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Single-RF MIMO: From Spatial Modulation to Metasurface-Based Modulation

Qiang Li, Miaowen Wen, Marco Di Renzo

arXiv:2009.00789v1cs.ITmath.IT

TL;DR

Conventional MIMO typically requires multiple RF chains, while single-RF SM has implementation challenges. This article proposes metasurface-based modulation for single-RF MIMO and evaluates its BER performance. It concludes that metasurface-based modulation is a competitive alternative to antenna-based modulation.

  • Problem

    Single-RF SM faces implementation challenges, including the need for high-speed RF switching, while conventional MIMO uses multiple RF chains for simultaneous streams.

    Source: INTRODUCTION · p. 2

  • Method

    The article proposes RIS-based single-RF MIMO, discusses implementations, and evaluates their BER performance.

    Source: CONCLUSIONS · p. 16

  • Results

    Metasurface-based modulation provides a competitive alternative to antenna-based modulation for single-RF MIMO.

    Source: CONCLUSIONS · p. 16

  • Takeaways & Limitations

    The article presents metasurface-based modulation as an alternative design direction for single-RF MIMO.

    Source: CONCLUSIONS · p. 16

    RSM requires NT RF chains and channel state information at the transmitter.

    Source: EVOLUTION OF SM · p. 8

Abstract

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In multiple-input multiple-output (MIMO), multiple radio frequency (RF) chains are usually required to simultaneously transmit multiple data streams. As a special MIMO technology, spatial modulation (SM) activates one transmit antenna with one RF chain and exploits the index of the active antenna for information transfer at each time slot. Recently, reconfigurable metasurfaces have emerged as a promising technology that is able to reconfigure the wireless propagation environment by altering the amplitude and/or phase of the incident signal. In this article, we aim for the implementation of single-RF MIMO by shifting the focus from SM to metasurface-based modulation. Specifically, the principles of SM and metasurfaces are first presented. After reviewing the evolution of SM, we elaborate on the idea of metasurface-aided single-RF MIMO and discuss some implementations for it in line with notable variants of SM. A comparison between antenna-based and metasurface-based modulation is made to highlight the advantages of using metasurfaces. We finally investigate the research challenges and opportunities in the context of metasurface-based modulation.

INTRODUCTION

SM offers single-RF transmission by encoding information in antenna activation, but its variants and hardware face implementation constraints. The article shifts attention to metasurface-based modulation and outlines its principles, implementations, comparison with antenna-based modulation, and research challenges.

INTRODUCTION

  • SM conveys information through the active antenna index and the transmitted symbol, using one RF chain and one active antenna per channel use.The antenna index carries spatial information in addition to the symbol.Source: INTRODUCTION · p. 2
  • SM variants can require multiple RF chains, while single-RF SM/GSM/QSM need high-speed, low-loss RF switching that increases complexity and cost.Antenna positions must also change at each channel use in SM, GSM, and QSM.Source: INTRODUCTION · p. 2
  • Antenna-based SM depends on distinct channel signatures, which may require rich scattering or many receive antennas that are not generally available.Limited device size can make it difficult to support many distinguishable antennas; antenna correlation can obscure states and degrade performance.Source: INTRODUCTION · p. 3
  • The article proposes metasurface-based modulation for single-RF MIMO, presenting SM and RIS principles, reviewing SM variants, and discussing implementations, comparisons, challenges, and opportunities.It positions RISs as a way to configure the propagation environment and motivate the shift from SM.Source: INTRODUCTION · pp. 3–4

PRINCIPLES OF SM AND METASURFACES

This section presents the principles of SM and metasurfaces as preliminaries.

PRINCIPLES OF SM AND METASURFACES

Principle of SM

SM sends spatial information through antenna selection alongside a modulated symbol. Metasurfaces instead configure reflected signals by adjusting element reflection amplitude and phase, which can also encode information.

Principle of SM

  • SM uses one RF chain to select one active transmit antenna per channel use, conveying spatial information through its index alongside an M-ary symbol.The index conveys ⌊log2 NT⌋ bits, and the receiver detects both the symbol and active antenna.Source: Principle of SM · pp. 4–5

Principle of Metasurfaces

  • Under local design, each RIS element’s reflection coefficient is β_n = a_n e^jφ_n, with amplitude a_n ∈ [0, 1] and phase φ_n ∈ [0, 2π).In practice, amplitude and phase values are discrete.Source: Principle of Metasurfaces · p. 5
  • An RIS controls reflected signals by adjusting element coefficients across cascaded incident, reflection, and outgoing links.A smart controller tunes element amplitude and phase; a PIN diode example imposes 0 or π phase shift depending on bias voltage.Source: Principle of Metasurfaces · p. 5
  • Setting all element amplitudes to 1 and aligning phases can enhance intended-user received power, while changing amplitudes or phases according to bits conveys RIS private information.Source: Principle of Metasurfaces · p. 6

EVOLUTION OF SM

The evolution review covers GSM, QSM, RSM, and RQSM, including enhanced SM variants proposed to increase spectral efficiency. It also compares antenna-based and metasurface-based single-RF MIMO at the transmitter level.

Comparison scope

  • The comparison of antenna-based and metasurface-based single-RF MIMO focuses on transmitter design unless otherwise specified.The review names GSM, QSM, RSM, and RQSM as representative SM variants.Source: EVOLUTION OF SM · p. 7

GSM

  • Single-RF GSM activates N_A of N_T antennas and sends one M-ary symbol through the active antennas.The active-antenna count is selected from the available transmit antennas.Source: EVOLUTION OF SM · p. 7

QSM

  • QSM independently applies SM to the I and Q dimensions, doubling spatial information bits while retaining the single-RF property.The I- and Q-component antenna selections may require one or two active antennas per time slot.Source: EVOLUTION OF SM · p. 7

RSM

  • RSM conveys spatial information through receive-antenna indices using transmit precoding, but requires N_T RF chains and transmitter channel state information.RSM is also reported to provide high beamforming gain and low receiver complexity.Source: EVOLUTION OF SM · pp. 7–8

RQSM

RQSM combines QSM and RSM, using receive-antenna indices twice to convey spatial information. The section also outlines six single-RF metasurface-based schemes, including MBM and RIS variants.

RQSM

  • RQSM combines QSM and RSM, selecting both precoding vectors from a common set and using a receive-antenna index twice for spatial information.Compared with RSM, RQSM increases spectral efficiency by ⌊log2 N_R⌋ bpcu.Source: RQSM · p. 8

Metasurface-based modulation

  • The six proposed single-RF metasurface schemes are MBM, RIS-MIMO, RIS-SM/GSM, RIS-QSM, RIS-RSM, and RIS-RQSM.Their system models are presented in Fig. 3.Source: RQSM · pp. 8–9

MBM

  • MBM encodes N source bits through ON/OFF RF-mirror states alongside an M-ary symbol sent from the RF chain.The RF mirrors can be viewed as metasurface reflecting elements with binary amplitude and zero phase.Source: RQSM · p. 9

RIS-MIMO

RIS-MIMO uses a single-RF transmitter to illuminate a reconfigurable intelligent surface, which modulates the reflected signal. The number of simultaneously transmitted information streams equals the number of RIS meta-atoms.

RIS-MIMO

  • RIS elements can implement amplitude and phase modulation, including PSK and QAM, by changing their reflections.A single-RF transmitter illuminates the RIS with an unmodulated carrier and applies modulation at each reflecting element according to the source data.Source: RIS-MIMO · pp. 9–10
  • The scheme encodes source data through element-wise reflection modulation while using only one RF chain at the transmitter.The transmitter illuminates the RIS with an unmodulated carrier signal.Source: RIS-MIMO · pp. 9–10
  • RIS-MIMO can simultaneously transmit as many information streams as the RIS has meta-atoms.The streams are transmitted via reflections.Source: RIS-MIMO · p. 10

RIS-SM/GSM

RIS-SM/GSM encodes information through the positions of active reflecting elements, while RIS-QSM reactivates inactive elements so all elements contribute to beamforming. RIS-QSM improves receive SNR but has the same spectral efficiency as RIS-SM/GSM.

RIS-SM/GSM

  • RIS-SM/GSM maps one source-data part to an M-ary symbol and another to the positions of active reflecting elements.An element is active or inactive through its reflection-amplitude setting; the number of active elements is between 1 and N−1.Source: RIS-SM/GSM · p. 10
  • RIS-SM/GSM underutilizes the RIS because some reflecting elements remain inactive.With RIS channel information available, element phase shifts can be tuned for BER minimization.Source: RIS-SM/GSM · p. 10

RIS-QSM

  • RIS-QSM assigns active and re-active states to elements, using their positions to convey spatial information while employing all elements for beamforming.The two states use phase shifts tied to the channel phase; the passage reports improved receive SNR and the same spectral efficiency as RIS-SM/GSM.Source: RIS-QSM · p. 10
  • RIS-QSM improves receive SNR, while its spectral efficiency remains the same as RIS-SM/GSM.Reactivating otherwise inactive elements distinguishes RIS-QSM from RIS-SM/GSM.Source: RIS-QSM · p. 10

RIS-RSM

RIS-RSM conveys information through receive-antenna indices while using one transmit RF chain and passive metasurface beamforming. RIS-RQSM extends this approach with separate I- and Q-branch data streams.

RIS-RSM

  • RIS-RSM uses the receive-antenna index alongside an M-ary symbol to convey information, with spectral efficiency matching RSM.It uses one transmit RF chain and passive metasurface beamforming rather than active precoding.Source: RIS-RSM · p. 10
  • RIS-RSM tunes reflection coefficients to maximize received power at the selected receive antenna.This uses metasurface beamforming in place of active precoding.Source: RIS-RSM · p. 11

RIS-RQSM

  • RIS-RQSM divides the RIS into two halves controlled by independent data streams on the I- and Q-branches, whose detection can be performed independently.Its spectral efficiency is 2(1 + log2 NR) bpcu.Source: RIS-RQSM · pp. 11–12
  • With M-PAM, RIS-RQSM achieves 2 log2 NR + log2 M + 1 bpcu by encoding information across the two branches.The I-branch phase is fixed to avoid phase ambiguity, resulting in a loss of 1 bit of spatial information.Source: RIS-RQSM · p. 12

COMPARISON AND PERFORMANCE EVALUATION

The evaluation compares antenna-based and metasurface-based modulation schemes, including BER performance and resource requirements. The results show reliability trade-offs across the schemes and identify metasurface modulation as a single-antenna, single-RF-chain option.

COMPARISON AND PERFORMANCE EVALUATION

  • At BER 10^-3, QSM and RQSM achieve about 3 dB SNR gain over SM and RSM, respectively; single-RF GSM achieves about 1.5 dB SNR gain over SM at the expense of two more transmit antennas.Source: COMPARISON AND PERFORMANCE EVALUATION · p. 14
  • Metasurface-based modulation can transmit information using a single antenna and a single RF chain.Table I summarizes the compared schemes by RF chains/transmit antennas, spectral efficiency, complexity, and CSI requirements.Source: COMPARISON AND PERFORMANCE EVALUATION · p. 13
  • The compared metasurface schemes vary remarkably in BER, and the evaluation verifies the feasibility of MBM and RIS-MIMO.Fig. 5(b) presents BER performance for the metasurface-based schemes.Source: Figure 5 · p. 13, COMPARISON AND PERFORMANCE EVALUATION · p. 14
  • RIS-RSM/RQSM significantly outperform RIS-GSM/QSM in BER, at the cost of using more receive antennas.The authors attribute the difference to RIS-RSM/RQSM beamforming toward the selected receive antenna, which favors antenna-index detection.Source: COMPARISON AND PERFORMANCE EVALUATION · p. 14

THE ROAD AHEAD

The road ahead identifies open design questions for metasurface-based single-RF modulation. Proposed directions include hybrid schemes and improved element grouping, alongside further study of I- and Q-branch element selection.

THE ROAD AHEAD

  • The authors identify numerous potential challenges and open research problems for metasurface-based single-RF modulation.The article frames these as directions deserving further investigation.Source: THE ROAD AHEAD · p. 14

Hybrid Modulation

  • Hybrid schemes could combine RIS-GSM active-element indexing with RIS-MIMO, RIS-QSM, RIS-RSM, or RIS-RQSM, trading BER performance for improved spectral efficiency.The paper also proposes investigating direct links and balancing their contribution with the reflecting link.Source: Hybrid Modulation · p. 15

Reflecting Element Grouping and Selection

  • Element grouping affects spectral efficiency and BER, so group size and the selection of elements for RIS-QSM/RQSM I- and Q-branches warrant study.For RIS-SM/GSM, selecting reflecting elements is also proposed when the relevant value is not a power of two.Source: Reflecting Element Grouping and… · p. 15

Differential Modulation and Space-Time Coding

The paper points to differential modulation and RIS-based space-time coding as research directions, alongside other metasurface modulation designs. It concludes that metasurface-based modulation is a competitive alternative to antenna-based modulation for single-RF MIMO.

Differential Modulation and Space-Time Coding

  • Large RIS element counts make channel estimation difficult for coherent detection, making differential modulation without CSI an appealing research direction.An RIS may also reconfigure reflection coefficients to perform space-time coding and achieve diversity gains.Source: Differential Modulation and Space-Time… · p. 15

Other Metasurface-Based Modulation Techniques

  • Other proposed designs vary the number of active RIS elements with source data or use an RIS to enable subcarrier index modulation.The latter applies to an orthogonal frequency division multiplexing signal.Source: Other Metasurface-Based Modulation… · pp. 15–16

CONCLUSIONS

  • The article presents metasurface-based modulation as a competitive alternative to antenna-based modulation for single-RF MIMO.Its scope includes reviewing SM, presenting metasurface-based implementations, comparing modulation schemes, and discussing challenges and opportunities.Source: CONCLUSIONS · p. 16
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