Energy-Efficient Access-Point Sleep-Mode Techniques for Cell-Free mmWave Massive MIMO Networks With Non-Uniform Spatial Traffic Density

Abstract

Cell-free massive multiple-input multiple-output (MIMO) is a novel beyond 5G (B5G) and 6G paradigm that, through the use of a common central processing unit (CPU), coordinates a large number of distributed access points (APs) to coherently serve mobile stations (MSs) on the same time/frequency resource. By exploiting the characteristics of new less-congested millimeter wave (mmWave) frequency bands, these networks can improve the overall system spectral and energy efficiencies by using lowcomplexity hybrid precoders/decoders. For this purpose, the system must be correctly dimensioned to provide the required quality of service (QoS) to MSs under different traffic load conditions. However, only heavy traffic load conditions are usually taken into account when analysing these networks and, thus, many APs might be underutilized during low traffic load periods, leading to an inefficient use of resources and waste of energy. Aiming at the implementation of energy-efficient AP switch on/off strategies, several approaches have been proposed in the literature that only consider rather unrealistic uniform spatial traffic distribution in the whole coverage area. Unlike prior works, this paper proposes energy efficient AP sleep-mode techniques for cell-free mmWave massive MIMO networks that are able to capture the inhomogeneous nature of spatial traffic distribution in realistic wireless networks. The proposed framework considers, analyzes and compares different AP switch ON-OFF (ASO) strategies that, based on the use of goodness-of-fit (GoF) tests, are specifically designed to dynamically turn on/off APs to adapt to both the number and the statistical distribution of MSs in the network. Numerical results show that the use of properly designed GoF-based ASO strategies under a non-uniform spatial traffic distribution can serve to considerably improve the achievable energy efficiency.INDEX TERMS Cell-free massive MIMO, energy efficiency, access-point switch on/off techniques, millimeter-wave communications, goodness-of-fit.

Energy Efficiency of User-Centric, Cell-Free Massive MIMO-OFDM with Instantaneous CSI

“…In Problem (19), the total transmission rate of each UE is replaced by a constraint on the average transmission rate. By constraining the transmission rate on each subcarrier to be greater than the average transmission rate on each subcarrier, the function of the logarithmic sum is replaced by a logarithmic function.…”

The D-OMA Method for Massive Multiple Access in 6G: Performance, Security, and Challenges

“…As a neoteric rate-enhancement milestone, researchers have initiated the utilization of a new radio (NR) high transmission bands for mobile wireless services (within the 24-300 GHz rang) [14,57,58]. Furthermore, the adoption of mmwave with cell-free network has received a recent attention in the literature [19,59,60]. Interestingly, cell-free network were found to provide an efficient solution for the poor scattering nature and high path loss of mmWave bands [59].…”

“…Furthermore, the adoption of mmwave with cell-free network has received a recent attention in the literature [19,59,60]. Interestingly, cell-free network were found to provide an efficient solution for the poor scattering nature and high path loss of mmWave bands [59]. Due to the propagation issues related to mmWave channels such as high pathloss an poor scattering, massive multiple-input multiple-output (mMIMO) systems are usually used in mmWave-supported wireless devices [20,61].…”

“…Along with cell-free network architectures, mmWave transmissions can be used to improve network capacity [19,59]. Interestingly, cell-free networks were found to provide an efficient solution for the poor scattering and high path-loss problem of mmWave transmissions [59].…”

“…Along with cell-free network architectures, mmWave transmissions can be used to improve network capacity [19,59]. Interestingly, cell-free networks were found to provide an efficient solution for the poor scattering and high path-loss problem of mmWave transmissions [59]. Due to the propagation issues related to mmWave channels, massive multipleinput multiple-output (MIMO) systems are usually used for mmWave-supported UEs [61].…”