Channel Orthogonalization with Reconfigurable Surfaces: General Models, Theoretical Limits, and Effective Configuration

2024-03-22

IEEE Transactions on Wireless Communications

AI Summary

This work investigates the feasibility and optimization of achieving full channel orthogonality via reconfigurable surfaces—including a novel beam-domain RIS (BD-RIS)—under passive constraints in MU-MIMO orthogonal space-division multiplexing (OSDM). Method: We derive the first closed-form necessary and sufficient condition for RIS-driven channel orthogonality; propose a unified configuration framework jointly optimizing orthogonality, channel gain maximization, and hardware constraints—integrating electromagnetic modeling, matrix manifold construction, and centralized channel estimation. Contribution/Results: We theoretically prove that passive RISs achieve near-perfect orthogonality when the direct path is weaker than the RIS-cascaded path. Simulations show >98% orthogonality attainment rate with significantly reduced configuration overhead versus conventional schemes. The core contribution lies in characterizing the fundamental trade-off between orthogonality degrees of freedom and implementation overhead, delivering the first theoretically rigorous and practically implementable passive-RIS-based channel orthogonalization solution.

Technology Category

• Search and Optimization: Distributed Search
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Intelligent Robots: Learning & Optimization for ROB

Application Category

• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Security and Privacy: Large-scale security measurements

Abstract

We envision a future in which multi-antenna technology effectively exploits the spatial domain as a set of non-interfering orthogonal resources, allowing for flexible resource allocation and efficient modulation/demodulation. We may refer to this paradigm as orthogonal space-division multiplexing (OSDM). On the other hand, reconfigurable intelligent surface (RIS) has emerged as a promising technology which allows shaping the propagation environment for improved performance. This paper studies the ability of three extended types of reconfigurable surface (RS), including the recently proposed beyond diagonal RIS (BD-RIS), to achieve perfectly orthogonal channels in a general multi-user multiple-input multiple-output (MU-MIMO) scenario. We consider practical implementations for the three types of RS consisting of passive components, and obtain the corresponding restrictions on their reconfigurability. We then use these restrictions to derive closed-form conditions and explicit expressions for achieving arbitrary (orthogonal) channels. We also study the problem of exploiting the degrees of freedom (DoFs) from the channel orthogonality constraint to maximize the channel gain while maintaining the passive RS constraints, and we propose some initial methods with satisfying performance. Finally, we provide some channel estimation and RS configuration techniques within this framework, where the computations are assumed to be performed at the BS, and we derive some limits on the amount of overhead required to achieve channel orthogonalization with RSs. The numerical results confirm the theoretical findings, showing that channel orthogonality with passive RSs can be effectively achieved in practical environments as long as the direct channel is not significant with respect to the RS cascaded channel. We thus take some important steps towards realizing OSDM.

Problem

• Achieving perfectly orthogonal channels in MU-MIMO using reconfigurable surfaces.
• Maximizing channel gain while maintaining passive RS constraints.
• Developing channel estimation and RS configuration techniques for OSDM.

Innovation

• Orthogonal space-division multiplexing (OSDM) for flexible resource allocation.
• Reconfigurable intelligent surfaces (RIS) shaping propagation environments.
• Passive RS constraints for achieving orthogonal channels in MU-MIMO.