors
envisioned in this paper. proposed in this paper. Section 4 evaluates the
performance of the proposed method by computer
On the other hand, a distributed arrangement
means a very large number of patterns in the way simulation and compares it with conventional
that the control parameters of each IRS reflector can clustering methods. Section 5 performs an
be combined. Most of the previous literature evaluation using experimental data. Finally, Section
assumes that the amplitude and phase parameters 6 concludes the paper.
are continuously controlled for each element of the
IRS reflectors [10-12]. However, the practical 2. MASSIVE DISTRIBUTED IRS AIDED
difficulty lies in the amount of time required for WIRELESS COMMUNICATION WITH
controlling and optimizing these and thus increases ON/OFF SELECTION
the operational complexity of IRS reflectors itself 2.1 System model
[14]. In addition, the effects of interference caused
by an increase in the number of propagation paths As shown in Fig. 1, this paper considers a system
must be considered in actual environments. To the concept where Base Stations (BS) and User
best of our knowledge, this is the first work that Equipment (UE) communicate via IRS reflectors.
performs combinatorial optimization of a large IRS reflectors are massively distributed, as they are
number of IRS reflectors while considering this installed on the roofs and walls of buildings. This
interference. environment is modeled as shown in Fig. 2. The
same frequency is used here. This model consists of
sender S, receiver D, interfered receiver P, and IRS

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