Cell-Free Massive Massive Multiple-input Multiple-output Systems Research Articles

Pilot Allocation and Power Control in Cell-Free Massive MIMO Systems

In this paper we consider a cell-free massive multiple-input multiple-output (MIMO) system which is composed of a large number of access points (APs) and a much smaller number of users. Each AP obtains the channel estimate from the uplink pilot and apply maximum ratio transmission in the downlink communication. Under the assumptions that the channels between the APs and users are Rician fading and the number of users is larger than that of orthogonal pilots, we derive the closed-form expression for the downlink achievable rate. Then, in order to maximize the achievable rate, we propose pilot allocation and power control algorithms for cell-free massive MIMO. Simulation results show that the proposed algorithms can achieve good system performance.

Secure Communications Over Cell-Free Massive MIMO Networks With Hardware Impairments

This paper investigates the effect of hardware impairments on the physical layer security for a cell-free massive multiple-input multiple-output (MIMO) network in the presence of pilot spoofing attack. By employing a classical additive hardware distortion model, the joint hardware impairment effects brought by both access points (APs) and user equipments have been taken into account, whereas the eavesdropper is assumed to be equipped with perfect hardware. Thereby, we derive a closed-form lower bound for the ergodic secrecy rate in the presence of imperfect channel state information. To obtain further insights, we investigate asymptotic secrecy performance under different hardware scaling factors. It proves that the hardware-quality scaling effect vanishes as the number of APs increases in the considered secure cell-free massive MIMO system with active attack. Furthermore, by using continuous approximation and path-following algorithms, the optimal power allocation scheme is obtained to maximize the achievable secrecy rate. Numerical results validate the derived results and the efficiency of the proposed power allocation scheme.

Cell-Free Massive MIMO: A New Next-Generation Paradigm

Cell-free (CF) massive multiple-input-multiple-output (MIMO) systems have a large number of individually controllable antennas distributed over a wide area for simultaneously serving a small number of user equipments (UEs). This solution has been considered as a promising next-generation technology due to its ability to offer a similar quality of service to all UEs despite its low-complexity signal processing. In this paper, we provide a comprehensive survey of CF massive MIMO systems. To be more specific, the benefit of the so-called channel hardening and the favorable propagation conditions are exploited. Furthermore, we quantify the advantages of CF massive MIMO systems in terms of their energy- and cost-efficiency. Additionally, the signal processing techniques invoked for reducing the fronthaul burden for joint channel estimation and for transmit precoding are analyzed. Finally, the open research challenges in both its deployment and network management are highlighted.

Performance Analysis and Power Control of Cell-Free Massive MIMO Systems With Hardware Impairments

As an interesting network architecture for future wireless communication systems, cell-free (CF) massive multiple-input multiple-output (MIMO) distributes an excess number of access points (APs) with single or multiple antennas to cooperatively communicate with several user equipments (UEs). To realize CF massive MIMO in production, hardware impairments become a crucial problem since cheaper and low-quality antennas are needed to ensure economic and energy feasibility. In this paper, we propose a framework for performance analysis in the CF massive MIMO with classical hardware distortion models. For both uplink and downlink, closed-form spectral and energy efficiency expressions are derived, respectively. Based on these results, we provide significant insights into the practical impact of hardware impairments on CF massive MIMO. For example, the impact of hardware distortion at the APs asymptotically vanishes. Furthermore, in order to ensure uniformly good service to the users, we propose a max–min power control algorithm to maximize the minimum UE rate. Via analytical and numerical results, we prove that CF massive MIMO can tolerate hardware impairments without performance reduction.

Precoding and Power Optimization in Cell-Free Massive MIMO Systems

Cell-free Massive multiple-input multiple-output (MIMO) comprises a large number of distributed low-cost low-power single antenna access points (APs) connected to a network controller. The number of AP antennas is significantly larger than the number of users. The system is not partitioned into cells and each user is served by all APs simultaneously. The simplest linear precoding schemes are conjugate beamforming and zero-forcing. Max-min power control provides equal throughput to all users and is considered in this paper. Surprisingly, under max-min power control, most APs are found to transmit at less than full power. The zero-forcing precoder significantly outperforms conjugate beamforming. For zero-forcing, a near-optimal power control algorithm is developed that is considerably simpler than exact max-min power control. An alternative to cell-free systems is small-cell operation in which each user is served by only one AP for which power optimization algorithms are also developed. Cell-free Massive MIMO is shown to provide five- to ten-fold improvement in 95%-likely per-user throughput over small-cell operation.