Fractional Programming-Based Uplink Transmission Power Allocation for User-Centric Cell-Free Massive MIMO Systems

In this paper, two centralized power allocation schemes are proposed for data transmission during the uplink phase in the user-centric cell-free (CF) massive multiple-input multiple-output (mMIMO) systems. The proposed schemes solve two non-convex power allocation problems of maximizing the summation of spectral efficiency (SE) (max-sum-SE) and that of maximizing the minimum SE (max-min-SE) to improve the overall SE and fairness performance while simultaneously reducing the per-user equipment (UE) transmission power. To solve the max-sum-SE problem, we utilize the fractional programming (FP) method to transform the non-convex problem into a series of convex problems. Furthermore, the max-min-SE problem is solved after reformulating it with the help of the FP method along with the alternating direction method of multipliers (ADMM) technique. The proposed schemes are computationally efficient as they solve the aforementioned problems iteratively by using only closed-form updates for the decision variables, which is one of their strongest features, and suitable for allocating power in large-scale CF mMIMO systems. Numerical results demonstrate that, compared to the no power control scheme, the proposed schemes improve the average SE performance by up to 47% while reducing the average transmission power by up to 95%.