Reconfigurable Intelligent Surface-Based Index Modulation: A New Beyond MIMO Paradigm for 6G (1904.06704v3)

Abstract: Transmission through reconfigurable intelligent surfaces (RISs), which control the reflection/scattering characteristics of incident waves in a deliberate manner to enhance the signal quality at the receiver, appears as a promising candidate for future wireless communication systems. In this paper, we bring the concept of RIS-assisted communications to the realm of index modulation (IM) by proposing RIS-space shift keying (RIS-SSK) and RIS-spatial modulation (RIS-SM) schemes. These two schemes are realized through not only intelligent reflection of the incoming signals to improve the reception but also utilization of the IM principle for the indices of multiple receive antennas in a clever way to improve the spectral efficiency. Maximum energy-based suboptimal (greedy) and exhaustive search-based optimal (maximum likelihood) detectors of the proposed RIS-SSK/SM schemes are formulated and a unified framework is presented for the derivation of their theoretical average bit error probability. Extensive computer simulation results are provided to assess the potential of RIS-assisted IM schemes as well as to verify our theoretical derivations. Our findings also reveal that RIS-based IM, which enables high data rates with remarkably low error rates, can become a potential candidate for future wireless communication systems in the context of beyond multiple-input multiple-output (MIMO) solutions.

• The paper proposes RIS-SSK and RIS-SM schemes that leverage reconfigurable intelligent surfaces to integrate index modulation with optimized phase adjustments for high data rates and low error rates.
• It formulates both maximal likelihood and greedy detection methods to balance performance with reduced computational complexity in practical deployments.
• Comprehensive analyses and simulations validate the schemes' exceptional performance in low SNR regimes, underscoring their potential as a breakthrough for 6G communications.

Reconfigurable Intelligent Surface-Based Index Modulation for 6G

The paper under examination introduces a novel communication scheme leveraging reconfigurable intelligent surfaces (RIS) to advance index modulation (IM) techniques within the context of beyond multiple-input multiple-output (MIMO) systems, potentially for 6G wireless networks. The core proposition involves integrating RIS with space shift keying (SSK) and spatial modulation (SM), leading to RIS-SSK and RIS-SM schemes, designed for enhanced signal quality and spectral efficiency in challenging wireless environments.

Key Contributions

The research extends the concept of RIS—which manipulates incident electromagnetic waves' reflection to improve signal characteristics—into the domain of IM. The proposed RIS-SSK and RIS-SM schemes utilize the RIS not only for optimizing signal reflection but also for encoding information through the selection of receiver antenna indices, improving efficiency and reliability.

1. RIS-SSK and RIS-SM Design:
   • These schemes ingeniously use the RIS to perform intelligent signal reflection and IM, allowing high data rates and low error rates without the need for complex RF hardware.
   • RIS-SSK maximizes the signal-to-noise ratio (SNR) at the intended receive antenna through optimal phase adjustments, while RIS-SM transmits additional data via conventional modulation formats.
2. Detection Methods:
   • The paper formulates both maximal likelihood (ML) and greedy detectors, where the latter reduces complexity by avoiding full channel state information (CSI), a common limitation in practical deployments.
   • Performance comparisons reveal the ML detector’s superior error performance due to its comprehensive index and symbol detection capability.
3. Theoretical Analyses and Simulations:
   • A thorough error performance analysis is conducted using both greedy and ML detection, supported by numerical and simulation results. These analyses confirm the potential for RIS-based schemes to achieve exceptional performance in low SNR regimes, which traditional MIMO systems struggle with.
4. Complexity and Trade-offs:
   • The research highlights a trade-off between detection accuracy and system complexity. While ML detection requires more CSI and computational resources, it offers improved error rates. Conversely, the greedy detector simplifies implementation, beneficial for large-scale deployments.

Implications and Future Directions

The implications of this research are extensive, both in theoretical and practical contexts. From a theoretical viewpoint, the integration of RIS with IM seeks to redefine wireless communication paradigms by harnessing environmental reflections and scattering. Practically, such developments can lead to simpler, yet more efficient, communication systems capable of meeting 6G requirements for ultra-reliable and low-latency communications.

The potential research avenues are numerous:

• RIS Optimization: Further exploration of RIS phase optimization under realistic channel conditions and imperfect CSI.
• Integration with Other Technologies: Studying compatibility and performance synergies with other advanced technologies like terahertz communication and cognitive radio.
• Real-World Constraints: Examining challenges associated with hardware impairments and deployment in realistic scattering environments.

This paper provides a significant exploration into RIS-assisted schemes, setting the stage for future breakthroughs in communication technologies. The RIS-SSK and RIS-SM schemes offer potential solutions to current wireless communication challenges, heralding a new era of efficient, high-performance networks.