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Physical-Layer Aspects of Repeater-Assisted MIMO

Diana P. M. Osorio, Kohei Ueda, Anubhab Chowdhury, Hiroki Iimori, Yuto Hama, Koji Ishibashi, Erik G. Larsson

arXiv:2609.29846v1eess.SP

TL;DR

This article examines how repeater-assisted MIMO and ISAC can use network-controlled repeaters while addressing practical hardware, channel, control, and sensing tradeoffs. It synthesizes design considerations and results, including conditions under which a delayed repeater path can add frequency diversity, and concludes that practical deployments need integrated designs.

Abstract

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Network-controlled repeaters (NCRs) are emerg- ing as low-cost, band-selective active scatterers that can re- shape the wireless propagation environment without backhaul or tight phase synchronization. We study physical-layer design for repeater-assisted multiple-input multiple-output (RA-MIMO) networks, such as hardware impairments, gain and activation control, duplexing, channel-state information acquisition, and wideband delay effects. We then discuss integrated sensing and communication (ISAC), where repeaters can extend non- line-of-sight (NLoS) visibility, improve target observability, and enhance weak-echo detection. These gains, however, are limited by amplified noise, clutter, feedback, and calibration errors, motivating joint optimization and gain control frameworks for swarm repeater deployments within ISAC.

I. INTRODUCTION

Network-controlled repeaters create amplified propagation paths that can extend coverage, macro-diversity, and channel rank, while repeater swarms can approach distributed-MIMO spectral efficiency without backhaul or stringent phase synchronization. This article examines practical propagation, duplexing, CSI, wideband-delay, and sensing challenges between that potential and deployable systems.

I. INTRODUCTION

  • NCR swarms are low-cost, low-power, frequency-selective active scatterers that can extend coverage, provide macro-diversity, and improve channel rank in cellular massive MIMO.These devices receive, amplify, and instantaneously retransmit wireless signals.Source: §I · p. 1
  • Repeater swarms can approach distributed-MIMO spectral efficiency without backhaul or stringent phase synchronization, while band selectivity supports multi-operator use.Unlike passive RIS, repeaters can isolate bands; swarm stability criteria address positive feedback, and gain and transmit-power optimization is possible within the stability region.Source: §I · p. 1

1) Propagation models for repeaters:

  • Repeaters receive, amplify, and retransmit signals, creating rank-one channel components for single-antenna configurations and requiring reciprocity calibration for transparent dual-antenna operation.The dual-antenna reverse-link gain multiplies the outer product of the over-the-air channels.Source: 1) Propagation models for repeaters · p. 1
  • The article focuses on practical duplexing and CSI mechanisms, wideband delay effects, and sensing tradeoffs, then closes with a research roadmap rather than a performance claim.It treats the same amplification that helps communication as a potential obstacle to detection.Source: 1) Propagation models for repeaters · p. 2

II. COMMUNICATION ASPECTS OF SWARM RA-MIMO

This section frames communication design for swarm RA-MIMO around practical foundations and recent advances. It connects hardware and standardization to repeater interaction, activation and gain control, duplexing, and wideband frequency diversity.

II. COMMUNICATION ASPECTS OF SWARM RA-MIMO

  • The section covers swarm repeater-assisted MIMO foundations and advances in interaction, activation and gain control, duplexing, and wideband frequency diversity.It first summarizes standardized and hardware foundations, then focuses on recent results and remaining communication-design challenges.Source: §II · p. 2
  • The propagation model introduced earlier is common to the communication aspects in this section and the ISAC aspects in Section III.Source: §II · p. 2
  • The section distinguishes practical foundations from recent advances and remaining communication-design challenges.Source: §II · p. 2

A. Practical Foundations and Recent Advances

RA-MIMO treats repeater-assisted paths as part of an effective channel for centralized BS processing, but this abstraction depends on real repeater responses. Practical design must jointly consider hardware constraints, deployment geometry, swarm stability, and control under limited information and coordination.

A. Practical Foundations and Recent Advances

  • The 5G-Advanced NCR standard provides a deployment baseline, while RA-MIMO models repeater-assisted paths as an effective channel for coverage, macro-diversity, and spatial multiplexing.The abstraction preserves centralized multi-antenna BS processing, but depends on physical repeater response; RF-chain mismatch can make effective uplink and downlink channels differ, requiring reciprocity calibration for transparent TDD operation.Source: A. Practical Foundations and Recent… · p. 2

B. Swarm Deployment, Stability, and Practical Control

  • Repeater value depends on geometry: well-placed devices can improve coverage and channel rank, while weakly connected or interference-exposed devices may mainly add amplified noise and unwanted signals.Placement and density planning should account for blockage, line-of-sight probability, angular diversity, traffic, and the quality of both path hops.Source: B. Swarm Deployment, Stability, and… · p. 2
  • Swarm placement, density, activation, and power control are coupled through positive-feedback stability, while activation also trades fairness and energy efficiency against active scattering paths.Fast adaptive control requires accurate, timely CSI and calibrated, linear hardware; longer-term cluster controls reduce signaling and complexity but may sacrifice spectral efficiency, fairness, or coverage. Robust policies must also consider incomplete CSI, delayed updates, and inter-cell or inter-operator interference.Source: B. Swarm Deployment, Stability, and… · pp. 2–3

C. Duplexing-Aware Swarm Control and Cross-Link Interference

Duplexing choices for swarm RA-MIMO trade calibration and feedback overhead against independent gain control and potential spectral-efficiency benefits. Dynamic duplexing also raises cross-link interference and timing-mismatch concerns that require joint control and analysis.

C. Duplexing-Aware Swarm Control and Cross-Link Interference

  • Calibrated TDD uses uplink CSI for downlink precoding without downlink CSI overhead, whereas FDD avoids reciprocity calibration through feedback that may be costly.FDD permits independent uplink and downlink repeater-gain optimization, which can improve spectral efficiency under certain power and overhead regimes.Source: C. Duplexing-Aware Swarm Control and… · p. 3
  • Dynamic TDD and in-band full duplex may improve time-frequency reuse, but repeaters can amplify cross-link interference along with desired signals.The design question is whether coordinated scheduling, precoding and combining, gain control, and interference-aware CSI acquisition can preserve repeater spatial and macro-diversity gains against that interference.Source: C. Duplexing-Aware Swarm Control and… · p. 3
  • Synchronizing repeater switching with the BS can create timing mismatch with users because of timing advance, potentially requiring a longer guard interval and reducing spectral efficiency.Source: C. Duplexing-Aware Swarm Control and… · p. 3

D. Wideband, Orthogonal frequency division multiplexing (OFDM), Delay, and Frequency-Diversity Effects

Repeater-assisted paths make delay a wideband channel-shaping factor: within the cyclic-prefix constraint, a distinct delayed path can add frequency diversity, while excessive delay can cause severe interference. Practical design must account for frequency-dependent gains, amplified noise, and receiver processing.

D. Wideband, Orthogonal frequency division multiplexing (OFDM), Delay, and Frequency-Diversity Effects

  • A narrowband SNR model is insufficient for practical wideband systems because artificial-path propagation and hardware processing introduce group and processing delays.These delays supplement the repeater's received-SNR gains.Source: D. Wideband, Orthogonal frequency… · p. 3
  • Repeater propagation, RF filters, baseband filters, and signal processing add delays that modify the effective OFDM channel in delay and frequency.These changes affect frequency-domain channel correlation, amplified-noise statistics, achievable rates, and error probabilities; gains and colored noise should be modeled with waveform numerology and receiver equalization.Source: D. Wideband, Orthogonal frequency… · p. 3
  • A sufficiently strong repeater path at a distinct delay can reduce frequency correlation and increase frequency diversity when the composite channel remains within the cyclic prefix.The benefit depends on path power, delay separation, the underlying power-delay profile, signaling, and receiver.Source: D. Wideband, Orthogonal frequency… · p. 3
  • When excess propagation and processing delay approaches or exceeds the cyclic prefix, delayed energy causes inter-symbol and inter-carrier interference, and a high-gain path can severely degrade performance.Wideband design must jointly optimize placement, gain, delay, waveform numerology, and receiver processing rather than only narrowband received power.Source: D. Wideband, Orthogonal frequency… · p. 3

III. SWARM REPEATERS FOR ISAC

Repeater swarms are presented as controllable active propagation nodes that may extend ISAC into obstructed or poorly covered regions and improve target observability. Their sensing role remains largely unexplored, and communication-optimized repeater settings may not suit sensing.

III. SWARM REPEATERS FOR ISAC

  • Repeater swarms may extend sensing to hot-spots, coverage holes, and line-of-sight-blocked regions that access-point-centric architectures may not easily reach.The proposed potential benefits include improved target observability and spatial resolution at low infrastructure cost; Fig. 3 illustrates potential repeater roles in ISAC.Source: §III · p. 3, Figure 3 · p. 4

A. The Role of Repeaters in ISAC

  • Repeaters can act as controllable active propagation nodes that reshape communication and sensing channels.The article introduces their possible roles in enhancing ISAC performance.Source: A. The Role of Repeaters in ISAC · p. 4

1) Repeaters for Coverage Extension, “See behind the corners”:

  • For NLoS sensing, target echoes are weaker than reflections from large obstacles, while repeater use introduces noise amplification, hardware-induced phases, and non-directional radiation.With careful calibration and signal processing accounting for radiation and amplification, repeaters could potentially overcome passive reflectors.Source: 1) Repeaters for Coverage Extension,… · p. 4

2) Repeaters as Distributed Illuminators, “Increasing Target Observability”:

Controllable repeater gains can support distributed sensing and weak-echo detection, but require channel characterization and control that account for feedback. For echo amplification, the gain optimum tends to favor the repeater closest to the target.

2) Repeaters as Distributed Illuminators, “Increasing Target Observability”:

  • Proper gain control can make repeater swarms a distributed sensing support layer that improves target observability and blockage robustness.This requires characterizing AP-repeater, repeater-user, repeater-target, and inter-repeater channels; when transparent repeaters lack pilot transmission or local channel estimation, channel charting, blind estimation, or network-side inference is needed.Source: 2) Repeaters as Distributed… · p. 4

3) Repeaters as Echo Amplifiers, “Enhancing Weak Target Detection”:

  • Repeater gains and activation or deactivation can be optimized to maximize sensing SINR while accounting for positive-feedback effects that may degrade performance.Echo amplification can be deployed in monostatic or bistatic settings.Source: 3) Repeaters as Echo Amplifiers,… · p. 4
  • Optimal solutions tend to assign a dominant gain to the repeater closest to the target.This result is reported for optimizing repeater gains and activation to maximize sensing SINR while accounting for positive-feedback effects.Source: 3) Repeaters as Echo Amplifiers,… · p. 4

B. Communications-Sensing Tradeoff in RA-MIMO

RA-MIMO ISAC faces coupled communication-sensing tradeoffs: repeater-created multipath can help communication yet complicate sensing, while indiscriminate amplification creates mutual interference between the two functions.

1) Multipath and propagation artifacts:

  • Frequency-dependent phase shifts and internal repeater delays can appear as ghost targets, biased range estimates, and degraded Doppler estimates in sensing.Stable, known delays can be calibrated for enhanced sensing, but unknown or time-varying delays must be estimated alongside the sensing scene, increasing complexity.Source: 1) Multipath and propagation artifacts · p. 4

B. Communications-Sensing Tradeoff in RA-MIMO

  • Repeater-introduced multipath can provide communication diversity and coverage but complicate sensing target discrimination.Communication can absorb frequency-dependent phase shifts and internal delays as effective multipath, whereas sensing may interpret them as propagation artifacts.Source: 1) Multipath and propagation artifacts · p. 4

2) Dual-function interference:

  • Repeaters indiscriminately amplify data signals, target echoes, interference, clutter, and noise, generating mutual interference between communication and sensing receivers.Sensing echoes can leak into communication receivers, and communication signals can leak into sensing receivers.Source: 2) Dual-function interference · p. 4

3) Gain- and density-dependent trade-offs:

Repeater gain and deployment density create opposing communication–sensing trade-offs: denser deployments can improve communication spectral efficiency but harm sensing under communication-centric gain. These trade-offs motivate joint control across communication, sensing, and system constraints.

3) Gain- and density-dependent trade-offs:

  • The severity of the trade-offs depends on repeater gains, deployment density, access-point precoders, geometry, and available channel-state information.Source: 3) Gain- and density-dependent… · p. 4
  • Low gains limit interference, noise amplification, and feedback instability but reduce coverage and sensing-aperture gains; high gains and dense deployments improve received power and sensing resolution but risk degrading communication SINR.Source: 3) Gain- and density-dependent… · p. 4
  • Joint optimization should account for communication rate, sensing accuracy and detection probability, interference leakage, repeater power, stability, and limited processing and pilot/CSI acquisition.Source: 3) Gain- and density-dependent… · p. 4
  • With denser deployments, communication-centric gain improves spectral efficiency with diminishing improvement while sensing SINR degrades sharply; sensing-centric gain keeps sensing SINR high, especially at high density, while spectral efficiency remains low and nearly flat.Source: 3) Gain- and density-dependent… · p. 5
  • Communication benefits from aggregate amplified paths, whereas sensing benefits from a single well-placed, high-gain path, so favoring one objective inherently penalizes the other.Source: 3) Gain- and density-dependent… · p. 5
  • Communication spectral-efficiency gains diminish beyond a certain density, so adding repeaters then consumes power and hardware without commensurate benefit.Source: 3) Gain- and density-dependent… · p. 5

C. System & System-level Design

Repeater-assisted ISAC couples communication and sensing impairments with network operation and resource allocation. Joint optimization and scalable processing remain open challenges as mobility, target dynamics, and dense deployments increase tracking and training demands.

C. System & System-level Design

  • Active RF chains amplify useful signals along with thermal noise, clutter, interference, and hardware distortion, degrading communication SINR and sensing detection performance.Source: C. System & System-level Design · p. 5
  • Phase noise, PA nonlinearities, I/Q imbalance, and reciprocity-calibration errors can reduce coherent integration, distort waveforms, bias range-angle estimates, and degrade sensing resolution.When repeater-noise covariance is known, spatial processing can partially mitigate repeater-originated noise.Source: C. System & System-level Design · p. 5
  • Activation, gain, TDD configuration, AP precoding/combining, synchronization, and channel training should be jointly optimized as AP roles and repeater functions vary across bistatic and multistatic ISAC.Source: C. System & System-level Design · p. 5
  • Mobility and target dynamics require continuous channel tracking, while dense deployments increase training overhead and computational complexity, making scalable joint processing and resource allocation an open direction.Source: C. System & System-level Design · p. 5

IV. OUTLOOK & ROAD AHEAD

The article reviews RA-MIMO physical-layer foundations and repeater-swarm support for ISAC, then identifies associated communication–sensing trade-offs. It frames a research roadmap while noting that practical deployment remains constrained by simplified assumptions and the technology’s early stage.

IV. OUTLOOK & ROAD AHEAD

  • The article reviews RA-MIMO deployment, hardware, repeater control, duplexing, channel-state information acquisition, and wideband effects, alongside repeater-swarm roles in extending sensing coverage, target observability, and weak-echo detection.Source: §IV · p. 5
  • Repeater swarms can support ISAC but introduce communication–sensing trade-offs from amplified noise, clutter, interference, hardware impairments, and stability constraints.Source: §IV · p. 5
  • The technology remains at an early development stage, and many existing results rely on simplified assumptions about hardware, propagation, synchronization, channel knowledge, and network operation.Source: §IV · p. 5
  • Table I organizes open questions and promising approaches across communication RA-MIMO and sensing, including ISAC design directions.The roadmap targets practical, scalable, intelligent repeater-assisted networks for future 6G systems.Source: §IV · p. 5, Table I · p. 6
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