Non-Stationarities in Extra-Large Scale Massive MIMO (1903.03085v2)

Abstract: Massive MIMO, a key technology for increasing area spectral efficiency in cellular systems, was developed assuming moderately sized apertures. In this paper, we argue that massive MIMO systems behave differently in large-scale regimes due to spatial non-stationarity. In the large-scale regime, with arrays of around fifty wavelengths, the terminals see the whole array but non-stationarities occur because different regions of the array see different propagation paths. At even larger dimensions, which we call the extra-large scale regime, terminals see a portion of the array and inside the first type of non-stationarities might occur. We show that the non-stationarity properties of the massive MIMO channel changes several important MIMO design aspects. In simulations, we demonstrate how non-stationarity is a curse when neglected but a blessing when embraced in terms of computational load and multi-user transceiver design.

• The paper details how extra-large scale massive MIMO systems exhibit spatial non-stationarities, where different array segments interact with terminals via distinct visibility regions.
• It analyzes how these non-stationarities challenge conventional channel models and necessitate new theoretical foundations and practical implementations for XL-MIMO.
• The research proposes novel low-complexity transceiver designs that leverage the spatial non-stationarity and visibility regions to optimize computational load.

Non-Stationarities in Extra-Large Scale Massive MIMO: A Summary

The exploration into the realms of massive multiple-input multiple-output (MIMO) technology, particularly in the context of extra-large scale deployments, unveils a set of unique challenges and opportunities. The paper "Non-Stationarities in Extra-Large Scale Massive MIMO" by Elisabeth De Carvalho et al., explores the intricacies of such advanced MIMO systems where spatial non-stationarities play a pivotal role.

Massive MIMO, as a cornerstone of 5G technology, enhances spectral efficiency by deploying numerous antennas at base stations to serve a multitude of terminals concurrently. Typically, massive MIMO systems have been conceptualized for moderately sized arrays. However, as the scale of such arrays expands to extra-large dimensions, the assumptions underpinning traditional MIMO paradigms encounter significant alterations. Such scenarios arise when the physical dimensions of MIMO arrays extend across several tens of wavelengths, leading to distinct spatial non-stationary characteristics.

Key Contributions and Findings

The paper distinguishes between large-scale and extra-large scale massive MIMO, outlining how non-stationarities manifest at these scales:

1. Spatial Non-Stationarity: As antenna arrays grow in size, different segments of the array face varied propagation environments, causing spatial non-stationarity. Notably, extra-large scale (XL-MIMO) systems entail scenarios where terminals interact with discrete segments, or visibility regions (VRs), of the array due to propagation path variability and physical obstructions.
2. Impact on MIMO Design: The research critically analyzes how spatial non-stationarities affect fundamental MIMO channel assumptions, notably questioning the efficacy of conventional channel models that assume wide-sense stationarity. The variability caused by VRs necessitates revisions in both the theoretical foundations and practical implementations of MIMO systems.
3. Simulation Insights: Through simulations, the paper illustrates that non-stationarity can either present challenges or opportunities based on its acknowledgment in system design. By factoring in the non-stationary nature, computational loads in multi-user transceiver design can be optimized.
4. Transceiver Design Innovations: The paper proposes innovative low-complexity transceiver designs that leverage the non-stationary properties of XL-MIMO channels. Specifically, the research suggests leveraging VRs to decrease the computational burden in the transceiver algorithms, thereby making the implementation of XL-MIMO systems more feasible.

Implications for Future Research

The analysis conducted within this paper suggests several pathways for ongoing and future research:

• Channel Modeling Enhancements: Developing robust channel models that accurately mirror the non-stationary characteristics of XL-MIMO environments is paramount. Factoring in dynamic VRs will necessitate more sophisticated models that surpass the traditional stochastic approaches.
• Hardware Design for Non-Stationary Conditions: With VRs playing a significant role, hardware architectures—particularly hybrid analog-digital structures—must adapt to the non-stationary influences for optimized performance and cost-efficiency.
• Near-Field Communication Considerations: Embracing electromagnetic principles, especially regarding spherical wavefronts near large arrays, will be critical in refining both theoretical models and practical engineering solutions for XL-MIMO.

Conclusion

The exploration of non-stationarities in XL-MIMO arrays spearheads a move towards accommodating and capitalizing on the unique attributes of such systems. By embracing the opportunities presented by spatial non-stationarity, future wireless communication systems can achieve unprecedented levels of efficiency and performance. The insights from this paper not only contribute substantially to the theoretical landscape but also guide the design of practical, robust systems capable of handling the challenges of extra-large-scale deployments.