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Is Repeater-Assisted Massive MIMO Compatible with Dynamic TDD?

Martin Andersson, Anubhab Chowdhury, Erik G. Larsson

TL;DR

The paper tackles the compatibility of repeater-assisted dynamic TDD with cross-link interference in massive MIMO networks by deriving closed-form DL/UL SEs that account for a complex repeater gain $\alpha_{\mathrm{r}} = r_{\mathrm{r}} e^{i\phi_{\mathrm{r}}}$ and proposing an iterative optimization to maximize SE. It introduces an algorithm that jointly optimizes the amplification and phase shift of the repeater while updating beamforming vectors, yielding practical SE gains in the DL and UL and highlighting the importance of phase optimization. Key contributions include closed-form SE expressions, a tractable SINR-based optimization with global stationary points for a given phase, and numerical evidence that, with appropriate repeater placement (near users and avoiding CLI amplification), repeater-assisted dynamic TDD can outperform baseline static schemes. The results provide deployment guidelines and motivate further work on networks with multiple repeaters and integrated optimization of repeater gains, power control, and precoding/combining.

Abstract

We present a framework for joint amplification and phase shift optimization of the repeater gain in dynamic time-division duplex (TDD) repeater-assisted massive MIMO networks. Repeaters, being active scatterers with amplification and phase shift, enhance the received signal strengths for users. However, they inevitably also amplify undesired noise and interference signals, which become particularly prominent in dynamic TDD systems due to the concurrent downlink (DL) and uplink (UL) transmissions, introducing cross-link interference among access points and users operating in opposite transmit directions. This causes a non-trivial trade-off between amplification of desired and undesired signals. To underpin the conditions under which such a trade-off can improve performance, we first derive DL and UL spectral efficiencies (SEs), and then develop a repeater gain optimization algorithm for SE maximization. Numerically, we show that our proposed algorithm successfully calibrates the repeater gain to amplify the desired signal while limiting the interference.

Is Repeater-Assisted Massive MIMO Compatible with Dynamic TDD?

TL;DR

The paper tackles the compatibility of repeater-assisted dynamic TDD with cross-link interference in massive MIMO networks by deriving closed-form DL/UL SEs that account for a complex repeater gain and proposing an iterative optimization to maximize SE. It introduces an algorithm that jointly optimizes the amplification and phase shift of the repeater while updating beamforming vectors, yielding practical SE gains in the DL and UL and highlighting the importance of phase optimization. Key contributions include closed-form SE expressions, a tractable SINR-based optimization with global stationary points for a given phase, and numerical evidence that, with appropriate repeater placement (near users and avoiding CLI amplification), repeater-assisted dynamic TDD can outperform baseline static schemes. The results provide deployment guidelines and motivate further work on networks with multiple repeaters and integrated optimization of repeater gains, power control, and precoding/combining.

Abstract

We present a framework for joint amplification and phase shift optimization of the repeater gain in dynamic time-division duplex (TDD) repeater-assisted massive MIMO networks. Repeaters, being active scatterers with amplification and phase shift, enhance the received signal strengths for users. However, they inevitably also amplify undesired noise and interference signals, which become particularly prominent in dynamic TDD systems due to the concurrent downlink (DL) and uplink (UL) transmissions, introducing cross-link interference among access points and users operating in opposite transmit directions. This causes a non-trivial trade-off between amplification of desired and undesired signals. To underpin the conditions under which such a trade-off can improve performance, we first derive DL and UL spectral efficiencies (SEs), and then develop a repeater gain optimization algorithm for SE maximization. Numerically, we show that our proposed algorithm successfully calibrates the repeater gain to amplify the desired signal while limiting the interference.
Paper Structure (11 sections, 2 theorems, 13 equations, 3 figures)

This paper contains 11 sections, 2 theorems, 13 equations, 3 figures.

Table of Contents

  1. INTRODUCTION
  2. SYSTEM MODEL
  3. and Data Transmission
  4. and Spectral Efficiencies
  5. OPTIMIZING THE REPEATER GAIN
  6. Maximizing the SINR in DL and UL
  7. Proposed Method for Repeater Gain Optimization
  8. NUMERICAL EVALUATIONS
  9. Achievable SEs in DL and UL with Optimal Repeater Gains
  10. Real-Valued Versus Complex-Valued Repeater Gains
  11. CONCLUSIONS AND FUTURE DIRECTIONS

Key Result

Proposition 1

An achievable SE for each DL user $k$ is given by $\mathtt{SE}_{\text{d},k} = \log_2 \left( 1 + \mathtt{SINR}_{\text{d},k} \right)$, where

Figures (3)

  • Figure 1: The two-cell repeater-assisted massive MIMO system operating with dynamic TDD. The CLI channels are denoted by red arrows. We let $\alpha_\text{r}$ be the repeater gain of the forward path.
  • Figure 2: The median DL and UL SEs for a varying repeater position.
  • Figure 3: DL and UL SEs with optimal and real-valued repeater gains.

Theorems & Definitions (5)

  • Remark 1
  • Proposition 1
  • proof
  • Proposition 2
  • proof