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Repeater Swarm-Assisted Cellular Systems: Interaction Stability and Performance Analysis

Jianan Bai, Anubhab Chowdhury, Anders Hansson, Erik G. Larsson

TL;DR

This work analyzes repeater swarm-assisted cellular systems where many low-cost, full-duplex repeaters act as active scatterers to boost coverage in massive MIMO networks. It develops a generalized Nyquist stability framework to prevent positive-feedback instability from inter-repeater interactions and couples this with an uplink performance optimization that jointly tunes repeater gains, user powers, and receive combiners via a WMMSE-like iterative algorithm. The paper provides explicit stability conditions, analyzes special cases (two repeaters and circle deployments), and demonstrates substantial uplink capacity gains in both sub-6 GHz and mmWave bands through detailed numerical results and deployment guidelines. The findings show that careful repeater placement and stability-aware optimization can nearly double the sum-rate relative to repeater-free systems, highlighting practical pathways for leveraging repeater swarms in future cellular networks.

Abstract

We consider a cellular massive MIMO system where swarms of wireless repeaters are deployed to improve coverage. These repeaters are full-duplex relays with small form factors that receive and instantaneously retransmit signals. They can be deployed in a plug-and-play manner at low cost, while being transparent to the network--conceptually they are active channel scatterers with amplification capabilities. Two fundamental questions need to be addressed in repeater deployments: (I) How can we prevent destructive effects of positive feedback caused by inter-repeater interaction (i.e., each repeater receives and amplifies signals from others)? (ii) How much performance improvement can be achieved given that repeaters also inject noise and may introduce more interference? To answer these questions, we first derive a generalized Nyquist stability criterion for the repeater swarm system, and provide an easy-to-check stability condition. Then, we study the uplink performance and develop an efficient iterative algorithm that jointly optimizes the repeater gains, user transmit powers, and receive combining weights to maximize the weighted sum rate while ensuring system stability. Numerical results corroborate our theoretical findings and show that the repeaters can significantly improve the system performance, both in sub-6 GHz and millimeter-wave bands. The results also warrant careful deployment to fully realize the benefits of repeaters, for example, by ensuring a high probability of line-of-sight links between repeaters and the base station.

Repeater Swarm-Assisted Cellular Systems: Interaction Stability and Performance Analysis

TL;DR

This work analyzes repeater swarm-assisted cellular systems where many low-cost, full-duplex repeaters act as active scatterers to boost coverage in massive MIMO networks. It develops a generalized Nyquist stability framework to prevent positive-feedback instability from inter-repeater interactions and couples this with an uplink performance optimization that jointly tunes repeater gains, user powers, and receive combiners via a WMMSE-like iterative algorithm. The paper provides explicit stability conditions, analyzes special cases (two repeaters and circle deployments), and demonstrates substantial uplink capacity gains in both sub-6 GHz and mmWave bands through detailed numerical results and deployment guidelines. The findings show that careful repeater placement and stability-aware optimization can nearly double the sum-rate relative to repeater-free systems, highlighting practical pathways for leveraging repeater swarms in future cellular networks.

Abstract

We consider a cellular massive MIMO system where swarms of wireless repeaters are deployed to improve coverage. These repeaters are full-duplex relays with small form factors that receive and instantaneously retransmit signals. They can be deployed in a plug-and-play manner at low cost, while being transparent to the network--conceptually they are active channel scatterers with amplification capabilities. Two fundamental questions need to be addressed in repeater deployments: (I) How can we prevent destructive effects of positive feedback caused by inter-repeater interaction (i.e., each repeater receives and amplifies signals from others)? (ii) How much performance improvement can be achieved given that repeaters also inject noise and may introduce more interference? To answer these questions, we first derive a generalized Nyquist stability criterion for the repeater swarm system, and provide an easy-to-check stability condition. Then, we study the uplink performance and develop an efficient iterative algorithm that jointly optimizes the repeater gains, user transmit powers, and receive combining weights to maximize the weighted sum rate while ensuring system stability. Numerical results corroborate our theoretical findings and show that the repeaters can significantly improve the system performance, both in sub-6 GHz and millimeter-wave bands. The results also warrant careful deployment to fully realize the benefits of repeaters, for example, by ensuring a high probability of line-of-sight links between repeaters and the base station.

Paper Structure

This paper contains 28 sections, 4 theorems, 67 equations, 13 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. Two Fundamental Aspects
  3. Contributions and Organization of the Paper
  4. Motivating Example
  5. Best-Case Scenario with Orthogonal Channels
  6. Non-Ideal Scenarios
  7. System Model
  8. The Repeater Swarm
  9. Uplink Model
  10. Downlink Model
  11. Stability Analysis
  12. Bounded-Energy Stability
  13. Simplified Condition for Interaction Stability
  14. Special Cases
  15. Two Repeaters
  16. ...and 13 more sections

Key Result

Theorem 1

Under Assumptions as: analyticity and as: channel amplitude gain, if the image of $\det({\boldsymbol{I}}_N - {\boldsymbol{D}}_{{\boldsymbol{a}}(j\omega)} {\boldsymbol{H}}^\mathtt{R}(j\omega))$ does not encircle the origin, then for ${\boldsymbol{G}}(s)$ defined in eq: effective response of repeaters exists for all $\sigma \geq 0$ and is independent of the choice of $\sigma$. Additionally, $\boldsy

Figures (13)

  • Figure 1: A motivating example.
  • Figure 2: Averaged sum rate for different repeater locations.
  • Figure 3: Block diagrams of repeater (redrawn from willhammar2024achieving).
  • Figure 4: Illustration of a repeater swarm-assisted cellular system.
  • Figure 5: Block diagram of the positive-feedback system.
  • ...and 8 more figures

Theorems & Definitions (16)

  • Definition 1
  • Definition 2
  • Remark 1
  • Remark 2
  • Remark 3
  • Theorem 1
  • Remark 4
  • Remark 5
  • Proposition 1
  • Remark 6
  • ...and 6 more