Distributed Versus Centralized Precoding in Cell-Free Systems: Impact of Realistic Per-AP Power Limits

In cell-free massive MIMO, centralized precoding is theoretically known to remarkably outperform its distributed counterparts, albeit with high implementation complexity. However, this letter highlights a practical limitation often overlooked: widely used closed-form centralized precoders are typically derived under a sum-power constraint, which often demands unrealistic power allocation that exceeds hardware capabilities. When two simple heuristics (global power scaling and local normalization) are applied to enforce the per-AP instantaneous power constraint, the centralized performance superiority disappears, making distributed precoding a robust option.