Abstract
The irruption of the cell-free (CF) massive multiple-input multiple-output (MIMO) network topology has meant taking one step further the concept of massive MIMO as a means to provide uniform service in large coverage areas. A key property of massive MIMO networks is channel hardening, by which the channel becomes deterministic when the number of antennas grows large enough relative to the number of serviced users, easing the signal processing and boosting the performance of simple precoders. However, in CF massive MIMO, the fulfillment of this condition depends on several aspects that are not considered in classical massive MIMO systems. In this work, we address the presence of channel hardening in both CF massive MIMO and the recently appeared user-centric (UC) approach, under a spatially correlated Ricean fading channel using distributed and cooperative precoding and combining schemes and different power control strategies for both the downlink (DL) and uplink (UL) segments. We show that the line-of-sight (LOS) component, spatially correlated antennas and UC schemes have an impact on how the channel hardens. In addition, we examine the existent gap between the estimated achievable rate and the true network performance when channel hardening is compromised. Exact closed-form expressions for both the hardening metric and achievable DL/UL rates are given as well.
Highlights
Universal frequency reuse has become the dominant planning strategy in cellular networks to maximize spectral efficiency, a condition imposed by the ever-increasing user data rate demands
In this work CF and UC massive multiple-input multiple-output (MIMO) networks under a spatially correlated Ricean fading channel have been analyzed under different precoding and combining strategies
It has been shown how the newly proposed UC based approaches imply a slight degradation of the channel hardening under the traditional deployment area of 1000 × 1000 m, but this degradation may considerably worsen if the coverage area is small
Summary
Universal frequency reuse has become the dominant planning strategy in cellular networks to maximize spectral efficiency, a condition imposed by the ever-increasing user data rate demands. Only a particular UC scheme was considered Both in [13], [14] an uncorrelated Rayleigh fading channel model was assumed, though practical channels in massive MIMO are correlated [15] and often exhibit a line-of-sight (LOS) component. This is experimentally demonstrated in [16], where it is shown that the commonly used independent and identically distributed (i.i.d.) Rayleigh fading channel turns to be too optimistic in terms of channel hardening in practical scenarios and spatial correlation should be considered for avoiding misleading conclusions in theoretical works. The existence of a gap between the achievable rates and the true network performance derived from the channel hardening ratio is discussed
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