Intelligent Reflecting Surface Aided Multicasting With Random Passive Beamforming

Abstract

In this letter, we consider a multicast system where a single-antenna transmitter sends a common message to multiple single-antenna users, aided by an intelligent reflecting surface (IRS) equipped with N passive reflecting elements. Prior works on IRS have mostly assumed the availability of channel state information (CSI) for designing its passive beamforming. However, the acquisition of CSI requires substantial training overhead that increases with N . In contrast, we propose in this letter a novel random passive beamforming scheme, where the IRS performs independent random reflection for Q ≥ 1 times in each channel coherence interval without the need of CSI acquisition. For the proposed scheme, we first derive a closed-form approximation of the outage probability, based on which the optimal Q with best outage performance can be efficiently obtained. Then, for the purpose of comparison, we derive a lower bound of the outage probability with traditional CSI-based passive beamforming. Numerical results show that a small Q is preferred in the highoutage regime (or with high rate target) and the optimal Q becomes larger as the outage probability decreases (or as the rate target decreases). Moreover, the proposed scheme significantly outperforms the CSI-based passive beamforming scheme with training overhead taken into consideration when N and/or the number of users are large, thus offering a promising CSI-free alternative to existing CSI-based schemes.Index terms-Intelligent reflecting surface, multicast, random passive beamforming, outage probability I. INTRODUCTION Intelligent reflecting surface (IRS) is a new emerging paradigm for the fifth-generation (5G) and beyond wireless communication networks [1]-[3]. Specifically, an IRS is able to alter the wireless channel by shifting the phases of the impinging signals via a large number of passive reflecting elements, for enhancing desired signal power or suppressing undesired interference. Being also cost-effective and energyefficient, IRS has received fast-growing research attention recently. To maximize the performance gains brought by IRS, it is of paramount importance to properly design the IRS reflection coefficients, which is also termed as passive beamforming. In the literature, passive beamforming design has been studied under various system setups with different performance metrics [4]- [12]. Particularly, the existing works on IRS have mostly considered a "fully intelligent" reflecting surface with the availability of full channel state information (CSI) for all the IRS-associated links (see, e.g., [4]-[11]). In practice, such CSI needs to be acquired in each channel coherence interval by the transmitter/receiver at the cost of channel training overhead and/or feedback complexity that increases with the number of IRS reflecting elements, N , Q. Tao and C. Zhong are with the

Double-IRS Aided MIMO Communication under LoS Channels: Capacity Maximization and Scaling

“…. Hence, t 2L in (11) and s L in (17) are almost the same, so are r 1R in (15) and s R in (18). Therefore, we have…”

Semi-Passive Reconfigurable Intelligent Surfaces Enabled Communications: A Comprehensive Survey

“…Even though passive RIS is applicable in the FDD and TDD systems, the latter ones are relatively more advantageous because the cascaded channels need to be measured only in either unlink or downlink sessions due to the channel reciprocity phenomena [13]. Moreover, it should be pointed out that, irrespective of the transmission session, CSI knowledge is the main pre-requisite for the effective design of passive beamforming techniques [40], [41] In general, the uplink data transmission protocol for passive RIS comprises three stages. First, the users send pilot signals to BS to evaluate the direct user-to-BS and cascaded user-to-RIS-to-BS channels considering reflection coefficients of RIS that vary from time to time.…”

A Survey on Channel Estimation and Practical Passive Beamforming Design for Intelligent Reflecting Surface Aided Wireless Communications

“…• Propose to randomly change IRS reflection pattern multiple times in each channel coherence interval without CSI in multicast systems [169] • IRS randomly sets its reflection, while the BS employs the proportional fair scheduling to exploit the multi-user diversity gain [170] Heuristic method Rician fading channel Received power/SNR • Use the particle swarm optimization method to find the near-optimal IRS reflection based on the received SNR at the user [171] As summarized in Table VIII, compared to the beamforming design with statistical or instantaneous CSI only, the design based on hybrid CSI is able to flexibly balance the trade-off between the performance and overhead. However, the latter approach usually requires a tractable approximation to the system utility and incurs a higher complexity in optimization.…”

“…3) Other Approaches: Besides the above two approaches, there are also other methods that can be used to design the IRS passive beamforming without explicit CSI. For example, random IRS passive beamforming is an efficient approach that does not need any CSI and thus is free of heavy channel estimation overhead, while it sacrifices the full passive beamforming gain [169], [170]. Specifically, the authors in [169] considered an IRS-aided multicast system and proposed to generate random IRS reflections over time for reshaping the distributions of all users' channels.…”