On Fairness Degradation in Cell-Free Multi-Stream Massive MIMO With Mobile-Grade Antennas

Abstract

For the commercial deployment of cell-free (CF) massive MIMO, mobile-grade antennas are envisioned for use in low-cost, low-complexity access points. It is essential to consider the practical constraints of these mobile-grade components when comparing the performance of CF and cellular networks. In this context, this paper focuses on the following main aspects: 1) we consider a generalized CF system with multiple APs and UEs, each equipped with multiple antennas and supporting multiple data streams per user. We exploit mobile-grade antennas with lower per-antenna transmit power constraints and compare their performance with conventional cellular networks, which are typically less constrained in this regard; and 2) we study a novel optimization problem aimed at achieving performance fairness in the presence of multiple data streams per user. To solve this problem, we derive convex formulations for the number of data streams, power allocations, and fairness weights. Based on these formulations, we propose a multistep alternating optimization (mAO) algorithm. To reduce the computational complexity of the mAO algorithm, we also propose a second algorithm based on closed-form solutions, which does not require convex solvers. Given the same total power budget in CF and cellular systems, the numerical analysis highlights the impact of mobile-grade antennas and shows that the fairness difference between cellular and CF is not as significant as portrayed in idealized scenarios. Specifically, we show that to achieve a sufficient fairness gap, CF requires up to 20 times more print-quality transmit antennas, i.e., radio stripes, than cellular systems. To assist future comparative studies, we also provide a computational complexity analysis of the proposed solutions.