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

Changsheng You, Rui Zhang

arXiv:2106.10963v2cs.ITcs.NI

TL;DR

The paper studies how active versus passive IRSs should be deployed to assist AP–user communication. It optimizes placement separately for downlink and uplink, then for their weighted sum-rate, finding that passive IRSs can be superior under sufficiently many elements or low active amplification and are more likely to win jointly.

  • Problem

    The paper examines how active and passive IRS deployment should be optimized and compared for separate and joint uplink/downlink communication.

  • Method

    The authors optimize IRS placement for rate maximization separately in downlink and uplink, and for weighted sum-rate jointly, under a single-antenna AP–user system.

  • Results

    Active IRSs should move closer to the receiver as amplification power decreases, while passive IRSs should be near the transmitter or receiver; passive IRSs can outperform with many elements or low amplification power.

  • Takeaways & Limitations

    For joint uplink and downlink optimization, passive IRSs are more likely to achieve superior rate performance because their near-transmitter-or-receiver placement is near-optimal for both links.

Abstract

from arXiv · show

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.

I. INTRODUCTION

IRSs assist blocked wireless links by reconfiguring signal reflections, using either passive reflection or active amplification. This paper compares their deployment and rate implications under uplink, downlink, and joint communication settings.

  • I. INTRODUCTION: IRSs reconfigure wireless propagation by tuning the reflection amplitude and/or phase shift of many reflecting elements.Their lightweight profile allows installation on environmental objects.
  • I. INTRODUCTION: Passive IRSs reflect without amplification or processing noise, whereas active IRSs use low-power reflection-type amplifiers to amplify incident signals.Active IRSs incur amplification noise and modestly higher hardware and energy costs.
  • II. SYSTEM MODEL: The system studies a single-antenna access point and user whose direct link is severely blocked, with an active or passive IRS deployed between them.The assumed deployment establishes line-of-sight links from the AP to the IRS and from the IRS to the user.
  • II. SYSTEM MODEL: Passive IRS elements use unit reflection amplitude, while active IRS elements use a common amplification factor no smaller than one and generate amplification noise.The active reflection matrix is represented as an amplification factor multiplied by a phase-shift matrix.
  • II. SYSTEM MODEL: Under the considered LoS model, active-IRS elements can share a common amplification factor, whereas general fading channels may require different factors across elements.Using different factors under general channel models has a higher implementation cost.

III. DOWNLINK COMMUNICATION

This section formulates downlink IRS placement optimization for both active- and passive-IRS systems. The objective is rate maximization under the considered downlink communication setting.

  • III. DOWNLINK COMMUNICATION: The downlink communication problem optimizes IRS placement for rate maximization.
  • III. DOWNLINK COMMUNICATION: The optimization compares active- and passive-IRS aided wireless systems.
  • III. DOWNLINK COMMUNICATION: Both IRS types are evaluated through their placement choices in the downlink communication setting.

A. Active IRS

The active-IRS design jointly optimizes phase shifts, amplification, and placement for rate maximization. Its optimal placement moves toward the receiver as amplification power decreases, while achievable-rate scaling is limited by amplification noise.

  • Amplification constraints: The active IRS amplifies both the received signal and noise, and its maximum amplification power P_F is practically limited relative to conventional RF amplifiers.The amplification factor must also satisfy η ≥ 1, requiring the incident signal power at the active IRS to remain below its maximum amplification power.
  • Active-IRS optimization: The active-IRS problem maximizes downlink rate by jointly optimizing phase shifts, a common amplification factor, and IRS placement.The phase shifts align the cascaded AP–IRS–user channel, and the active-IRS power constraint is active at the optimum.
  • Optimal placement: The optimal AP–IRS distance x*_AI decreases with maximum amplification power P_F and does not decrease with the number of reflecting elements N.This placement trend follows from the dependence of the feasibility boundary and objective on P_F and N.
  • Optimal placement: With smaller amplification power, the active IRS is deployed farther from the transmitter so its elements can use a higher amplification factor η > 1.The placement compensates for the reduced available amplification power by changing the incident-signal power at the IRS.
  • Rate behavior: For small x0, the optimized active-IRS downlink rate scales linearly with N, inversely with D^2, and increases with P_F.This contrasts with the quadratic scaling order stated for the passive-IRS case.

B. Passive IRS

The passive IRS uses unit reflection amplitudes and optimized phase alignment. When the AP–IRS height is small, placing it above either the transmitter or receiver is near-optimal.

  • Passive-IRS optimization: The passive-IRS downlink optimization maximizes achievable rate by selecting the IRS phase-shift matrix and placement.The optimal phase shifts align the cascaded AP–IRS–user channel.
  • Optimal placement: For small IRS height H, a near-optimal passive-IRS placement is x_AI = 0 or x_AI = D.Thus, the IRS is deployed above either the transmitter or the receiver.

C. Active IRS versus Passive IRS

With optimized placement, the passive IRS tends to outperform the active IRS when active amplification power is too small or the number of reflecting elements is large. Reduced per-element amplification may fail to offset amplification noise.

  • Performance comparison: The passive IRS tends to achieve higher downlink receive SNR than the active IRS when active amplification power is too small and/or N is large.This comparison assumes small IRS height H and separately optimized placement for each IRS type.
  • Performance comparison: The comparison is attributed to a reduction in each active element’s amplification factor, which may not compensate for amplification noise.The active IRS amplifies both the received signal and noise.

IV. JOINT UPLINK AND DOWNLINK COMMUNICATION

The joint uplink–downlink analysis optimizes IRS placement for active and passive systems using weighted sum-rate as the objective.

  • Joint uplink and downlink communication: The paper optimizes IRS placement in both active- and passive-IRS systems to maximize the weighted sum-rate of uplink and downlink communications.

A. Active IRS

For active IRSs, weighted sum-rate placement is obtained by one-dimensional search and generally balances conflicting uplink and downlink placement preferences, depending on rate weights.

  • A. Active IRS: The active-IRS weighted sum-rate problem can be efficiently solved by a one-dimensional search over x_AI.
  • A. Active IRS: For small amplification power, the active IRS is generally placed closer to the AP for uplink and closer to the user for downlink.
  • A. Active IRS: The joint uplink-downlink optimum balances these opposing placement preferences and depends on the rate weights.

B. Passive IRS

For passive IRSs, phase alignment yields a weighted sum-rate optimization in which placing the IRS near either endpoint is near-optimal for both links, making passive IRSs more likely to outperform active IRSs jointly.

  • B. Passive IRS: Passive-IRS weighted sum-rate optimization is formulated after designing phase shifts to align the cascaded line-of-sight channel.
  • B. Passive IRS: When H is small, placing the passive IRS above either the AP or the user is near-optimal for both uplink and downlink communications.
  • B. Passive IRS: When H is small, the near-optimal AP-IRS horizontal distance for the weighted sum-rate is x*_AI = D.
  • B. Passive IRS: For joint uplink and downlink communication, passive IRSs are more likely to achieve superior rate performance because endpoint placement remains near-optimal for both links.

V. NUMERICAL RESULTS

The numerical study compares active and passive IRS placement and rate performance under a specified simulation setup.

  • V. NUMERICAL RESULTS: The simulations use N = 400 reflecting elements and deploy the IRS at altitude H = 1.5 m.
  • V. NUMERICAL RESULTS: The carrier frequency is 0.75 GHz, with reference path-loss β = (λ/4π)^2 = −30 dB and wavelength λ = 0.4 m.
  • V. NUMERICAL RESULTS: Other stated simulation parameters include D = 50 m and AP transmit power P_A = 20 dBm.

A. Downlink Communication

In downlink communication, the optimized active-IRS placement moves closer to the user as amplification power decreases, while passive-IRS rate growth eventually surpasses active-IRS performance.

  • A. Downlink Communication: As active-IRS amplification power P_F decreases, the optimal placement moves closer to the user, indicated by larger x_AI.
  • A. Downlink Communication: The downlink rate of passive IRSs grows faster than that of active IRSs because passive IRSs have power scaling O(N^2) versus O(N).
  • A. Downlink Communication: For P_F = 5 dBm, passive IRSs achieve higher downlink rates when N ≥ 300, while for P_F = 0 dBm the threshold is N ≥ 250.

B. Joint Uplink and Downlink Communication

For joint uplink and downlink communication, passive IRSs tend to achieve higher weighted sum-rates than active IRSs under optimized placement. The active IRS must balance uplink and downlink performance, whereas passive IRS placement near either endpoint is optimal for both.

  • The passive-IRS weighted sum-rate increases with the downlink weight because the AP transmits at higher power than the user.
  • With PF = 0 dBm, the active-IRS weighted sum-rate first decreases and then increases as the downlink weight rises.
  • Active-IRS placement must balance uplink and downlink rate performance when their weights are comparable.
  • Passive IRSs tend to achieve higher weighted sum-rates than active IRSs with respectively optimized placement.
  • Passive-IRS placement near the transmitter or receiver is optimal for both uplink and downlink communication.
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