Wireless Communication Aided by Intelligent Reflecting Surface: Active or Passive?

Abstract

In this letter, we consider an intelligent reflecting surface (IRS)-aided wireless communication system, where an active or passive IRS is employed to assist the communication between an access point and a user. First, we consider the downlink/uplink communication separately and optimize the IRS placement for rate maximization with an active or passive IRS. We show that the active IRS should be deployed closer to the receiver with the IRS's decreasing amplification power; while in contrast, the passive IRS should be deployed near either the transmitter or receiver. Moreover, with optimized IRS placement, the passive IRS is shown to outperform its active counterpart when the number of reflecting elements is sufficiently large and/or the active-IRS amplification power is too small. Next, we optimize the IRS placement for both active and passive IRSs to maximize the weighted sum-rate of uplink and downlink communications. We show that in this case, the passive IRS is more likely to achieve superior rate performance. This is because the optimal active-IRS placement needs to balance the rate performance in the uplink and downlink, while deploying the passive IRS near the transmitter or receiver is optimal regardless of the uplink or downlink.

Active Intelligent Reflecting Surface Assisted Secure Air-to-Ground Communication with UAV Jittering

“…And if the fading coefficient is large, the signals from this longer reflection link lose more power than that from the short direct link, resulting in a limited secrecy performance gain compared with the one without IRS. Recently, a new concept of active IRS has been proposed [20]- [24] which can combat this "double fading" effect. Compared with passive IRS, the active IRS directly reflects the incident signal with the power amplification in the electromagnetic (EM) level, and it still takes the advantage of IRS that no complex and powerhungry radio frequency (RF) chain components are needed.…”

“…Therefore, the physical size of the active IRS can be reduced, making it more suitable for UAV communication limited by the payload. Furthermore, it offers more freedom to design secure beamforming by optimizing the amplitudes of the reflection coefficients instead of just phases, resulting in more flexibility to reconfigure the wireless propagation environment [24], [25]. As a result, the secure UAV communication enhanced by active IRSs is a promising solution for providing high-efficient communication services.…”

Resource Allocation for Secure IRS-Assisted Multiuser MISO Systems

“…Moreover, we observe from (2) that the noises at the IRS can be divided into two categories, i.e., dynamic noise and static noise [9]. In particular, the dynamic noise is generated due to the power amplification [11], where d ∈ C N T ×1 is modelled as additive white Gaussian noise (AWGN) with variance…”

“…To circumvent these issues, the authors of [9] recently proposed a new IRS structure, namely, active IRSs. In particular, equipped with reflection-type amplifiers [10], [11], active IRSs can not only reflect the incident signals by manipulating the programmable IRS elements, but also amplify the reflected signal with the support of an extra power supply. We note that active IRSs are fundamentally different from full-duplex amplify-and-forward (FD-AF) relays in terms of hardware architecture and the mode of transmission.…”

Intelligent Reflecting Surface Aided Wireless Energy and Information Transmission: An Overview

“…While we have shown that the passive IRS is able to improve the WPT/SWIPT/WPCN performance at low energy and hardware cost, a large number of reflecting elements is practically required due to the product-distance path loss caused by IRS's passive reflection. To overcome this drawback, a new type of IRS, called active IRS, has been recently proposed [89], [90], as shown in Fig. 12(d).…”