Extremely Large-Scale MIMO: Fundamentals, Challenges, Solutions, and Future Directions
Published in IEEE Wireless Communications, 2023
Abstract: Extremely large-scale multiple-input-multiple-output (XL-MIMO) is a promising technology to empower the next-generation communications. However, XL-MIMO, which is still in its early stage of research, has been designed with a variety of hardware and performance analysis schemes. To illustrate the differences and similarities among these schemes, we comprehensively review existing XL-MIMO hardware designs and characteristics in this article. Then, we thoroughly discuss the research status of XL-MIMO from “channel modeling”, “performance analysis”, and “signal processing”. Several existing challenges are introduced and respective solutions are provided. We then propose two case studies for the hybrid propagation channel modeling and the effective degrees of freedom (EDoF) computations for practical scenarios. Using our proposed solutions, we perform numerical results to investigate the EDoF performance for the scenarios with unparallel XL-MIMO surfaces and multiple user equipment, respectively. Finally, we discuss several future research directions.
Index Terms: Future wireless communications, XL-MIMO, near-field communications, degree-of-freedoms
Fig. 1: Application scenarios for the XL-MIMO system.
Fig. 2: General XL-MIMO schemes and their characteristics. △ is the horizontal and vertical antenna spacing. A is the element size. M_V and M_H are the numbers of antennas per row and per column, respectively. Besides, the isotropic patch or point antennas are assumed in this paper. Note that the CAP-based XL-MIMO with spatially-continuous aperture and the other schemes with discrete antennas are different on the practical design and the signal processing protocol. However, to comprehensively embrace the XL-MIMO schemes, we also discuss the promising CAP-based XL-MIMO and give some intuitive mathematical and physical relationships between the CAP-based XL-MIMO and the other XL-MIMO schemes.
Fig. 3: Fundamentals of the XL-MIMO channel modeling.
Fig. 4: Key challenges and respective solutions. Challenge 1: How to model the accurate and tractable channel for the hybrid propagation with the LoS component and the NLoS component? Challenge 2: How to analyze the EDoF performance with practical scenarios (such as unparallel transmitter and receiver as (a) or multiple receivers as (b))? Challenge 3: How to design the processing schemes with acceptable complexity?
Fig. 5: EDoF for the system with unparallel XL-MIMO surfaces against the number of antennas per side.
Fig. 6: EDoF for the XL-MIMO system with multiple users.
Recommended citation: Zhe Wang, Jiayi Zhang, Hongyang Du, Wei E. I. Sha, Bo Ai, Dusit Niyato, and Mérouane Debbah, "Extremely Large-Scale MIMO: Fundamentals, Challenges, Solutions, and Future Directions," IEEE Wireless Communications, accepted, 2023. https://arxiv.org/pdf/2209.12131.pdf