Abstract
In this article, we consider the problem of downlink power allocation in a cell-free massive multiple-input multiple-output (m-MIMO) communication system under spectral efficiency (SE) constraints for the users. From the perspective of green communications, the power allocation is formulated as an optimization problem where the aim is to maximize the sum SE as the objective function, while limiting the transmission power of each access point (AP) and imposing lower and upper bounds on the achievable SEs of different users. The resulting optimization problem is non-convex since the objective function is non-concave and the upper bounding constraints on user SEs are non-convex. To address these difficulties, we first derive a closed-form lower bound on the sum SE (objective function) and prove that it is a quasi-concave function. Then, we relax the unwieldy upper bounding constraints on the user SEs by replacing them with linear functions, which renders the optimization problem convex. An optimal solution to the relaxed problem is finally obtained by solving a sequence of convex feasibility programs. We evaluate the performance of the proposed downlink power allocation scheme through Monte Carlo simulations under the uncorrelated and correlated shadow fading models. The results show that for both models, the proposed algorithm can lead to a significant reduction in total power consumption compared to a benchmark approach, while accurately allocating power to the APs so that the SE constraint of each user is satisfied within the imposed bounds.
Highlights
Cell-free massive multiple-input multiple-output (m-MIMO) systems have been recently proposed as a promising technology for the generation of wireless communication systems [1]
The mutual coupling effect, which is a common issue in collocated m-MIMO systems [3]–[5], can be alleviated since the large-scale array gain is achieved by a large number of separated access point (AP) which are normally equipped with only a small number of antennas
The results show that for both models, the proposed suboptimal algorithm can lead to a significant reduction in total power consumption compared to a benchmark approach, while accurately allocating power to the APs so that the spectral efficiency (SE) constraint of each user is satisfied within the imposed bounds
Summary
Cell-free massive multiple-input multiple-output (m-MIMO) systems have been recently proposed as a promising technology for the generation of wireless communication systems [1]. Considering that the per user downlink SE is expected to reach 30 bits/s/Hz in fifth generation (5G) wireless networks [13], the limited backhaul capacity represents a significant bottleneck for distributed applications To address this issue in cell-free m-MIMO systems, the problem of optimal power allocation under the constraint of limited-capacity backhaul was formulated and solved using different objectives, including SE and EE maximization, in [14]–[19]. Motivated by the above considerations, we address in this work the problem of downlink power allocation for cellfree m-MIMO systems in which the achievable user SEs are lower and upper bounded This optimization problem can be labeled as a ‘‘green power allocation’’ for these systems since its solution leads to a total power consumption that is proportional to the users’ data rate requirements. The greatest (resp. smallest) integer less (resp. greater) than or equal to x is denoted by x (resp. x )
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