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Holographic MIMO Communications: What is the benefit of closely spaced antennas?

Antonio Alberto D'Amico, Luca Sanguinetti

TL;DR

This paper considers a simple uplink scenario with two side-by-side half-wavelength dipoles, two users and single path line-of-sight propagation, and shows that the channel gain and average spectral efficiency depend strongly on the directions from which the signals are received and on the array matching network used.

Abstract

Holographic MIMO refers to a (possibly large) array with a large number of individually controlled and densely deployed antennas. The objective of this paper is to provide further insight into the use of closely spaced antennas in the uplink and downlink of a multi-user Holographic MIMO system. To this end, we utilize multiport communication theory, which ensures physically consistent uplink and downlink models. We first consider a simple uplink scenario with two side-by-side half-wavelength dipoles, two users, and single-path line-of-sight propagation, and show both analytically and numerically that the array gain and average spectral efficiency strongly depend on the directions from which the signals are received and on the array matching network used. The numerical results are then used to extend the analysis to more practical scenarios involving larger arrays of dipoles (arranged in a uniform linear array) and a larger number of users. The case where the antennas are densely packed in a space-constrained factor form is also considered. It is found that the spectral efficiency increases with decreasing antenna spacing only for arrays of moderate size, e.g. in the order of a few wavelengths. In comparison, larger arrays with closely spaced antennas show only marginal improvements in spectral efficiency compared to half-wavelength arrays.

Holographic MIMO Communications: What is the benefit of closely spaced antennas?

TL;DR

This paper considers a simple uplink scenario with two side-by-side half-wavelength dipoles, two users and single path line-of-sight propagation, and shows that the channel gain and average spectral efficiency depend strongly on the directions from which the signals are received and on the array matching network used.

Abstract

Holographic MIMO refers to a (possibly large) array with a large number of individually controlled and densely deployed antennas. The objective of this paper is to provide further insight into the use of closely spaced antennas in the uplink and downlink of a multi-user Holographic MIMO system. To this end, we utilize multiport communication theory, which ensures physically consistent uplink and downlink models. We first consider a simple uplink scenario with two side-by-side half-wavelength dipoles, two users, and single-path line-of-sight propagation, and show both analytically and numerically that the array gain and average spectral efficiency strongly depend on the directions from which the signals are received and on the array matching network used. The numerical results are then used to extend the analysis to more practical scenarios involving larger arrays of dipoles (arranged in a uniform linear array) and a larger number of users. The case where the antennas are densely packed in a space-constrained factor form is also considered. It is found that the spectral efficiency increases with decreasing antenna spacing only for arrays of moderate size, e.g. in the order of a few wavelengths. In comparison, larger arrays with closely spaced antennas show only marginal improvements in spectral efficiency compared to half-wavelength arrays.
Paper Structure (28 sections, 3 theorems, 72 equations, 13 figures, 2 tables)

This paper contains 28 sections, 3 theorems, 72 equations, 13 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Review of Multiport Communication Theory
  3. Signal generation and power
  4. Impedance matrices
  5. Losses in the antennas
  6. Noise sources
  7. Input-Output Relation
  8. Transmit power and noise covariance matrix
  9. Matching network optimization
  10. Computation of the Mutual Coupling Impedance Matrix
  11. Impedance matrix ${\bf Z}_{{\rm AR}}$
  12. Impedance matrix ${\bf Z}_{{\rm ART}}$
  13. Holographic MIMO Communications
  14. Uplink data transmission
  15. Downlink data transmission
  16. ...and 13 more sections

Key Result

Lemma 1

Consider the uplink with $M_{\rm BS}=2$. If a full matching network is used at the BS, then in single path LoS propagation the array gain (compared to a single antenna BS) for UE $k$ is with $\psi_k = 2\pi \frac{d_H}{\lambda}\cos(\theta_k)\sin(\phi_k)$.

Figures (13)

  • Figure 1: Physical model of a multi-antenna communication system, based on the circuit theoretic concept of linear multiports Nossek2014.
  • Figure 2: Normalized eigenvalue distribution of ${\bf Z}_{\rm AR}$ and ${\bf U}$.
  • Figure 3: Behaviour of the normalized mutual coupling coefficient $\mu$ for two half-wavelength dipoles with sinusoidal current and side-by-side configuration as the spacing $d_H$ varies. The radiation resistance is $R_{\rm r} = 73$Ω while the dissipation resistance is $R_{\rm d} = 10^{-3}R_{\rm r}$.
  • Figure 4: Uplink SNR of UE $1$ (in dB) with MMSE as $\phi_1$ varies with and without a noise matching network at the BS and ${M_{\rm BS}}=2$ antennas. The UE is located at a distance of $50$ meters and that the BS array is at an height of $10$ meters. Different values of $d_H$ are considered. The SNR with a single antenna is also reported as a benchmark.
  • Figure 5: Uplink SNR of UE $1$ (in dB) with MMSE with and without a noise matching network at the BS and $M_{\rm BS}=2$ antennas. The UE is located at a distance of $50$ meters and the BS array is at an height of $10$ meters. Different values of $\phi_1$ are considered. The SNR with a single antenna is also reported as a benchmark.
  • ...and 8 more figures

Theorems & Definitions (6)

  • Remark 1
  • Remark 2
  • Lemma 1
  • proof
  • Corollary 1
  • Lemma 2