RIS Selection Scheme for UAV-Based Multi-RIS-Aided Multiuser Downlink Network With Imperfect and Outdated CSI

Abstract

In this paper, we explore the use of reconfigurable intelligent surface (RIS) in unmanned aerial vehicle (UAV) based multiuser downlink communications, where a flying UAV serves multiple single antenna users through multiple RISs mounted on various buildings. More specifically, we consider the selection of RISs based on the outdated and imperfect channel state information (CSI) of the composite UAV-RIS-User channels at the UAV. After selection process, the UAV communicates to the user via the selected RISs and also with the direct link. Particularly, we derive an infinite series based expression for selection probability of RISs under both the outdated and imperfect CSI of composite channels based selection scheme. We also derive the statistical distribution of instantaneously received signal-to-noise ratio (SNR) under outdated and imperfect CSI conditions of both the direct and composite links at the user. Next, using the derived statistics, we analyze the network's performance in terms of the average coverage probability (ACP) and average bit error rate (ABER) over the complete UAV flight time. Moreover, we discuss the behavior of ACP and ABER for very small and very large values of UAV transmit power, respectively. It is depicted through numerical results that selecting more RISs from a group of small-sized RISs may not be as advantageous as selecting fewer RISs from a group of largesized RISs. Moreover, we also demonstrate the effect of several system parameters such as number of RIS reflecting elements, number of selected RISs, the severity of UAV-RIS and RIS-User links, and the severity of imperfect and outdated CSI on the network's performance. The analytical results are corroborated with Monte-Carlo simulations.Index Terms-Reconfigurable intelligent surface (RIS), unmanned aerial vehicle (UAV), outage probability, nultiuser downlink communication, outdated channel state information (CSI), imperfect CSI, RIS selection schemes. communications.The integration of UAV and RIS has recently come up as an important framework to enhance the energy efficiency and spectrum efficiency in B5G and 6G communications [24]-[30]. In [27], an algorithm has been proposed to jointly design active and passive beamforming and UAV's trajectory in RISassisted UAV networks. While the secrecy rate maximization problem has been formulated in [28] for jointly optimizing the transmit power, UAV trajectory, and RIS phase shifts, an iterative algorithm has been proposed in [30] to maximize the sum-rate of the RIS-assisted UAV orthogonal frequencydivision multiple access (OFDMA) communication systems. Recently, considering practical models and designing a robust beamforming in RIS-assisted communications becomes very important. Accordingly, in [31], an algorithm for robust beamforming design has been proposed for RIS-aided communication where a multi-antenna access point serves a singleantenna user with the aid of an RIS. Similar to [31], the authors in [32] have jointly optimized hybrid access point energy beamforming, time allocation, ...

Multi-IRS-Assisted mmWave UAV-BS Network for Coverage Extension

“…The UAV functions as a BS/RS and communicates with the IRS deployed on the side of buildings in this related research. Ankur et al [ 28 ] proposed an RISs selection scheme based on the composite UAV–RIS–user channel gains with the possibility of an imperfect and outdated CSI of the composite links. It has also been deduced from the results that it is always better to select fewer large-sized RISs than more small-sized RISs for a fixed number of total reflecting elements in the network.…”

“…IRS collaborated with UAV-BS [28] Proposes RIS selection scheme for UAV-based multi-RIS-aided multiuser.…”

Joint Phase Shift Design and Resource Management for a Non-Orthogonal Multiple Access-Enhanced Internet of Vehicle Assisted by an Intelligent Reflecting Surface-Equipped Unmanned Aerial Vehicle

“…Due to the high mobility of the IRS-equipped UAV and CEVs, the transmission channels vary rapidly. Therefore, we use the outdated channel estimation method proposed in [22] to reduce the pilot overhead. The signal-to-interference-plus-noise ratio (SINR) γ v of the v-th CEV is…”

“…Facing the above challenges, the authors [21,22] adopted an intelligent reflecting surface (IRS)-equipped UAV to create ITS-related services for vehicle platooning in obstructed rural highways covered by 5G/B5G. First, the IRS technique can enhance 5G/B5G signal coverage in rural areas, especially in situations with a complex terrain or buildings that block signals [23,24].…”

“…However, it is challenging to integrate the IRS-equipped UAV and NOMA technique into IoV while optimizing the resource management and phase shift design. First, the works in [21,22,30] make an implicit assumption that spectrum-efficient resource management can be achieved. Actually, in dynamic IoV, the subcarrier allocation and power control policies need to be delicately designed to improve the communication performance.…”