Multi-Antenna Users in Cell-Free Massive MIMO: Stream Allocation and Necessity of Downlink Pilots

2025-05-05

AI Summary

In cell-free massive MIMO systems with multi-antenna users, conventional pilot-free schemes fail due to the lack of channel hardening in the effective channel matrix, severely limiting downlink spectral efficiency (SE). Method: This work first rigorously proves the non-hardening property of the channel matrix in multi-antenna user scenarios, establishing the fundamental necessity of downlink pilots. It then proposes a pilot-based framework jointly optimizing channel estimation, stream allocation, and precoding/combining, and derives a novel closed-form SE expression for correlated Rayleigh fading channels. Contribution/Results: The proposed distributed stream transmission strategy effectively balances fronthaul overhead and SE gain, achieving substantial downlink SE improvement under typical configurations. Systematic performance analysis quantifies the impact of user count, access point (AP) count, and antenna configuration, and explicitly reveals the intrinsic trade-off between SE and fronthaul load.

Technology Category

• Search and Optimization: Distributed Search
• Multiagent Systems: Mechanism Design
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems

Application Category

• Security and Privacy: Large-scale security measurements
• User Modeling, Personalization and Recommendation: Practical large-scale studies of user experience
• Systems and Infrastructure for Web, Mobile and WoT: Data management and stream processing for Web, mobile and wireless applications

Abstract

We consider a cell-free massive multiple-input multiple-output (MIMO) system with multiple antennas on the users and access points (APs). In previous works, the downlink spectral efficiency (SE) has been evaluated using the hardening bound that requires no downlink pilots. This approach works well for single-antenna users. In this paper, we show that much higher SEs can be achieved if downlink pilots are sent when having multi-antenna users. The reason is that the effective channel matrix does not harden. We propose a pilot-based downlink estimation scheme, derive a new SE expression, and show numerically that it yields substantially higher performance when having correlated Rayleigh fading channels. In cases with multi-antenna users, the APs can either transmit the same or different data streams. The latter reduces the fronthaul signaling but comes with a SE loss. We propose precoding and combining schemes for these cases and consider whether channel knowledge is shared between the APs. Finally, we show numerically how the number of users, APs, and the number of antennas on users and APs affect the SE.

Problem

• Evaluating downlink spectral efficiency for multi-antenna users in cell-free MIMO
• Proposing downlink pilot estimation to improve spectral efficiency
• Analyzing stream allocation and precoding for multi-antenna user scenarios

Innovation

• Proposes pilot-based downlink estimation scheme
• Introduces precoding and combining schemes
• Evaluates spectral efficiency with multi-antenna users