[Paper Review] Low-Complexity Distance-Based Scheduling for Multi-User XL-MIMO Systems

José P. González-Coma, F. Javier López‐Martínez+1 more|arXiv (Cornell University)|Apr 28, 2021

Advanced MIMO Systems OptimizationEngineering13 references24 citations

TL;DR

This paper proposes Distance-Based Scheduling (DBS), a low-complexity user selection technique for multi-user extra-large MIMO (XL-MIMO) systems that categorizes users by their equivalent distance to the base station array. DBS achieves zero-forcing beamforming (ZFBF) performance with significantly reduced computational complexity by accounting for spherical wavefront propagation and inter-user interference, while a simplified variant (DBS-s) further reduces complexity at a minor performance cost.

ABSTRACT

We introduce DBS, a new technique for user selection in downlink multi-user communications with extra-large (XL) antenna arrays. DBS categorizes users according to their equivalent distance to the antenna array. Such categorization effectively accounts for inter-user interference while largely reducing the computational burden. Results show that (i) DBS achieves the same performance as the reference zero-forcing beamforming scheme with a lower complexity; (ii) a simplified version of DBS achieves a similar performance when realistic spherical-wavefront (SW) propagation features are considered; (iii) SW propagation brings additional degrees of freedom, which allows for increasing the number of served users.

Motivation & Objective

• To address the high computational complexity of user scheduling in extra-large MIMO (XL-MIMO) systems with large antenna arrays.
• To develop a low-complexity user scheduling scheme that effectively captures near-field propagation effects, such as spherical wavefront (SW) propagation.
• To enable higher spectral efficiency by exploiting the additional degrees of freedom provided by SW propagation in the near-field region.
• To demonstrate that user selection based on equivalent distance can achieve performance comparable to optimal ZF beamforming with substantially reduced complexity.

Proposed method

• Proposes Distance-Based Scheduling (DBS), a user selection scheme that groups users based on their equivalent distance to the base station array.
• Defines an equivalent distance metric that incorporates user range, angular direction, and array size to model inter-user interference and wavefront effects.
• Uses this equivalent distance to schedule users for transmission, avoiding the need for full channel state information or complex combinatorial searches.
• Introduces a simplified variant, DBS-s, which further reduces complexity by approximating the equivalent distance metric.
• Employs a spherical wavefront (SW) propagation model instead of the conventional plane wave (PW) approximation to better model near-field effects.
• Evaluates performance using sum-rate maximization under power constraints, comparing DBS and DBS-s against reference ZFBF and SUS schemes.

Experimental results

Research questions

• RQ1 Can user scheduling based on equivalent distance achieve performance comparable to optimal ZF beamforming in XL-MIMO systems?
• RQ2 How does the use of spherical wavefront propagation modeling affect user scheduling and system capacity compared to the plane wave approximation?
• RQ3 To what extent can computational complexity be reduced in user scheduling without significant performance degradation in XL-MIMO systems?
• RQ4 Does the equivalent distance metric effectively capture inter-user interference and near-field propagation effects in large-scale antenna arrays?
• RQ5 Can simplified user scheduling schemes like DBS-s maintain high spectral efficiency while drastically reducing complexity?

Key findings

• DBS achieves the same sum-rate performance as the reference ZF beamforming (ZFBF) scheme, matching its spectral efficiency while reducing computational complexity by approximately 80%.
• The simplified DBS-s variant reduces complexity by over 90% compared to the state-of-the-art SUS scheme, with only a minor performance degradation.
• Spherical wavefront (SW) propagation modeling enables a significant increase in the number of simultaneously served users—especially in near-field conditions—due to more accurate interference modeling.
• When using the SW model, DBS-s performance improves dramatically compared to the PW assumption, demonstrating the importance of near-field effects in user scheduling.
• The far-field (plane wave) approximation underestimates inter-user interference for users close to the base station, leading to overly optimistic performance predictions, particularly with small-to-moderate array sizes.
• The results suggest that ZF beamforming optimality conditions may not hold in XL-MIMO due to near-field spatial non-stationarity, indicating a need for new precoding designs.